Dead cylinder at idle

[Guest]

I noticed last season that my idle was rough. I then noticed that the exhaust pipe on cylinder 4 is cold when the bike has been idling. I am assuming from this that it is not firing. I confirmed a good spark, and figured it must be the carb.

This winter I tore down the carbs and fully cleaned them, including dipping in Berryman's, and spraying all of the key passages to ensure they are clear.

I put them back on last weekend and it started right up. With the choke pulled, it was running fine, and I checked the pipes with my infrared thermometer, and all 4 got hot. Once it warmed up, I pushed the choke back in, and let it run for a few minutes. The idle was rough again, and once again, cylinder 4 cooled down, while the others remained hot.

I am uncertain how to proceed. I'm looking at getting a colortune so I can see what is really happening with that cylinder.

I could also pull the carbs and teardown number 4 again, and see if I missed some blockage or cleaning step on that one.

What else could cause this symptom? Valve clearance? I checked and adjusted the valves when I got the bike two years ago, and have not checked since. I don't put a lot of miles on, so it's probably 1,500 or 2,000 miles since they were done.

I checked the spark plug. The gap was a bit tight, so I regapped it.

Last year I checked compression, and all 4 cylinders were close. I don't remember the numbers, but I think I posted them on here. I can look for it if need be.

Ideas?

Thanks,
Glen

[78550GS]

O-rings on the carb to head boots. Replace all 4.

Did you replace the carb O-rings as well? You can strip and dip all you want, but if the O-rings are brittle, they need replaced.

Personally, I would strip and dip the carbs again. And then check to make sure you don't have any holes in the diaphragm on that carb.

[gustovh]

My first thought is O rings as well.
You can check the mount O rings with a little spray of wd40 on the boots where they bolt up, if the r's pick up then you know. Doesn't always work but worth a try.

Something that I would do before pulling the carbs, is unscrew the spark plug caps and snip about 1/4" off the ends of the wires and screw the caps back in. This freshens the connections, don't forget to look into the caps for condition of the little screw that screws into the wire, it should be bright and clean.

V

[tkent02]

Runs fine with choke?
Doesn't without?
Something is still clogged in the pilot circuit.
Could be o rings too.

[glenwill]

Thanks to both of your for replying.

When I got the bike 2-3 years ago, I did a thorough reconditioning. On the carbs, I replaced the carb boots, including the o-rings. I cleaned the carb and replaced all of the o-rings and pilot plugs, set the float level. I replaced the 1 bottom gasket that was shot. I inspected all of the other components, and they looked good.

I'll try the spark plug wire idea first, and if it doesn't help, I'll pull the carbs and clean again. I'll inspect the diaphragm at the same time.

Anyone use a colortune? Would it help me confirm no combustion vs very little combustion at idle?

Glen

[tkent02]

Cold pipe?
Its confirmed.

[glenwill]

Runs fine with choke?
Doesn't without?
Something is still clogged in the pilot circuit.
Could be o rings too.

Thanks. I'll check the pilot circuit again more carefully. Where all do I look for clogging? The cleaning guide mentions two passages to spray into, and where it should come out. Both look related to the choke, not the pilot. I did inspect/clean the pilot jet, which I can do again. Is there another passage I need to look for? I'm thinking it would be between where the pilot jet screws in, and where the pilot adjustment screw is located.

[tkent02]

Could also be carb synch, if one isn't open at all it will do as you describe.

[Skinner]

What about a carb sync? If number 4 carb was closed at idle as compared to 1,2 and 3 would this not be the symptom? The OP stated that he stripped and dipped but did not mention a bench or vacuum sync.

[glenwill]

I did not sync them when I put them back together. Probably a mistake. Last year I bought a Motion Pro SyncPro and synced the carbs. This year when I cleaned them, I figured the positions were locked in, and I just put them together. I should have at least bench synced them before putting them on the bike, to make sure they were still good.

I may sync them before I take them off the bike, and see if that is the issue.

[Fjbj40]

Take a propane torch, turn on gas but don't ignite. Move it around the carb to check for vacuum leak. When the leak draws in the propane the rpm's will rise.

if it is not a vacuum leak then tear into the pilot circuit.

[jsandidge]

Try to synch it first. It's a lot less work, and at least you could rule that out as the problem.

[chuck hahn]

Tom wins...post 8.

[mike_of_bbg]

I did not sync them when I put them back together. Probably a mistake. Last year I bought a Motion Pro SyncPro and synced the carbs. This year when I cleaned them, I figured the positions were locked in, and I just put them together. I should have at least bench synced them before putting them on the bike, to make sure they were still good.

I may sync them before I take them off the bike, and see if that is the issue.

Yeah, what he said. You've got the tool, man! Syncs should be at least done annually, whenever you do carb work, and when you adjust the valves.

[Steve]

Anyone use a colortune? Would it help me confirm no combustion vs very little combustion at idle?

Yes, I have used a Colortune. Yes, it can help confirm proper (or NO) combustion at idle.

However, as others have mentioned you should use a Carbtune (or your Motion Pro) first.

If you already have a Colortune, feel free to use it, but don't count on it.
I used one on a bike with VM carbs and was VERY excited about the results, so I took it home to use on my wife's bike. I saw absolutely NO change in the color throughout the adjustment range. The only thing I saw change was the frequency of the nice, blue flame. As I leaned the mixture screw (on the BS carbs), the flame would occasionally stutter, then finally go away. As I richen the mixture back up, the flame would stutter into existence, then stay steady. Richening the screw farther made no change to the color of the flame.

I posted the results here and asked about the experiences that others have had with a Color tune on BS carbs and the results were about 50/50. Some saw changes, others only saw the stuttering that I saw, so I chose to not get one for my own toolbox.

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