This has been bugging me for years. When I pull up to a light, hit neutral and wait with the throttle twisted off, my engine takes a half-minute to a minute to return to a loping idle.

The butterfly is definitely closing all the way instantly, yet the revs come down very slowly.

When it finally slows all the way, it's pretty even idle.

The bike is a 1976 74 cu. in. Shovel, w/ stock pistons, heads, pipes, and Keihin carb, with an aftermarket 'donut' K&N air filter and solid lifters replacing the hydraulics. It has a 'mild' cam (that's all I know about it), and I've polished and done a 'little' opening up of the intake and exhaust ports. The Keihin has a 100 slow jet and a 185 main.

[However, this problem was occuring before I did the port work and changed the cam, and increased the main from 170 to 185.]

Any ideas?

 

As long as you are sure that the throttle valve of the carb is returning to the idle stop I would suggest that it is internal so it may be time for a carb kit !

 

I've had this carb on the bench twice in the last few months, with a new kit each time. This carb has no visible signs of damage that I can discern.

There's no question at all that the butterfly (valve) is closing all the way, I can see, feel and manipulate the throttle lever easily.

 

Does the throttle valve shaft have play in it(bushing wore out) and how about the accelerator pump adjustment.Thoose are the next items I would recheck.
Good Luck !

 

Air intake leak?????

 

all-one,
are you running stock ignition or an electronic that still uses the stock ignition advance weights?
If you are, it could be your advance mechanism needs cleaning and lube or the springs for it are failing, or both.
I used to install new springs every time I replaced the points for a tune-up because the heat in there does get to them after a while.

 

 

I've got the stock ignition and advance weights. But it's all in great shape. I've cleaned everything, lubed it, and checked for wear and rough movement. I'm sure the mechanism itself is working fine, insofar as moving freely. The springs are new.

Yet, I'm almost certain it's in the ignition advance. After my first post, I started really paying attention, and I've noticed that the behavior is extremely "precise" in that when the engine finally slows down, getting to a particular speed, every time, it then falls off abruptly to a proper idle.

In fact, if I drag the clutch I can immediately get it to idle properly every time.

I put a crimp in the straigt section of the springs to pull them a bit tighter, and it seems it has made a tiny difference .. but maybe it's my imagination. If anything the rpm at which the springs return the weights MAY be a little higher now. Without a tach, I can't be certain.

I have ordered stronger springs. I know it's going to screw with my advance curve, I'll be running with more retard all through town (which is where I do 95% of my riding) but I have to try it.

 

Have you checked adjustment on the throttle cables? You need a little free play so the butterfly closes easily.

 

This was a suggestion that never got answered. Had the same problem several times in the past, most times it was a small intake leak, other times it was the timing too far advanced. When the bikes running at idle, spray some starting fluid on the intake manifold where the seals are and see if your idle changes when you spray. If that's not it, check your timing. I like mine to be about 8 degrees advanced, they run real sweet there, but if you go any farther it will be hard start and the strange idle return.