I have a really weird idling issue. Makes no sense.
 

I have a 89 sportster that’s carbureted. I have a weird high idling issue I can’t seem to figure out. After riding and the engine is hot, the idle will go from 900rpm to 13-1500 rpm’s. It will hang there for a couple minutes then suddenly drop to 900 and will continue to idle at 900rpm until I ride it again. I’ve verified throttle cable is not hanging up during the concern. I also don’t think it’s a vacuum leak since it’s not continuous at idle. It stinks cuz when the idle is high like that it’s hard ride in stop and go, but if I adjust the idle down, when it does decide to Randomly my go back to normal- the bike will die. I’m at a loss.

Intake manifold leak, common.

wouldnt a intake manifold leak cause a high idle at all times when hot? Instead of Just idling high for a couple minutes then suddenly falling back down to normal idle?

Check your vacuum piston, perhaps it's torn and hanging up from dropping back down? When is the last time your manifold seals were refreshed? And idle return cable improperly adjusted can cause idle speed issues. Have you turned the handlebars while it's idling to see if the cable is causing it?

John

I rebuilt the carb recently, the vacuum piston seal was inspected for any abnormalities. The manifold seals were replaced 2 weeks ago. The idle return cable is slightly loose in resting position, I also pushed on the butterfly to verify the throttle isn’t hanging up and is resting against idle screw stop at all times.

Well, the mystery continues. Did this coincide with the carb rebuild and seal changes? Those manifold seals to the heads can be problematic., perhaps with aftermarket seals? IDK. I have not had issues, but have read several threads where getting them to seal was a pain. I used H-D seals just to be safe when I last did mine three years ago. I used red grease, although I guess the service manual says dish soap to lube them up good before install. Have not had an issue with red grease.

John

I’m not sure. The bike hasn’t 15-20 years, I did the carb rebuild and intake seal replacement out of necessity. The intake seals were from HD. I sprayed some brake cleaner around them and it didn’t seem to affect anything. I would still find it odd that an intake seal leak would cause an intermittent issue vs a constant issue.

do you think it could be a timing issue?

In addition to the above, it could be a fuel flow issue:

1. Check all the vacuum lines to your petcock. Edit, 1989 did not have a vacuum operated petcock
2. Check your fuel line to ensure it is in good shape.
3. Remove your gas tank once the fuel is low and on reserve and check your filter screen for any debris and such. If it has never been replaced it might be a good time to replace it.
4. Try running the bike on reserve with a partial fill fuel tank, if it does not have the issue the problem is the fuel screen or crap in the tank covering the fuel flow.

Ref:20
62166-81A
FILTER, fuel valve - with reserve tube
Price: $16.33
https://cimg4.ibsrv.net/gimg/www.hdf...299d0266c6.png

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What brand carb is on your bike? Certain brands that were used in the past, with their tiny passages, were considered junk.