gs1000g flat spot

[dbd32]

My GS1000G 1981 has a flat spot at 3thou rpm after that is ok and ticks over ok Checked float heights-ok sincronised carbs-ok fit new carb to head rubbers-ok mixture seams okish
Any ideas

[Nessism]

Make sure the airbox end caps, and the filter frame, all have good sealing foam. Make sure the stock jetting is installed, and the jets are real deal Mikuni. If that doesn't work, you can try lifting the needle. To do that, you must reduce the thickness of the shim on TOP of the needle. Don't put washers under the needle, because that does nothing for those carbs. To reduce the thickness of the washer, you can shave it down through sanding. Or set it aside, and replace it with a small stack of thin and tiny washers. Target something half the original thickness of the plastic shim.

[dpep]

Ed is there any chance his problem could be firing related?

Even after all the work we did on those carbs you squared away for me I still had this small flat spot around the 3700-4100 rpm range, not enough to interrupt my ride but there. It got worse and I tried a little experiment.

I went out on my test ride route and did a mile or two deliberately running up and down through that range and feeling the stumble. As I stopped for a light at the end there was a loud pow and the engine shut down. It started right back up and I made it home okay but I am thinking that the only way I could get that much unburned fuel built up in a cylinder was if the plug wasn't firing or firing properly. Always in that exact rpm range regardless of which gear, engine temp, load, weather, etc.

What say the guru?

[Nessism]

Ed is there any chance his problem could be firing related?

Even after all the work we did on those carbs you squared away for me I still had this small flat spot around the 3700-4100 rpm range, not enough to interrupt my ride but there. It got worse and I tried a little experiment.

I went out on my test ride route and did a mile or two deliberately running up and down through that range and feeling the stumble. As I stopped for a light at the end there was a loud pow and the engine shut down. It started right back up and I made it home okay but I am thinking that the only way I could get that much unburned fuel built up in a cylinder was if the plug wasn't firing or firing properly. Always in that exact rpm range regardless of which gear, engine temp, load, weather, etc.

What say the guru?

Hum, Don. Sorry to hear the bike still doesn't run to your liking.

If the entire engine flames out with a bang, that seems to point to the ignition. You might want to check and lubricate the centrifugal advance mechanism. Also, remove the plug caps, check them for resistance (should be about 5k ohms), then snip off a small piece of ignition wire and screw the caps back on. Another thing, check for voltage at the coils. Should be at least 10.5 volts, hopefully more.

Oh, and what kind of air filter do you have? Is it clean and properly oiled? If it's an old foam filter, it's possible that the foam is deteriorated. They don't last forever.

Good luck and hope you get it sorted.

[dpep]

If the entire engine flames out with a bang, that seems to point to the ignition. You might want to check and lubricate the centrifugal advance mechanism. Also, remove the plug caps, check them for resistance (should be about 5k ohms), then snip off a small piece of ignition wire and screw the caps back on. Another thing, check for voltage at the coils. Should be at least 10.5 volts, hopefully more.

Oh, and what kind of air filter do you have? Is it clean and properly oiled? If it's an old foam filter, it's possible that the foam is deteriorated. They don't last forever.

Thanks, Ed, for (sort of) confirming my suspicions.

I went over the air box & filter back when we were sorting the carbs. In the past when I have had air intake issues they weren't restricted to a narrow rpm range. I could usually tell right at idle.

I am going with the electrical/ignition prognosis for now. I will be taking the tank off and trying to get some readings while it is running roughly in the bad rpm range. That may have to wait until the weather cools a bit. Ungodly temps and humidity here right now. .

[salty_monk]

I had a big flat spot on my 1000G that turned out to be a bad ignitor..... Symptom was it would just get up to 5,000 or so RPM and then just stop accelerating. Those ignitors are getting old and crabby!