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.

 

[pdqford]

-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.
...

Just wondering what type of fuel filter you installed.
(I assume that stock 850s didn't have an in-line filter?)
Could it be plugged after the fuel system disaster?

Anyone know if there is a spec for the amount of fuel that should flow for a given amount of time?

Heavy throttle at higher RPMs requires "a lot more" (technical term) fuel flow than a lightly loaded system.

 

[Grimly]

Anyone know if there is a spec for the amount of fuel that should flow for a given amount of time?

0.5lb / HP / Hour

 

[rphillips]

Now that's confusing. 0.5lb per HP = 40lb, from a gravity flow system. Not sure about the HR either.

 

[pdqford]

0.5lb / HP / Hour

Technical terms?

Okay, thanks dave.

If my math is correct,
that's about 0.008lb per HP minute.

And when it's putting out say ~60HP,
that would be about 0.5lb fuel per minute.

Say fuel weighs about 7.5lb per gallon
so 0.5lb would be about 1/15th of a gallon a minute.

At 128 oz/gal, that would be about 8.5 oz/minute,
or 2/3 of a soda pop can in a minute,
while laying down 60 HP.

So can O.P. (Reckhard)'s gravity fuel system come close to filling a soda pop can in a minute?

 

[tom203]

”I've nearly convinced myself that the issue is with the main jets. I may have messed them up when I cleaned the carbs“

while you’re having carb fun, check the vacuum diaphragms for integrity and for correct orientation

 

[Reckhard]

I came with an inline K&N fuel filter. I swapped it for a much larger one with no change.

Vacuum diaphragms all look good and are in the right way.

As for handling the fuel flow - Suzuki designed the bike this way - I presume they knew what they were doing and the bike as designed was able to get to the 78hp it was rated at.

New jets arrive next week - carbs are sitting on my bench waiting...

 

[rphillips]

No doubt, hope this fixes the problem, but got to be skeptical. I wondered since reading post # 21, How can you mess up a main jet other than the screwdriver slot or threads? Thinking it'd be hard to accidently make the tiny hole that meters the fuel smaller or larger... Do hope this fixes the problem.

 

[Rich82GS750TZ]

I came with an inline K&N fuel filter. I swapped it for a much larger one with no change.

As for handling the fuel flow - Suzuki designed the bike this way - .

No, they didn't. Your bike may have come to you with a fuel filter, But Suzuki didn't put it there. There's a filter screen on the petcock, and on the the float needle valves inside the carbs. But your bike did not come from the factory with an in-line fuel filter.

 

[pdqford]

I came with an inline K&N fuel filter. I swapped it for a much larger one with no change.

Vacuum diaphragms all look good and are in the right way.

As for handling the fuel flow - Suzuki designed the bike this way - I presume they knew what they were doing..

I agree, Suzuki knew what they were doing, but your fuel system is now 42 years old and has had a fuel system disaster.
If it was me, I'd pull the fuel line, stick it in a beer can, put the petcock on prime and see how long it takes to fill the can. If fuel flows well, go for the mains. If it doesn't, see Rich's post #29 above.
But that's what I'd do.

 

[Isetta]

based on my experience with 2 GS550 (about 1982) bikes which had similar problems, i believe it needs larger main jets than the original old ones. petrol (gasoline) is not as potent as it was decades ago. keep the original airbox in place. Put larger main jets in. I can't be exact on what size you need. maybe 2 sizes bigger and see how it goes. I tried so many things on mine trying to get it right, wasted hours and hours, just needed some larger main jets.

 

[Isetta]

Actually what I wrote above must be wrong. If it runs better with no air filter then it can’t be because the main jets are too small. If they were too small it would get worse with no air filter. Still sounds like a main jet problem due to the problem being at higher revs under load.

 

[Reckhard]

Okay, got new main jets. The bike is almost 100% now. It runs great at higher rpms but doesn't pull as well as before up to 4K rpm. I'm going to tweak the air screws to see if that makes a difference. Also the vacuum valve in the petcock seems to be functioning as it should now. My theory is that I was a bit too aggressive in cleaning out 32years of varnish in the mains and made them bigger. Too rich a fuel mixture caused the stumble when the bike hit around 5K, which created all the issues.

 

[pdqford]

I've nearly convinced myself that the issue is with the main jets. I may have messed them up when I cleaned the carbs.

There you go! Good call!

Are the new jets the same size as the old jets?

 

[Reckhard]

There you go! Good call!

Are the new jets the same size as the old jets?

I went with the factory spec'd 115 mains.
I've also now ordered a new set of pilot jets just so I can get the carbs back to original. Once I get those I'll need to rebalance and everything should be perfect just in time to park the bike for winter

 

[luftkoph]

Is it possible that when you oiled the uni filter you used motor oil? or did you use filter oil? maybe the motor oil causes restriction in the foam media that filter oil does not, just wondering, hope you sort it out.