Lean running Combat

I know this issue is pretty played out but here goes. 1972 Combat 750 fully rebuilt. Original 12000 mile 932 Amal’s. Slides don’t have any noticeable play. I vapor honed the bodies, replaced the jets, needle, needle jet, gaskets, float needle and floats using stock Amal new parts. Cleaned out the pilot jets. Starts on first or second kick and idles very well. Pulls hard on throttle when rIding and main jet seems fine on larger throttle openings. BUT - it stumbles slightly at 1/4 - 1/2 throttle and wants the air slides about 2/3 down or it stumbles more even when warm. I checked float levels and they are set about .050 below the rim. Right pipe has turned a golden color and the left pipe has not but the right float was significantly low when checked before last adjustment. Carbs tickle fine but not gushing. New trispark ignition strobes at 28 degrees btdc fully advanced. I’m thinking that the needles need to be raised or one or both of the needle jets is clogged. I will pull the plugs tomorrow and photograph. Any guidance will be gratefully accepted as gospel. Ben

I set my floats level with rim, inverted- will run a little richer, raise needle- will run richer. One or both should fix you. I recently built a Triumph 650 that acted like yours, had to put next size richer throttle valve, to fix leanness and popping, and after a few hundred miles, had to switch it to stock- became too rich

Slide slop.
Air/fuel mix continually varies allowing it to be consistently inconsistent.

What does ‘stumble’ mean? Can you be more descriptive?

The reason I ask is that what I’d call a stumble in that range is normally an indicator of richness, not weakness.

And, you say it gets worse when warm... again, that sounds like an indicator of richness (think how a choke works, it enrichens the mixture, and when the engine is warm it doesn’t like having the choke on).

Weakness can show as a cough or spitting back through the carb. Or a general loss of power at a certain throttle opening. It will normally be worse when cold and might take longer for the engine to warm up to the point where it runs well.

Richness can show as a momentary miss-fire, or 8 stroking, hence I’d normally call it a stumble. The engine can sound heavier, tick-over might sound lumper.

A really good tip is to mark your twist grip in 0, 1/4, 1/2, 3/4, fully open, positions. Then you can ride at a steady state at a precise opening, gently rolling the throttle slightly more / less around that specific point. Which jet circuit is in use at any point in time is governed by the throttle position. During normal riding, we are often opening and closing it constantly, riding around or masking an issue.

I’m always wary about changing needles and needle positions etc on a stock bike. If the bike is stock, then stock settings should make it run well. I have gotten myself down deep rabbit holes in the past by changing the wrong thing and subsequently over compensating somewhere else etc!

But... if I were you, the first thing I’d do is try NEW spark plugs... I was really thrown a curved ball by duff (new) plugs recently.

Note: my advice is not based on claims of being an expert. On the contrary, it’s based on the many times I’ve struggled, got it wrong, disappeared down rabbit holes and generally pulled my hair out!!

Thanks all. I’ll start with the plugs, mark the throttle and then if necessary pull the carbs and check slides and needles. The “stumble” is consistent whether warm or cold. Sort of a spitting at low off idle throttle then goes away as throttle is opened. Worse if air slides are opened (cables tightened) and won’t go away as throttle is opened. I’ll check float levels again also.

PM sent

"Right pipe has turned a golden color and the left pipe has not"

How do you synchronize your carb?
As is known a smooth idle is relatively meaningless once the throttle cables are being used to lift the slides. 99% of home tuners get hit or miss synch results...mostly miss.
It's only a matter of degree of mis-synchronization that if large enough will result in one cylinder pulling and the other is loafing. I see it all the time...
That's why I bought a brake dyno...to MEASURE the results of a tune up and do the corrections.

dynodave said:

"Right pipe has turned a golden color and the left pipe has not"

How do you synchronize your carb?
As is known a smooth idle is relatively meaningless once the throttle cables are being used to lift the slides. 99% of home tuners get hit or miss synch results...mostly miss.
It's only a matter of degree of mis-synchronization that if large enough will result in one cylinder pulling and the other is loafing. I see it all the time...
That's why I bought a brake dyno...to MEASURE the results of a tune up and do the corrections.

Click to expand...

air leak on RH carb inlet?????

Bernhard said:

air leak on RH carb inlet?????

Click to expand...

That's the difference of having test equipment, and doing measurements to verify faults or thowing darts at a dart boad to get possible faults out of a list..

