Lean Burn Jetting LM4

[wirespokes]

The patient - 1985 LM4. Asleep since 1991 with 4K on the clock. Never modified, even still had the original foam grips.

Two thousand miles ago I brought it back to life and have been working through various issues. At first it was getting 35mpg - pretty poor considering my 87LM gets 45 most of the time, and has even seen 50 once.

I've found and corrected poor connections in the ignition circuit and plug leads. It's running the Millich jet recipe. Consumption hovers around 40mpg. It runs well, but know it's running rich. It requires choke at cold start up, but comes off pretty quickly. Most recently I changed the needle jet from a 265 down to 260. It ran well, but off-the-line sucked. Had a pucker moment crossing traffic when the engine practically stalled,in the middle of the lane with a car bearing down on me, before taking off. Changed back to the 265 and then checked the fuel pump priming thinking they might not be working as they should. Found one of the blanking plugs had a deteriorated o-ring. Maybe that was it? Who knows.

So in doing some research, a five year old thread from WG came up about Lean Burn carbs and dealing with them. I read through the whole thread, and it was all about the 850 LMIII with the 36mm Dells. I've had the carbs apart cleaning them when I first got the bike and had noticed the odd brass bit poking into the venturi. Took another look today, and sure enough, they're lean burn carbs. But they came with the standard US jetting.

So before I launch into the whole rejetting experience, I thought I'd check with you guys. Have any of you rejetted these Lean Burns and gotten better mileage?

Another question - since all I've seen so far is a discussion on 36mm lean burns - what was the difference in jetting between Lean Burn vs not? Perhaps I could downsize all of my jets appropriately.

I'm looking into purchasing an oxygen sensor to hook up to a volt meter as a way to sort this all out. That seems like an inexpensive way to go. My headers have sensor bungs (off of an 1100 Cali) so wouldn't need a heated sensor. A heated sensor would be nice for those bikes that don't have a bung requiring sniffing at the muffler. Up till now i've managed to get close enough and bring consumption into the mid to high 40s with other bikes I've worked on, but this time I'd rather short circuit the process if possible. I'm no expert at jetting, and besides I don't have a full set of jets to play with.

Does the lean burn require leaner jets all through the range or just at idle? i know this has been discussed heavily in the past, so if there's a thread detailing solutions for the 40mm PHM ND/NS lean burn carbs, point me there.

[pressureangle]

First thing, verify that the rubber in the chokes isn't deteriorated and leaking fuel while closed.

[wirespokes]

I had the chokes apart when I cleaned the carbs last year. I'll take a look, but you know, I don't ever recall Dells having any rubber seals in there - just a paper gasket on the mounting surface. I made sure of free play so they are returning to the rest position.

[moto-uno]

 The bottom of the brass choke plunger has a rubber seal built into it , noticeable if you turn the plunger upside down .
 My 990cc Le Mans2 has the 40 mm dellortos and it's run and started well with the #60 idle jet , K5 needle in the richest
position (lowest clip notch) , #60 slide and 265 needle jet and 155 main jet .It's got K&N air filters (larger oval units , not
the conical ones , too restrictive ) and a BUB exhaust system . Just recently installed 266 needle jets and passing power
is improved . I don't know how relevant this may be to the later Le Mans . YMMV as they say !  Peter

[Cdn850T5NT]

Hey this may not be relevant - but I want to provide this account of the 30mm PFH DellOrto's that are OEM  / stock on my 1985 850 T5.  I might add that they are Euro-jetted, as my T5 is a Eurospec bike.  My Dell's had a factory flaw.

