Cold Cylinder!

[MPittma100]

1979 XS11 Special (SF) has cold (header warm to touch) #1 cylinder.

#1 cylinder has 170 PSI compression. Others are same. Other headers are "hot as a firecracker" at idle. New Dyna coils and wires. Carbs were off and cleaned most recently - specifically the pilot jets and pilot circuits. Pilot jets are correct 2-1-2-1. There are filters on carbs. The #1 idle mixture screw does not affect idle at 1100 RPMs. Nor does removing the vacuum cap affect idle whereas all 3 others do. The spark plug on #1 is black where the others are brownish in color. Switching plugs does not change anything.

Running wise, it stumbles briefly off idle and then resumes good/full power afterward through RPM range.

So what have I missed? It appears that this is a carb (pilot circuit) related issue but I am now lost.

Your thoughts??

Thanks, MP

[Gallaecus]

Im pretty well convinced at this point, that carb cleaner, sea foam or what ever else you spray in the idle circuits when cleaning does not really remove 30 years of crud, and if it sat for years like most of them have, its worse.
my guess would be that you need to clean carbs again,

but you could try unscrewing the plug cap and cutting a quarter inch off the plug wire and screwing the cap back on.

[James England]

you could try unscrewing the plug cap and cutting a quarter inch off the plug wire and screwing the cap back on.

He has new wires though, so it's unlikely to be that.

More like the carb, as you said. MPittma100 says he's cleaned the carbs off the bike. It very much depends on the definition of 'cleaning', doesn't it? Many people claim to have cleaned the carbs merely by running a product through them in situ. Seafoam, Redex and any other product will not beat stripping a carb and cleaning it thoroughly with all the jets removed, airways and fuel ways blown through with compressed air etc. The only thing I would not remove would be the butterflies. Nor would I split the carbs as it's not necessary for a thorough strip and clean.

[BA80]

Fuel, compression and fire...........one of those 3 is missing at idle.

You say you have spark and compression so your missing fuel.

There are several things that could cause no fuel at idle. The first thing that comes to mind is a broken tip on a mixture screw because that is the most common. Are you certain that there isn't a I/M screw tip broken off and stuck in the idle port?

A clogged pilot circuit. All pilot galleys clean and flowing? There is a transfer port between the main circuit and the pilot circuit that is REAL easy to miss if you not familiar with the 79 carbs. (BTDT) It's just below the plug screw in the pilot well and the main jet in its well. You'll have to take the emulsion tube out to really check it.

Those are the most common problems.

[PalmSpringsXS]

I have a stupid yet obvious idea to try. Do all the carbs have the idle circuit enrichment tangs in one piece? I had dealt with a similar problem years ago and one of the tangs (I don't know what else to call them) had rusted and snapped off so that one of the exhaust headers wouldn't heat the same way that the other three did until the bike was ridden for about 10 minutes. If this has happened on the number 2 or 3 carbs, you may have to pull them to tell.

[BigRed]

Have you tried swapping the spark plugs and wires between #1 and #4 cylinders? I agree that this is likely a carb problem, but it only takes about five minutes to make the swap and be sure.

Good luck!

[DGXSER]

The #1 idle mixture screw does not affect idle at 1100 RPMs. Nor does removing the vacuum cap

Kind of telling right there, sounds like a vacuum leak to me.

The spark plug on #1 is black where the others are brownish in color. Switching plugs does not change anything.

The black plug is kind of contradictory to a vauum leak though. I had a very similar problem with my bike recently, I found I was missing the mixture screw though. Reality was that the spring in there was very weak and it would loosen up and run like crap and then the cylinder would backfire through the carb and blow out the mix screw.

[MPittma100]

Cold Cylinder No Longer!

I think that you will be very interested in what I finally found. This was one of those #@!!!%&^#!! problems. A guy told me the other day that - if you can't fix it with a hammer, it's an electrical problem?

No. 1 cylinder was the cold one as you recall. It had good spark and plenty of compression. Even though I just had the carbs apart, I went back in to number one carb (without taking all of them off). I removed the bowl, main jet, plug, and pilot jet. Did not see a problem-no clogs.

So I put it all back together, turned on the fuel and started it up. It mysteriously is now running on all 4. All pipes are getting hot, even number one. Then suddenly, about 30 seconds in, I see fuel running out of the air box drain?

Shut it off and turned the fuel back off and was scratching my head? As it turns out, if I left the bowl screws kinda loose for testing, the overfueling would happen, but it would run on all 4. If I tightened the bowl up, no fuel spilling, but the pipe was cold. When the pipe was cold, I could take the bowl drain plug out and no fuel came out?

So it had too much fuel or none. Sounds like the float was hanging on the bowl, right. Well you are correct, it was. The float is exactly what was causing the problem - even though it was a new float. I put a used float back in, adjusted it, and all is well - FINALLY! Who would a thunk it?

The new float was not bent or banged up. As I looked at it closely beside of the used float, I saw that the 2 floats on the new one were soldered differently to the swingarm - they protruded more than the used one did and this was allowing interference with the bowl.

Anybody ever have this happen to them?

Thanks for your help

[jetmechmarty]

Where did you source the new floats?

[MPittma100]

New Floats

I believe that I bought them off of one of the Ebay stores several months ago. Will have to do some digging to see exactly who it was.

MP