2.2 MFI Backfire

[50/50]

Looking for some pearls of wisdom.
My 2.2S has had a consistent popping both on and off throttle for a couple of seasons. The engine make plenty of power.
So far:
- the pump timing alignment has been checked
-the linkages have all been calibrated
-the fuel solenoid has been replaced
-the aftermarket exhaust and headers have been checked for leaks

None of this had a significant effect and a took it to another shop which did the following:

-and most recently the 40 year old Permatune CDi was replaced with a MSD Ignitiion and coil.
- all electrical connections were examined and cleaned.

After replacing the Ignition the car ran with almost out incident. And after refreshing the older fuel (which was a mix of ethanol 93/stabil and a few gallons of 110 race fuel) it
ran fine.

I took it out yesterday (a week later) put in another 6 gallons of pump 93 fuel and it is popping as before.
Which is baffling. Why would it run great after replacing the ignition, then a week later revert to the backfiring?

So what's left? residual old fuel contamination?, throttle bodies?, pump?

Any Ideas?

[Mr9146]

Sounds like you answered your own question with the fuel...

The blended fuel was higher octane.

Richen the mixture and you'll probably be fine...

[Flieger]

What kind of popping? Same type on or off throttle? Rev range?

I would think if it were light pinging it would go away off the throttle, otherwise the pinging would be so bad the engine would expire before long.

[G69]

How old are the spark plugs? Mine looked great but when replaced engine really settled down.

NGK BR8 ES.

Check for your specific heat range.

[50/50]

Thanks for the suggestions Gents. Will try the spark plugs and maybe dump the fuel also.

[vicrola]

Plugs might be the reason. I recently changed my '72 911E (w/ MFI), to Iridium NGK BPR7EIX. Try those.

Additionally, I'd re-examine the gasket between the heater boxes and aftermarket muffler. After doing that, my backfiring stopped.

Vic Rola

[cmcfaul]

Valve adjustment. Too tight would definitely cause back firing.

Chris

73E

[csbush]

The answers on this thread are all over the place- but could all be right. You really have to follow the manual, and go through the entire system to find the problem. It can be one thing or a myriad or little things.

In my experience with my 70S, backfiring was caused by an over rich fuel mixture necessary to compensate for worn throttle bodies, and other mis-adjustments.

[raspy2point2]

My 2.2 S had some popping on overrun. Had a slight adjustment to one of the stack/ throttle body ; that fixed it. But mine is also in need of valve guides and rings. Chris

[50/50]

Thanks for all the input. We have been working it.
After a balky warm up ,it is down to a slight popping between 2800-3000 rpm,usually off throttle but sometimes on throttle.
Most recently also added Stabil Marine formula.
Any thoughts about the consistent issue at a narrow rpm range?