The idle mixture outlet exits the downstream throat of the carb at a battered angle relative to the centreline of the throat.  This drilling is such that the idle mixture screw's steep conical section - can actually be seen when looking at the outlet at (obviously) the same battered angle as the outlet... i.e. you are looking down the bore of the outlet.  Well, on one of the carbs - indeed - you could see the idle mixture screw's cone-portion / seat (if memory serves)...  Well, on the other carb, all I could see was shank-portion of its mixture screw.  So - the drilling / idle mixture outlet was mal-positioned (laterally) in the bore.  The net effect was that the shank of the idle mixture screw, in part, along with the conical portion of the mixture screw (at the end of the shank-portion) both were contributing to the mixture setting.  So, opening the mixture screw the standard 1.5 turns - applied to the correctly drilled carb - but not so to the incorrectly drilled carb.

What I did was to chuck the mixture screw at its threaded portion (in copper foil) - into a regular 3/8" drill, and I offered-up the shank to a fine bench grinder stone (while spinning it with the power drill) thereby turning-down the shank.  If memory serves I used a forked piece of wood as a steady-rest.  So now my 1.5 turns open idle mixture screw is close to what it should be... as fuel now properly flows along the narrowed shank of the mixture screw relatively unimpeded (and the cone and seat, mostly, control the mixture).

Now, is this relevant?  All I am saying is that DellOrto of that same era (1984, 1985) screwed up one my carburetors.  It took doing a detailed side-by-side inspection of one carb versus the other to find this.  I might like to suggest you do similarly, to see if they screwed up one or both of your carbs, in some fashion.

[wirespokes]

moto-uno  - I removed the choke and saw the rubber seal. Not sure how to check if it seals or not, but it looked ok. Is the only way to check involve replacing them?

I think your jetting formula is similar to the Milich recipe, which makes sense because that's what you're running. My 87LM4 has the Milich recipe and get's decent mileage, so set this one up the same, but gets crappy mileage. So something else is going on. I even changed the rear caliper because if felt like it was sticking (which could reduce mileage) a little now and then. I could have rebuilt it, but instead put on a spare.

Cdn850 - Those sort of faults can be tricky to find. it requires looking closely at everything. It also requires knowing how it's supposed to be, otherwise if there's no obvious damage like ragged metal, scrapes, gouges or missing bits, you wouldn't know anything's wrong.

Unfortunately this one isn't that easy. I'm pretty sure I've stumbled onto the source of the poor mileage - it's a LEAN BURN carb so needs different jetting. In my searching I found no reference to 40mm lean burn carbs, only the LMIII 36mm Dells.

That also raises this question - if the lean burn shroud noticeably helps fuel consumption, why aren't all the newer ones the same? Why did they discontinue the shroud? My 87 doesn't have them, but the 85 does. ???   

[SED]

I struggled with the jetting on my LMIII 36s for loooong time.  It was a US bike, but had Euro jetting with mismatched atomizers and super rich mains. Knowing what I know now I would do the following.  (most of which you've probably done)

1. Re-jet everything back to stock.  For the LMIII, Guzziology recommended Euro jetting which was already mostly installed.  This was not simple as there are 3 different variants of the Euro jetting with 262, 265 and 268 atomizers.  If there are multiple factory specs, jet to the richest.   If there is a "tweener" idle jet (specs call for a 52, but only 50 and 55 are available) go with the richer version.  I also replaced parts with new jets, atomizers, needles, floats and float needles and new stock slides and new rubber manifold sleeves.

2. Make sure the float level is correct and matches side to side.

3. Refit the stock airbox and filter.

4. Retime the engine to stock (or just 1-2 degrees less) and make sure both cylinders are in time.  Replace the advance springs if needed.

5. If you've got the '80s plastic twist grip, throw it away!  (carb balance changes every time you open the throttle because the plastic deforms).

6. Clamp the carbs side by side in a vice with a pan to catch gas and check that both accelerator pumps spray for the same time.

7. Start the bike, set the idle mix and balance the carbs.

8. With the engine idling, screw in 1 idle speed screw so that you raise the idle on one carb.  If the engine starts to miss the mix is too lean.  Return that carb to idle and check the other side.  I found that one side was way leaner than the other.  My solution was to increase the size of the idle jet so that there is a 55 on the right and a 60 on the left (52 is stock by the specs.).  It is also likely that raising the needle 1 notch may solve the problem on the lean side but didn't try that.

