...for the most insane backfire known to man!
('74, 1.8, dual 40's, stock ignition, 2l sshe's, bursch, 3lbs fuel pressure, compression: 1-25, 2-26, 3-55, 4-65 but they hold those numbers)

i finally have the carbs dialed & running good. (some people know this engine and it's car sat for years [decades] under a willow tree on a farm). fresh valve adjustment done last weekend.

last night i was running it and modulating the throttle from 1/4 open to 3/4 open to try and draw out a backfire (this has been happening on decel) but i couldn't get it to explode, so i thought my last adjustment was good.

i had been running somewhat rich, so last night's run was to try and lean it out as much as possible. i got home and shut the car off and i thought i was going to find the muffler it pieces after what sounded to me like a frickin' cannon shot about 5 seconds after shut down.

i've already decided i'm taking it easy on this engine, but i need to resolve this. and i'd rather not have the neighbors think i'm firing shotguns in the garage.

paul

Well here,s my two cents worth...It should be obvious that excessive fuel is causing the backfires,and explosion (s)/ after shutting off the car.You did say it was running RICH. the rich fuel condition could be ....OLD gas which always causes problems,and sometimes makes you THINk the engine is rich..,,with lots of water ,and contamintation..flush the gas tank,and check the strainer..more than likely the carb seals are dried out,the accelator pump diaphrams are cracked /and some jets are clogged with dried fuel "tarter"and crud. a defective ,dried out accelator pump diaphram will allow a flooding condition...and don't forget that the float needle valve,and float could have taken a also.If the timing,valve lash, electrical system and basically everything else is good....then rebuild the carbs.

Backfires on decel are most certainly caused by an exhaust leak(s). Even the slightest of leak can cause one. Check ALL exhaust connections to be sure the sealing surfaces are flat and true. Then change all gaskets.

It could have been that the muffler had a tiny hole and now it toasted itself. . .

25-65lbs compression? Why bother. You will "dial in carbs" for nothing. Way too low and way too mismatched. I've seen "cores" for less than 200 bucks with higher compression. My guess is burnt valve (s),

Tear it down now. Your gonna have too anyway. Rebuild the carbs then.

T.C.

thanks for the leads, guys.

respecting the carbs. they're new and clean, no issues there. the gas is good, i've been driving it for 1500 miles (in various states of tune). i actually think part of the issue is timing. i have no marks and only an old timing light, so i'm limited on that to "by feel" mostly.

it WAS rich, but i leaned it as far as it would go. however i'm running the idle screws a little open so that's something to correct.

the popping seems to come and go with my carb adjustments. i'll double check the exhaust seats gladly.

i'm on the verge of swapping the heads as i have a set of newly rebuilt ones from the 2056.

I'm far from an expert, but if those numbers you mentioned earlier are actually the compression numbers, swapping the heads is probably a poor idea. To get compression numbers that low, across all four cylinders, the motor is simply ready to go -poof-.

Pull the motor, and build the 2056 those heads were planned for...

-Josh2

did you check your compression warm? because those numbers are hella low!




even if it's a burned valve, there's probably an underlying cause. so I'd build the 2056

For a quick and dirty, you can pull the motor, hone the cylinders and get new rings, and have the valves reground. It's not a rebuild, but it'll only cost you a couple hundred bucks and it'll bring your compression numbers back up (albeit somewhat temporarily).

i think it only explodes after running for an extended time and is hot (it's running 160-175, but it was about 185 or a maybe little more last night).