Need help with Dellorto phm 40 carbs

[JesperN]

Hi. I have a question about float height on Dellortos. I'm reading a carb manual that claims that the float height affects whether the engine runs lean or rich. This seems strange to me. If the float height is too low, the engine could stall as it would run out of fuel. If its too high the carbs can overflow. But it is the jets and needles that decide whether the engine is rich or lean, right? Float height should not affect this, or?

My Lemans 4 (-87) runs really uneven around 4 000 rpm, I have tried all combinations of jets with little or no changes. Therefore, I suspect that something else is wrong. But my floaters are a bit worn and bent. Thinking of buying a new plastic pair. (7450.1) but they are pretty expensive.

[wirespokes]

Welcome to the forum, Jesper.

What you're asking isn't specific to Dellortos. The same principle that allows an airplane to fly is the reason a carburetor works. The basic idea is that when air speeds up, pressure goes down. The air traveling over the top of a wing is forced to travel farther and compressed into less space, so it travels faster. Because of that, the pressure on the top side of the wing is less than the underside. Nature wants to equalize, so the air under the wing pushes up.

In a carburetor, the main passage through the middle is restricted - closed down by a few millimetres - which reduces the area for the air to pass through. The same amount of air that entered the carb has to make it through the restriction, so speeds up. Due to that, pressure goes down.

The next thing to understand is that the float bowl, and the gas in it, is open to atmosphere. It's affected by normal atmospheric pressure. The area where the jets exit in the Venturi has less pressure.

Atmospheric pressure PUSHES the fuel up the jets in an attempt to equalize the difference. It's not that the gas is sucked up. But either way, if the fuel level is high, then it's easier to push the fuel up through the jet, so you'll get more. If the level is low, it's harder, so there will be less fuel. It's as simple as that.

I've gone through the jetting issues with my 85 and 87 LM but didn't have any issues at 4K. What jetting combination are you running? Do you have the data how to set the floats?

[Groover]

Here a few links to further understand carbs, parts. and their functions:

http://www.ducatimeccanica.com/dellorto_guide/dellorto.html

https://www.fix.com/blog/motorcycle-carburetor-float-height-adjustment/

Have fun!

I would check the low end needle, maybe tweak that, in or out depends, though maybe out a 1/2 turn from what it sounds. You'll have to clarify the symptoms at the range you mention.

[Antietam Classic Cycle]

Thinking of buying a new plastic pair. (7450.1) but they are pretty expensive.

If those floats are the white ones, then save your money, they're crap.

[guzzisteve]

In my experience w/PHF or PHM's it's the float needle that lets fuel in. Make sure it's large enough or it works too much and wears the pin & spring.
The new white floats are junk, just letting you know. You can adjust the floats to be perfect again if they have metal arms. White ones are plastic and leak.

Tuning info
http://www.thisoldtractor.com/dellorto_a_guide_1-1_to_the_choice-_setting_and_use_of_tapered-needle_motorcycle_carburetors_--.html

[JesperN]

Thanks for good info. I'll stay away from those white plastic floaters. Regarding my jetting: I have tried the recommended combo with 50/3 slides, 128 main jet k4 needles, AB 265 atomisers and I think it was 65 idle jets. My Lemans 4 ran like crap with this kit.

My bike has an open exhaust, stainless with 45 mm outer diameter, cross under the gearbox and short stainless mufflers with an inner diameter of 52 mm, no cones or db killers just straight through. The heads are mildly ported and the valves were renovated last winter, they seal perfectly now. I have separate oval Kehin air filters. Ignition is Lucas-Rita. I have problems setting the timing, I have no marks left on the flywheel, they disappeared when making it lighter in the mill..

My problems started after the valves were renovated, but the stainless mufflers were added att the same time. At the moment it has the original k 19 needles, AB 268 atomisers, 145 main jets and 60 idles. One of the floater needles broke some years back, I changed both of them and the seats, but I'm can't remember what size I bought. I'll check this weekend. Any suggestions on what size on seats to use?

I think the bike is running lean around 4000 rpm, not rich. I have ordered AB 270 atomisers and will try bigger idles, 70:ies this weekend. I'll also check how open the throttle is when the problem starts, and check the valves play. Thanks again folks, good answers. I love those beefy looking phm 40:ies, but they sure are hard to tweak!
Jesper

[SED]

Throttle opening is more important than rpm to mixture because the opening determines how far the needle is out of the atomizer. the classic method is to mark a line on the throttle clamp, then wrap a piece of tape around the twist grip and mark it 0, 1/4, 1/2, 3/4, 1.  You then ride the bike and find where it runs poorly (with a steady throttle).

