1979 gs750l #3 cylinder problem

[crowfoot]

#3 cylinder has cold pipe and seems to not be burning all the gas, it warms up a little after being ridden. All plugs firing as should, compression test was #1-165 #2-150 #3-150 #4-160 took carbs off, cleaned and rebuilt bottom part of carb, new jet, float valve etc. Same problem cont. Bought bike this year and put over 4500 miles on it with no problems until now, #3 cylinder pipe is not as discolored as the rest, possibly inticating theres been a continuing problem. Can anybody help me figure this problem out? Thanks.

[tfb]

Which carbie intake does your fuel tap vacuum pipe go to?

[spyder2120]

crowfoot, i also have a 79 750l and mine did the same thing.im not sure which one of these, or combination of these fixed the problem, but i dipped and rebuilt the carbs using mr. barrs o-rings, replaced the o-rings on the intake boots (very important, mine were like hard plastic so they couldnt seal), put new air tubes in the airbox (hard and not sealing), plus sealed the airbox (per basscliffs website), i also had a leaky exhaust system. between all these leaks nothing ran right. i also cleaned all the connections in my wiring harness. im no mech. by any means and learn as i go. tfb asked which carb the fuel tap vacuume line goes in. its #3. if you have a modified petcock valve like mine (thanks to the po), then plug the line, carbs #2+4 have vent line that originate fron the same part of the carb; make sure these are not obstructed. and the most basic thing is make sure you have good plugs in it, advance carry ngk's for a couple of bucks apiece. also pull the #3 plug and ground it on the engine to check for spark. it could be something else that i havent experienced, so maybe the cafe kid or someoneelse with his knowledge will happen across this thread. good luck with it buddy. a side tip... if you havent done so already; when you need new handgrips try progrip gels. they are about $10.00 and made a world of difference on my bike.

[tfb]

As the vacuum line comes off cylinder #3, then, it makes sense to check:
1. that the vacuum line doesn't have a leak/split in it somewhere, so that it's sucking in air; or
2. that the petcock diaphragm doesn't have a small hole in it, so that extra fuel is being sucked down the vacuum line to #3.

[crowfoot]

As the vacuum line comes off cylinder #3, then, it makes sense to check:
1. that the vacuum line doesn't have a leak/split in it somewhere, so that it's sucking in air; or
2. that the petcock diaphragm doesn't have a small hole in it, so that extra fuel is being sucked down the vacuum line to #3.

Thanks for your help tfb, im going to look into that.

[crowfoot]

crowfoot, i also have a 79 750l and mine did the same thing.im not sure which one of these, or combination of these fixed the problem, but i dipped and rebuilt the carbs using mr. barrs o-rings, replaced the o-rings on the intake boots (very important, mine were like hard plastic so they couldnt seal), put new air tubes in the airbox (hard and not sealing), plus sealed the airbox (per basscliffs website), i also had a leaky exhaust system. between all these leaks nothing ran right. i also cleaned all the connections in my wiring harness. im no mech. by any means and learn as i go. tfb asked which carb the fuel tap vacuume line goes in. its #3. if you have a modified petcock valve like mine (thanks to the po), then plug the line, carbs #2+4 have vent line that originate fron the same part of the carb; make sure these are not obstructed. and the most basic thing is make sure you have good plugs in it, advance carry ngk's for a couple of bucks apiece. also pull the #3 plug and ground it on the engine to check for spark. it could be something else that i havent experienced, so maybe the cafe kid or someoneelse with his knowledge will happen across this thread. good luck with it buddy. a side tip... if you havent done so already; when you need new handgrips try progrip gels. they are about $10.00 and made a world of difference on my bike.

Thank you also spyder, I certaintly will take anything into consideration at this point, im going to go see what I can come up with, thanks for your info.

[crowfoot]

problem solved

Hi, just wanted to let all know that my bike is running again and found that all the stinken problem was, was the petcok. I just didnt think it would be bad, because the guy I bought the bike from had put a new one on. Oh well, I'm just glad to be on the road again.
A couple of things to point out, the bad diaphram in the petcok would cause a little gas to drain in the crankcase, so if you change your petcok you would want to do an oil change before riding the bike.
And for my model there's no rebuild kit, and the whole petcok is discontinued. Found one at a bike shop for a 1980 gs 450l, It fit perfectly.
I thank you all for your input and help.
Crowfoot

[dlanes]

Not trying to hi-jack, but the carb line off #4 goes to my petcock. Should I change it to #3? That's how it was hooked up when I first got it. I've only cleaned out the carbs and replaced the fuel filter and fuel line. Oh and my petcock is pretty useless and they had a shut off valve on the fuel line which I use only if the bike is sitting for a long time. Also with some reading on this site I was able to check my floats and adjust them and now the bike runs awesome. I'm a noob and learning a lot quickly. This is my first bike

[tlepac]

Dlanes

Need to know what bike --model and year--that you have to answer that question but from my reading the majority of the VM carbs have the Fuel line going to the #3 carb from the petcock. Also would help to know what Carbs you have

[dlanes]

I have a 78' GS750 I think

[rustybronco]

Not trying to hi-jack, but the carb line off #4 goes to my petcock. Should I change it to #3?

I have a 78' GS750 I think

The vacuum line needs to be connected to #3, unless someone has put mis-matched carbs on your bike.

[tfb]

It doesn't matter which carb the vacuum line goes to; just so long as it's connected to a manifold -- any manifold -- that's the main thing!

My first GS1000S had the vac. line connected to #2; the second GSthou had it connected to #3. It's all 6 o' one, 1/2 a dozen of the other.

[rustybronco]

It doesn't matter which carb the vacuum line goes to;

It sure does matter which carb. on VM equipped bikes the #3 carb has a B-B closing off the bowl vent hole, allowing it to be used as a vacuum port.

http://s32.photobucket.com/albums/d44/rustybronco/?action=view&current=PICT2564.jpg

[dlanes]

K, thanks I switch it to 3. I'm learning new things everyday.

[rustybronco]

K, thanks I switch it to 3. I'm learning new things everyday.

Make sure there's vacuum at the #3 nipple. I've had people mix and match the carb bodys around.

Does the bike only run with the pectcock in the prime position?