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LeMans III: off-idle throttle response is poor

KnowFear

KnowFear

Cruisin' Guzzisti
Joined
Feb 2, 2011
Messages
103
Problem: the engine does not progress smoothly from idle, I need to blip the throttle several times to get the engine to rev past the progression area. It becomes annoying to get bogged down at most traffic stops. Once moving, the carburetion is great and the bike pulls very nicely all the way to redline.

Bike: 1983 LeMans III, with stock airbox & exhaust. The carbs were fully disassembled, cleaned, and re-assembled with new jets/needles about a year ago. The accelerator pumps have been disconnected.

Jetting:
- atomizers = AB268 (owners manual calls for AB265)
- K18 needles, 3rd notch
- idle = 50 (screws @ 1.5 turns out)
- main = 118 (manual calls for 115)
- 60/3 slides
- the choke cables are loose

So, the only deviations from original are the slightly larger mains, and slightly larger atomizers.

I'm scratching my head, wondering what I should do next. Any ideas?
 
Go back to the 265 atomizer. It will do better. The slightly larger main shouldn't be a problem. I prefer to use 52 pilots, but the idle mix adjustment seems to closed. Back them out to 2.5 turns. Turn in slowly until engine rpm dips a little, then open 1/2 turn. She how she does then. The idle mix adjustment may be all you need.
 
I went to 265 atomizers, also adusted the idle mixture screws to 2 turns on one side, and 2.5 on the other, as per your suggestion. It seems marginally better, but not by much. Now, the engine rpm falls slowly when the throttle is fully closed. I have experimented by opening the choke at various points and the result is always worse, so I'm pretty sure that I'm in a rich condition virtually everywhere.

It is still noticeably boggy in the transition zone. Once on the main jet (3/4 throttle and above) it runs very cleanly. I'm wondering if the next step is to lower the needle by one notch?

Thanks again John -- I really appreciate your help.

Chris
 
Try it. From now on, just change one thing at a time and check the results. If better, drop the needle again. If you need to lean more, you will need needles with less taper. Maybe a K22. Also a 50/3 slide may increase air speed and turbulence for a better mix at low openings. I'd try a different slide last as they may be a little expensive.
 

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I have a set of 50/3 slides that are a bit worn, would it be worth trying those before moving the needle? In fact, those slides came with the bike. I never put them back because the jetting the bike came with was all wrong according to the manual.
 
Whether you move the needle or change slides, just do one or the other. If you do put the 50/3 in, leave the needle in the same position and try it. Personally I'd try the needle first with the 60/3 slide.
 
I assume you made sure the ignition timing was correct to start with. Once you are sure of the timing correctness I would check throttle sync.

As John says "do one thing at a time" and check the result.
 
I'm running the Sachse distributorless ignition system, so nothing really to check other than the static timing. The carbs are in sync above idle, but I should probably redo the idle sync since I've been messing around with the idle mixture.
 
KnowFear said:
I'm running the Sachse distributorless ignition system, so nothing really to check other than the static timing. The carbs are in sync above idle, but I should probably redo the idle sync since I've been messing around with the idle mixture.
Click to expand...

Double check the high speed sync as well. Every time you take the slide out, or move the carb body (rotate) the cable position can change which will impact the sync.
 
Lowering the needles by one notch ruined the 1/4 to 3/4 throttle zone.

Now I suspect that it is a pilot jet issue, or slide cutaway. Next experiment will be 52 pilots (gotta get some), or installing the 50/3 slides.

I was pretty sure that I had a rich condition since the addition of choke causes an immediate stall. Is it possible that the adding of the choke overwhelms a slight lean condition so massively and quickly that it only "seems" like the transition zone is already rich enough (yet it is still too lean)?
 
Maybe a long shot after all the other suggestions but try the accelerator pumps connected That is what they are supposed to do. minimise the flat spot down low when blurping the throttle. Why are they disconnected?
Cheers
 
YESTERDAY:

I started by installing the # 52 pilot jets. I synchronized the carbs, and had it idling nicely. Great. So next I went into the house, put on my gear and got ready for a test ride... after thirty minutes of despair, trying to restart the bike, I gave up. The bike would not run without a HUGE amount of throttle and/or the chokes fully engaged. Seemed like a very lean condition, like something had suddenly plugged the idle jets...


TODAY:
I decided to put the # 50 pilot jets back in, and instead, install my 50/3 slides (60/3 were currently installed.) After putting in the 50s, I decided to check the idle mixture screws just to confirm how many turns out they were... LO AND BEHOLD, they were fully closed! I had counted the turns yesterday, but then had forgotten to turn them back out. No wonder the bike would not start after the synchronization. I'm such an idiot.

Anyway, today's test ride was awesome. The 50/3 slides have solved two problems: the off idle stumble is 100% gone, and the mid-throttle flat spot @ 5000 rpm is almost not noticeable. I get an occasional off-idle fart, but not nearly a problem. Maybe its running slightly rich in that off-idle zone controlled by the cutaway, but I think I will leave it alone. I could convert my 60/2 slides to 55/3 as an experiment, but I'll run it as is for now.

Anyway, thanks for all your help!
Chris
 
OK, the off idle stumble is fixed. If I'd seen this I would of immediately said slide.

The mid range woolyness may well simply be down to worn needle and atomiser. When you are 'Cruising' on a low/mid throttle the needle rattles in the atomiser and over time the annulus gets bigger. Just try new atomisers and needles.

Pete
 
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