9. I also raised the needles 1 notch and have been running 1 size larger on the main jet.  (ethanol burns slightly leaner) 

It is step 8 - raising the idle speed side to side - that revealed the lean transition.  In my experience the accelerator pump does not make up for a lean transition.

Raising the needles 1 notch just brightened up the acceleration at cruising speed. 

Gas mileage is better (from about 40 to mid 40s) and power is better with it richer.  Plugs look better.  It still should be dyno'd with a gas analyzer but it's running better than it ever has.  As they say, ymmv

Let us know what you find.

[SED]

Unfortunately this one isn't that easy. I'm pretty sure I've stumbled onto the source of the poor mileage - it's a LEAN BURN carb so needs different jetting. In my searching I found no reference to 40mm lean burn carbs, only the LMIII 36mm Dells.

That also raises this question - if the lean burn shroud noticeably helps fuel consumption, why aren't all the newer ones the same? Why did they discontinue the shroud? My 87 doesn't have them, but the 85 does. ???   

I thought all later 36s and 40s were lean burn.  Guzziology has both Euro and USA jetting specs - do you have them?  Any way to find out if they both had shrouds?

[Cdn850T5NT]

SED - Very comprehensive and instructive. Thx!

[SED]

SED - Very comprehensive and instructive. Thx!

Thank you - hope it helps.    It seems we need to know when the shrouded needle jet (lean burn) was fitted.

[wirespokes]

I have no data whether all carbs from the LMIII and newer are lean burn. I'm pretty certain the 87 isn't lean burn, but now i'd better check. Since it's running the same jetting (Milich) but getting 45+ mpg, I have to assume it's not lean burn. What I've read says that jetted rich, the lean burn carbs get poor mileage. I'm guessing rich could be just the stock jets for non-lean burn bikes.

SED - how do you accurately check fuel level? I'm considering making a custom float bowl retainer nut with an embedded bit of brass tube protruding from the inside all the way out - long enough to attach some clear tubing. Then I'd know for sure what the level is. Last time I checked it seemed to be in range.

Thanks for the tip on determining a lean idle condition. Good trick!

The carbs are well synched and run very smooth. No bar-end weights needed - the bike didn't have any when I got it and haven't felt the need to replace. No complaints the way it's running, just uses more fuel than it should. Not only does it cost more to run it, the extra fuel washes down the cylinder walls resulting in a worn out cylinder sooner.

Yes, Guzziology has the lean burn jettings for the 36mm Dells, but doesn't label as such. It's just one of the variations of LMIII jetting. For the LM4, it's just got the standard US and euro jets. In the whole carburetor section there was no mention of lean burn carbs.

I think my plan of attack will be to use the LMIII jetting as a baseline and factor in the 12% increase in displacement. That brings the idle jet to 57, main to 128 and then I could try the 260 which was the standard needle jet in the first place. I'm really starting to think the accelerator pumps weren't working very well yesterday and that's why the hesitation off the line. When I checked the pumps a week or two ago, they seemed a bit weak.

[moto-uno]

If the float needle and seat are good , then a caliper is all that's needed to check the float level ( carb throat perpendicular to the ground ) and measure from gasket surface to the furthest edge of float and compare to specs . Checking your jetting ideas , mains seem rather low ( do you have the stock exhaust and air filter ) . If you're going to use that rather small needle jet (atomizer ) , what jet needle number are you using with it ? I don't know what slide # you have , but keep in mind that some use a much later accelerator pump starting point , there's a number on the slide that indicates the accelerator ramp . Also the slide #cutaway influence low speed throttle response . If you'd like I have an original dellorto manual and can email you a photocopy of the relevant parts and specs .  Peter

[SED]

Accelerator pump made no difference off the line for me - only roll-on from about 1/4 throttle.  Some guys disable accelerator pumps and still get good performance.  Better performance off the line was richer idle jet (idle speed trick), 268 atomizers and raising the needle a notch.  And replacing ignition advance springs that were trashed. 
 
