Dead cylinder #4!

[ligamentx]

Hi everybody. I was hoping to get some opinions on how to troubleshoot a dead #4 cylinder. I recently got a GS550T, 1981. First thing I did was clean the carbs, replace the boots and o-rings, and make sure there weren't any air leaks. I rode the bike a few days, thinking it was all good until I checked the plugs. Cylinders 1,2,3 were whitish, and 4 was brown. Thinking I didn't have my mix screws right, I adjusted them.

After riding the day after, I noticed that cylinder #4's exhaust was pretty cool. I replaced plug #4 with a new one and made sure it was firing by taking off the coil and plug and grounding the plug on the engine, watching for spark. There was a thick blue spark. I readjusted the mix screws, but it seemed that cyl 4's screw wasn't doing anything. Felt the pipe, still cold. Then I tried to pull cylinder 4's boot while the bike was firing to see if the RPMs would drop, got a big shock, but no change.

When I pulled the plug, it wasn't wet at all so it seems that that cylinder may not have been getting any fuel. However, the first plug I had in there fouled and turned black after running for a day, so there must have been fuel flow at one point... The original plug was from the PO, but was white and was firing I looked when I bought the bike)

I haven't adjusted the valves, but I have checked them. There were a few that were tight at .03mm, but still in spec. I plan on adjusting them this weekend, but I want to solve this dead cylinder issue first....

What do you guys think as for next steps in troubleshooting? I was thinking of cracking #4's carb bowl drain plug to see if theres fuel in there and perhaps checking for a sticky float valve.... Don't want to pull the carbs again and clean then, as I did a good job and followed Cliff's guide.

Hmmm....oh and, I checked compression... it was good- around 140 if I recall correctly.

Thanks,

Nick

 

[850 Combat]

Don't want to pull the carbs again and clean then
Nick

I can't blame you, but...

That sounds like where the body is buried.

 

[JEEPRUSTY]

Hehheeheh
got a shock you say
leaking hi tension lead arcing to the engine block and not firing the plug sufficiently or intermittently resulting in a sooted up plug.

Swap you plug caps from a good working cyl to the #4

 

[ligamentx]

I'm going to try to pull the #4 bowl and check for fuel... do float valves all of a sudden get sticky? I also forgot to mention I dropped the bike on my first ride doing some slow speed manuvering :\. Embarassung, but it has been a whole winter since Ive ridden! It died while tipped over but started back up with 10-20 second of intermittent cranking after the fall- maybe I could have fouled the plug on that first ride and then gotten the float valve stuck? I'm hoping it's that easy...

 

[fishfarm]

Loosen the drain plug to see if fuel is in the bowl to eliminate a stuck closed float valve.

edit: I see we had the same idea at the same time.

 

[ligamentx]

Hehheeheh
got a shock you say
leaking hi tension lead arcing to the engine block and not firing the plug sufficiently or intermittently resulting in a sooted up plug.

Swap you plug caps from a good working cyl to the #4

Thanks, I'll try this tomorrow- is this repairable or would It require a new boot/coil?

Thanks for the help... would this result in a dry plug? Or maybe idling the bike for a while didn't get enough fuel in the cylinder to have a wet enough plug to notice...

Nick

 

[JEEPRUSTY]

Thanks, I'll try this tomorrow- is this repairable or would It require a new boot/coil?

Thanks for the help... would this result in a dry plug? Or maybe idling the bike for a while didn't get enough fuel in the cylinder to have a wet enough plug to notice...

Nick

Caps go bad NGK makes cheap HQ replacements.
You will have to refer to NGKs partsfinder page for the correct resistance cap etc.

I have had insane probs cured by caps.

 

[850 Combat]

So was all this carb cleaning done last year?

 

[ligamentx]

No, I just purchased this bike non-running this year... got it to this point. I had a little honda rebel that I sold last year... big difference

 

[cowboyup3371]

You might want to clean those carbs thoroughly again.

 

[SqDancerLynn1]

The CV carbs can not be too clean. The #4 carb still has a problem

 

[rustybronco]

I replaced plug #4 with a new one and made sure it was firing by taking off the coil and plug and grounding the plug on the engine, watching for spark. There was a thick blue spark.
Hmmm....oh and, I checked compression... it was good- around 140 if I recall correctly.

Compression, spark and valve clearance isn't your problem.

did you bench sync the carbs when you reassembled them? have you vacuum sync'ed them yet?

 

[ligamentx]

I haven't done any itype pf synching. I have a vacuum guage... Would this help a lot?

 

[JEEPRUSTY]

In spite of the disparate claims made so far my plug cap check is free and prevents a lot of other very hard work if correct.

 

[ligamentx]

Just traded 1 and four plug caps, then the plugs. No change, 4 is still dead

 

[ligamentx]

Problem solved.

Thanks for all the help guys... In the end, it was a plugged pilot jet in Carb 4. I feel silly, because thats the number one thing everybody usually says. I guess I didn't let it soak long enough/didnt look through it when I put it back in!

Anyway, I ended up doing an entire up an down, troubleshooting each part until I narrowed it down- by taking off the carbs. Also adjusted my valves while I was working today.... unfortunately I mysteriously stripped a valve cover bolt- seems like I stripped it on the way out... (is that possible?) But I ran over to home depot and got a 6mm x 50mm metric bolt and a nut and was able to fit it through the head and the valve cover- clamping it down from both sides (It was one of the bolts along the front edge of the engine.

All in all, a very satisfying wrenching day!

Next week- vacuum sync time!

Thanks again everybody.