Cylinder 3 still not working?

[robgusto]

Okay, I checked the float bowls and no leaks. I have decided to change the coil. I have spark confirmed several ways but I was told that there is resistance and even when I switch from 2 to 3 on the wires that the signal may follow path of least resistance. This could lead to a weak spark on cylinder 3. Longshot but worth a try, I picked up a coil for $25 on ebay and will cross my fingers. Next step if that doesn't work is replacing the motor. Also, I had another friend/mechanic/shop owner take a look this afternoon, he is STUMPED too. The silver lining is that it is funny to see people tell me that they can figure it out then look at the bike with hands on hips shaking their heads!!!! Another benefit is that I'm getting really good wrenching on this bike!!! Staying positive!!!

[MisterCinders]

If you get spark at higher rpms but not low, you should check the forward ignition timing. Could be that your timing slightly off such that it won't fire properly on the forward end but is close enough on the advance end to catch at >2,500 RPM.

HTH

[robgusto]

Sounds like a good thing to check out. I am unfamiliar with how to check the forward ignition timing. I think it may be fixed on my bike, it's an 81 gs750L. If I can adjust the timing on can someone tell me where to start. I looked on bikecliff and did not find a how to link.

[laverda1200]

Hello, Sorry I have not posted in awhile because my friend was looking at my carbs as a favor. I have rebuilt carbs and rejetted with Dynojet for pods and pipes no megaphones. Compresstion at 120 to 124. Leak down test shows normal. Valve clearances adjusted to .004, gasket replaced, cam looked normal for age lobes looked good. Switched number 2 and 3 coil to confirm spark, used colortune to confirm spark also. Switched plugs multiple times. Adjusted air/fuel many times. I pulled plug after running and it was dripping wet with gas, pipe also has gas coming out. I hand cranked and the piston is moving normally. Myself and my mechanic and everybody else who has looked at bike are stumped???? Any ideas???

81 GS 750 L, KN pods, rejetted mikuni carbs, 2 into 1 pipes.

Hi Rob

was the bike having this problem with cylinder 3 BEFORE you made your recent modifications?

if not, you have to expect that it is a carb problem, relating to any of the following:

- removing the air box and going to crappy individual pod filters which typically don't flow enough air to support proper running, and don't provide the calm and tuned intake flow a stock air box does. Individual pod filters are garbage, especially if they are the cheap crap Emgo things, and even if they are the higher end K & N's or similar. To even get an engine to start with those crappy pod filters you usually have to make a major adjustment to the idle circuit.

-going to open pipes with no back pressure. Exhaust system tuning is an art, open pipes simply don't work properly, a lot of noise for very little forward progress.

-a combination of open pipes and crappy pod filters.

-intake manifold leak on number 3

-a problem in the carb rebuild, carb setup, or with the carb rejetting kit itself. I set up a bike for someone a few weeks ago after a"complete carb rebuild and rejetting" by the owner, sucker wouldn't run right on one cylinder. When I pulled the float bowls to check float height and float valve sealing, I found that the owner/carb rebuilder failed to install a main jet on one of the carbs..........

-incorrect valve clearance on cylinder 3. Check and adjust the valve clearance.

-carb float height not adjusted properly, float valve worn and not sealing completely.

if cylinder 3 was running fine before the recent modifications, it is NOT the engine, although check the valve clearance to be sure.

very best case diagnostics, borrow a known good set of carbs and try them on your bike, and better yet, put the stock air box and a decent set of pipes back on the bike while you are at it..........

Paul

[robgusto]

The problem has existed from the beginning, I agree that the optimal way to have the bike is in stock condition however I purchased the bike with pods and straight pipes. I upgraded to K&N and rejetted with Dynojet kit to try to get #3 running. I rebuilt carbs and double checked #3. We have adjusted the float bowl and made sure there was no gas leaking past O rings post rebuild. Valve clearances were adjusted to .004, also did leak down test to confirm. I may try to put another set of carbs on the bike if I can find them.

[chef1366]

Is the #3 header clear?

[robgusto]

Yes the header was clear last time I had them off aprox 6 months ago. Also the gas that is not combusting comes out the pipe.

[chef1366]

Before buying a new motor put a Dyna S and Dyna green coils on the bike and ride it.
What is the actual compression on #3?

[Flyboy]

Check that your choke, or more correctly, your enricher circuit is shutting off all the way and not allowing fuel to by pass.

[robgusto]

I purchased a replacement coil for 2 and 3 and put on the bike today. We were thinking that it could be a resistance thing however cyl. 3 still not running. I'm pretty sure that it's not a coil probem. The compression on #3 is the same as the rest of the cylinders at 120.
The choke is working properly as far as I can tell. I may pull the carbs off again and switch the needle and choke assembly and start eliminating carb parts as the problem.

[robgusto]

update on the evil bike

Just thought I'd update progress on my repairs!! The motor has been taken down to the head and I just got the gasket to replace. I was also thinking (because this issue is rediculous!!! and still stumping everyone I talk to) that if it doesn't work I will replace the motor. I hate to not figure this issue out but if you look at the beginning of this thread I have not been riding for a LONG TIME!!! Sorry it's been awhile but I've been waiting on parts and trying to have a bunch of people take a look at the bike and it continues to stump EVERYONE!!!!

[tkent02]

I smell a parts bike!

[robgusto]

I have at least another couple months of effort left. Then it's parts time!!

[5azzmonkey]

Sounds like the internal passages in the carb are still buggered up. If you spray carb cleaner into the air pilot jet (use the little spray straw and stick it in deep) does it come out the 3 little passages under the butterfly? Check the volume that comes out against one of the other working carbs. If it only trickles out or does not come out at all you will need to soak the carbs again.

-Dave

[robgusto]

All carb passages were clear and we have been through them/soaked and they were checked again when rejetted. However because I have been through the carbs many times and still have not figured out I may just start over with a new one. I have been looking on Ebay but can only find a set of 4. I'm not going to build my own from carb body because it could be a screwy part. The carb was injecting gas however the bike would not backfire? I figured I would see at least a little fire out of the pipe. There was gas leaking out of the pipe? We did make sure that the bowl was not overflowing but even if it was I would expect the occational backfire?