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High RPM's WTF!?

  • Thread starter Thread starter B Evil
  • Start date Start date
B

B Evil

Guest
1981 GS 550 L

OK, so I've done the carb dip and rebuild with new O rings and adjusted the valves and installed new gaskets. Finally got her put all back together (other than the tank as shes still getting painted). So I was getting ready to sync the carbs. Set up an aux gas tank plugged the vacuum line and set the idle to 2000 RPM's to sync the carbs. Hooked up the home made manometer and fired her up. The bench sync didn't get me no where close but no problem, I'll just adjust them accordingly. I really never would have thought they would be as sensitive as they are. So I've been at it for about 45 minutes (keep in mind this is my first time trying to sync carbs) and the tank ran out of gas. I put a little more gas in there and it took her a minute to get going again but she finally fired up and ran like a bat out of hell. Now she's running at 5500 to 6000 RPM's and I couldn't bring the idle back down. Any thoughts?

She did run the best I've ever heard her run up until this point so thanks for all the tips along the way. Now I just have to figure out what's causing the high idle. Any help is greatly appreciated.

Thanks
-Bradley
 
You have a vacuum leak. Did you replace the rubber intake runner "boot" o-rings? You also need to plug the vacuum line that normally runs to the petcock.

Edit: sounds like you have a lean mixture, possibly a result of your aux. fuel tank running dry when trying to sync. Same result as a vacuum leak; idle will increase. To reinforce what psyguy said, make sure to put a large fan, or two, blowing on the engine when sync'ing otherwise the engine will overheat.

Good luck.
 
Last edited:
Yeah the intake O rings were replaced and the petcock line is plugged. If it is a vacuum leak, why didn't it do this the first 45 minutes I had it running?
 
be careful not to overheat the engine - i hope in your 45min run you had the engine periodically off to let it cool down
this may as well account for the high rpm espec if the carbs are not adjusted well - but the rpm would drop once the engine cooled down
 
Could be that you've got a sticking float needle and 1 or more carb bowls aren't filling. Try blowing down the fuel tube before connecting up the fuel line again.

I agree with the sensitivity of 550 carbs in particular for synchronising - never ceases to amaze me how much difference only a very small adjustment makes.
 
psyguy said:
be careful not to overheat the engine - i hope in your 45min run you had the engine periodically off to let it cool down
this may as well account for the high rpm espec if the carbs are not adjusted well - but the rpm would drop once the engine cooled down
Click to expand...

Yeah I had an industrial fan blowing on the engine and it never really ran for more than 5 minutes at a time because the oil in the home made manometer would start rising too much so I'd have to shut her off and let the levels balance out.
 
i think i would want to go for a good run first after rebuilding the carbs before doing fine adjustments
the aim would be to see that the fuel/air system works fine in general and that there are no major problems (like sticky needles, fuel starvation, etc that was suggested above)

also, did you have any filtration of the fuel (since using an alternative tank)?
 
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