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Idle speed variance before and after a ride

  • Thread starter Thread starter with_problems_like_these
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with_problems_like_these

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Riding my '78 GS 750 and have some questions about how much variance I should see in the idle speed before and after light and heavy riding. It's my first older motorcycle and I've been riding it regularly for about 2.5 years now; hopefully I don't sound like too much of a beginner for asking this :-\\\. I have an aftermarket speedo/tach that is a bit wonky when reading RPM so the estimations below are exactly that, estimations based on a wonky tach and listening to the bike.

After I warm the bike up and hop on it idles fine (between x700 to x1000). After light riding on city streets the idle speed increases slightly, and the more I ride, the more it increases. After any sustained riding at 55 MPH and above, the idle speed will pretty much double.

This phenomenon doesn't really impede on my riding experience at all but I'm concerned that I'm using way more gas than necessary and have potentially made some correctable errors while maintaining and tuning the bike.

Any insight into the following would be greatly appreciated:

- Is idle speed variance of this magnitude normal?
- Could it be indicative of any correctable problems with my bike?
 
Carb boot o rings are often the problem when the idle increases after fully warmed up......
 
I know if the idle mixture is a little rich they'll idle slower after they get hotter, so I'd guess if it was a little lean it might idle higher as it got hotter.
 
You have an air leak. New carb boots and/or Boot O rings needed
 
I gain about 100-200 rpm after getting the bike good and warm. If its just around town it will sit pretty much at 900-1000 but if I get on it some or take a 10+mile ride on the highway if settles in at 1100. It will idle steadily there the rest of the day purring just fine.

Before the boots and all the rest of the carb finageling .. it would climb to 2500. Not good.
 
Welcome to GSR.

with_problems_like_these said:
.........

After I warm the bike up and hop on it idles fine (between x700 to x1000). After light riding on city streets the idle speed increases slightly, and the more I ride, the more it increases. After any sustained riding at 55 MPH and above, the idle speed will pretty much double. .............
Click to expand...

You have just described the symptoms of air leaking into the intake. Most likley at the intake boot between the carb and the head, the o-ring in the boot up against the head being a likley suspect. It will get worse.
Other remote suspect, from my limited expereince, is the o-rings in the carb enricher (choke) mechanism. But that rquires to complety break down the carbs.
The o-ring in the boot is the leading suspect.

See Cycle o-rings for:
- carb boot orings
- carb boot replacment screw kit
if still have the problem then maybe the - carb o-ring kit (might as well get it now for later sometime).
 
Just a thought.....had a 'rising idle speed' after warmup on my GS 1100E. Realize different carburetor types....but... after a strip and clean and new O rings etc. and carb sync...would idle high when at full operating temp...anyway....throttle cable slack insufficient when cold. Went to none when warm / hot. Oversight on my part....adjusted properly....all is well.
 
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