If you have the K19 needle run the 268 atomizer.  USA specs say K33 needle with 260, but 260 sounds lean.  It is possible that Guzzi was trying to meet EPA tests that were done at constant rpm and steady throttle so leaned out the midrange (atomizer) beyond best drivability.  For comparison USA V65SPs were fitted with 30mm PHPB carbs with 265 or 268 atomizers and a primitive accelerator pump.  Guzziology recommended scrapping the accel pump and drilling the atomizer to 271 (#36 drill bit) - WAY BIG!  It runs brilliantly and gets 50+mpg.   

My LMIII was down on power and getting crappy MPG - even the plugs looked too dark.  Leaning the mix just made drivability worse and never improved mpg.  Richening the mix (268 atomizer, raised needle, larger idle jet) the engine actually ran hotter (I could feel it on my knees) - apparently making more power more efficiently.  The plugs got hotter and look leaner after richening the mix. 

BTW I think the overly rich "runs like a munter" complaint was when LM1 main jets were used in LMIII lean burn carbs.  Don't think it referred to changing atomizers.

Float setting is factory specs.  I've installed brown, black and white floats (and new needles and seats) and none of it made much of a difference.  Just lay the carb on its back on the bench to measure the float like Moto-Uno says and it is close enough.  Some floats have seams so ignore those, but it shouldn't make a difference.  One tip I've heard is that you know your float level and idle setting is close if you can turn off the taps and the idle speed does not change even as it uses up the gas in the float bowls - it then just dies.

[wirespokes]

Thanks Moto and SED. You've reminded me I did check the float level, but had a question when doing it. I measured 18mm with the float needle compressed, not when lightly touching. To get that measurement I already had to raise one of them a ways. It looked like the floats were angling up pretty good, and since I know it's running rich (black exhaust, low mpg) left it at that. Maybe I need to revisit and set them lightly touching the float needles, and adjust everything from there.

The bike only had 4K on the clock and I don't get the idea anything was altered, except for scratches and scrapes. It even had all three throttle springs - MAN, it took superman to work that throttle! The old foam grip would slip on the tube allowing the throttle to close unless gripped even tighter! LOL

How could a factory ever allow such a monstrosity out its doors??? I would have had second thoughts (or seven or eight) buying something like that new. It must have been the Italian mystique that sold these bikes back then.

Ok - here's some hard data:

The bike came with -
1. K33 Needle
2. 145 Main
3. 62 Idle
4. 50/3 Slide
5. 260 Atomizer

I immediately replaced all of those with Ed's formula, since the 87 LM runs well with this combo:
1. K4 needle
2. 128 main
3. 68 idle
4. 50/3 (left it the same) - my 87 has the 60/5
5. 265 atomizer (needle jet)

I have since reverted to the 62 idle jet and it runs great.

Returning to the 260 needle jet worked well, except for off-the-line performance. But now I'm thinking the float level might be to blame for that.

SED - your experience flies in the face of all logic. How can larger jets DECREASE consumption? Obviously there's more going on here we don't know about. I'm curious what an air/fuel meter  would have shown.

Thanks for the tip on turning off the gas and idle speed not changing. With the 87 when coming home, there's a spot a mile away I turn off the petcocks (while riding) and can make it home before the engine dies just as I get there. I haven't noticed the idle speed changing on that one.

With the 85, I run out of gas maybe three blocks away, so obviously don't go as far per bowl. But I don't recall idle speed changing. I'll keep that in mind. What's been your experience using that data?

