Odd '77 GS750 Carb Issue??

[DAS750]

First off, first post so bear with me. I checked out all the historical as well as information on other sites but none seem to hit the nail on the head. I have re-incarnated my old '77 GS 750. I'll get photo's up soon. New electrics, LED lights, custom made light bar, crash bars sissy seat, repainted, bone yard Harley fender, wide bars, all done in the garage. I've had the bike since '94 and it sat for a while in the garage, with gas in it, for about 5 years. I know shame on me, had kids what are you going to do. Anyway to the issue. Start the bike, runs fine. Idles smooth and runs great for about 10-15 min. after that will randomly bog down like the engine is getting flooded when I hit the throttle. Carb issue I would think.. sounds simple. Issue is tried to use the old SeaFoam in the tank approach, no good. Basic cleaning of carbs, no good. Pull carbs apart and soak, no good. Look pretty well sync'ed, like I said will run great through all gears for about 10-15 min until it gets pretty warm. Will idle all day if I let it. Some black smoke out of exhaust for #3-4 if I really crank it. Float level OK, intake boots and O rings OK, needle valves seal well, main jets and air mix seem OK for firs 10 min, etc. Could take apart and soak again but really don't want to have to sync again if i can avoid it. Suggestions???

[tempebru]

When was the last time you checked the valve clearances? And at what rpm does it bog? How new is the air filter? Does it have new points and condensors? Have you checked the timing and advance?

[Coady]

Sounds fuel related. Running pretty rich to blow black smoke. Was it running well before you parked it?

[DAS750]

Bike was running well when it was originally parked, Air filter is new, Timing was checked and looks good. Points were replaced about 750-1000 miles ago and timing was adjusted then but still checks good. I have not checked the valve clearances but I would think tha twould show up more consistently. It runs great for the first 10-15 min if I am either simply idling or actually out riding. as far as when it bogs down it seems to happen at a range of rpm's but only occurs when the throttle is increased. It will bog down like it is flooding, if you play with the throttle a few times the rpm's will race and it will settle for a minute or so (litteraly) and then act up again. It makes for a long slow return from a mile or two from home.

[Nessism]

Sounds like a petcock issue. You might want to check to see if you are getting good flow.

[DAS750]

Nessism..Just rebuilt the petcock, new seals, new fuel lines. If anything I might be getting to much flow but it only seems to be comming from one side. This should limit it to Carbs #3 and/or #4 I would think. After a while this exhaust pipe also seems to run a bit hotter then the other.

[Steve]

Nessism..Just rebuilt the petcock, ...

Just because it was rebuilt, does not guarantee that it's working properly. (especially a petcock)

Please test and report back.

Besides, did you notice Nessism's signature line? "To measure is to know."

.

[DAS750]

OK,
Sorry forr the long delay, Was waiting for a new petcock kit and some time. Christmass and all. All new gaskets and the like. Checked the vacuum. Also cleaned the carbs again and inspected CLOSELY. All looks OK. I seem to be getting good flow from the petcock under vacuum. The carbs also do the same thing if the valve is on the res setting. I belive this setting takes the vacuum out of the situation. Any other thoughts. Seeing as it is Winter in NE Ohio getting road time is tough but I'm open to Idea's.

[Larry D]

I'm not familiar with your carbs, but, it's seems like it's running really rich to me. Your off idle mixture sounds way off. Where are the fuel and air mixture screws on the carbs set ?

[don]

I don't even pretend to think I know about all of the different GS's in existance like most of these guys do, but I do know mine pretty good. I had a verry simmilar situation with my 83' 550. If you do a search on " gas in airbox " you will find a quite lengthy thread by me. (I basicly spent about 1-1/2 seasons trying to figure it out with the help of these GS folks.)

The symptoms you describe are all to farmilliar to me. In the end it was my petcock. The one on my bike has 2 diaphrams in it. One is the big one that closes and opens the fuel passage, (which comes in the rebuild kit with all of the other various o-rings, gaskets, etc), the other one is a non-replaceable one. It's covered by a small white plastic disk with a few small holes in it. It ammounts to a simple flapper valve in the vacuum inlet side of the petcock. The one in my petcock completely decintegrated. It's purpose is to open when the piston, (in the engine), draws vacuum, but close when the piston is coming back up and the intake valve is closed. This keeps a constant vacuum on the larger diaphram so it stays open while the engine is running. Without this vacuum check flapper thing, the large diaphram opens and closes with the vacuum pulses of the engine, acting more like a fuel pump than a valve. The end result is a preasurized fuel system, (even with the petcock set to prime).

I would try to run your bike somehow without the vacuum line hooked up to the petcock. If the problem still exists without vacuum on the valve you can rule out the petcock for sure.

Sorry for the long winded post but it's kind of hard to describe ........