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Cylinder 3 still not working?

  • Thread starter Thread starter robgusto
  • Start date Start date
R

robgusto

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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.
 
the cylinder does come on at higher rpms. I can feel it kick in.
 
take that pilot jet out and make sure you see light through it.
 
I / my moto mechanic friend and I , had carbs soaked and rebuilt 2 x and a rejet. The problem has persisted throughout. Each time we take them apart we thoroughly clean all jets.
 
Have you switched your plug leads between two and three to see if it is the wire or cap breaking down. They fire together.
 
Yes I have confirmed spark by changing 2 and 3 wires, confirmed with colortune, and confirmed by grounding plug to block and actually looking at it. It is getting strong spark. I am at the point where I may replace the motor 1st then carb #3 because this is such a crazy problem. The mechanic that worked on this bike said he has never seen anything like this problem. The other people (2 mechanically inclined friends) said that they will be able to figure it out, after a few hours they walk away scratching their heads and I'm out a 6 pack!!! I am naming the bike the ghost because the only thing I can think of is that the bike is haunted!!!
 
I had a similar problem with my No. 3 cylinder. It is not totally sorted out yet because there are some lingering compression issues in that cylinder. One thing that worked for me was checking and adjusting the ignition timing. When I tightened that up, it ran much better, though No. 3 is still a bit slow to warm up at idle.
 
Good to know I'm not the only one with this problem!! We are going to give it one more shot this weekend then it's to plan B, NEW MOTOR... I found a couple on Ebay for around $600. Any other places to pick up a motor? I may need to start looking for one...
 
I had had intermittent problems with firing until I switched to the Dyna green coils. The stock coils were new when I started my project, but one of the pipes would run a lot cooler than the rest. It would fire at higher rpm's but drop out at idle. Once the coils and wires were changed, the problem went away.
 
I did confirm solid consistant spark several different ways but I may try new coils before I replace the motor. I just have a hard time doing it when the spark for #3 fires #2 perfectly.
 
The #2 carb has the vacuum plug on it, if it was that one i would suspect a failing petcock.
The fact that at higher rpm the cylinder kicks in means that your ignition is not the problem.
I would suspect maybe the floats arent shutting off the gas properly. Or maladjusted float heights. I would check that the fuel shuts off with the floats lifted up.
 
All floats heights have been adjusted or checked when rebuilt. One was bent a little bit but it was for carb #1, it was corrected. The prev. owner eliminated the vacuum style petcock and now the current petcock does not have the vacuum hose, it has a cap on the #2 carb to prevent the leak. I have been told that this should not be an issue but at this point I'm considering everything!!!
 
robgusto said:
All floats heights have been adjusted or checked when rebuilt. One was bent a little bit but it was for carb #1, it was corrected. The prev. owner eliminated the vacuum style petcock and now the current petcock does not have the vacuum hose, it has a cap on the #2 carb to prevent the leak. I have been told that this should not be an issue but at this point I'm considering everything!!!
Click to expand...
Ok, but have you checked that the float/seat seal properly and shut off the fuel when the floats rise?
Pull the carb bowl, hold up the floats and turn on the petcock, no fuel should flow at all.
 
I will double check tomorrow and post again. I have replaced all O rings and I believe everything is seated well. I'm not getting any overflow or leaks when petcock is left on, but I will definitly give it a check tomorrow.
 
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!!!
 
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
 
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.
 
robgusto said:
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.
Click to expand...

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..........:confused:;):):rolleyes:

Paul
 
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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.
 
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