The bike is running a K&N filter. It's something custom I put together. I found a couple of filters at a yard sale and they must have been off of a car or something. They're each much too large for one carb, but one is perfect spanning the two. The curved plastic tubes leading from the airbox to the carbs I kept and ran them into each end of the filter. One fit without issue into a hole cut in one end. The other didn't fit as well but was close. Some hot glue fixed that. The filter spans the distance between the carbs and fits well. It even looks good.

I am a little concerned the mufflers could be part of the issue. They're Keihan LaFranconi copies. I've ridden quiet bikes (stealth mode, under the radar...) since the 80s, so these sounded ultra loud. Everyone I've asked assures me they're ok - not Harley loud. But still, they're a straight-through design and they're not quiet. The bubs, also a straight-through design are much quieter.

The 87 is running Bubs (took a while figuring out the trick to packing the things) and the oval K&Ns. It's getting mileage right around 45 consistently in comparison.

Thanks for the offer of the Dellorto manual moto, but I do have it around here somewhere. Often it's easier calling it up on line.

[SED]

SED - your experience flies in the face of all logic. How can larger jets DECREASE consumption? Obviously there's more going on here we don't know about. I'm curious what an air/fuel meter  would have shown.

Not if the logic is that it is running so lean on transition that you need to open the throttle more to maintain speed.  (i.e., metering fuel through main atomizer rather than transition port and idle jet.)  I've not had it on a gas analyzer, but it runs stronger at lower throttle openings, gets off the line better and is more tractable in town. It also feels like more heat is coming from the cylinder heads and not in the exhaust. Raising the idle speed screws one side at a time should let you know if the transition is too lean.

One thing I've learned with the PHFs and the Mikuni on the bacon slider is that the transition from idle jet, transition port (under slide, but fuel still comes through idle jet) to the main atomizer is controlled by the position of the needle in addition to the cutaway and idle jet.  The higher the needle the sooner the engine transitions from the transition port to the atomizer.  Before the start of the taper clears the top of the atomizer the engine is running on fuel from the transition port and the little bit that gets between the strait part of the needle and the atomizer.  The Dellorto manual says this, but it didn't interpret it that way until recently.

The Dellorto manual describes setting the float level with the float lightly touching the spring pin on the end of the float needle.  It's why they have you put the carb on its back rather than upside down on the bench.  If the float level was set with the spring pin compressed it is likely shutting off the fuel much too early so fuel level is too low.  It would explain running out of fuel earlier than the '87 and may explain the lean transition.

It looks like your close to solving it!   

[moto-uno]

  ^ +1 . As of yesterday , I've became even more pleased with my Le Mans running , I placed a washer (probably from some H-D dynojet kit) under the K-5 needles I'm running, and it's drive-ability is even better . The washers are the same thickness as the jet needle grooves , and to be clear , this is with these needles in their richest position . This better running from what are richer mixtures, is not as of yet reducing my mileage .  Keep in mind that the better performing motor needs less throttle to produce the necessary power to maintain the road speed you drive at .

[Bazil]

Great discussion.
My mark iv has always had terrible fuel consumption. It has lafranconi pipes and did have Lynx Ramflo pod filters, and runs the milch recipe
., with K4 needle and AB265 atomisers.
I just ditched the Ramflo filters, for a couple of reasons - I had to replace a split rubber manifold which I think was partially caused by the weight of the filter, I was concerned with how well they filtered and I suspected it contributed to poor fuel consumption. My guess was that the pods flowed more air and therefore dragged more fuel into the head than could be fully burnt (dual plugging would probably fix that). What do you think?
Anyway, went back to stock air box and fuel consumption has improved. Still need to play with jetting a bit, as it seems a bit rich, and has a flat spot at 3750 rpm. Accelerator pumps are disabled, and the flat spot may disappear if reconnected.
Still seems to use more fuel than I would expect, so the search continues...
Bazil

[moto-uno]

  This flat spot you mention , is it while gently accelerating or at heavy throttle acceleration. Easy throttle application kind of driving doesn't require accelerator pumps . I've gone through a couple of pairs of LaFranconis over the years and loved the sound , they weren't as loud as the Bub pipes . Usually , a restrictive air filter ( very dirty ) leads to a richer
running motor ( if it was jetted correctly in the beginning) . Dual plugging is a matter for another discussion .

