Fast idle

[Julian Mullins]

Hi all.

 

For some time, my 6 has suffered from a rough uneven idle. With the advice from members here, (thank you.), I have been trying to get a nice smooth idle and after months of fettling, I think Im nearly there.

 

Its a CP running on twin SUs with electronic ignition. I have no idea what cam is fitted but its not standard. Engine and carbs rebuilt.

 

I have advanced and retarded the timing, richened and weakened the mixture loads of times and now have a fairly good tickover, (one Im happy with,), nice coloured plugs and a pail blue spark with a colour tune plug on both no2 and no5, no pinking or running on and all seems well.

 

Except, when you lift off the throttle the engine settles at about 1400 revs and slowly, very slowly, drops back to about 900 revs.

 

Any suggestions ?

 

Thanks in anticipation.

 

Julian

[Fireman049]

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Edited April 20, 2018 by Fireman049

[Julian Mullins]

Hi Julian ~

 

I take it that the throttle return springs are strong enough?

 

Tom.

Thank Tom

 

Yes I think they are, if I open the throttle by hand and release it, everything seems to go back to where it should be, it just takes a while (40 - 50 seconds), for the revs to settle back to its idle speed !

 

Julian.

[John L]

Do you have a distributor with the vacuum connected, I think it could be that the vacuum is holding the advance and so the higher idle, you could try disconnecting the and blocking the vacuum pipe and seeing what happens.

Have you checked the diaphragm in the distributor for a hole, do a tongue test and see if it holds vacuum?

Are you sure that the distributor you have is for that engine?

John

[roy53]

throttle cable . If it goes back quick when you operate by hand? then cable sticking.

[Julian Mullins]

Do you have a distributor with the vacuum connected, I think it could be that the vacuum is holding the advance and so the higher idle, you could try disconnecting the and blocking the vacuum pipe and seeing what happens.

Have you checked the diaphragm in the distributor for a hole, do a tongue test and see if it holds vacuum?

Are you sure that the distributor you have is for that engine?

John

throttle cable . If it goes back quick when you operate by hand? then cable sticking.

John.

 

Thanks for that, I will look at the vacuum and diaphragm etc. I hadn’t thought that it may not be the right distributor for the engine. How would I check that ?

 

Roy 53.

 

Good thought, I will check that tomorrow. Thanks.

[John L]

Does the distributor have 2 vacuum connections, one is for advance the other retard, are they connected correctly?

When you say electronic ignition, is this just a hall effect unit instead of points, tell us which system you have.

There is a number stamped on the body of the distributor with a date code also, may helpful to get this.

May also be worth a check, have a look at the throttle butterflies do they have a spring loaded stud in the them, they can leak and continue to draw air in and so keep the idle speed up, they can be changed for solid butterflies.

There is a lot of info in workshop manual, There is one note that says that the action of vacuum advance deleted and blanked off

John

[peejay4A]

With the air cleaners off lift each of the SU pistons with a finger then let them drop. They should descend and make a metallic clonk when they have fully dropped. Do they both drop at the same rate, is one or both sticking?

[Julian Mullins]

With the air cleaners off lift each of the SU pistons with a finger then let them drop. They should descend and make a metallic clonk when they have fully dropped. Do they both drop at the same rate, is one or both sticking?

 

Does the distributor have 2 vacuum connections, one is for advance the other retard, are they connected correctly?

When you say electronic ignition, is this just a hall effect unit instead of points, tell us which system you have.

There is a number stamped on the body of the distributor with a date code also, may helpful to get this.

May also be worth a check, have a look at the throttle butterflies do they have a spring loaded stud in the them, they can leak and continue to draw air in and so keep the idle speed up, they can be changed for solid butterflies.

There is a lot of info in workshop manual, There is one note that says that the action of vacuum advance deleted and blanked off

John

Thanks John

 

The distributor is a type D22 (so only one vacuum connection), but I dont know the date code off hand. I took the distributor apart today, the diaphragm is ok and blocking the advance pipe makes no difference to the revs dropping slowly.

 

Ignition is a luminition unit, already fitted when I bought the car. It looks professionally fitted.

 

The carbs have got solid butterflies which I fitted when I rebuilt them. On the bench the butterflies are a good snug fit.

 

Pete.

 

The piston both seem to drop at the same rate and stop with a clonk. They dont appear to to stick. But thanks for the suggestion.

[Nick Jones]

It's getting air from somewhere - but a limited quantity. Brake servo related? Easy to temporarily disconnect and blank off to check. Beyond that I'm still inclined to suspect linkage/butterfly fit. It really doesn't take much air to raise the idle speed a bit.

 

Nick

Edited April 20, 2018 by Nick Jones