N.K.-simple things first.Check the valve lash to make sure there aren't any tight valves.Tight lash on one valve could cause it to hang open and cause your back fire.

 

Try richening up your mixture on the back carb by a 1/2 turn and I bet your problem will be cured.Also check your airflow balance.

Later,Norm

 

I would richen it up like Norm said.
If you replace the intake/exhaust gasket then you should take care of any possible vacuum leaks there too.

 

But Bill, all the compressions were over 150, do you still think it has a tight valve?? NK

 

Norm, you'll be suprised how little the valves being out can affect the performace of the SU carbs. When the vacuum from the valves being opened is "altered" it will affect how the high the pistons rise in the carb. So, I would adjust your valves first (cold) then make the needed adjustments to the airflow on the carbs then also you can take the tops off your carbs and close the mixtures all the way and measure with depth gauge on a dial caliper and you'll see that the nozzles are usually a little off, mine were about 1/4 of a turn different from one to another. Find out on yours where they will sit and then you can reset your mixtures to where your car operates best. Best of luck!

 

It takes 2 seconds to adjust the mixture. If you want to spend an hour adjusting the valves first that's your choice, but common sense troubleshooting says try the 2 second fix first.

BTW, I have never seen a valve adjustment cure an SU backfire. I have seen fuel and timing adjustments fix SU backfires everytime that I have tried it though.


Later,Norm



Post Edited (Jul 9, 8:17pm)

 

"I have seen cars backfire 'cause of tight valves BUT the cylinders with these tight valves had less than 100 psi and in some cases lower than that! I feel like an idiot for not trying to richen up that back carb. I'll try it and see. I'll keep the valve adjustment and my 17mm crowsfoot in my back pocket," Thanks to EVERYONE on this one! Norm K.

 

The ignition timing could be the culprit as well, but normally both SU carbs would be backfiring.Also make sure airflow into both carbs is synced.

 

"I still have my Uni-Sync from the 70's" NK eer, now to use it. NK