High Idle Speed

Hello All:

I have a 1981 550ltd that was given to me. I just had the carbs rebuilt and now have them back on the bike with new vacuum tubes. The bike ran when I got it, but idled high and leaked fuel out of one of the carbs. It now has all new gaskets and a new (well used but better) float bowl because the overflow tube in that one was cracked.

My issue is that the bike idles at about 4000rpm, maybe a little higher.

The idle screw is out all the way and the throttle cables are loose enough that I wouldn't feel comfortable driving it because they could work their way loose--that is they are a tiny bit slack. The carbs have not been synced yet.

Do I have a timing issue? Did the carb job get botched?

Any and all comments are much appreciated.

Cheers

smørbrød wrote:

Hello All:

I have a 1981 550ltd that was given to me. I just had the carbs rebuilt and now have them back on the bike with new vacuum tubes. The bike ran when I got it, but idled high and leaked fuel out of one of the carbs. It now has all new gaskets and a new (well used but better) float bowl because the overflow tube in that one was cracked.

My issue is that the bike idles at about 4000rpm, maybe a little higher.

The idle screw is out all the way and the throttle cables are loose enough that I wouldn't feel comfortable driving it because they could work their way loose--that is they are a tiny bit slack. The carbs have not been synced yet.

Do I have a timing issue? Did the carb job get botched?

Any and all comments are much appreciated.

Cheers

new vacuum tubes --- which tubes would those be?

Some of the possible causes for too high idle include:

-- not having the idle thumb screw in mid-position during the bench sync;

-- advancer unit sticking in advanced position;

-- air leak (perhaps deteriorated carb holders).

Imo, likely not sync issue, unless far out of sync and no bench sync.

Good Fortune!

Sounds like stuck slides/throttle in the carbs.
Or a massive air leak.

With the idle screw all the way out, the slides should be completely closed and the bike would not start/run.

Check the carbs carefully for strange settings/anything missing - like the mixture screws, or vacuum port plugs.

GL.

Patton wrote:

smørbrød wrote:

Hello All:



new vacuum tubes --- which tubes would those be?

Sorry, not vacuum tubes but the intake tubes that connect the carbs to the air box.

The carbs have not been synced yet. I took them off the bike to have them rebuilt and just reinstalled them. My plan was to drive the bike to a shop to have it synced.

Also it does not appear that the slides are stuck. They seem to move back as far as they should.

With the idle screw all the way out and the throttle all the way down the engine should kill? So is it possible that they are way out of sync and therefore not all closed at idle, and some are letting in lots of fuel still?

Patton wrote:
"Some of the possible causes for too high idle include:

-- not having the idle thumb screw in mid-position during the bench sync"

That's where I would look first. It's an easy mistake for the carb guy to make if he's not careful. Ed

smørbrød wrote:

The carbs have not been synced yet. I took them off the bike to have them rebuilt and just reinstalled them. My plan was to drive the bike to a shop to have it synced.

Also it does not appear that the slides are stuck. They seem to move back as far as they should.

With the idle screw all the way out and the throttle all the way down the engine should kill? So is it possible that they are way out of sync and therefore not all closed at idle, and some are letting in lots of fuel still?

plan was to drive the bike to a shop to have it synced --- with idle at about 4000rpm, maybe a little higher, and idle screw out all the way, sync is likely impossible.

If carbs were properly bench synced before refitted to engine, the high idle might result from air leak (perhaps deteriorated carb holders, leaky vac plugs, etc.) or advancer unit sticking in advanced position.

Would copiously spray WD40 all around common leak sites while engine is running. If WD40 is ingested through an air leak, the exhaust will smoke.

Am thinking the ignition timing on this bike isn't designed to be adjusted. But if ignition is adjustable, could be checked statically (engine not running), and lube and manipulate advancer to help assure it's properly functioning.

Good Fortune!

Sometimes the carb holders look pretty good on the outside, but might look like this on the inside where supposedly connected air-tight to engine.

Good Fortune!