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Causes for idle to go up

  • Thread starter Thread starter Killer2600
  • Start date Start date
K

Killer2600

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Sorta like the title says what are the causes for the idle to go up when the bike warms up?

This is what's happening: start bike cold, warms up, turn off choke, idles good at 1150 rpm, runs good, sounds good, blips good, get on bike and ride for a mile or two, notice idle is getting higher, throttle blips seems to fall slower than cold, by the time I get home (~4 mile round trip) the idle is close to 2000 rpm.

Aside from the idle and blip stuff the bike runs and pulls good aside from hiccupping at times when taking off from the light at a stop.

I've torn apart and cleaned the carbs (albeit just using spray and not dipping, they were clean before and their even cleaner now), adjusted the valves less than a year ago, pulled and put back the carbs 3 times this year so I'm thinking I would have gotten the boots sealed good at least once (although the clamps were bent a little when I pulled he carbs for the winter and I've had to bend them back to as round as possible), the tank of gas is fresh this year.

I'm at wits end here, short of riding the bike in the artic I don't know what I should do.

Update: took the carbs off again and checked the intake boots as suggested and two of them had cracks so I replaced the two even though I don't think the cracks were big enough to make a difference. Also replaced all the clamps with new ones so everything is tight. So everything is good there. I re-bench synced the carbs and reset the idle mixture screws. Noticed that one of the mixture screws wasn't actually seated when it felt seated so I probably fixed a long time problem there. Threw the carbs back on and....

Same issue occurred but this time the bike seemed to be lean(ish) off the bat. Being sure that the intake boots HAD to be sealed good or at least better this time I put my attention to the mixture screws and idle speed adjustment. I turned the mixture screw out an extra turn to a total 3 turns out and turned the idle speed down, the bike seems better this way but I've yet to see how it acts when cold and throttle just off closed has a stumble so I'm thinking I still have more work before things are fixed.
 
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Typically air leaks are the main culprit of an increasing idle...

Things to check:

Make sure the airbox is sealed (assuming you are stock)

Spray some carb cleaner around the carb intake boots while it's running & check for leaks (drop in RPM).

Did you dip the carbs & replace the o-rings inside??
 
Also, Its possible that your symptoms are caused by a rich state of carb tune. If your adjustment screws are too far open, as the bike warms up, the idle will richen further, causing the idle to rise.
 
Take off the intake boots and push each end together and see if they are cracked on the inside.
 
Yea the bike is stock. Air filter is seated in it's place, don't think my setup has the sealing issue as the air filter is paper (the cylinder shape one) and has it's own foam part to seal against the box. I will try the carb spray and boots thing, just need to get a fire extinguisher first. :D Just for completeness, no I didn't dip the carbs just used the spray and compressed air--they were pretty clean when I started and even cleaner when I finished, I'm pretty sure all the passages are clear, the pilot jet's small hole wasn't even near clogged when I started and that's a small ass hole. Oooo I typed ass hole :lol:

TCK: I never knew that, I have been told the bike is on the rich side of things but everythings stock so I guess it's in the screws...anyone in michigan got a colortune handy? :lol:
 
Killer2600 said:
Yea the bike is stock. Air filter is seated in it's place, don't think my setup has the sealing issue as the air filter is paper (the cylinder shape one) and has it's own foam part to seal against the box.
Click to expand...

Make sure the bow itself is sealed well - you may need to use some weather stripping to get the lid sealed well.

Did you ungang the carbs when you cleaned them?? When were they last sync'ed?
 
hikermikem said:
Make sure the bow itself is sealed well - you may need to use some weather stripping to get the lid sealed well.

Did you ungang the carbs when you cleaned them?? When were they last sync'ed?
Click to expand...

yea they were disassembled and unganged, I bench synced them before putting them back in, but to answer your question I vacuum synced them last year and the problem was present before I tore them apart. I would have vacuum synced them today but my motion pro sync tool lost fluid to evaporation over the winter or something and now works like **** (the bike at low idle pulls enough vacuum to suck the remaining fluid out) so now I gotta rebench sync, fix my tool, or get another tool to fixed my f'ed up sync today.
 
Killer2600 said:
2 turns out from lightly seated
Click to expand...

The 550T I used to own had a hanging idle and didn't run properly until the screws were set at 3.5 turns. You might want to try 3 turns and see if the bike runs better.
 
TheCafeKid said:
Also, Its possible that your symptoms are caused by a rich state of carb tune. If your adjustment screws are too far open, as the bike warms up, the idle will richen further, causing the idle to rise.
Click to expand...

I have the same engine in an 81 GS750L. I replaced all the idle air screws and the idle air screws are set at 3/4 of a turn out. The only thing I can figure is the new screws have a greater taper than the stock ones did and therefore needs to be turned in a little farther.

Anyway, this what I did. Keep in mind this after the bike is fully warmed up....Had the idle air screws set at 2 turns out. The bike would bog and stumble terribly when rolling on the throttle. I kept turning the idle air screws in by a quarter turn. Each time the idle would rise when I did this. So I just kept turning the knob down so it would idle around 1100 rpm. After doing this a few times, the bike idles nicely and accelerates well. The screws ended up at 3/4 of a turn out. Which by all accounts is lean. I'm keeping an eye on the plugs.......The bike is running better than ever.:)
 
idle increase

idle increase

i had the same problem with my bike (81-850) last year. i noticed my oil had a gas smell to it. i replaced the petcock and it has run great ever since.
 
I replaced all the idle air screws and the idle air screws are set at 3/4 of a turn out. The only thing I can figure is the new screws have a greater taper than the stock ones did and therefore needs to be turned in a little farther.
Click to expand...

Larry if you have larger pilot jets installed in the bike, you will not need to have the screws opened as far. If you were to swap the 42.5 for a 45 or larger the screw settings would not stay the same as with the stock pilot.
 
I am just throwing out something to check.....I had an older (74) cb750 that did the same thing, and I was going nuts. I replaced the throttle cable and it was mint after that( this is after ripping into everything else)......Might be something to look at??????.....Ps....If this sounds stupid just tell me to go back in my hole LOL:cool:
 
It is a possibility, for sure.

I had to replace mine last year as it was doing the same thing, even after the silicone treatment and a little light oil. New cable and all was well. ;)
 
I had the same issue, richening the idle mixture (turn screws out) solved the problem. If the mixture is on the lean side it will hang at a high idle for a while before settling down.
 
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