backfire

Blown v8 · Post by **Blown v8** » Wed Jun 11, 2008 5:40 pm

5.0l 4-71 supercharged- just replaced the blower to manifold gasket, warmed it up gentlly till the oil cooler was functioning. Gave it some throttle, as it came off the throttle ,a massive backfire, blowing said gasket again, and not firing on number 7 ! checked static cylinder pressures and A OK any 1 got any ideas before i rip the engine out for the upteenth time in 22 years ! 1 one day i will get it right

RoverP6B · Post by **RoverP6B** » Thu Jun 12, 2008 11:31 am

Backfires can usually be attributed to lean running.

How do your plugs look?

Ron.

Ian Anderson · Post by **Ian Anderson** » Thu Jun 12, 2008 12:05 pm

Possibly incorrect timing causing it to fire when Inlet valve is open.

Is it Carb feeding the supercharger? If so it could be sucking in a load of fuel in above the throttle and then it managing to dump it in the plenum and getting raw fuel through and detonating.

Ian

Blown v8 · Post by **Blown v8** » Thu Jun 12, 2008 2:15 pm

plugs look good. holley 650 double pumper above blower. Had engine dynoed a couple of years ago when engine was built.Cars been laid up for 4 months . drained tank and replenished with fresh petrol, maybe a look into the carb to see whats going on is needed !

kiwicar · Post by **kiwicar** » Fri Jun 13, 2008 2:14 pm

Pretty good chance that you are getting fuel puddling in the inlet when you lift off, a small air leak after the carb would give you a nice big load of charge in the inlet ready to go pop, and as you have the thing on over run then the timing is heavily advanced and it fires with the inlet valve still just open bang!

May be wrong but I would get the WD 40 out and start hunting air leaks post carb above the SC (or in the body of the SC).
Mike