[wirespokes]

I've got several dual plugged airheads that came to me that way. Not sure I'd go that route if trying to stop pinging. There are other solutions, but they do run well. Like any modified bike I've gotten, it took a bit of effort sorting them out - ignition and carb adjustments mostly.

I'm used to the Bings where all that's necessary to check or adjust float level is remove the bowl, hold the float up against the needle, turn on the fuel and notice where it starts to flow and where it stops. Pay attention mostly to where it stops, although it's logical they should be the same. The casting line on the float should be parallel to the bottom of the carb. I'm thinking that might be one way to do it with the Dells - fuel flows or stops at 18mm.

There's another method, and this is what I did before, clamp the inlet fuel line, remove the bowl and measure the amount of fuel it contained. It's supposed to be 55ccs. It's difficult catching all the gas with the transmission in the way.

When I did it today, I set them at the point where the needle valve just closes at 18mm. With a section of new fuel line i blew into the inlet and noticed where the flow was blocked. Adjusted that to 18mm. We'll see how that works out. One of them was off by a bit, the other was at about 20mm. So the level was low.

I think the accelerator pumps on our bikes don't activate until past 1/8 or 1/4 throttle. I'm still looking for that data. If that's the case, then my issue is a lean idle situation, nothing to do with the pumps, and nothing they could do to help. When I gave it a lot of throttle, the engine would then rev which kind of suggests the pumps are working just fine.

You know, I had a feeling the needle has some affect on low-end revs, but the charts don't say that. Thanks for bringing that up SED.And thanks for explaining your situation - that does make sense.

Moto - agreed, an engine runs best when it's getting the correct amount of fuel; not too much or too little.

Bazil - what's your consumption? (I'll probably have to convert from metric for it to mean anything to me) Both of my LM1000 had crappy mileage when I got them - mid thirties. Like 10.6 kpl. I've left my pumps connected on the 87 and it gets 45mpg. I got it with the bubs, and don't think they'd ever been repacked. Let tell you, the were  L O U D !!!! Really loud. After the packing rapidly blew out in six or nine months even though correctly packed, I looked for another solution. It involves stainless steel wool, a fine screen, and some dense kiln insulation close to the outer edge. I think that also helped consumption. Mine has never bogged at 3700 with the Milich recipe. Actually, come to think of it, I had to play with the jetting, and it's running either 60s or 62 idle jets.

After a while I lose track and have to refer to the notes.

Yeah, Moto - the bubs when they needed packing were much much louder than the LaFranconis. Now, with good packing, they're a lot quieter.

Bazil - I'm curious - are your carbs the lean burn variety?

[Bazil]

Hi Guys;

my carbs are not lean burn ( ie - dont have the brass "shroud" in the airflow ( dont think we got them in Australia)

The current flat spot is noticeable when just gently rolling on the throttle, as well as when running it higher up the rev range. ( I did have a hunch that pumpers might not be the cause). So that means back to jetting.

Fuel consumption - just put in 20 litres at 260 km, so about  13 km per litre. That's a bit worse than recently, but I have been playing in the shed  - probably lost a bit while tuning. Recently it's been getting 270 / 275 k to reserve, which I'm not too unhappy with. When I started, I was getting 220 to 240 km to reserve.

I like the idea of measuring volume of fuel in the bowls - must try that.

I'll continue playing, but I'm in no hurry - it's riding season down here!

Baz

[wirespokes]

That's awful, Baz! That converts to 30.5 mpg!

