No Combustion in #3, getting spark. Tune carbs? New plugs?

[Icculus88]

So my 79 gs 750 just got back from the shop (getting vacuum synced) and it ran well on the ride home. The spark plugs are a bit old, over a year, and got a bit gummed up from working on the bike (trying to start without pods or air filter). It ran well from the shop then after a few days of sitting she bogged a lot then i felt exhaust and #3 was not HOT. although when i warmed her up it sounded like all 4 were going then when i started riding i noticed a lot of bog and no power. then i felt the exhaust.

any thoughts? is this a carb tuning issue or electrical?

[tkent02]

Petcock issue perhaps.

[greg78gs750]

icculus, huh? ITS IN THE BOOK!! READ THE BOOK, OK!!!

sorry, had to get that out, heh. could be valves, have you adjusted them? how did the shop clean the carbs? shops have a reputation for shortcuts. do you have spark? you've been a member for a year now, you must have perused cliff's little site, done everything there yet, maintenance wise? we're gonna need some more info.....

[Icculus88]

can you explain further how it could be the petcock?

[Icculus88]

i rebuilt them, the only thing they did was vacuum sync and adjust floats. Greg, good one i wish i had a phriendly book for this GS. Bike CLiff comes close enough! yes i get spark, i'll go check again.

Also i want to get new plugs anyway, any recommendations or should i just get the NGK

[tkent02]

can you explain further how it could be the petcock?

Vacuum line from #3 carb. If the petcock diaphragm is leaking, carb #3 will suck a ton of fuel straight down into the carb and into the engine, it can be so much fuel that the cylinder cannot run at all.

Not saying it is the problem, but it might be. Whenever it's the cylinder with the vacuum line that has a problem, the chances are good the petcock has something to do with it.

[Icculus88]

Ok that makes sense. i have overflow tubes, if #3 was getting too much fuel wouldn't it overflow? I pulled the plug just now and it is pretty black and wet with fuel.

I pulled the plug and it did not spark. Popped in an old dry plug and it sparked. going to get new plugs now and we'll see how she reacts.

[tkent02]

Ok that makes sense. i have overflow tubes, if #3 was getting too much fuel wouldn't it overflow? I pulled the plug just now and it is pretty black and wet with fuel.

I pulled the plug and it did not spark. Popped in an old dry plug and it sparked. going to get new plugs now and we'll see how she reacts.

No, the overflow tubes are for the float bowls. This fuel comes straight down into the carburetor bore right in front of the intake port, it runs straight down into the engine from there. Just take the vacuum line off, plug the vacuum port on the carb, run the engine with the petcock in PRI and see what happens.

With a good petcock, no fuel will ever come out of the petcock vacuum line.

[MisterCinders]

Listen to tkent. He is wise.

Also, even with the new plug, check that vacuum line. You may have a petcock problem that fouled out the old plug and continues to flow gas where it should not.

Check your oil. If it is extra full and/or has any gassy smell, your petcock flooded the crankcase. If there is gas in that oil, don't start the bike until you correct the flooding and change the oil and filter.

[Icculus88]

Ok no gas smell in oil and its not too high. Threw in a new plug in #3 and bike runs great! I rode about 2 miles and am back. Any suggestions on reading the plugs or any further signs of petcock issues?

[tom203]

Ok no gas smell in oil and its not too high. Threw in a new plug in #3 and bike runs great! I rode about 2 miles and am back. Any suggestions on reading the plugs or any further signs of petcock issues?

Put some clear vinyl tubing on petcock vacuum fitting and suck on it- any signs of gas headed your way ?

[MisterCinders]

Ok no gas smell in oil and its not too high. Threw in a new plug in #3 and bike runs great! I rode about 2 miles and am back. Any suggestions on reading the plugs or any further signs of petcock issues?

See if the new #3 plug is extra rich.

Check for any gas or moisture in the vacuum line.

Pull the tank and run a fuel line into a container. Leave it in the ON position overnight. See if any fuel leaks into the container via a slow drip.

[Icculus88]

I have come to the bike after it has sat for a little while and the fuel line has air in it, as if fuel had slowly flowed into the carbs and they let a little air up. Would that have anything to do with petcock or is that just fuel evaporating from the carbs?

[koolaid_kid]

The second. If it sits for a bit, put the petcock on PRI until you get it started so the vacuum function will work. Then switch it back to ON.