Petcock issue or carb?

[oldgsfan]

So, my bike, a 1980 GS850G has been bogging down up around 60-65mph.

I just did a valve job this past weekend and am pretty comfortable that I got all the valve settings within specs. The only issue I had was at one point cranking the engine without the shim in place and scoring one lobe.

But this bogging issue was cropping up even before the valve job. Now, though, it seems a bit worse.

It reminds me alot of an issue I dealt with about 20 years ago on another GS I owned back then, and that problem was solved with a new petcock.

So I'm thinking that I should replace the petcock on my bike, which looks like it's probably the original.

My question is, could an issue with my carbs also cause the engine to bog down like this? I've read some postings that seem to suggest this, but it just wasn't clear to me.

I did a carb rebuild last summer and was very happy with the result, but I never did a vacuum synch on the carbs, just a bench sync.

Could that be the problem, that the carbs aren't properly synched?

I am thinking I should get the new petcock anyway 'cause the one on the bike just seems so old.

The only bummer to this is that I was hoping to finally put on some shiny new exhaust pipes. I'm thinking I need to resolve this issue first.

 

[Guest]

What do you have to lose? That's one of the first things I did to my GS.

 

[Nessism]

Bogging like you describe is not likely to be petcock related. That would only be the case if the carbs are being sucked dry.

Some possible causes are the carbs are sucking false air from the intake boot O-rings or around some deteriorated airbox seals, coils are not getting full voltage, spark plug caps have developed resistance, carbs are dirty, or the carb jetting is lean for some reason (such as from installing aftermarket carb kits, or modifying the bike away from stock). Also understand that installing that new exhaust system will mess up the carburation if it's more free flowing than the stock system. Going away from stock has benefits for some people but it also can have drawbacks.

 

[tlepac]

I had a situation this spring where I was riding and experienced the severe bogging and then the bike just quit on me. I let it sit on the road side and after a few minutes it started and I was able to ride it but then the same sequence happened again. Finally I was able to limp into town--never going over 4000 on the tach and when I got home and checked things out the vacuum wasn't having much if any effect on the gas flow from the petcock. I tested this by putting a long hose to the vacuum port of the petcock and leaving the gas line open to air. When I sucked on the vacuum line--> No gas. I replaced my petcock

So my recommendation is to do this little test and if it fails then replace the petcock. You will be glad you did----BTW the metal round plate inside the old petcock was broken

Good luck

 

[oldgsfan]

Bogging like you describe is not likely to be petcock related. That would only be the case if the carbs are being sucked dry.
Some possible causes are the carbs are sucking false air from the intake boot O-rings or around some deteriorated airbox seals, coils are not getting full voltage, spark plug caps have developed resistance, carbs are dirty, or the carb jetting is lean for some reason (such as from installing aftermarket carb kits, or modifying the bike away from stock). Also understand that installing that new exhaust system will mess up the carburation if it's more free flowing than the stock system. Going away from stock has benefits for some people but it also can have drawbacks.

Thanks. I'll do some work this weekend and see if I can get to the bottom of the bogging issue. I just cleaned the carbs last summer, and they weren't that dirty when I took them apart to begin with, so I wouldn't expect foul carbs to be an issue.
You mentioned the airbox seal - I replaced both the intake boots with new O-rings and the boots for the airbox, but I don't think I did anything with the seal, which, as I recall, was not in great condition.

I'll look into that and I'll try that petcock test tlepac suggested.

Thanks for the suggestions.