I am experiencing a great deal of confusion with the dashpot piston (or suction piston) lifting method to tune the mixture of the SU carburetors.

I've read that lifting on one carb helps diagnose mixture on the other carb. I've also read (according to Scott Fisher) that lifting the piston of one carb helps with diagnosis of that carb itself.

What's most interesting is that there is a lot of support for both cases but I am receiving the most factual support for the first case - lifting on one carb helps diagnosis of the other.

Support:

Naturally you would think that lifting of the piston of one carb affects that carb only and helps one diagnose mixture for that carb. Not to mention that most of the sources I reference refer to this technique including Scott Fisher and a good Z friend of mine in Cincinnati.

However, I've also read that lifting of the piston of one carb cancels that carb by opening the venturi too much to allow fuel pickup. This in turn forces the engine to rely wholly on the other carb. If it stalls the mixture on the other carb is too lean if it runs and maintains idle the mixture is rich. If it reduces idle but doesn't stall the mixture is stoichiometric. Furthermore, I've verified this on my car, but I'm still having tuning problems with this method as well as the other.

Any help would be greatly appreciated, and if you need more clarification I would be more than happy to oblige.

Thanks.

I guess I should have clarified a question to be answered.

I need to know which method is the proper method. Which carb should I be diagnosing for mixture when lifting the suction piston of say the front carb and listening for the changes in idle?

Oooops! http://www.classiczcars.com/forums/showthread.php?s=&threadid=12741

Found it, and I didn't have to buy the damned video!

Thanks for asking the original question, Deadflo, and thank you for your well informed answer and direction, Beandip. This forum is an outstanding help.

The Doctor

And here is the tech article by Mr. Beandip himself.

http://www.classiczcars.com/forums/showthread.php?s=&threadid=9477

Thank you for posting that anyway helps me as I always forget myself. I have a Unisyn for tuning the airflow now so I will give that a try I've been doing it all by feel so far.

I might invest in a colortune kit also but so far so good.

Gav,