gs 850 idles and runs to 3000 rpm then cuts out.

[G.Roy]

Hello,
I'm new to this forum as a member but have read it for a few years I have had my 1981 gs 850. My bike which I am trying to get running correctly has a consistant problem. I've cleaned the carbs as per the advice in the forum. Used only mikuni jets and set floats to spec 22.4 mm. new o-rings in carbs. I have a dyna ignition and coils. I checked the timing with a timing light and it advances to the marks at 2000 rpm. Sync carbs to spec. I have new manifold intakes and o-rings as well as air box manifolds. Also I have shimed valves to spec.
The problem I have is that the bike idles well and runs up to 3000 rpm but then acts as if its starving for fuel and dies down to idle and stalls most of the time. It can be started right back up and settles down to a steady idle. The plugs when taken out after the stall are carbony black- the electrode is brownish. Any help will be really appreciated I'm kind of stuck now as to how to proceed.

[tom203]

A lurker! welcome! sounds like you have done your homework regarding carb cleaning and tightenning up intake. My first guess would be over oiled air filter element. But does your petcock flow gas nicely in prime spot (it's nice to rule out petcock problems quickly) ?

[G.Roy]

Yes, I'm a lurker! My airbox has a k&n stock filter in it. I just did a compression test and got No. 1-100 psi, No. 2- 90 psi, No. 3- 89 psi, No. 4- 90 psi. All out of range. Would this affect my problem? I know these are out of range. Could this be rings or valves?

[eil]

Was the compression test done with the engine hot or cold? The readings are generally not accurate unless done on a hot engine. (But a cold test can be helpful for troubleshooting a no-start problem, for example.)

Those may not be bad numbers if the engine was cold. But if those are warm readings, it is likely that something is amiss. When was the last valve clearance adjustment?

[tom203]

Does it hit this 3000 rpm "wall" immediately after idling nicely? How about removing filter element and see how that changes things. The CV vacuum diaphragms should be starting to lift smoothly about here.

[Steve]

I've cleaned the carbs as per the advice in the forum. ...
The problem I have is that the bike idles well and runs up to 3000 rpm but then acts as if its starving for fuel and dies down to idle and stalls most of the time. ... The plugs when taken out after the stall are carbony black- the electrode is brownish.

Sounds like you did all the "right stuff".

Has it had this problem ever since you rebuilt the carbs?

Sounds like the rubber plugs are missing over the pilot fuel jet.



.

[jwhelan65]

I had this exact same problem on an 80 GS 750 and it was the plugs that Steve mentioned above.

[G.Roy]

Thanks for the replies.

I have rebuilt the carbs recently and I am sure the rubber plugs are in place. The compression test was done on a cold engine and was not done as per the manual. I will redo the test when I have time at the end of the week. I did take the air filter off and looked at the slides when the engine was running. At 2500-3000rpm the slides do not move only vibrate up and down to the frequency of the motor. I took the slides out and looked at them in a dark room with a flashlight shining behind and did not see any cracks or holes-the diaphrames are pliable. I'm going to take the cam cover off and make sure the cam timeing is correct. This is where I am right now. G.Roy.

[tom203]

G.Roy said....
"At 2500-3000rpm the slides do not move only vibrate up and down to the frequency of the motor"

That's interesting- I've never looked at Mikunis CV in operation . But on a 62 volvo with similar style carbs, they didn't vibrate up and down ( but I believe those carbs had oil damping). Of course, the intake stroke comes every other rev but I would still think the slides would hold reasonably steady even with filter off.

[tom203]

I was sorta hoping that someone would direct me to a movie showing inner workings of CV carbs in action as load,rpms, and throttle positions were changed.

[chef1366]

I had a bad Dyna S act like that. Any ignitions to swap to check?

[G.Roy]

The rubber plugs in my carbs were really hard so I ordered a new set and will try them. As for the ignition I guess I could find the old setup and put it back to see if there is a change. Thanks for all the input. One way or another I'll get her running. G.Roy.

[tom203]

Don't forget to update us as I'm curious as to what's happenning.

[BassCliff]

Greetings and Salutations!!

Hi Mr. G.Roy,

Lurker or not, you still get one of these.

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Here is your very own magical, mystical, mythical, mind-expanding "mega-welcome". Please take notice of the "Top 10 Common Issues", "Top 15 Tips For GS Happiness", the Carb Cleanup Series, and the Stator Papers. All of these tasks must be addressed in order to have a safe, reliable machine. This is what NOT to do: Top 10 Newbie Mistakes. Now let me roll out the welcome mat for you...



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More links to helpful threads in the forum:
Help! Your Bike Won't Start
DON'T DO THESE THINGS
Help! Your Bike Won't Run Well
Oh God! Pods!


Thanks for joining us. Keep us informed.

Thank you for your indulgence,

BassCliff

[G.Roy]

Hello All,

Just to give you guys a update on my little running problem. I changed to idle jet plugs to nice new rubber and rechecked the float height. Reinstalled on bike and got same running condition Idles nice and revs to 2500-3000 then dies. Starts right back up and idles nice again.
I did have an interesting thing happen when I checked my compression (which by the way is fine at 150psi across all cylinders) when I put the plugs back in the bike it would not start. I checked the spark and it was very weak. I switched the feed wires to the coils and the bike started and ran fine. This I can not understand. My timing is fine as checked with a timing light.
The next thing I will try is to shim up the needles to richen the mid range. I have replaced the foam filter with a k&n paper filter and they may flow more air?
How much should the petcock flow on primary?

Thanks for the input,
G.Roy