Disappointing dyno run and poorly tuned carbs- PHF36 on CX-100

[Dukedesmo]

My LM2 with 950cc/B10 cam/PHF 36 is running with 148 main jets and, whilst I've not had it on the dyno feels fairly strong to me.


Actually, when I bought it, as an 850cc, it was running the 148 jets. I fitted the big bore and left the jets as were, I did intend to get it on the dyno but never got round to it, it certainly isn't running lean though.

[Frenchfrog]

Amedeo from Raceco UK said many moons ago that 34 mm carbs were optimal for the small valve heads in terms of power

[blackcat]

I have 36's on my CX and it's been fine and I never opened the intakes to accommodate the larger carbs.

[Turin]

It all comes down to the combination of components. The first mod I did to my 850T was to install 36 Delortos with delrin manifolds. The results were disappointing. (nothing lost buy not much gained) Installing an X9 cam, increasing compression/.040 squish, and minor porting really make for a really sweet running 850.

If I it had a wilder cam and bigger valves, It would need 40's, but that's another bike.
 

[Mayor_of_BBQ]

I’m thinking the phf 36’s are too big too, I think the vhb’s are all you should need

Pretty common to hear someone try bigger carbs as an upgrade

May very well be true, but I am not a tuner... And I'm not even a decent mechanic. I can do my normal maintenance etc.. But I bought this thing because I thought a knowledgeable enthusiast put it together, and everywhere I have looked, it's cocked up. I still like the bike, but wow have I learned a valuable lesson. Not only did I overpay by at least $1k, but my hopes of a turn-key custom hotrod have pretty much been kaput. I would have been better off buying a late model V7 or waiting 8mos and getting the V100 Mandello.

Spent the weekend doing spring yard & garden prep.. I did get my jets and some other bits... Hopefully the rain stops and I will take it out for a rip with the new mains and see what it does.

[Tony F]

I’m thinking the phf 36’s are too big too, I think the vhb’s are all you should need

Pretty common to hear someone try bigger carbs as an upgrade

If you are happy with the performance with the vhbs fitted, that's fine, no point in expending time and money changing things.

But if you fancy a little more grunt then the larger carbs and ideally porting to match is a good start. You also need to remember that these engines are getting on in years so the condition of bores, rings. valve guides, cam, lobes and valve sealing may leave a bit to be desired and could impact performance whatever route you take.

[Tom]

Turin & Blackcat have touched on the subject of timing & ignition.  I'll throw in also sparkplugs.  You're getting fuel but probably not a complete burn when the throttle is opened.

[John A]

I had one like that, it turned out that the squish band was too wide. Deleting the base gasket and using flange sealant cured it without any machining. It came out at .040”. So if everything else checks out, I’d try that. It didn’t ping , which was a concern.

[DC950]

almost everyone thinks that bigger jets are the answer to more power.  This is often wrong and is particularly wrong on an engine with good head work. 

Conversely to what one would think, an engine with free flowing heads needs smaller jets (and even carbs at times) because it is now flowing fuel and air so much more efficiently.  The better air flow actually pulls fuel up efficiently now as opposed to just burbling up from a bigger jet before the head work.

I used to roadrace a Sportster.  When I made it a 1200 with Buell heads, I naturally put in a bigger main jet.  It bogged on WOT.  I put in the next size jet.  More of the same.  Repeat for every large jet I had.  The same.  I even took a jet an bored it out even bigger.  The worst running yet.  I was all set to buy new cams.

In desperation, I used the smallest main I had and went on the the track.  It ran like a raped ape with zero hesitation when the throttle was whacked open.  Power everywhere and a stupid amount of torque.

Where one has the accelerator pump set to activate also plays a role.

[MattP]

I,m thinking cam off a tooth

[blackcat]

I,m thinking cam off a tooth

Didn't think of that, certainly worth checking if all else fails.

[Tony F]

almost everyone thinks that bigger jets are the answer to more power.  This is often wrong and is particularly wrong on an engine with good head work. 

Conversely to what one would think, an engine with free flowing heads needs smaller jets (and even carbs at times) because it is now flowing fuel and air so much more efficiently.  The better air flow actually pulls fuel up efficiently now as opposed to just burbling up from a bigger jet before the head work.

I used to roadrace a Sportster.  When I made it a 1200 with Buell heads, I naturally put in a bigger main jet.  It bogged on WOT.  I put in the next size jet.  More of the same.  Repeat for every large jet I had.  The same.  I even took a jet an bored it out even bigger.  The worst running yet.  I was all set to buy new cams.

In desperation, I used the smallest main I had and went on the the track.  It ran like a raped ape with zero hesitation when the throttle was whacked open.  Power everywhere and a stupid amount of torque.

Where one has the accelerator pump set to activate also plays a role.

I has a similar experience with my SP1000 but with the pilot jets. It picked up from okay from low revs/low throttle openings but not as well as I thought it should. Conventional wisdom was that the pilot jet was right and a smaller one wouldn't work. But a smaller pilot jet transformed it!

[Mayor_of_BBQ]

OK!

I kinda put this out there and left it hanging for a couple weeks....  But I have a hellish work situation right now, and the first two weeks of May is when you put your garden in here- I had a lot of work to do- so no riding.

1. I am doing one change at a time so I can track results

First thing was a simple jet swap. I pulled the 140's and put in 145's

THE CHANGE IS AMAZING!

