Engine will not return to idle

[c-152]

I have a 1979 xs1100f that I have spent the summer getting running. The engine has a fresh top end, I cleaned the carbs to like new condition and the bike starts up without a problem and idles at 1000 rpm great. I synched the carbs with vaccum gauges and it revs clean. The problem occurs after the bike is warm. after acceleration the bike does not want too return to idle. this makes it hard to ride in town and not all that safe. My first thought was vaccum leak. So I coated the intake boots with permatex and replaced the gasket. No help. Vaccum advance line works fine. Sealed the vaccum caps, replaced card holder clamps. Nothing has worked and not sure where to go from here. I have read other posts and used some of the info to no avail. Here might be some other relevent info - I have 4 into 1 exhaust, and had to remove the old fuel petcocks. I now use a simple on/off fuel valve attached to a piece of plate screwed into the void where the old petcocks were (I caped the inside carb vaccum lines) It is not the throttle cable (I disconnected it and ran it with the same results) Timing is fine
My next steps
1. reinstall stock exhaust (do not think this will change anything) Back pressure?
2. Replace carb slide springs
3. Pony up for new inake boots
4. ?

What am I missing?

THanks for reading

[thewiz]

this slow return to idle ...

can be caused by carbs out of slightly synch and also a float level or two that are high(too much fuel). I would check to be certain that all the float levels and drops are the same and within spec. Also recheck the needle clip (should be center) positions and that all slides are working smoothly.

Non carb related problems that can make this happen are sticking advance plate or sticking mechanical advance on the snout of the crank ... in which case lube or replace if sticking.

You have the vac advance attached to the #2 carb and not the intake port, right?

There's a few roads to go down before spending any hard cash ....

[Ken Talbot]

Check for vacuum leak where the carb boots attach to the head, and where the butterfly shafts poke through the sides of the carbs. You can do this by using an unlit propane torch to waft a small amount of unburned gas in those areas. IF there is a vacuum leak, the gas will get sucked in and you will hear the engine speed increase.

Also, check the vacuum advance hose for cracks.

[Narcisist]

Choke?

My 79 special is my first bike, so I didn't know a whole lot when I first started learning to ride it. I had this same thing happen to me, and only later after fiddling with it a little, learned that I had the choke mixed up. Thought having it pulled all the way out was off. So it started up and idled great, went for a ride and when it warmed up the idle would stay up around 3k. Oops.

Hopefully yours is the same thing, cause it doesn't get more simple. . .

[catdna]

I had this happen on my trike - would idle fine and when revving the engine (while trike was stationary), it would return to idle perfectly.

It was only when out for a ride that the revs would gradually build up and I'd have to hit the kill switch when coming to a stop with the engine revving at about 4000+ !

Scratched our heads for ages about this one and nothing seemed out of the ordinary - it was only when I got my XS11 *bike* and noticed the 'idle adjuster' was a bit stiffer, that I realised there's a damn spring on the thing ! (trike idle adjuster was a 'bit' stiff because of 'gunk' build up, but not enough to stop it moving).

The idle adjuster on the trike had been winding down at certain revs causing the issue. Put a spring on there and the problem vanished immediately

Just a thought... sometimes the simplest things cause the biggest problems !

[Ivan]

Are you certain that you have all the brackets in place on the choke rod? I had inadvertently got one out of the divot and it held the other three choke plungers open just a hair and caused the same condition you are speaking of.

Another thing that came to mind is the clamps on the inner boots have screws long enough to bind the throttle linkage, and need to be positioned out of the way. That wouldn't be temperature related though.

[c-152]

Thanks for all of your replies. I have checked the idle spring and it holds at temmp, I recheck how everything was pluged in (vaccum line wise) and everything was fine. I will double check the position of the choke rods. I also ned to make sure the vaccum advance is not sticking, then I will get into the carbs again and double check the floats. Thnks everyone for the replies, this has definatly given me some suggestions, I think I am going to print this post off and go through the bike one by one. I will keep everyone informed as to my progress. Work will keep me from working (irony) on it until wednesday. Thanks again

[c-152]

Update - I have not had much time to check things out. but here is were I am at. Currently I have rechecked the timing and checked the vaccum advance. Everything seems fine. Thats all for now, I will hopefully start again this weekend