Since it's not a lean burn, which I believe is the same as my 87, with the Milich jetting it should be at least 40 if not 45mpg.

Has it always been that bad? Something must be really wrong to be using that much gas.

The closer the ignition and carbs are synchronized the more they complement each other. As they go out of synch they work against each other.

Maybe a brake is dragging?

Perhaps the coils aren't getting full voltage? Doesn't take much of a drop in voltage for the spark to get weak.

i could understand it if you're hauling a 400 lb pillion - otherwise there's something really wrong. 

[moto-uno]

  As a point of reference my much modified and enlarged Le Mans 2 usually gives 18 Kilometers per litre , frequently more  .
It tends to drop after 120 kph .  Peter

[wirespokes]

18K/L works out to a little over 42mpg - that's much better, but think it could possibly do better.

[wirespokes]

It's 50 today, 10 to the rest of you, and sort of misty. Good morning for a test ride. There's a pretty good hill six miles away and that's where I went.

With the float changes I revisited the 260 needle jets. No other changes.

It ran a little bit poorer at start up, choke required, of course. However it was rideable fairly quickly. It did run poorly, didn't want to accelerate very well, sputtered and popped, but that's fairly normal for a cold engine running without choke engaged. The choke as it is, is either on (while I hold it) or off. So that was a good sign actually. 

After a few miles acceleration was fine though it seemed to require more twist than before. Heading up the steep hill, I passed a slow car and the pick-up left nothing to complain about.

The next thing I'll do is experiment with the #62 idle jet. By my crude calculation (based on the LMIII lean burn jetting) I ought to be able to run a 58, so I'll get a selection of jets - 55, 58 and 60 and see what that gets me. Considering how quickly I was able to ride off cold without the choke, I'm thinking it's still rich at idle. And most of my riding is no more than 1/4 or 1/3 throttle.

[Furbo]

It's 50 today, 10 to the rest of you, and sort of misty. Good morning for a test ride. There's a pretty good hill six miles away and that's where I went.

With the float changes I revisited the 260 needle jets. No other changes.

It ran a little bit poorer at start up, choke required, of course. However it was rideable fairly quickly. It did run poorly, didn't want to accelerate very well, sputtered and popped, but that's fairly normal for a cold engine running without choke engaged. The choke as it is, is either on (while I hold it) or off. So that was a good sign actually. 

After a few miles acceleration was fine though it seemed to require more twist than before. Heading up the steep hill, I passed a slow car and the pick-up left nothing to complain about.

The next thing I'll do is experiment with the #62 idle jet. By my crude calculation (based on the LMIII lean burn jetting) I ought to be able to run a 58, so I'll get a selection of jets - 55, 58 and 60 and see what that gets me. Considering how quickly I was able to ride off cold without the choke, I'm thinking it's still rich at idle. And most of my riding is no more than 1/4 or 1/3 throttle.

WS,

to be clear, this is a LM IV, 1000cc w/Del 40's yes?

3 things:

- spend some $$ on real pod airfilters. I always prefered Unifilters to K&N's but that's just me.

- If you have not done so already, replace the diaphrams in the access pumps. These get old & stiff and don't function well

- I like Ed, but 128 main seems way too small for that application.

[SED]

The next thing I'll do is experiment with the #62 idle jet. By my crude calculation (based on the LMIII lean burn jetting) I ought to be able to run a 58, so I'll get a selection of jets - 55, 58 and 60 and see what that gets me. Considering how quickly I was able to ride off cold without the choke, I'm thinking it's still rich at idle. And most of my riding is no more than 1/4 or 1/3 throttle.

This is where my aha! moment with raising the idle speed occurred.  You can set the idle mix to perfect with a jet that is too small by opening up the mixture screw and too large by closing it down. 

