Pboden

911s - 74.
Spent the weekend adjusting the distributor to find the right timing. Also messed with fuel/air ratio and I believe I have done everything according to the book, but it keeps on backfire.

If the timing is about OK-ish, what could cause the backfire ?
To much fuel, bad injectors or maybe air leak....

Thanks

r911

out the exhaust or out the intake?

Pboden

It's from the exHaust, but few times through the intake. ( that's smoke) a burp.
I have now taken out the throttle body and searching for possible leaks. I have ordered new injectors with the additional seals.
I have tighten the seals from the intake, the WUR is a non vacuum unit, so there is not many places to look for vacuum leaks. Also ordered new inlet manifold gaskets.
New spark plugs and wires.
Also invested in a new electrical ignition from Hot-spark, but for diagnostically purposes, I return the the old one back.

Any other places that's needs attention?

theiceman

well i would not have messed with the mixture .. but a few things we can check .

if its backfiring out of exhaust tou could have a too rich mixture and its unburned fuel, an air leak at the exhaust that is causing combustion in the pipe or a combination of things.
Check your head studs and make sure none off them are broken alowing the head to lift off.
check your cams for pitting/excessive wear
check your valve clearances and do a valve adjust
Check for broken exhaust studs.
Could also be mechanical cam timing if your chains/ramps are worn.

i assume your wires are hooked up correctly and you have checked the basics for cracked cap or rotor.

Also why were you playing with the distributor , was the car running fine then it suddenly started backfiring ? we need more history , it could even just be bad gas.

Pboden

Well, before I do a total engine tear down, I'll check if the new injectors will be of any help.
I have tighten the nuts for the studs.
I have adjusted the valves.
New exhaust manifolds and muffler.

The backfire came gradually, if I remember right, but I have not been able of start and running it much. That was initially the problem.
And yes, I have messed with the mixture.

If nothing else will work at the end, the engine is coming out.

r911

also... how many tanks of fuel have gone thru it while it is backfiring?

and have you changed both fuel filters?

Pboden

Not changed the fuel filter.
There are only one filter an one accumulator in the engine bay.
The fuel tank is new, so there should not be much debris.

r911

the 2nd filter is a screen near the tank outlet - if new, it may be fine

do the other one & get some new fuel in there before doing more expensive items