Carb issue [FIXED]

So, my bike is idling erratically, either up to 4-5k rpm or choosing to slowly stall out and die, and I suspected a vacuum leak on the engine side of the carb. I took some WD40 and sprayed it on the rubber carb holder boots and on the throttle shaft seals around carbs 3 and 4, and the bike ran JUST FINE... at least until the WD40 stopped holding the seal.

I pulled off carb holders 3 and 4 and filled in any cracks with black high-temp RTV sealant and ensured that on putting the carbs back on the bike, that everything was secure. The result is that the issue is not corrected.

I'm thinking that I should replace the carb throttle shaft seals. If they are truly the problem, would I be seeing idle speeds that are either happy up at 4-5k rpm or that instead choose to die out (both at the same idle adjustment screw position with no throttle)?

Should I suspect my enrichment circuit seals as well? Are there o-rings or other things that seal that up inside of my carb?

After all of this LOVE that I've given the carburetor, it should be fixed by now!!!!


(Some of my previous carb work: 2 X carb cleaning, float bowl adjustment, patched small holes in rubber diaphragms with liquid electrical tape, and tightened on of the throttle tension springs - which likely added too much tension to the old throttle seals and messed them up.)

I'm going to try loosening that spring that I wound an extra time around and hope that the seal closes while I'm waiting for some of your guys' feedback.

 

Re: Carb issue

You can seal the manifolds with rtv by running a bead around them at the engine. Propane works better than WD40 and way less messy to find leaks.

So tell us about your idle. You start with choke on or no choke? Immediate rise to 4 or 5 k hang for a while than down to stall? Cold start or does it do it even when warmed up?

 

Re: Carb issue

Oh! Thanks for the reply

Ummm, I find that it's harder to start since I sealed diaphragm holes with liquid electrical tape and set the proper float levels (they were all too high by a couple of milimeters before). It hasn't wanted to start in the morning here since pulling the carbs off and reinstalling them after making these modifications, and this place (Albuquerque, NM) isn't even cool this time of year. I've had to go buy starting fluid for the first time ever. In the past, I've needed very little choke to start it in the morning.

With starting fluid, I start it with no choke, and I don't necessarily get the 4-5k hanging idle from the start. It seems to randomly switch between 4-5k hanging idle or the stalling out idle, no matter if the engine is cold or hot.

 

Re: Carb issue

Even on 90 degree days mine likes a bit of choke to cold start then you can cut it off when the engine warms up.

Sounds like it could be air leaking but it's odd it only revs for a bit them dies off - could be a leak that is not leaking all the time. Or it could be fuel it revs until it pulls the bowl don then dies off. This would assume you are having a flow problem from the petcock. Have you verified it is fine and checked the vacuum line for leaks/cracks?

What happens if you put it on prime and start up?

 

Re: Carb issue

Hmmm, I do think that my petcock valve leaks when my engine is off. The vacuum line running from the engine intake to the petcock valve looks quite new and supple, so I don't suspect that. I do think that when parked,the leaking petcock valve has caused gas leakage while parked a few times. I've been planning to rebuild the petcock valve, but never got around to it.

I think that I have also run this thing yesterday with the PRI setting, but am not very sure. I do remember fiddling with it, even after I used it to fill my bowls up after I put the carbs back on. Perhaps I should do that as well, but to the best of my memory, even in the prime setting it runs like crap.

I do notice a weird thing with my carbs, however. After removing the carbs from the bike, I notice that fuel continues to be delivered to the carbs when I disconnect the petcock line from the filter/carb fuel line, almost like the bowls continue to fill up. I'm not even sure where that fuel goes to, but I am sure that when I check the float levels with a plastic tube, they are exactly where I set them. Odd stuff, eh?

 

Re: Carb issue

For sure - it sounds like the float needles are leaking but you should see high bowl levels if that were so.

I would try the propane trick and see if you have any leaks - resolve the leaks and go from there.

 

Re: Carb issue

Hmmm, propane trick? I wonder where I can get a cheap propane torch. would running an unlit cigarette lighter work as well? (My girlfriend calls me cheap, but it's hard not to be when you're a student eating ramen all of the time)

 

Re: Carb issue

I think they run about $20 most places. Works very well and could help you find any leaks. Can you borrow one?

 

Re: Carb issue

Hmmm, actually the lab that I work in has a ton of torches laying around come to think about it, and they aren't too anal about us borrowing tools as long as we return them...

 

Re: Carb issue

UPDATE: Alrighties, I've used high-temp RTV sealant to seal the carb boot holders onto the engine intake AND moved the throttle cable holder that is on top of the carburetor to its proper place. It was bumping into the engine head, and was installed wrong for so long before I bought the bike that it chewed a little dent into the head!

The result is that my erratic idling is now over! It now runs like it used to, but it kinda has a tendency to go to sleep slowly and die during idle, a problem that lessens as the bike warms up. I think that setting my idle adjust screws, checking my valves, and synching my bike should clear this up?

ALSO: I spent all yesterday showing a friend how to clean his Suzuki GS500 carbs, and the BBQ and beer was amazingly good fun in the process. He left his bike sitting all winter (like he does every winter), and it just wouldn't start. The result of his carb cleaning is that his bike started!


NEXT STEP: do the valves, adjust idle screws, sync the carbs.

 

Re: Carb issue

Ah, another thing... I did the propane torch check that you guys recommended, and I couldn't get ANY change in my rpm, which I think is a really good thing! No change at all... vacuum leak solved for sure.

 

Re: Carb issue

Yup that should help you out. Glad you got it nailed. I really enjoy working on the bikes - keeps me out of trouble.

 

I hear you guys talking about that propane trick to test for leaks.How does that work exactly,never heard about that before???Curious me!!!!

 

You use an unlit (but "on") propane torch to probe an idling bike for leaks in the area of the intake manifolds, throttle shaft seals, etc; if there's a vacuum leak it will suck in the propane and cause the idle to go up. It DOES work; it won't however diagnose all vacuum leaks.