Idle Speed Increases after a Blat

[virden]

When cold my X/F ticks over at a lumpy 800 rpm, get it up to temperature and this rises to a smooth 1000rpm, but after a blat, it increases to a smooth 1500rpm. Is this a heat sink problem. Otherwise the thing is running beautifully.

PS throttle cable does not stick, return spring is new.

[Jonathan Kay]

What's the ignition system?

 

Changed anything recently?

 

Jonathan

[Roger King]

Almost certainly needs attention to either throttle cable stickiness and/or carb balance and idle mixture settings. You must also have a little slack in the throttle cable at idle. Unfortunately, a new spring is unlikely to deal with a sticky cable

 

The only way to be 100% certain of the cable is to manually make sure that the carbs shut completely every time that you lift off the throttle. If they don't, disconnect cable from carbs and operate them by hand. If they now shut reliably, you need to deal with a sticky cable. If they are OK with the cable connected, you need to look at balance and idle mixture screw settings.

 

It is very common for idle speed to be all over the place if the above are incorrect. Engine temperature can affect this quite dramatically. Typically, balance and mixture need to be set once a month for a road car, but this can vary dramatically from car to car.

[Paul Deslandes]

could be the mechanical advance mechanism in the distributor sticking in an 'advanced' state when hot. I've never seen it but the additional advance would cause the idle revs to rise.