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

[Reckhard]

Hi all,
I am not the type to ask for help but I'm about to drive my bike into a lake...

Back story and the problem:
I bought a 1982 GS850GL (19,000km) earlier this summer that had been parked since 1992. I was told it was 100% functional when it was put away and stored indoors for most of that time. This checks out with everything I've seen to date. I got the bike to the point where I can road test it and have been fighting this issue - when the bikes rev's to about 5K rpm under load it seems to bog down, almost as though it's hitting a rev limter. It will rev freely to the red line in neutral, the issue seems to only exist when the bike is pulling.

To get the enginer running I've done - in order:
-rebuilt the carbs - cleaned by pinesol dipping, and carb cleaner, new gaskets, and new pilot jets since I had to drill a couple out. I was not able to get the needle jet seats out so left the old ones in, new gaskets and O rings.
-fuel system was a disaster - tank was thick with rust, cleaned everything. New petcock. replaced all the fuel lines and fuel filter.
-bought a new foam air filter, all the old foam bits had turned to dust.

Test fire - bike ran like crap. I bought an ultrasonic cleaner and thought I'd redo the cleaning but more thoroughly - turns out the issue last time was some air box foam detached and jammed one of the carbs. But since I had the new cleaner I went through with another complete disassembly and cleaning.

Test drive - the bikes only rev's to about 5K rpm under load, almost as though it's hitting a rev limter. It will rev freely to the red line in neutral, the issue seems to only exist when the bike is pulling. As well when the petcock is in the on position the bike gets starved of fuel while driving but recovers when i switch to PRI.

I thought it seemed like a fuel flow issue - took apart the petcock, verified that the vacuum valve worked as it should - stumped. While in the PRI position there is tons of fuel flow.
Took off the carbs again - replaced the float needles and seats this time. Re-calibrated the float heights. No change to how the bike runs.

I then verified valve gaps were all within specs - they are (at the low end but above the spec). Also balanced the carbs. Plugs looked good but put in a new set. Still no change to the the WOT problem.

So now i'm thinking it's a vacuum problem since it seems like the main jet circuit is where the problem lies and it would also lead to the petcock problem. The bike starts and idles great, seems to be 100% until about 5K rpm and then turns to crap. I'm pretty sure the carbs are clean. I can't find any vacuum leaks. I really don't know what else to check.

Any thoughts from the braintrust at GSresources - not sure where to go...

[KiwiAlfa156]

If it were me, I'd start by checking that the timing marks advance to full advance at 2750 rpm with a strobe.

I'd also check the nitrile rubber plugs that cover the pilot jets are sealing properly.

[rphillips]

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"?

[tom203]

Does this model have the ignition mechanical advancer…or the ignitor with built in electronic advancer ?

[Reckhard]

If it were me, I'd start by checking that the timing marks advance to full advance at 2750 rpm with a strobe.

I'd also check the nitrile rubber plugs that cover the pilot jets are sealing properly.

The plugs over the pilot jets are new and sealing as they should. Unfortunately I don't have a timing light but also the bike doesn't have points so not sure how to verify timing advance.

[Reckhard]

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"?

the issue exists on PRI as well.

[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.

[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...

[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 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.

[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.

[850 Combat]

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

[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.

[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 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.