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Oil in cylinder #3 - 1979 GS750

  • Thread starter Thread starter Farnworth
  • Start date Start date
F

Farnworth

Guest
Hello everyone,

I have a 1979 GS750E, bought last winter.

I've been trying to pinpoint an issue with my spark plug fouling on cyl #3.

I suspected it was the carbs that was allowing too much fuel into the cylinder (I've completely rebuilt the carbs, but still same problem), but now I realize it's really oil fouling. If I put new plugs in, the 3rd cylinder will always smoke when started, I originally thought the smoke was caused by old gasoline. It will only stop smoking once it's been fouled I suspect, the bike slowly starts to lose power and feel sluggish after several kilometers. Also the engine causes the whole bike to vibrate a lot (could this be caused by cyl #3 not firing? Or is it something else?).

I can clearly see oil on the spark plug end and everywhere on the top of the piston #3. All the other cylinders are dry.
I've done a compression test and my results are: Cyl1:90psi, Cyl2:90psi, Cyl3:120psi, Cyl4:90psi
I suspect the 3rd cylinder gets a boost because of the oil leaking into it.

Is there a way for me to find out if the problem is the valve stem or the piston ring?

Would a valve stem leak really cause such evident issues?

Thank you.

PS, love your site, extremely helpful!

Alex
 
Check your petcock, #3 is the one with the vacuum tap for the petcock. If the vacuum diaphragm leaks it will put a lot of fuel in that cylinder, which can wash the oil off the rings. This causes them to wear. If this happens for a long time the rings are running the whole time without much oil, it will wear out the rings to the point that they don't seal very well..... But your #3 has the highest compression, so it's not too far gone. Check the petcock by sucking on the vacuum tube and see if fuel comes out, if it does that's your problem. It should be bone dry in the vacuum line. If it's got gas in it replace the petcock and run it a while, like a 1000 miles or so and see if the oil usage fixes itself. It might, and it might not.

And please let us know how it works out.
 
Could be an oil leak as he states and the oil is sealing the rings on #3 as well. Valve stem oil seals will leak down into the combustion chamber when the bike sits for awhile and smoke on start up. Could also be the cylinder head gasket leaking a bit if it is oil. It may not be oil and be the petcock as already mentioned. The vacuum petcock was put in place because some owners thing floats and float seats/float needles are supposed to cut gas off when the bike is parked. The purpose of those things is to meter gas when the bike is running and are not always great at stopping a puddle on the garage floor when the bike is parked on side stand (usually carb #1) and the seat/needle are overwhelmed by the gas in the other carbs pouring into that bowl. The earlier bikes had manual shut off petcocks but some folks forgot to shut them off. One probably went into his garage and tossed his cigarette on the floor and POOOOOF! Lawyers to the rescue and the vacuum petcock was born (my theory only)... first thing I did when I got my project 1000 was scrap the vacuum petcock and put a Pingel adapter plate on and screw in a manual petcock for this reason. They are much more reliable if you remember to turn them off when parked; especially on side stand.

If you ever have poor compression (90 psi is marginal I guess) and want to test rings vs valve seal, after you pull your plugs, put ONE tea spoon of oil in each cylinder in turn AFTER checking compression without the oil. DO NOT put oil in all the plug holes at once unless you remember to wear a raincoat and goggles. Don't worry, if you forget and fill them all you won't have to be reminded a 2nd time!
 
Sure could be any of these things, but the petcock is the easiest to test, and might avoid taking the engine apart needlessly. Anytime the cylinder with the vacuum port is the one with the problem, that's the first thing I would suspect. I'm usually right.
 
Hi Kent,

That's exactly what I thought the problem was this afternoon. I noticed there was gas in the vacuum tube, so I know for sure the diaphragm is leaking, but I didn't see how that could be related to having oil in the cylinder.

There's still a ton of smoke once I put new plugs in however, I don't understand how so much oil would get into the 3rd cylinder. Even if the diaphragm was leaking, how is the oil getting into the cylinder?

tkent02 said:
Check your petcock, #3 is the one with the vacuum tap for the petcock. If the vacuum diaphragm leaks it will put a lot of fuel in that cylinder, which can wash the oil off the rings. This causes them to wear. If this happens for a long time the rings are running the whole time without much oil, it will wear out the rings to the point that they don't seal very well..... But your #3 has the highest compression, so it's not too far gone. Check the petcock by sucking on the vacuum tube and see if fuel comes out, if it does that's your problem. It should be bone dry in the vacuum line. If it's got gas in it replace the petcock and run it a while, like a 1000 miles or so and see if the oil usage fixes itself. It might, and it might not.

And please let us know how it works out.
Click to expand...
 
Too much fuel can wash the oil off the cylinder walls, and out of the rings. Without this oil the rings wear. Once they wear too much, oil could get past the rings in large quantities, enough to smoke or foul plugs.

Did you fix the petcock, or at least run it with the vacuum hose from the carburetor disconnected and plugged? Put the petcock on Prime, this will let it run without the vacuum line. Don't forget to turn it back when you're done!
Running it like this will tell you more. Run it like this a while to see if the smoke and the fouling reduces. May take a while, longer than a mile or two. Generally bad valve stem seals will smoke when it's started up, maybe while it's running, but usually not enough to oil foul a plug that quickly. You can sometimes ride real smokers a long time without fouling any plugs.

