Decaying idle

[randy]

If the carbs are truly synched, and the idle still hangs, it's running lean. Like others posted, check for vacuum leaks or maybe you still have dirt in some pilot passages.

Also leaks at exhaust ports will cause lean running.

[psycoreefer]

Originally posted by mro

So is that from a cold start or after engines warmed up?
Do you start it with the "enricher circut" or not?

mro

Didn't see an answer to this one so I'm quoting it. Are you having this problem an a fully warm engine, or a cold one. Is it better/worse/same when cold vs when hot?

On my 79 when its cold and I first start it up, choke full on, it will idle at about 1200, if I rev it at this point it will drop down to 3k-ish quickly and then slowly back to 1200. As it warms up the idle increases until I turn the choke off.

more ideas on vacuum leaks...
did you check for holes in your slide diaphragms?
Did you check that the slides all rise and lower with about the same resistance and none are "sticky"?

I'm thinking one or more of the slides sticking in the "up" position and then returning down more slowly then the others could cause something like you describe.

[swiftwing]

Balancing

Mitch,
Thanks for "trying" to help. Balancing was perormed by the shop manual for this model year. I can't be more pleased with the smoothness of the idle. It just doesn't drop back immediately to 1000 rpm.

If I force the idle down with a little clutch drag and nothing else, it will stay there. I would think that vacuum leaks would tend to add roughness to the idle but still provide a constant idle. Smoothness of idle is great no matter what rpm (high or low).

[swiftwing]

Engine temp affects

When I start with a cold engine and full choke it will initially idle around 1200. As I start down the road and shift the idle will hang at 2500 or so.

When it is warm enough to run without the enrich, the idle will drop reliably to 1000. As it warms up to normal running temp, this is when the idle between gears and when I come to a stop that the idle remains high (2500 or so). Again, all I do when stopped is put a little load on the engine to drop the rpm's to 1000 and she will stay there after the load is removed. It will idle there all day until I rev it up again.

The diaphrams of the main jets are perfect. And there is no indication of drag in the movement of the body of the diaphram tube.

[trbig]

It just seems to me that everything we suggest.. you have already done perfectly before... There are no air leaks, the carbs have been cleaned and tuned correctly, yet everything you describe suggests either dirty carbs, improper carb synching, or both.

Not quite sure what else to tell you. Have you adjusted the valves or checked compression?

[thewiz]

If it were me,

I would pull the timing cover off and observe movement of the vac advance. See if, once the idle speed has elevated itself to the 2500rpm you're talkin' about, that when you let off the throttle the advance retards the plate to it's static position. If the vac advance arm is still contracted up into the diaphram (advanced) and doesn't return when you let off the throttle, then vac advance and/or plate is suspect. If this is the case, cause that to happen again only this time reach down there and MANUALLY pull the advance plate back and see if the idle returns to normal. If it returns, probably the vac advance unit or PU coil plate.

[swiftwing]

Timing

Great suggestion. Need another day to check it out!

[TopCatGr58]

Hey Swift,

You say it's a stock bike as far as you can tell! What we mean by stock is OEM airbox without holes drilled into the bottom, and stock 4-2 OEM exhaust pipes. MODS to either can contribute to leaning. At first you posted it would drop idle to stall, then after recleaning, synching, replacing pilot jets with mikuni 42.5's it will now idle, but hangs AFTER it gets warmed up.

You didn't answer a while back how you tuned the PILOT SCREWS? Standard 1-1/2 turns out from gentle seat, or colortuned, or tuned by ear??

IF the vac. adv. hanging doesn't pan out, try turning the pilot screws out 1/2 turn each and see how it behaves, sounds like it is still getting lean as it warms up, and that would help to richen it up a bit! You may need to turn back the MAIN central idle screw as turning the pilots out may increase the overall flow and raise the base idle a bit. JAT!
T.C.

[psycoreefer]

Originally posted by TopCatGr58
Hey Swift,

You say it's a stock bike as far as you can tell! What we mean by stock is OEM airbox without holes drilled into the bottom, and stock 4-2 OEM exhaust pipes. MODS to either can contribute to leaning. At first you posted it would drop idle to stall, then after recleaning, synching, replacing pilot jets with mikuni 42.5's it will now idle, but hangs AFTER it gets warmed up.

You didn't answer a while back how you tuned the PILOT SCREWS? Standard 1-1/2 turns out from gentle seat, or colortuned, or tuned by ear??

IF the vac. adv. hanging doesn't pan out, try turning the pilot screws out 1/2 turn each and see how it behaves, sounds like it is still getting lean as it warms up, and that would help to richen it up a bit! You may need to turn back the MAIN central idle screw as turning the pilots out may increase the overall flow and raise the base idle a bit. JAT!
T.C.

+1 I was going to suggest the same thing. Even if your plugs look good still give it a try 1/8 to 1/4 of a turn makes a difference but can be hard to see on the plugs.

[NemisusDM]

Hello,
I had a similar problem just last week and it turned out to be an intermittent wire on the vac advance. Check the vac hose too.
Might want to check for a clogged breather on the fuel cap. As the level drops and no air gets in, the fuel flow will slow down.

[XSPastor]

Are the carb diaphragms in good shape?

[Toolmaker Tim]

When you cleaned the carbs, did you clean the spooge hole in the bottom of the float bowls. That keep me from being able to adjust my idle. I had to run my idle at 1500-1600 rpms otherwise it would die. I noticed it was possible, if I remember correctly, to cover the hole that allows air to vent into the carb bowl with the bowl gasket if you installed it backwards. Just my 2 cents

[swiftwing]

You say it's a stock bike as far as you can tell! What we mean by stock is OEM airbox without holes drilled into the bottom, and stock 4-2 OEM exhaust pipes.

There are no holes in the air box bottom other than the 4 bolts for wingnuts. The pipes are original 4 to 2. Mufflers might be after market but are baffled with good noise reduction.

The pilot screws are set at exactly 1.5 turns. Haven't had a chance to test the timing recovery yet. Fuel tank is coming off for another good look at the linkages and a touch of lubrication on each pivot pin.

[Toolmaker Tim]

Pilot screws

Be very very careful when checking the number of turns in and out. The tips of the screws are very fragile. When they break off they stay stuck in the tapered holes they fit in. You might want to take them all the way out and check the tips. If they are broken off ( they should be pointed) they can cause problems. They are a real pain to get unstuck from the orifice holes. They get seized up and it takes an act of congress to get them out.