Lean running Combat

dynodave · Feb 28, 2020

Just thinking out loud:
Curious why my combat with stock carb settings with chokes installed, running 220 jets, for well over 10,000 miles never needed new plugs, made 47rwhp, runs as sweet as a peach at a light cruise or full throttle and only suffers from cold oil drag @ start up low idle? (due to lucas rita advance curve)
My guess is your 270's would not get more than 38-40hp and contributes to your other symptoms.
Leaking intake manifolds will possibly affect idle which is making high vacuum therefore leakage % volume is more significant for the low idle a/f mass, but insignificant at WOT.

maylar · Feb 28, 2020

dynodave said:
Curious why my combat with stock carb settings with chokes installed, running 220 jets, for well over 10,000 miles never needed new plugs, made 47rwhp, runs as sweet as a peach at a light cruise or full throttle

My buddy George (you know him) would say the same about his 72 Combat Interstate. Stock Amals, Boyer MKIII ignition. Starts with one kick, pulls to redline like a freight train. Idles superbly.

bill · Feb 28, 2020

first off I would NOT toss the chokes I keep telling people if you dont need the chokes you are TOO RICH period. this goes back to people having to replace plugs in in a short time and poor fuel milage. the last time i replaced plugs was because the outside looked like shit from rust not because of starting or running, but they did have 40,000 miles on them. from your description if you add choke and it is better it is a lean stumble .( see you can also use the choke as a diagnostic tool )

Scout63 said:
Tossing the air slides is definitely on the horizon but I really want to figure the problem out first.

I truly appreciate the feedback.

maylar · Feb 28, 2020

bill said:
from your description if you add choke and it is better it is a lean stumble .( see you can also use the choke as a diagnostic tool )

My sentiments exactly.

Scout63 · Feb 29, 2020

I dug into it today and pulled the carbs, checked the needles (top position), adjusted the floats (again), checked the throttle stops and cables, set the air screws at 1.5 turns and went for a ride to get it warm. Then I adjusted the throttle screws at just faltering and adjusted air screws to optimize idle. It ran better but still wanted about 1/2 air slide to run better at about 1/3 throttle. All in 31F weather (no Harley’s out today). Then I read the oldbritts carb article that said if it runs better with the slides on, the needle should go up a position. Makes sense. The Norton manual calls for second position with “no mute.” I’m running Interstate pipes that sound pretty much open, so hopefully this is the fix. I’ll post tomorrow.

htown16 · Feb 29, 2020

You have a very simple problem. It is lean in the transition to needle. Running better with the choke on is a sure sign, because the choke has most effect with the slide up some. Raise the needles one notch, middle groove. That will richen the mixture. You may even have to go to the bottom groove. My 72 likes the bottom the best. Think higher needle more gas flow. Leave the chokes in they help on cold days. What do the plugs look like.

Fast Eddie · Feb 29, 2020

Scout63 said:
It ran better but still wanted about 1/2 air slide to run better at about 1/3 throttle.

It would seem that one of you problems here is ‘placebo’...

You’re convincing yourself of cause and effect that’s not there...

If your choke slide is 1/2 down... and your throttle is 1/3 open... then the choke slide is doing NOTHING to the mixture... it’s still buried inside the throttle slide.

I agree with the other hypothesis though, it does sounds like the needle is too low. That’s an easy thing to trial.

Did you change the plugs?

One tip when struggling with carbs is to ensure you’re dealing with a properly warmed up engine. It’s easy to be impatient and try fine tuning a cold engine, which A) isn’t possible and B) will result in a badly tuned warm engine !