While it could happen...since 1992 when I got my dyno I never had or detected a leaking manifold.

mis synchronized cables or different cylinder to cylinder timing = 70%

re; "While it could happen...since 1992 when I got my dyno I never had or detected a leaking manifold."
there's always a first time, and your dyno will not show that up, it's not that good!!!!

Bernhard said:

re; "While it could happen...since 1992 when I got my dyno I never had or detected a leaking manifold."
there's always a first time, and your dyno will not show that up, it's not that good!!!!

Click to expand...

It's not the dyno but the skill of the operator who diagnoses the running engine under load without running next to the bike 30-40-60 MPH
Most of the leaking intakes I have discovered is from not being assembled correctly. and the engine does not even have to run...

I don't even have to run next to my bike to diagnose a leaking manifold . . . .

Have you tried adjusting the air screws 1/4 turn in and or out? The old Amal way was to adjust the air screws sweet spot with the tick over set high. I'm pretty sure it was to mask any issues in the primary circuit.

Did you say you start it right up first second kick and it idles well? Did you set your idle hot after a decent run or cold? I can never get a British bike to idle well right at start. I agree on carb synch. I battled this even after the PO had it tuned by an "expert" - or at least he charged like one. If it runs like garbage but idles when you start it up I guess you an imagine which one I'd rather have. I'd just start your tune over. Set your air screws at one and a half and dial it back in and try to get it where it runs well. You could be too lean on one side and too rich on the other which could cause your hiccup at 1/4 so go overly rich on both and see if that solves your problem. I'm not an expert either and maybe I missed your point- just spent a lot of hours screwing with this stuff.

Eric
The added load of the cold oil when first started is the drag that makes the bike not want to idle at first. The shape of the advance curve affects this phenomenon. Points with a flat advance up to about 1250 is way better than one of the boyer or rita curves that have a steep curve right off low idle.
I'll bet when the engine heats up, it speeds up, and will then idle at the carb setting.
The later microprocessor ignitions have reverse acting curves that have an antistall effect. Not hard to do when the RPM is measured and then the timing picked out of a table.
too slow...pick more advance

I meant that on startup it idles well with the throttle set up a little so it won’t stall. It doesn’t stumble. Even when warm it doesn’t stumble at low throttle settings, only at partial throttle, and worse if the slides are open. I need to find a few quiet hours and start from scratch as suggested. So many variables I need to go in order. Thanks all. I’ll report this weekend.

First thing Id do is bin, or shelve the B F # ! Choke Cables slides & anything else to do with it I could find to do with them .

Then Main Jetts . Maybe 270 wouldnt be oversize . Dont be scared of re jetting .

Scout63 said:

I meant that on startup it idles well with the throttle set up a little so it won’t stall. It doesn’t stumble. Even when warm it doesn’t stumble at low throttle settings, only at partial throttle, and worse if the slides are open. I need to find a few quiet hours and start from scratch as suggested. So many variables I need to go in order. Thanks all. I’ll report this weekend.

Click to expand...

Totally agree on one variable at a time. It's funny I was having all of these problems when I just got my bike after it had been tuned professionally and being newer to British bikes I was terrified to change it. But after tweaking it a thousand times and it just getting worse and being a slave to the previous setting I just said screw it and started over and it solved all the problems. It had been set up too lean and was popping on the right side intermittently at 1/4 throttle. About 1 million separate anecdotes but you never know, good luck!

dynodave said:

Eric
The added load of the cold oil when first started is the drag that makes the bike not want to idle at first. The shape of the advance curve affects this phenomenon. Points with a flat advance up to about 1250 is way better than one of the boyer or rita curves that have a steep curve right off low idle.
I'll bet when the engine heats up, it speeds up, and will then idle at the carb setting.
The later microprocessor ignitions have reverse acting curves that have an antistall effect. Not hard to do when the RPM is measured and then the timing picked out of a table.
too slow...pick more advance

Click to expand...

Very interesting stuff.

Matt Spencer said:

First thing Id do is bin, or shelve the B F # ! Choke Cables slides & anything else to do with it I could find to do with them .

Then Main Jetts . Maybe 270 wouldnt be oversize . Dont be scared of re jetting .

Click to expand...

Tossing the air slides is definitely on the horizon but I really want to figure the problem out first. I have a Madass linkage that will go in at some point after I get the jetting right.

I’m always surprised at how I can rebuild an entire motorcycle but fail to properly attend to or diagnose simple things. I lived with an intermittently stalling /5 for six months before realizing that one choke arm was stuck halfway up.

I truly appreciate the feedback.