GS850GL plea for help - bike won't pull past 5000 rpm

[Reckhard]

When the engine goes above 5k under light throttle it sounds reasonably normal but giving it more throttle does little but cause the stumble.

I've nearly convinced myself that the issue is with the main jets. I may have messed them up when I cleaned the carbs. I'm ordering a new set so will be giving that a shot and posting after they arrive and I have a chance to try them out.

[tom203]

Yes, all the 82’s had ignitors, but earlier models had separate mechanical advancers. Later versions had the advance curve built into ignitor itself. In any event, advance would be complete by 4K for both types. When you’re going down this hill lightly loaded, what happens at 5k …..any sense of misfire? ….no throttle response?

[Reckhard]

I don’t like the consistency of not pulling past 5k…..can you find a safe long downhill and see what happens when it’s lightly loaded under full throttle?
in my opinion, the main jet is doing most of the mixture delivery by 5k…..I’m assuming you got the rubber plugs over the pilot circuit.
I asked before if you had the mechanical advancer setup…..apparently some 82’s had the electronic gizmo ignitor.

I can get it to rev over 5K under light load - like down a hill. It caps at around 5 only while accelerating. My bike has the electronic ignitor - no points. Plugs are over the pilot circuit.
As I mentioned the bike pulls strong without the air filter so timing should not be an issue.

[rphillips]

Thinking the pilots won't have any noticeable effect at 5K.

[tom203]

I don’t like the consistency of not pulling past 5k…..can you find a safe long downhill and see what happens when it’s lightly loaded under full throttle?
in my opinion, the main jet is doing most of the mixture delivery by 5k…..I’m assuming you got the rubber plugs over the pilot circuit.
I asked before if you had the mechanical advancer setup…..apparently some 82’s had the electronic gizmo ignitor.

[Reckhard]

Why can't you go all the way to redline? Don't have a first gear?

I don't have sufficiently sized testicles to keep the throttle pinned when the bike is begging for a shift...

[Reckhard]

I just had another idea that might be my issue - I'm hoping that someone that knows a lot about carbs and jetting might be able to chime in.
When I cleaned the carbs I had purchased a cheap carb kit from Amazon but tried to use as many of the stock pieces as possible. What I used from the kit was: gasket, float needle and valve and the pilot jets. (I didn't want to replace the pilot jets but damaged one, I had to drill it out, and thought the a matching set was better than one that might be different than the others)

The carb kit didn't detail the specs but led me to believe that it was basically stock specs.

since my issue seems to happen during the transition where the pilot jet is the main player in supplying air/fuel to when the main jet is the big player I'm thinking the issue might be a mismatch between the pilot and the main jet. Unfortunately I have no way to measure the jets to confirm their size. My question is; is my logic sound? If so I should probably get new pilot jets where I know the spec...

The part that confuses me though is that it runs great up to 5K rpm when the unknown spec pilot jet is in play but not after 5K when the known OEM main jet takes over.

[Rob S.]

I'm truly surprised that the very small amount of restriction the air filter...

When I replaced the stock intake on my 11EZ with APE pods, I had to cover about 90% of each filter with tape before it would run good enough to ride it to the mechanic for re-jetting on a dyno.

Why can't you go all the way to redline? Don't have a first gear?

[Reckhard]

I just ran a bit of an experiment. Took the bike for a spin without the air filter and then with the air filter noting where and when the performance faltered.

Without the filter the bike struggled to rev to 5K rpm under load - but once it got to 5K it pulled hard and rocketed to red line (well it probably would have but I let off the throttle before I got there)

With the air filter the bike runs well up to 5K rpm and then falters and does not got much past.

I've dug into the details of the carb operation and it's unclear to me what might be causing this. I'm truly surprised that the very small amount of restriction the air filter introduces would cause such a significant effect. At first I thought that since the slide and the petcock were both vacuum operated that those problems might be tied together but now I'm thinking that is a red herring, the petcock issue will be dealt with in time - this performance thing is killing me.

[850 Combat]

These bikes with the CV carbs need the airbox or to be rejetted.

[Rob S.]

...the pilot screw is only used to meter air/fuel mixture at idle and very low throttle so don't believe it will help in this case.

That's my understanding of it.

[Reckhard]

I have a 1980 GS850GL and rebuilt my carbs and stock airbox. When I put them back on at 2 to 2 1/4 turns out on the pilot screw, turned out to be way too rich. Reset them all to 1 1/2 turns out and all was good.

If it runs good without the airbox, maybe you should adjust those pilot mixture screws in a bit.

Or, perhaps there is a leak in your vacuum hose? Or a loose vacuum nipple on the carb or petcock.

Thanks for the reply - I did a pretty thorough check for vacuum leaks and cannot find any including the fittings on the carb and petcock.
The bike runs perfect all the way up to about 5K rpm when it doesn't. My understanding of the mikuni cv carbs is that the pilot screw is only used to meter air/fuel mixture at idle and very low throttle so don't believe it will help in this case. You likely had different carbs.

[dgates1]

I have a 1980 GS850GL and rebuilt my carbs and stock airbox. When I put them back on at 2 to 2 1/4 turns out on the pilot screw, turned out to be way too rich. Reset them all to 1 1/2 turns out and all was good.

If it runs good without the airbox, maybe you should adjust those pilot mixture screws in a bit.

Or, perhaps there is a leak in your vacuum hose? Or a loose vacuum nipple on the carb or petcock.

[Reckhard]

I had the thought last night that the issue could also be that the bike isn't getting enough air. I just took it for a spin with the air filter out and amazingly the top end is better. I've been so fixated on fuel since that's what was totally messed up that I didn't even consider that it might not be getting enough air. I just took the air box off and will now clean it out as well as the filter. It's a brand new filter so I'm wondering if I clogged it when I did the oil soak...

So now I just finished taking the air box off and thoroughly cleaning it. I soaked and squeezed the filter in gasoline to clean it. Put it back in the bike before oiling to see if that helped. It didn't. Same issue is back. It's a UNI filter that is supposed to be a direct replacement for the OEM. I'm pretty sure oiling it isn't going to help the air flow...

[Rob S.]

With pet cock in "on" the bike gets starved for fuel but "recovers when switch to "PRI"... Does it go past 5K while on "PRI"?

That's what I was going to say. Because if it revs properly on PRI, that would indicate it's the petcock.