Rough Idle and Sputtering

[Dave Russell]

Sharon -
In my experience this type of problem indicates that the throttle bushes in your carbs are worn and when the car gets hot the throttle bodies expand faster than the brass throttle pin, and opens up an air leak gap, causing the mixture to lean out ... causing the idle to drop.

If you have a BJ8, the fix is pretty easy, just get the teflon bushes the Nocks sell at British Car Specialists in Stockton, CA. If you have older carbs, you will have to have your carbs rebushed, reamed and matched to a throttle shaft. This should be done by an SU carb specialist like Joe Curto.

To put this in perspective:
Based on the two different metals coefficients of expansion, a temperature change from 30 degrees F to 200 degrees F would cause the aluminum throttle body to expand 0.0003" (three ten thousandths) more than the brass shaft.

Considering that a "tight" shaft would already have three times this much clearance, or 0.001", the additional heat expansion would hardly be enough to cause a noticeable increase of air leakage. The cost of rebuild or new seals would be much better spent on something more productive.
D

 

[Dave Russell]

I should like to add somthing here which I am sure has nothing to do with your problem but it should catch the eye of owners who are thinking about it. On advice from a very experienced car (60yrs!) he suggested I lighened the oil in my '63 BJ7's carb. dashpots from the Manual recommended engine oil. I put in transmission fluid: remarkable improvement both in acceleration and shut off.He had heard of brake fluid being used for racing.

Hi John,
A properly working damper/dashpot is a one way valve. It resists air valve opening but "should" have no affect on closing. Changing fluid viscosity could very well affect acceleration but not shutoff. Unless of course, the valve is defective.
D

 

[healeygal]

Hello all,

Sorry I haven't been paying attention and done a follow up. I think the main problem was the mixture. The car seems to be running well now - did the checklist suggested by Tracy and adjusted the mixture, test drove it a few times, then adjusted the mixture a little more. Thanks to everyone for all their help!

Sharon
BN1