Slight backfire after key is turned off.... why?

[Tangmere]

Ok, fellow 02'ers. I am stumped here on the basics of tune ups with this. I am new to the 02 world, and am just not getting it. Here is the setup. a 75, stock other than a brand new 32/36 water choke Weber. The air fuel is set at 2 and 1/4 turns out. Timing seems to be a mystery, as from what i have read, all of them are different, so there is no real degree that works for everyone? So i set it a a good sound, no rough idle at about 950 rpm. It is not advanced to the point of no power at higher rpm, UNTIL about 4000 rpm. So my issue is i have a weak spot, not really a dead spot at about 4000rpm, or about 70ish mph. I have fiddled with timing and air fuel for about a month now and just cant seem to get it right. Should i set the timing at revving the motor to about 3500 rpm, to make sure the distributor is fully advanced, then set to smooth running? Why does it slightly backfire after i shut the motor off? Oh, and i just re adjusted the valves to make sure i didn't have any issues there. I am guessing the timing is still off, for the back fire, but why after the ignition is off does it backfire? Too rich and fuel left over out of the exhaust igniting in the exhaust manifold?

Thanks in advance for your responses, this is probably a simple issue, and i am just too much of a noob with these to get it....

[Guest Anonymous]

do you still have points? When I did, the adjustment of them made the difference on backfiring - don't remember which way, but a little off and the car backfired. Higher octane gas might help too.

[Tangmere]

Sorry, forgot to mention that. No i installed a pertronix. Can that be the issue? I kind of heard that its either on or wont fire, so if it worked, i figured i had it right, using the plastic gauge, I have not fiddled with the pertronix gap since i set it. Oh, and it is easy to start in the morning here, morning temp about 28, but it wont stay running, will die due to low RPM. Although i can start it easy, fires right up, i have to keep my foot on it till it warms up or it will die. Once warmed up, idles at 950, fairly smooth.

[roadhog0]

My car does the same thing warming up. As far as the backfiring check your jets and make sure they are the correct ones. Also if you dont have idle cut off solenoids the car can diesel a bit sometimes. Mine does from time to time as well. No biggie really.

[Tangmere]

So here is a 64k question. What should my jets be? This is a Rock Stock car, nothing extra. I realize it might be different for each situation, but is there a guide line you use?

Thanks!

[c.d.iesel]

set your timing @1400 rpm, "BALL" mark aligned in the

timing inspection hole to the rear of the starter motor.

If your car has a vacuum advance - disconnect the vacuum

hose at the advance pot while setting the timing.

your 'step' or 'flat spot' in power delivery at 4000 rpm is a function

of the air correction jet sizes. Try my suggested

jetting for the 32/36 and you'll see no more 'flat spot'.

the backfiring on shut off can be caused:

-by a leaking fuel shut off solenoid at the

base of the carb - if yourshas one fitted?

-Valved adjusted with too little clearance?

-Float level too high?

-idle mixture way off - too rich?

-spark plugs wrong - too hot heat range?

-ignition timing too retarded?

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

...my WEBER prescription:

32/36

Float Level 40mm

PRIMARY

idle 60

main 140

air correction 145

SECONDARY

idle 55

main 170

air correction 175

AND -PLUG THE SECONDARY ENRICHMENT HOLE AT THE TOP OF THE CARB AS MY DIAGRAM SHOWS . DO IT AND SEE THE HUGE DIFFERANCE IN ACCURATE FUEL METERING THROUGH OUT ALL RPM AND THROTTLE OPENINGS.

Float Adjustment Instructions

[Tangmere]

So here is a 64k question. What should my jets be? This is a Rock Stock car, nothing extra. I realize it might be different for each situation, but is there a guide line you use?

Thanks!

[c.d.iesel]

see above - it's your starting point

[Tangmere]

Thanks for all the info on the carb and timing settings. One quick question, what degree adv or ret do i set the timing light on to see the ball? Or are you saying use a standard timing light.

Thanks!

[BillWilliams]

Timing light set to 0 degree advance.

The BB (ball) on the flywheel is set at your degree of advance.

The BB is in red in this photo. The white line covers the TDC mark. So you can see the degree of difference