Idle speeds up after warm up

pahako

pahako

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I understand that it is normal for the idle to speed up after the engine is warm, but sometimes when I am down shifting and pulling up to a stop the engine starts reving very fast. If I hold the brake and let out on the clutch some while I am at a stop it will go back down to a normal idle speed, but this is beginning to be an annoying problem. Also, the right cylinder has frequent sputtering or mild back firing. I have an 81 special and except for these problems starts and runs very good. I have cleaned the carbs, have had them off a couple of times recently. Could it be that they need to be synced?

Thanks

Paul
 
I posted a thread on here a while back ago about running temps at the cylinder head and I got so many guys saying their bikes run around 230 degrees. I am a perfectionist most of the time and hate that my bike does what you explained as well but..... I do believe it is normal. Yours may be a little on the lean side causing it to run hotter than mine but I basically set my carbs so my bike runs like 800 rpm cold almost to the point of dying.....then it runs a perfect 1200 to 1300 after warming.

Funny thing is I bought a Trail Tech spark plug temp gauge

http://trailtech.net/tto_temperature.html

and when my bike idles at 1200rpm.....I'm running about 320 at the spark plug!!!:eek: My oil is around 180 to 200 which I am ok with but the 320 scared me. I read on some aircooled VW sites and some Harley sites and the 320 , amazingly was pretty normal. Sure I'll get guys disagree with me on here but temps literally an inch from combustion are super high!

Anyway, now I know how hot aircooled engines can run and I am ok with the higher rpms when she's warm. For $40 that TTO temp sensor really is nice!
 
the same exact issue was happening with my 1981 special before I tore it down. From what I could figure out, it really seemed to be related to battery charge. are you sure your battery is getting the proper charge from the charging system?

Also, you can adjust the idle screw on your carbs while you're at the light if the idle starts to rev up too high. I was constantly thumbing my screw though, so it's obviously not a permanent fix.

Make sure there is no air leaks around the carb intake
 
Funnily enough I was just about to post the exact same scenario!

I had to put some Harley mufflers on my bike to quieten it down to pass inspection, I also didn't put the lead replacement solution in the last tank of gas.

Now I didn't notice this issue prior to those to changes. I alos notice if I open the throttle full 110% it will stick unless I actually rotate it back. Which would be the natural case when slowing down.

It has been suggested it could be a sticky needle, and when warm expansion tightens it up a bit.


Interested to see if we get any decent answers or just more "Mine Too!"
 
Look for an exhaust leak. I looked everywhere for an air leak and couldn't find one. It turned out the exhaust valve was too tight, making it act like a small air leak.

Simply adjusting the valve clearance properly ended about a month of head scratching and working on the carb. Changed it from a barely rideable bike into a perfect running one.
Click to expand...

Found the culprit! My left carb's vacuum plug on the boot was torn and almost completely off. I replaced both of them with some fresh rubber and went ahead and changed the clamp bolts to allen heads. She's running great again
Click to expand...

Here are the results from another thread same issue!
 
pahako said:
If I hold the brake and let out on the clutch some while I am at a stop it will go back down to a normal idle speed,
Click to expand...

I had this on a bike. Everybody was saying it was an air leak at the intake somewhere, but I couldn't find one. After I finally adjusted the valves it went away. The exhaust valve was real tight. It might have been staying open during part of the intake stroke, causing an air leak. It was a single cyl bike. When you're able do drag it back down like you're doing and it stays there it's telling you something.
 
I will try adjusting the valves before I do anything else. I did it once earlier this summer when I got the bike, but it was my first attempt, and I may not have got them quite right. It seemed pretty easy at the time, so it seems like a good place to start. I'll let you know how it turns out.
 
I've found this will happen if the mix screw is not adjusted correctly, carbs not insync or the incorrect pilot jet is used. Over adjusting the mix to make up for the wrong pilot jet (like 4+ turns out) will always cause this in my travels.
This can be best tested, quickly, by cracking the throttle and then seeing if the engine hangs for a second or drops. It should drop right back down as soon as you let off the throttle, no hanging.
I have also noticed this only happens after the engine is warmed up.
 
Littlebill31
My throttle does not always drop back down as quick as I want it to. I have always wondered about that. Also, I didn't think there was a mixture adjust screw on an 81. Could the carbs not being insync cause backfiring out of one cylinder? It does frequently just on the right side.
 
Thanks for the article. I believe all the info I need to fix my problem is there. First thing I did was check the cam chain, it was right on. Next I checked all the valve clearances. The intake and exaust were both to tight on one cylinder. The other side was right on. Reset the side that was to tight and rechecked all 4 valves. Started the bike and it was very loud coming from the engine. Sounds like the valves are banging way louder than normal. I shut off the bike pretty quick because I didn't like the sound of it. I will let it cool down and recheck them again. Just wondered if anyone had any thoughts on why it would be so much louder.
 
You have an '81?
These are the measurements you should be using.
0.06mm / 0.0024in for the intake and 0.15mm / 0.0059in for the exhaust.
And you are setting the pistons to TDC on the compression stroke for the correct valves?
 
OK I made a dumb mistake. Forgot to turn the engine one complete revolution before I adjusted the other side. Got it back to sounding the way it did before starting. On the intake I set it to .003in because I remember a couple of posts on here about how .0024in was to tight and some are setting there intake at .004in. If that is not ok, let me know. Next I'll attempt to adjust the mix screw, then try to sync with a manometer. Oh yeah, mine is an 81.
 
Last edited:
I use the stock measurements and they work well for me. I've set my at .003 and it sounded horrible. Just depends on the bike. A new set of tappets will be tighter than a worn out, dimpled set.
Always adjust one valve at a time, one side at a time. Never just set it at TDC, compression stroke and adjust all 4. I rotate it for each one.
Also, I've said this a bunch of times, but after you set the specific valve, I like to rotate the engine around (back to TDC, compression) and then re-check that valve again. I find, often, that it is a little out. Usually loose.
 
did this get figured out? noone said anything about sloppy advance weight springs. do yours snap back like they should. i could be completely wrong.

 
This is the first I've heard about checking the advance springs. Might be worth looking at. I am making progress. I was able to recheck the valves today, and sync the carbs. They were pretty far off. I wasn't able to take it on a test ride yet, but just from syncing the carbs I think I got rid of the "spitting" noise that was coming from one side, and the throttle seemed to come back down quicker. Haven't had time to check the mix screws yet , but will see how it runs tomorrow on the way to work.
I gotta say, I love this forum. The homade manometer was the easiest and cheapest tool to to make, and worked great.
 
If you have the original ignition then you have no advance unit or advance springs. Those parts are only installed on the '79 and older points equipped bikes or if you have a Pamco ignition.
 

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