Running Again, Almost?

[DaveP]

It has been a while (1 year +), but the 1981 GS650L is running again for the first time tonight. I had done most of the work on getting it back in running order previously. Everything from valve adjustments and carburetor cleaning to re-building the brakes. Everything was running well until the rear cam chain guide broke in half and I had to completely tear down the engine to replace the guide. The number 3 intake valve was also bent when the timing jumped with the broken guide.

Everything is back together now, valve clearances double checked/adjusted for the valve replacement, and the engine is back in the frame. I did a cold compression test just to make sure everything seemed to be in order. Very consistent across all four cylinders with 140-150 psi. Assuming would be better with a warm engine. Carbs and air box were already installed and throttle held wide open to check the compression.

Started the bike tonight. Didn't start easily, but now sounds good, idling smoothly, revving nicely..., but the number 2 exhaust header is ice cold compared to 1, 3, and 4. A damp paper towel against 1, 3, and 4 will sizzle. Nothing on 2.

Confirmed that there is spark on number 2.

Any thoughts why the number 2 cylinder appears to be running cold compared to the others?

Thanks,
Dave

[Guest]

Since you confirmed spark and compression, I would check fueling next, maybe a stuck float? Blocked jet? At least you have it narrowed down to 1 carb to diag.

[Big T]

Blocked carb, or the vacuum diaphragm on the petcock has failed and flooded the #2 cylinder

[tom203]

Blocked carb, or the vacuum diaphragm on the petcock has failed and flooded the #2 cylinder

Yeah, petcock failure is likely - mine failed like this....you can test it or just get replacement for about $50.

[DaveP]

Currently running on a portable fuel tank assuming that I was going to be checking the carb sync again after everything was running properly. Once I am back to using the bike's fuel tank, the petcock was also replaced in the earlier work. With the portable fuel tank, a golf tee is currently plugging the vacuum line. The portable tank does have a basic fuel valve inline. The valve is closed when I am not trying to run the engine.

When I removed the number 2 spark plug to check for spark, the plug was dry.

So blocked carb seems most likely.

I'll have to look later, but probably not much I can do without removing the whole gang of carburetors again??

Thanks,
Dave

[Rijko]

How about a squirt of brake cleaner into the vacuum line ?

[Guest]

Currently running on a portable fuel tank assuming that I was going to be checking the carb sync again after everything was running properly. Once I am back to using the bike's fuel tank, the petcock was also replaced in the earlier work. With the portable fuel tank, a golf tee is currently plugging the vacuum line. The portable tank does have a basic fuel valve inline. The valve is closed when I am not trying to run the engine.

When I removed the number 2 spark plug to check for spark, the plug was dry.

So blocked carb seems most likely.

I'll have to look later, but probably not much I can do without removing the whole gang of carburetors again??

Thanks,
Dave

Yeah, those will have to come off again. I think you could most likely start with leaving them ganged together though. I would first check the float for proper operation, sometimes they get sticky and wont flow fuel to the bowl, or if you accidentally installed it back wrong it could again not flow fuel to the bowl. If its not the float I would then re-clean the idle ciruit/pilot jet to ensure it is not blocked off. Since it is getting fuel past the #2 carb to the outside carb, I wouldn't suspect it would have anything wrong with getting the fuel from the tank to the carb ie the "T" connections. Good luck, let us know what it was when you track it down.

[Brendan W]

Space is tight in there but before removing them I would try to get the top off the #2 carb to see if the diaphragm is seated right.

[DaveP]

Just opened the float bowl drain. Gas is in the bowl, but not getting to the cylinder I guess. Disappointing to remove the carbs again since even now, one press of the start button and it fires right up.

[chuck hahn]

SYNC carbs. The throttle plate isnt as open as the others.


EDIT..take them off and ensure you have a good EVEN bench sync between all throttle plates.

[DaveP]

I'll try the sync before taking them off again. Wasn't thinking that would be the issue since they were synced when it was last running and shouldn't have been a major change. Picture is Carbtune from after the carb removal and cleaning.

IMG_4503.jpg

[chuck hahn]

Anytime you "clean" carbs or do a valve adjustment this changes the whole thing, so yes it also changes sync. I can also suggest you loosen the adjuster screw between 1 and 2 and gently turn the adjuster to see if the offending cylinder finds life. May have to turn the idle up a bit to prevent stalling while you test tweek the adjuster. If a tweek brings it back online then proceed with another sync...and try to get the bars a little tighter.

[DaveP]

Finally had a chance to check the carb sync, but did not improve the number 2 cylinder. Pulled the carb bank off as soon as things cooled down enough and opened up the float bowl on the number 2 carb. There was fuel in the float bowl, but interestingly the fuel across all 4 carbs has a distinct green tint. The fuel is new, treated with Seafoam and Stabil 360, and clear in the auxiliary tank... The real problem seems to be some crud that has already been generated in the carb. The main jet was completely plugged. Nothing solid, but enough to prevent a good flow of fuel. Easily cleaned with piece of wire and carb cleaner.

The fuel hose was replaced with Suzuki OEM hose. Any possibility that the fuel is deteriorating the hose that quickly to create this crud within the carburetor? There is nothing else that would be introducing this to the fuel that I am aware of of. The number 2 carb would be first to receive this crud before the other 3 carbs?

[tom203]

"The number 2 carb would be first to receive this crud before the other 3 carbs?"

unlikely, they'd all get a taste. The new fuel hose is not likely to deteriorate , but if it was laying around , critters could have found a hideout. Does #2 still have fuel bowl inlet screen?

[Big T]

There's a crystalline build up from evaporating gas causing the green tint.
While a full carb clean is generally recommended, it may not remove the problem

I recommend riding it and see if the problem reoccurs. Run a bit of Seafoam in every tank of gas