If you were paying attention, I said the bike starts, idles, and runs great at low and moderate speeds.  Well, I put in the bigger jets and took it for a real romp around the mountains yesterday.  It hauls ass. Really, like a different bike. I finally feel like it's doing what it should do!

I am going to do a couple other minor things like torque the heads, clean the air filters...  I had just set the valves and changed all the fluids, fixed an exhaust leak.

If I can line it up this summer, I will ride out to another event where they set up the dyno and try it out.

I don't know if it's perfectly tuned... for sure you can't change one set of jets and proclaim it fixed. BUT it is running the best it has and I am pleased as punch. I'm going to futz with it more in the future, but just larger jets has really transformed the bike. A nice starting point at least!

[PeteS]

A dyno with a sniffer will give you a better idea of the mixture throughout the rev range. A WFO run on a straight road or one up a steep hill at a lower gear will give a seat of the pants feel on the main jet. If you do a dyno run take some jets with you a step larger and smaller to better tell the direction you need to go if any.

Pete

[Mayor_of_BBQ]

A dyno with a sniffer will give you a better idea of the mixture throughout the rev range. A WFO run on a straight road or one up a steep hill at a lower gear will give a seat of the pants feel on the main jet. If you do a dyno run take some jets with you a step larger and smaller to better tell the direction you need to go if any.

Pete

yep,
Dyno with the sniffer was what indicated the problem in the first place

[guzzisteve]

If you read the tuning book there is a formula so the main jet is big enough to feed the atomizer/needle  area.
I'll bet it will be closer

[Mayor_of_BBQ]

If you read the tuning book there is a formula so the main jet is big enough to feed the atomizer/needle  area.
I'll bet it will be closer

I'll never read that book. No way I could get thru that. Same reason I would drive 8hr round trip to have you do Guzzidiag on my bike. I just dont have the knack for this stuff, it doesn't land for me.

I'd more likely drop it off with someone to tune and pick it up completed. I dont find wrenching soothing or enjoyable at all. It's just frustrating and irritating. I have a ton of respect for the people who have the mind for it, but it's not for me. I already barely have time to ride 1-2 times a week.. Working on the bike or taking it out of commission to spend hours on it in the garage just sounds like torture to me!

The important thing is, the bike is running awesome and is hugely improved with a simple jet change. I doubt it's at peak performance and superbly tuned, but I don't care enough to agonize over it.

[pressureangle]

...Working on the bike or taking it out of commission to spend hours on it in the garage just sounds like torture to me!

The important thing is, the bike is running awesome and is hugely improved with a simple jet change. I doubt it's at peak performance and superbly tuned, but I don't care enough to agonize over it.

So here's the easy path forward. Since it's a simple quick task, go up one size on the main jet and ride. If you feel an improvement, go up one more size next time. And again, until you make a change with no difference- then go back to the previous size main jet. You'll then be on the lean side of max power. Adjust the needles as necessary for drivability and enjoy. I've spent an enormous amount of time reading plugs, changing carburetors, and making dyno runs to discover at the end of the day that the 80/20 rule applies; you can get 80% of the benefit for 20% of the work, and on public roads you can't tell the difference. Unless you're racing or have a fever to maximize fuel mileage, there is no benefit to micro-tuning if you don't enjoy the tuning process itself.

[Mayor_of_BBQ]

So here's the easy path forward. Since it's a simple quick task, go up one size on the main jet and ride. If you feel an improvement, go up one more size next time. And again, until you make a change with no difference- then go back to the previous size main jet. You'll then be on the lean side of max power. Adjust the needles as necessary for drivability and enjoy. I've spent an enormous amount of time reading plugs, changing carburetors, and making dyno runs to discover at the end of the day that the 80/20 rule applies; you can get 80% of the benefit for 20% of the work, and on public roads you can't tell the difference. Unless you're racing or have a fever to maximize fuel mileage, there is no benefit to micro-tuning if you don't enjoy the tuning process itself.

Excellent advice, I can work on these terms

[JoeW]

I bought a 79 1000SP last winter. I think a previous owner was trying to make it a LeMans. It has LeMans foot pegs and mufflers and 36mm carbs and it ran like dog poop! I'm sure the motor is stock. I went back to VHB30 carbs and it runs great. I think 32mm carbs would be a nice upgrade though. Bigger isn't always better when it comes to carbs. My LeMans 4 powered sidecar rig hit a flat spot at 4200 RPM with the stock 40mm carbs, it would not pull past it. especially with a head wind. It runs much better with 36mm carbs. You can put a dial indicator on the rocker and measure lift, you may be able to figure out what cam is in it from that.

[Tusayan]

I bought a 79 1000SP last winter. I think a previous owner was trying to make it a LeMans. It has LeMans foot pegs and mufflers and 36mm carbs and it ran like dog poop! I'm sure the motor is stock. I went back to VHB30 carbs and it runs great.

A ‘79 1000SP would have ordinarily come from the factory with the same foot pegs and mufflers as a LeMans, except that the exhaust was chrome.  If I recall correctly the SP pegs and mufflers (and seat) were made more touring oriented in 1981, with what some call the NT version.

My SP and my current CX100 both ran a lot better after 36-mm PHFs were installed, as much as anything because you don’t have to twist the throttle a long way to get a given throttle opening.  Otherwise they were well suited to the engine and had/have no flat spots.  Lack of power or other issues would not in my experience be due to using 36-mm carbs, if they were jetted correctly.