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

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Originally Posted by HDSBC

comp cam110 degree normal lift longer duration cam

JMHO, 110 LSA is a little tight for the street, is this a THUMPER cam?

To answer everyone s questions it has a jegs duel plain aluminum intake I checked the torque on all the bolts has been gaskets and did the old propane torch wave to look for any leaks no signs of leaks.

I will try turning down the float levels that might help with the richness.

I can't document the timing threw the rpm range the only tool I am missing is an adjustable timing light I only have an old school light hoping to get one or borrow one soon.

Yes 8-1 stock but I put 64 cc heads raise the compression ratio up to more like 9-1 I didn't want to go to wild remember this is a heavy hauler not a drag strip racer.

Yes much agree signs of too rich senario but carb is brand new has less then 200 miles on it how could the power valve be gone already and if so what dose it look like when it's broken.

I agree vacuum is low but I can't pull any more then 11 inches no mater what I do and when I adjust it minimum is 10 inches of murcury. Everything I read it should be 12-14 but it just can't get there and I am not even running a vac brake booster. My brake booster is run off the power steering pump because my chassis was a g30 coe design didn't have enough room for a big booster under the hood.

Yes I did a royal purple brake in oil and did the proper brake in procedure when I rebuild the engine.

Compression tested all cylinders at wot lowest cylinder was 122 highest was 130 psi all checks out.

Cam is comp cam high energy line if I remember correctly.

Originally Posted by HDSBC

To answer everyone s questions it has a jegs duel plain aluminum intake

I will try turning down the float levels that might help with the richness.

I can't document the timing threw the rpm range the only tool I am missing is an adjustable timing light I only have an old school light hoping to get one or borrow one soon..

You can check the timing - use you timing light and by changing the idle screw and watching the tach, come up through the rpms by 100 and see where the timing is. Not the fastest way, but it does work. Generally most engines like total timing of 30 or 32 degrees by 2K to 2200. And don't forget, the vacuum chamber isn't working as the throttle is being opened and dead at full throttle, so don't confuse that operation by just disconnecting and plugging the hose. Once you know where the timing is at, you could maybe add some initial timing or start playing with weights and springs. There was a thread about posting pics. I'll see if I can find it.

Originally Posted by HDSBC

TI can't document the timing threw the rpm range the only tool I am missing is an adjustable timing light I only have an old school light hoping to get one or borrow one soon.

You can use timing tape with your light to see the timing curve.

Originally Posted by HDSBC

Yes much agree signs of too rich senario but carb is brand new has less then 200 miles on it how could the power valve be gone already and if so what dose it look like when it's broken.

I’m sorry, I didn’t mean to confuse you, I wasn’t suggesting the PV was blown, but rather the power valve with your low vacuum may be coming on too soon. Also, all carburetors need to be adjusted to match the motor they are installed on, no carburetor can come out of the box perfect. While idle mixture and main jets will usually suffice, some need more advanced tuning.

Originally Posted by HDSBC

I agree vacuum is low but I can't pull any more then 11 inches no mater what I do and when I adjust it minimum is 10 inches of murcury. Everything I read it should be 12-14 but it just can't get there

Check your gauge, it may be out of calibration.
JMHO, 600rpm idle is a little low for your cam, raise the idle to 700-800, should improve vacuum. Also too rich will lower vacuum, as will incorrect ignition timing.

Originally Posted by HDSBC

Yes I did a royal purple brake in oil and did the proper brake in procedure when I rebuild the engine.

Break-in oil and correct cam break-in procedure with a flat tappet cam is essential, as is high pressure lube (cam grease) massaged into /onto the cam lobes at installation, also confirming lifter rotation (push rods rotating). Unfortunately, still no guarantee the cam will not fail with todays oil formulas, especially a more aggressive cam.

Originally Posted by HDSBC

Compression tested all cylinders at wot lowest cylinder was 122 highest was 130 psi all checks out.

Check and make sure your valves are not adjusted too tight.
Did you break-in your rings?
While your rings will probably take a little more than 200 miles to fully seat (and raise compression test readings), they do require a seating process to assure it.
DENNY wrote an excellent post on the procedure and I’m sure if prompted he will link you to it.

As "34-40" stated, a little more initial timing will help, BUT you must be careful with a stock distributor, advance stops will be needed. Usually a stock distributor adds 20-25° of mechanical timing through its curve, 12° with the added high side of 25° is over the edge of detonation at TOTAL 37° for a SBC (34-36 max).

Without knowing what the total timing is at 4000 RPM, you're guessing and gambling. So without knowing, I'd say 12° initial would be max. Having said that, you can add vacuum advance to the idle by putting its source to manifold vacuum (vacuum at idle), which will probably help too.

IMPORTANT: what is your fuel pressure??

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.