What color is the smoke when you start it up? And are these fouled plugs flat black or gloss black? There is still something funny going on here.

The smoke could be gasoline burning off from inside the exhaust, gasoline that has built up there from the leaky petcock. Could also be all of this gas fouling the plugs.
 
Last edited:
I haven't tested it yet, I will try in the coming days.

It is definitely oil smoke, I was able to scrape off some of the grime off the top of the piston and it was definitely oil. The smoke is bluish/white. It will only smoke after I have either cleaned the spark plug with a brush, swapped plugs or installed a new one into cyl #3, I also notice a considerable increase in power after the plug has been replaced. Once the smoke starts going away (after about a 10-15km ride), I noticed the power will go down as well, as if there is no combustion. It only gets worse with time after this. The smoke happens on idle and while accelerating (at any speed), I will see a very large puff of smoke go into the cars behind me. Once the smoking stops, it never comes back unless I replace the plug again. It only smokes out of cyl #3.

When looking into cylinders 1, 2 and 4, they are visibly dry, #3 usually has a wet spark plug (oil and gas on it), and the piston is black and oily.


tkent02 said:
Too much fuel can wash the oil off the cylinder walls, and out of the rings. Without this oil the rings wear. Once they wear too much, oil could get past the rings in large quantities, enough to smoke or foul plugs.

Did you fix the petcock, or at least run it with the vacuum hose from the carburetor disconnected and plugged? Put the petcock on Prime, this will let it run without the vacuum line. Don't forget to turn it back when you're done!
Running it like this will tell you more. Run it like this a while to see if the smoke and the fouling reduces. May take a while, longer than a mile or two. Generally bad valve stem seals will smoke when it's started up, maybe while it's running, but usually not enough to oil foul a plug that quickly. You can sometimes ride real smokers a long time without fouling any plugs.

What color is the smoke when you start it up? And are these fouled plugs flat black or gloss black? There is still something funny going on here.

The smoke could be gasoline burning off from inside the exhaust, gasoline that has built up there from the leaky petcock. Could also be all of this gas fouling the plugs.
Click to expand...
 
Farnworth said:
I haven't tested it yet, I will try in the coming days.

It is definitely oil smoke, I was able to scrape off some of the grime off the top of the piston and it was definitely oil. The smoke is bluish/white. It will only smoke after I have either cleaned the spark plug with a brush, swapped plugs or installed a new one into cyl #3, I also notice a considerable increase in power after the plug has been replaced. Once the smoke starts going away (after about a 10-15km ride), I noticed the power will go down as well, as if there is no combustion. It only gets worse with time after this. The smoke happens on idle and while accelerating (at any speed), I will see a very large puff of smoke go into the cars behind me. Once the smoking stops, it never comes back unless I replace the plug again. It only smokes out of cyl #3.

When looking into cylinders 1, 2 and 4, they are visibly dry, #3 usually has a wet spark plug (oil and gas on it), and the piston is black and oily.
Click to expand...

OK, still give it a try with the vacuum line disconnected, just as a test.
 
Tom's half right

Pull the vacuum line from the carb and put a cap on it. Run the bike with the petcock on Prime and see if the problem goes away

Those are low compression numbers. When did you last adjust your valves? Has this bike sat idle for years?
 
I've only had the bike for a few months, I haven't done much engine tinkering with it as I was waiting for spring to come around so I could test ride it, so I've never adjusted the valves.

The previous owner had it for 4 years, and he rode it in the summer only, put few miles on it, but it has about 75,000km on the odometer.

Big T said:
Tom's half right

Pull the vacuum line from the carb and put a cap on it. Run the bike with the petcock on Prime and see if the problem goes away

Those are low compression numbers. When did you last adjust your valves? Has this bike sat idle for years?
Click to expand...
 
Farnworth said:
I've only had the bike for a few months, I haven't done much engine tinkering with it as I was waiting for spring to come around so I could test ride it, so I've never adjusted the valves.

The previous owner had it for 4 years, and he rode it in the summer only, put few miles on it, but it has about 75,000km on the odometer.
Click to expand...

Adjust the valves. Might not help your problem but it needs it anyway. I have only ever bought one GS in which all of the valves were in spec, one bike out of about 50 something.
 
I'm looking for a good valve adjustment guide on this forum, know where I could find one? Or some type of maintenance/common problem guide?
Do you know if there's some type of Haynes manual available for this model/year as well?

tkent02 said:
Adjust the valves. Might not help your problem but it needs it anyway. I have only ever bought one GS in which all of the valves were in spec, one bike out of about 50 something.
Click to expand...
 
Oh this is great, been looking for one for a while now.

Thanks a ton for your help so far!
 
Hi guys,

So I've tried plugging the vacuum hose and setting it to prime and so far it seems to have solved the issue. I haven't had time to check more in depth but will soon.
I've put over 100km on it so far with no issues.

I have noticed however that the engine causes a lot of vibrations, after a one hour ride my whole body is tingling, I've never had any vehicle vibrate this much. I suspect it might also be related to valve adjustment, but I will need to wait until later to find out.

Do you know if something else could cause vibrations as such? It vibrates especially between 5k and 7k rpm.

Thanks for your previous help, I'm very grateful.

PS, here's what the bike looks like at the moment.
IMG_20160514_164710073.jpg
 
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