Say you have a 55 idle jet and the mixture screw is out 2.5 turns and it idles great all day.  But then you go to accelerate off the line or pull away on a cold engine and it stumbles or pops through the carb.  Then go and raise the idle stop screw on one carb and you find it stumbles there too.  The solution (for me) was to put in a richer idle jet (60) set the idle mixture screw to 1.5 turns and no more stumble when raising the idle stop screw or off the line.  This happened because not enough fuel was coming from the transition port under the slide.

My perception is there is a balance between the size of the idle jet and the size of the atomizer and the height of the cutaway.  If you have a small atomizer, you can compensate a little by using a larger idle jet. 

BTW - the top of the needle, the straight part, is 2.5mm rod.  Dellorto jets are measured in 1/100s of a mm so a 260 atomizer has a 2.6mm hole through it.  A 268 has a 2.68mm hole through it.  (a 62 idle jet has a 0.62mm hole)  You can calculate the passage for the fuel  (before the taper clears the top of the atomizer) by calculating the x-sectional area of the hole and subtracting x-sectional area of the needle.  Comparing the fuel passage area between 260, 265 and 268 will give you some idea of how much more fuel is likely to flow in the larger passage (ignoring boundary effects).  FYI the stock Dellorto on the 18 hp bacon slider has a 270 atomizer and needle is still 2.5mm dia.

The lean burn PHF36s on the 850 LMIII have been most tractable with 268 atomizers.  You can likely get away with smaller atomizers because of the 50 cutaway. 

I'm curious how yours will run with the smaller idle jets - my guess is that it will stumble more off the line.  Have you tried it with just the 265 atomizers and correct float level?  Maybe I missed it 

Good Luck!

[wirespokes]

Furbo - yes, this is a 1985 LM IV. 950cc, 40mm Dells. Keihan LaFranconi copy mufflers.

Thank you on the tip about the pump diaphrams. I'll be sure to check on them.

I know what you mean about the 128 mains. With the airheads we've found that 130 or 135 was better than 150 or 160. So 128 isn't all that different. Also, consider this - the lean burn 36 Dellorto on the LM3 came with a 115 main jet.

SED - I put about 50 highway miles on the bike this afternoon and it runs great. The bike originally came with 260 needle jets and I put them back in. No, I didn't run the bike with the 265s after setting the floats. Two changes at once - a no no, i know. Originally the bike came with 260s but had a K33 needle. It now has a richer needle - K4, so should actually be richer than original. A lot of the time traffic was moving along at 70 and 75, sometimes more, and the bike felt plenty strong.

The idle and mixture needed adjusting (died instead of idling) from the changes, but had plenty of pickup during the ride and ran well otherwise. Keeping the throttle open a little at stop lights kept it idling without issue.

For now I'll run it as is through the next tankful - though I know already it's doing better. It's never made it to 200 miles before reserve. When I got home today I noticed mileage at 202 miles and not on reserve yet. Funny thing is, while adjusting the mixture and idle speed it was acting weird - I had to run the mixture screws out from where I'd just set them and it wasn't sounding right. It had just run out of gas and flipping to reserve brought everything back to normal. What timing, huh?

After tjhat I tried the test of letting it idle with fuel turned off. The idle never rose, but toward the end started to sound weaker. It idled a really long time on just what was in the bowls! Got tired of it when it started sounding weaker and called it good.

[moto-uno]

  Could someone explain what this lean burn carb business is ? Is it something other than the same carbs just very leanly ( is that a word?) jetted for US emissions ?  Peter

[wirespokes]

There's a WG thread from five or six years back that has some photos. I'll see if I can find it.

Essentially, the only difference, at least as far as I know, is there's a brass shroud around the needle jet that extends maybe 1/2" into the venturi.

Go down to post number 14 in this thread - the second bunch of photos. It shows pretty clearly.

https://wildguzzi.com/forum/index.php?topic=69557.0

[moto-uno]

  Thanks for the direction "wirespokes ". Appears vaguely familiar , possibly from Japanese carbs in the 70's or 80's .  Peter