'79 GS850 Power Sputter

[Abbondanza]

Hey everyone, I recently re-joined the GS owners family when I purchased a used '79 GS850 (had a GS450 awhile back). Soon after acquiring this 850, it began to power stutter (thats the best term I can come up with to describe how it feels) throughout the gears, as I accelerate..when I get on the highway and try to open it up, it barely reaches 60 mph full throttle in 5th gear, then it seems to eventually jump out to 75-80, and then stuttering back down to 55-60, then repeating the pattern.

The previous owner didn't indicate it was dormant for any extreme period, but of course you never know, I changed the plugs, and noticed they looked sooty and the threads had what appeared to be oil on them, although the bike does not leak oil....the new plugs didn't seem to make a bit of difference with the power inconsistency...another factor was recently after a 15 mile drive and a 3 hour stop, I re-started it and one of the hoses that come out of the bottom of the carb started spitting gas out at my feet...not sure if thats related to the power loss but thought I'd be thorough with all pertinent information...

I am intermediate in mechanical experience, so any initial help anyone could give me in things to investigate or test to isolate the problem would be greatly appreciated! I don't have the funds to pay a mechanic to do it so I will attempt all advice myself....

Thanks in advance for your help,

Jon

[Abbondanza]

just an addendum, I pulled each spark plug and replaced it into it's respective boot, did the whole crank the starter with it grounded to the head thing, each one sparked, air pumped out of each vacant hole, I put my thumb over the outside cyclinder holes (all I could reach) and if I pressed hard I could hold it with out the pressure forcing it off...does that mean the compression is too low? How do I check if I'm running on all 4 or not?

[Guest]

Sounds like you have a sticky float or a bad petcock. Do you have a vaccuum petcock? Sometimes the diaphragms go bad and they leak air so it does't offer good flow and cause gas starvation at highway speeds. It also causes the gas to flow when the bike isn't running and cause overflow. Try turning the petcock off while not riding.
A sticky float will cause gas starvation because it won't allow flow into the bowl as quickly as it should and could also cause the bowls to overfill while sitting causing overflow. I would check the petcock and clean and rebuild your carbs including the o-rings. Also check your intake o-rings and all boots on your airbox. Don't forget to adjust your floats correctly.
Just read your last post, you should get a compression gauge and hook it up to one cylinder roll throttle as for as you can and hit the starter button until the gauge stops rising and if it registers lower then 100 lbs you have a problem. Pour a little motor oil into the cylinder and try again if the pressure rises then you have bad rings, if it doesn't it is a valve problem

[JayH]

How does the inside of the tank look? Did you put fresh gas in the tank? I'll assume that the bike was sitting for a while and the gas inside was stale.

Also look to see if you have an inline fuel filter between the tank and carbs. This will prevent any small rust particles that get past the screen on your gas tanks petcock from going into the carbs.

[mark]

The easiest way to check if you're running on all cylinder is to feel your exhaust headers as the bike is warming up. Be careful...they warm up fast!

I have the same model bike as yours and I had pretty much all the problems that you describe. Post's above are pretty right on, just be systematic about it.

Check your petcock. On "run" or "res" no fuel should flow into the carbs when the bike isn't running. The smaller vacuum line shouldn't be drawing any fuel when vacuum is applied. Also, as said above, you might be sucking air into the fuel line from the vacuum line. You can test for that with a clear plastic line. Gas should flow freely on "Prime" even with the bike off.

Also, get a manual and go through your tune-up procedures. You have points ignition, so take off the cover while the bike is running....see if the points are arching (they shouldn't be), check the timing and point gap, just basic tune-up stuff.

One basic thing about these 850's is that your airbox needs to be sealed perfectly for the bike to run well. Check to make sure that your clamps are tight to the air box and also the intake boots. Check that the side cover gaskets on your airbox are in good shape and have a good seal.

You might have to remove and clean your carbs to make progress as well.

Intake manfold O-rings mentioned above often need to be replaced. They make a seal where your intake manifold meets the cylinder head. When the bike is running you can spray a little water on them....sucking noises or rpm changes indicate that they must be changed.

When you do a compression test, make sure you hold your throttle wide open, or your readings don't mean much.

Good luck messing around and let us know how it goes.