If your poor running happens between 0-1/4 it is likely the idle jet, slide cutaway, float level - though I have found that too small an atomizer or too low a slide needle can give problems here too.  If runs poorly between 1/4 and 3/4 usually the slide needle.  Poorly above 3/4 is usually the main jet.  If it always runs poorly at 4000 rpm regardless of throttle position, then it is ignition.

Let us know how it goes!

[Groover]

Based on the RPM range you mention and the suspicion of lean which also sounds right to me, I think about half turn more out on the low end needle on each side here would help. I think the rule of thumb for most dellortos is to screw it in all the way, then back it out 1.5 turns and it gets you "there" but if the engine runs hotter, then that circuit needs more flow (controlled by idle jet and low end mixture needle), so smaller engines typically a 1/4 turn at time is noticeable, but I'd try half turn out extra your case, then if you notice a change, then dial it in our out in less increments. If you hit 2.5 turns out and it still seems lean, then up your idle jet and start at 1.5 out again, and this is assuming all the other items within range (float levels, etc)





For the float valve assembly, you may need the 300. I believe the 250 are used for 30-36mm carbs, though hopefully someone else can chime in on that, could be your problem there.

[wirespokes]

If it runs well after 4K, then that would rule out fuel flow (float valve).

Without flywheel marks how do you match ignition timing side to side? I'd find and mark TDC for each side, then mark static and full advance. It's not hard to do.

Messing with jets and other 'solutions' comes after a proper tuneup.

Even with timing marks, when I've messed up setting one side, it ran ok, but not great. And the carbs sure didn't want to adjust properly.

[moto-uno]

  I believe every tooth on the ring gear was around 3.75 degrees , so tdc and back the number of teeth that gives you the correct timing (for your motor)
I've been running 40 mm dellorto's on my 990cc Le Mans 2 for decades and #70 idle jets sounds way large . I loved my Lucas Rita years ago and contemplated
replacing it , but they're long gone now . I have a Bub system also with the larger header diameter and the (very) low restriction tapered megaphones . Since
the Rita doesn't use the advance mechanism it's pretty easy to set the full advance timing ( fewer flopping about parts) . If interested I could recheck the jetting
I have in it ( also have larger oval K&N air filters).   Peter

[mtiberio]

float height is a poor way to change mixture (people did it in a pinch at the race track). Set it proper and jet proper.

[moto]

  I believe every tooth on the ring gear was around 3.75 degrees ...

Every tooth is exactly 3.75 degrees, since there are 96 teeth and 360/96 = 3.75. You should carefully count teeth from TDC and then permanently re-mark the flywheel, once and for all.

Moto

[wirespokes]

My 87 LeMans has Bubs with straight through glass packs. With a good packing it's not overly loud. It's also got K&N pod filters. Ignition is Dyna.

I've set it up with the Millich recipe, though it's got the euro slides. I think that's the only difference. I have no complaints how it runs and it regularly gets 45mpg or better.

[JesperN]

Hi everybody. A quick update. I checked the ignition, the timing was a little bit off. A mounted 270 needle jets, and now it seems to run well again. I also found microscopic cracks in the rubbers that connect the carbs to the heads, I am not sure if they go all the way through but I put som tape around them just in case. So, now I don't know what problem was, leaking rubber, atomizers or timing. I will change back to 268 atomizers, to see if the problem comes back.
Cheers

[jwinwi]

Hi everybody. A quick update. I checked the ignition, the timing was a little bit off. A mounted 270 needle jets, and now it seems to run well again. I also found microscopic cracks in the rubbers that connect the carbs to the heads, I am not sure if they go all the way through but I put som tape around them just in case. So, now I don't know what problem was, leaking rubber, atomizers or timing. I will change back to 268 atomizers, to see if the problem comes back.
Cheers

That's a fine motorbike!   
Cracks in that type of rubber *usually* do not go all way through... Hoping you get it running as good as it looks!

[JesperN]

OK, problems solved. It was not the carbs it was the ignition. I should have realized this from the start, the engine ran well last year. I'm pretty sure I burnt the electronics for the Lucas-Rita in the beginning of the summer. I couldn't afford a new more modern ignition system, but I found a British company, Rex's speed shop, that builds new modernized circuits for the Lucas- Rita. I bought one and new coils. Now its faster than ever, it feels like I have mounted a turbo! I will deal with the fine adjustments of both the carbs and the ignition when te autumn comes, now I just want to ride fast again, really fast!!
Thanks everybody for the input.

[Furbo]

In my experience the vast majority of Carb problems are ignition.....

[moto-uno]

  Love it when the follow up is positive  .  Peter

[JesperN]

In my experience the vast majority of Carb problems are ignition.....

Haha! good one.