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

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Ok here goes I have a mild built sbc in an 1981 Chev 1 ton chassis and a custom 1950 cab mounted to it as a work truck. Been a few year build constantly upgrading but here is where I am at now.
1981 sbc stock bore stock crank 16000original miles rebuilt it because as everyone knows bad year for camshaft.
Put new rings bearings honed cylinders
comp cam110 degree normal lift longer duration cam
Double roller chain stock pin on cam gear
New jegs stock length pushrods
New roller rocker stock ratio valve train
New jegs 2.02 aluminum heads
New champion plugs
New spark wires
new hei distributor
New680 quick fuel vacuum second
Jegs duel plain high rise intake
All brand new gaskets
Custom 1 5/8 right to 3 inch no restrictions stack mufflers
Turbo 400 trans
4.10 rear gear

Timing is set to 12 degrees
Carb has 11 inches of murcury max at 600rpm idel
Jetted the carb from quick fuel had it at 70 78 I have it down to 68 72 and the plugs are still black and some wet. Number 5 cylinder is just starting to turn dark brown.
Vacuum ported advance
6psi fuel pressure
Fuel is in middle of the sight glass on the bowls

Now here is the problem the truck is a total dog has no get up and go. And I have a dead spot from 2000-2500rpm where it backfires out the exhaust almost constantly and even will ocationaly stall and refire under light accelerations at slow speeds. It seams to run fine before 2000rpm and after 2500 it stops back fireing but feels as if it had a miss in a random cylinder every once in a while.

Should I be looking at the weights in the distributor or fuel delivery system. At a loss everything I try dose not seam to remidy the problems.

What intake manifold?

Total guessing from here... but yes, I think you need to document when/where your timing is during the rpm change.

I'd also set the fuel level to the bottom of the site glass.

OH, and welcome to the site. Nice to have you here. And we really like to see some pictures of the project we're discussing.

Originally Posted by HDSBC

Timing is set to 12 degrees

What is TOTAL timing (timing at 3500-4000rpm, with vacuum advance disconnected)

Originally Posted by HDSBC

Carb has 11 inches of murcury max at 600rpm idel

JMHO, vacuum is low

Originally Posted by HDSBC

the plugs are still black and some wet. And I have a dead spot from 2000-2500rpm where it backfires out the exhaust almost constantly and even will ocationaly stall and refire under light accelerations at slow speeds. It seams to run fine before 2000rpm and after 2500 it stops back fireing but feels as if it had a miss in a random cylinder every once in a while.

Classic signs of too rich. Check the power valve

JMHO, 1981 SBC truck motor had 8-1 compression which may not match well with your cam and head combo, requiring some serious tuning.

Originally Posted by HDSBC

1981 sbc stock bore stock crank 16000original miles rebuilt it because as everyone knows bad year for camshaft.

JMHO, the camshaft (flat tappet) problem was (is) caused by oil formula changes. If you went back with a flat tappet cam, BE SURE you break it in correctly and use the correct oil (and/or additives).

to me it sounds like it could be a lifter problem or cam. could be #5 but I would start there. I think that after 2500 the other 7 cylinders are carrying it. what is your compression on all 8?

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.

I don't know how to post pictures from my photo gallery if someone tell me how I will most definitely post some pics of the old girl

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.

So a little update tonight went through the valve train. Brought #1 to Top Dead Center

Exh.- 1 – 3 – 4 -8​
Int.- 1- 2 – 5 - 7 ​
​
Get #6 to Top Dead Center

Exh.- 2 – 5 – 6 – 7​

Int.- 3 – 4- 6 - 8

Brought it to finger tight on the pushrods and turned past 1/4turn and set the set screw on the roller rockers.
Still same vacuum inches of murcury
600rpm =10.5inches of murcury
800rpm=13 inches of murcury
And the higher I went on the rpms the inches of murcury increased
I have all 4coners of the carb idel screws set to 3/4turn out from bottomed
Even though it is an auto trans it seams to really like 600rpm idel.

Went for a ride down the highway it's still backfire ing out the exhaust probably not as bad but still doing it in the 2000-2500rpm range.