Very fast idling after run

[Don Vosper]

Just been for a short run on my 38 500 Sunbeam Lion.
On return the idle speed had gone up to well over an estimated 2000 revs. Plenty of slack in the throttle cable and screwed the slide right down. Didn't slow it down at all. Retarding the ignition fully slowed it down but only to a still fast tickover When cold it ticks over normally and starts and runs ok. Haven't disturbed anything for quite a while.
Any ideas before I start investigating the problem?
Cheers
Don

[Don Vosper]

I am told that one possibility is a worn slide in the carb. In the past I've seen ads in OBM of companies that do this, but a scan of a recent OBM doesn't show any. Anyone know who does this sort of thing these days. No problem with speedo or magneto repairers.
Don

[murdo]

Sounds more like an air leak in the intake somewhere. Maybe as the engine gets hot a clearance expands and allows air to enter. Could also be a valve clearance problem getting tight. Is the ignition advancer free and not sticking?

[Don Vosper]

Hi Murdo, thanks for the reply.
I have the carb off now and looking at it tomorrow.
Gasket looked ok.
Ignition advance ok but not looked at valve clearances yet.
Don

[rogerwilko]

Tight valve clearances wont make it idle fast???Make sure your slide is closing  properly even  though you said it was, otherwise it must be an air leak or radically worn slide. Idle speed should be set at full advance and when warmed up.

[murdo]

No, your right, my mistake. Tight valve clearances will slow down idle speed.

[mark2]

Idle speed should be set at full advance and when warmed up .............................. is this correct ? ive always set mine at fully ret when warm

[Don Vosper]

Thanks for all the comments.
The slide does have some wear as would be expected. Looks like it's plated brass.
I guess the idle speed should be set at full advance but so that it doesn't stall when fully retarded.
I shall clean out the jets etc  and put it back and see what happens. Looks like getting the carb re-sleeved isn't a cheap job looking at other forums. I see I can buy a new 276 for £300.00 or if I want to change the type, a concentric for about £100. But that's a last resort.
Don

[mark2]

if you set the idle at fully advanced it will stall when ret , or have I got this wrong , I must take another look

[Don Vosper]

The Sunbeam is a long stroke sidevalve. If I set the idle speed to what sounds normal on full advance, no rev counter, it will tick over very slowly, without stalling, when retarded. I have to say I've never read any instructions regarding this.
If you take a machine with advance weights in the distributor the the idle speed is set with ignition fully retarded anyway.

When riding, after starting with it fully retarded, I advance it fully and leave it there unless I stop for any length of time.

Don

[mark2]

why not set the tickover at fully ret ?

[Don Vosper]

It was still too fast when fully retarded in this case. It was reluctant to stop when the valve lifter was used.
It has worked fine the way I had it. As I said I have the carb off now so I'll have a good look at the slide and make sure it is going fully down as I can see things better with it off the bike.

Don