GS850 Flat spot between 3 and 4000 rpm.

[Chris Bohane]

My GS850 has a flat spot between 3000 and 4000 rpm.

The carbs have been cleaned and new o rings installed. I have standard 115 main jets.

I have a stock air box and new inlet manifolds from the engine to carbs and from the air box to carbs.

The exhaust is standard.

The carbs have been balanced with and electronic balancer.

ANY ADVICE AS TO WHERE TO START TO OVERCOME THIS FAIRLY MINOR BUT IRRITATING PROBLEM WOULD BE GRATEFULLY RECEIVED.8-[

Regards to all.

 

[Dave8338]

My GS850 has a flat spot between 3000 and 4000 rpm.

The carbs have been cleaned and new o rings installed. I have standard 115 main jets.

I have a stock air box and new inlet manifolds from the engine to carbs and from the air box to carbs.

The exhaust is standard.

The carbs have been balanced with and electronic balancer.

ANY ADVICE AS TO WHERE TO START TO OVERCOME THIS FAIRLY MINOR BUT IRRITATING PROBLEM WOULD BE GRATEFULLY RECEIVED.8-[

Regards to all.

Had the same issue on mine. Solution...raise the jet needles up one notch and re-test. If it seems to run too rich ie. exhaust smell, plug reads, you can lower them in half steps using small washers. (I think radio shack sells them and they are about perfect).

 

[mark]

Had the same issue on mine. Solution...raise the jet needles up one notch and re-test. If it seems to run too rich ie. exhaust smell, plug reads, you can lower them in half steps using small washers. (I think radio shack sells them and they are about perfect).

What year is your 850?

Dave8338's post is my experience as well. But before you tear your carbs apart, make sure other things are right first, like valve clearances, timing, points set correctly (if you have a '79), float position on your carbs are correct. Make sure your airbox is sealed perfectly.

Your problem is actually at a certain throttle position and not RPM, so keep that in mind. Main jets don't really kick in until you're almost at wide open throttle.

 

[Chris Bohane]

Thanks guys,

Valves, timing and points all done following the winter rebuild, less than 20 miles ago. I was aware of the airbox sealing issue and I think I have a good seal. The float heights have been checked.

The engine will start for a few seconds with or without the choke, dies, and, then just turns over and will not start. However, a couple of breaths into the carb breather pipes and she will start and run quite happily.

Pulling out the choke causes the engine to stall.

Do I have two problems or are they related?

I have no problem pulling of the tops of the carbs to get at the needles but I don't know how to adjust the seating height. (I polished the tops and fitted stainless cap heads during the rebuild.)
An explaination would be appreciated.

Thanks, in anticipation, for your time.

 

[mark]

I can't chime in much more until tomorrow night.

But here's something for you to look at. See page 7 (I'm assuming you have a '79 with VM carbs since you spoke of points)........
http://www.thegsresources.com/files/vm_carb_rebuild.pdf
Shows you the pieces your dealing with.

I have to say that if everything is stock, you shouldn't have to adjust your needles. But assuming that you don't have an air leak somewhere............

you're going to want to know what color you're plugs are at the specific area you're having a problem with. Run it at the RPM/throttle setting you're having trouble with for a minute or so, chop the throttle (hit the kill switch) pull the plugs, and see what color you're plugs are. Especially note the throttle position that you're at when you do your test.

Then if you make a change you can have a better idea of any progress you're making.

A couple of other things to consider......are your pilot idle screws (underneath at the front of the carb) adjusted correctly? Factory settings are about 5/8 turn from LIGHTLY seated. Air screws should be somewhere around 1 1/2 to 2 turns out.

Also, make sure your petcock is operating correctly of course.

 

[49er]

Thanks guys,

Valves, timing and points all done following the winter rebuild, less than 20 miles ago. I was aware of the airbox sealing issue and I think I have a good seal. The float heights have been checked.

The engine will start for a few seconds with or without the choke, dies, and, then just turns over and will not start. However, a couple of breaths into the carb breather pipes and she will start and run quite happily.

Pulling out the choke causes the engine to stall.

Do I have two problems or are they related?

I have no problem pulling of the tops of the carbs to get at the needles but I don't know how to adjust the seating height. (I polished the tops and fitted stainless cap heads during the rebuild.)
An explaination would be appreciated.

Thanks, in anticipation, for your time.

When you say that your engine will start with or without the choke on, are you meaning when the engine is cold? If so, you are far to rich on the pilot jets, either their size or settings. Your engine shouldn't stall with the choke fully on. It will start running erratically though, if left on when the engine is warm.

If your model is the '79 850, you should have VM 26 carbs fitted. When stock, your mains should be 102.5 or 105 maximum, not the 115's you say you have. With 115 mains, your transition from the pilot to the needle circuit will too rich, causing the engine to bog down in the 3000-4000rpm range as you mentioned.

Is it possible that the PO was running pods before you bought this bike?
That may account for such radical jetting, especially if he returned to using the stock airbox.

 

[Chris Bohane]

Thanks again for the replies guys.

I forgot to mention that the engine is 1979, points and kickstart with a 1980 head to give me the CV carbs. I also changed the airbox and choke cable for the 1980 versions!

115 main jet is standard I think.

Sorry for ommiting this information.