Thread: Looking for another opinion. What is causing my backfireing.

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So just a little clairification the motor is fully broken I. I rebuilt engine the first time 3 seasons ago and have about 2000 miles on it and redid the top end this winter with the proformance top end kit or heads intake new carb distributor that kinda stuff. And have only drove about 200 miles this year testing and tuning.so all the major components s like rings and what not have had time to seat by now.

How do I set up the power valve please give me some insight into that never have played with one before.

Vac guage is brand new right out of the box bought it for this project to set up the carb

Valves were set to snug then 1/2 turn

I have been conversing with an old stock car racer he said same as you need to know advanced timing but with the cam and heads combo he suggested going to the 12 degrees initial timing.and the motor is not nocking

Fule pressure is 6 psi right what the carb manufacturer recommend for street application

Originally Posted by HDSBC

Valves were set to snug then 1/2 turn

Tell me more about you did this process, I think your valves may be a little tight.

Rule of thumb with a power valve you take your idle vacuum and cut it in half, however that is not always the case. Do you know what number PV you have? And it's always good to check the gasket at the PV when there's a rich issue, occasionally the manufacturer makes mistakes.

By your description, the problem comes on after 2000rpm, which is just coming out of the transition circuit into the cruise (primary jets). With Holley type carbs, I jet the primary for cruise first and don't touch the secondaries until the cruise is right. Remember, WOT is the accumulative of the primary, secondary jets and the POWER VALVE. The Power Valve is HUGH in tuning these carbs for the street.

But first, we need to get the idle vacuum up or nothing is going to tune right. And, this is why I'm asking about the valve lash setting, it is REAL easy to set valves a little tight if you're not experienced with setting valves (no disrespect meant). Tight valves will cause all of the troubles you are experiencing. It may not be actually running rich, but rather not burning the fuel because the valves are not completely sealing.

Just to confirm, you could loosen all rockers 1/4 turn (really doesn't take much) and see if the vacuum comes up. If it does, we'll go over the process of setting valve lash in more detail.

I don't know the power valve size off by hand but I will take apart the carb and see.
I will most definitely reset the lash on the valvetrane see if that is my low vacuum problem. Most likely going to be on the weekend I will be able to do that task. Thank you for the information fingers crossed I can get this stumble out of the engine.