1970 911t tuned but some issues
 

So, my 1970 911t has had the top end rebuilt with 85mm JE pistons with 9.5:1 CR.

The car has had the zeniths rebuilt and fitted at a local vintage porsche specialist. The car is now running way better than before but "pops" when you lift off the gas.

The jetting has not been changed but everything else has been done (timing, valves, plus, distributor cap etc).

Any ideas what causes the popping?

Too rich of a mixture, vacuum leak,timing off.

Pops where? Exhaust or carbs?
Tony

First don’t chase carb issues unless you are sure compression, valve adjustment, and ignition timing are all correct. Otherwise you will only be chasing your tail. If those things are correct then look to the carbs.

It should go without saying to test fuel pressure, float levels, and ensure all jets/vents are free of debris before making any changes.

Popping on deceleration is a common issue for these carbs since they were set-up to meet emissions requirements This popping can be tuned out with some work. Are the Zeniths utilizing the emissions equipment or not? The enrichment devices are designed to trigger when you lift off the accelerator. If installed will most likely need adjusting To ensure it is functioning properly. If not installed then all the ports need to be properly tapped and plugged with cap screws to eliminate any air leaks. The stock jetting tends to lean towards emissions compliance. They respond pretty well to re-jetting and can be tuned fairly well but do not possess the same eager throttle response of a Weber. The Zeniths are very sensitive to heat and ethanol in fuel. You may want to run stabil’s anti-ethanol additive if you live in an area that adds ethanol to the fuel. You may need to run higher octane in summer months. I like the Zeniths ok but still find Webers to be better from a tuning perspective. That said they can be made to run well without popping so don’t give up.

What Hi Fi said.

But "pops when you lift off the gas'? Not sure what you mean. I am guessing you mean when you are decelerating with your foot off the gas it is popping out of the exhaust?

The thing about Zeniths, assuming it is a US car, is they were jetted very lean to meet emissions. If you make any changes to the motor you will have to adjust the carbs accordingly. First would be increasing the idle jet size.

Popping when decelerating usually is a result of too lean or vacuum leaks.

Just changing from the stock air cleaner or muffler can make this worse.

BTW, re-curving the distributor can help make these cars idle better. You want around 7* BTDC at idle instead of the 4* ATDC that these motors came with. But you don't want to increase the total advance, in fact you may want to limit the total advance to around 30 -32* with the CR you have.

Also, if you want to really wake up this motor change to E cams and use a sport muffler.

Thanks all, yes i think its popping through the exhaust. The emissions enrichment circuit is already plugged. I will check for leaks and try bigger idle jets.

Also, did you increase the venturing size? With the work done on your engine it may be needed along with jetting.

Do some searching as there are some really helpful Zenith threads here. On one an owner did all sorts of Air/Fuel ratio testing and tuning on his Zeniths. REALLY helped me when I was considering venturi and jet selection. My Zeniths run 100% better than when I purchased my T.

As far as plugging the enrichment circuit did you use cap screws with locktite? I've seen people try rubber vacuum caps or even worse bolts shoved into rubber vacuum lines and these set-ups always seem to leak air and lead to intermittent carb issues.

Could also be exhaust leak.

Great tips Gordon,I went to 30mm vents 55 idles and 130 mains and it went pretty good,but appeared a touch rich before and after transition, dropped down the 52 idles and 125 mains and now it starting/running great.
The carbs were rebuilt and I spent a lot of time sorting the linkages as many of the rod ball ends were worn out and this messed up the balance @ idle,once sorted balancing was easy.
IMO fuel pressure and float height are super important,found the PMO float tool rather handy.
Good luck to the OP.