Scout63 · Mar 1, 2020

It’s possible that the stumble is at a little higher throttle opening, maybe between 1/3 and 1/2. I should have marked the grip. Moving the air slide lever from off to 1/2 on definitely makes it run better. I rode for about 15 minutes before adjusting everything, but it was cold yesterday. I did change the plugs. Before the adjustments the left plug was lean and the right a little sooty. I’ll pull and photo the plugs from yesterday and report.

htown16 · Mar 1, 2020

It is possible to be lean on the needle and rich on the idle jet at the same time. Makes plug reading problematic. If you do a fair bit of idling can still color plug. Again, I recommend raising the needle based on what you have described. Fairly easy to do.

maylar · Mar 1, 2020

Fast Eddie said:
If your choke slide is 1/2 down... and your throttle is 1/3 open... then the choke slide is doing NOTHING to the mixture... it’s still buried inside the throttle slide.

I'm sure he's using relative terms, i.e. air slide control lever at 1/2. No way to know where the slide actually is in the carb. I have had my choke lever come loose while riding on the highway and drop the slide down partially. It certainly was noticeable in engine running.

Fast Eddie · Mar 1, 2020

maylar said:
No way to know where the slide actually is in the carb.

If you mark the twistgrip you know exactly where the slide is... and exactly which jets are in play...

maylar · Mar 1, 2020

Fast Eddie said:
If you mark the twistgrip you know exactly where the slide is... and exactly which jets are in play...

Hard to mark the choke lever though.

Fast Eddie · Mar 1, 2020

maylar said:
Hard to mark the choke lever though.

ahh, well if you mean the choke slide, that should just be fully up and out of the way! If necessary, an elastic band or similar should be deployed on the lever to keep it there!

maylar · Mar 1, 2020

Fast Eddie said:
ahh, well if you mean the choke slide, that should just be fully up and out of the way! If necessary, an elastic band or similar should be deployed on the lever to keep it there!

Normally, yes. But the OP says with the choke partially closed his motor runs better. That's the crux of this discussion.

Fast Eddie · Mar 1, 2020

maylar said:
Normally, yes. But the OP says with the choke partially closed his motor runs better. That's the crux of this discussion.

Well if he wants to use it as a diagnosis tool he really needs to mark it, and the throttle. And be precise about which is where.

Remember that the choke has ZERO effect unless it’s below the level of the slide.

Overall I’d get those choke slides out of the way, as they’re risking confusing the matter.

batrider · Mar 1, 2020

Make sure you don't have "choke on" and "choke off" lever postions reversed in your mind. The choke slides should be pulled all the way up with cables tight when in the off position for normal riding. Sorry if this sounds obvious but it is a common non-problem.

maylar · Mar 1, 2020

Fast Eddie said:
Overall I’d get those choke slides out of the way, as they’re risking confusing the matter.

They've already given diagnostic info.. we (collectively) have agreed that the problem is a lean stumble.

Fast Eddie · Mar 1, 2020

maylar said:
They've already given diagnostic info.. we (collectively) have agreed that the problem is a lean stumble.

I’m not sure what your point is to be honest. Yes I agree that it seems to be a ‘lean stumble’.

But that’s only the begging isn’t it? Now he needs to know where it is lean and consequently what needed changing... To do that he needs to know the throttle position where the leanness occurs.

And to monitor any corrections made, he needs to makes sure the choke slide is out of the bloody way when testing it.

If he were to just guess at random fueling changes, and not take care to ensure the choke is fully open, he ain’t gonna get very far is he ?!

htown16 · Mar 1, 2020

Carb diagnosis is always a bit of guess work. Take all your riding info and guess what the problem is and then likely solution. Make one change and one change only then test ride. If better you solved the problem if no change or worse reverse that change and try the next one. That's kind of a problem with forums, you get a whole bunch of suggested changes at once. My recommendation on this case is to start with moving the needle up one notch. Report back the results, if that doesn't fix it then move it back and try something else. It would be great to have unlimited dyno time to play around with different settings. But even putting it on a shop's dyno usually means only a couple pulls so you really don't have the time to properly sort things.

Fast Eddie · Mar 1, 2020

I totally agree htown. With the one caveat that the guesswork is greatly enhanced if the guesses are made based on an understanding of how the mixture is being affected. This diagram might help the OP with that:

Lean running Combat

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