[Fummins]

Like my title says having issues dialing in my 468...motor was a runner that came out of a buddys xamaro, i heard it run and drive. Im getting a backfire popping out of the card. New Holley Avenger 870, floats and accelerator pump checked and rechecked. New MSD rotor and cap installed, the old cap had a crack. Motor idles fine...when i roll into it hard it pops thru the carb with enough force to blow the vacumn fitting plugs right off the carb. Tried pcv and breather, no pcv with breather, timing set by vacumn. The fuel screws on both sides are at 3 turns out....seems like it would be plenty rich and smells it at idle, no visible smoke out the pipes and its not rich enough to burn your eyes but you can smell it. Tested the power valve by covering the carb horn breather and it didnt die out at idle. Full tank of 91 octane fuel too. Car SEEMS to be starving for fuel under hard load, light popping. Fuel pressure fluctuates between 5 and 7 psi. Plug wires are brand new, firing order checked and rechecked. I did have a bad carb leak at the base until i installed a spacer and new gasket, it cleaned up some after that work, but still isnt right. What the heck am i missing here?

 

[jeff swisher]

timing needs to be set by a light
But even at that i have had timing all over the place and no popping issues.

I would say lean mixture.
Does not matter what the mixture is at idle.. it will be different during cruising and power runs.
How do the plugs look?
..You could also have 1 fouled plug causing issues.
I have had bosch platinum and split fires give me issues..

 

[Fummins]

Thanks Jeff, plugs all look uniform and are light brown/tan in color. Im using Autolite #25 plugs.

 

[1224]

Any idea what the cam specs are?

 

[Fummins]

Any idea what the cam specs are?

Not exactly, my buddy (motor donor) just had shoulder surgery and hes out of it for a while. I recall the lift is in the low to mid 400's

 

[VinceS427bb]

Not exactly, my buddy (motor donor) just had shoulder surgery and hes out of it for a while. I recall the lift is in the low to mid 400's

that's a puny lift figure for a 468

did it backfire when in your buddy's car???

 

[Fummins]

that's a puny lift figure for a 468

did it backfire when in your buddy's car???

I could be wrong....wouldnt be the fist time, got a call into him on that. Nope, no popping in the Camaro it was in.

 

[1fast71elco]

3 turns out on the air/fuel mix screws seems very excessive at idle. Typically 1.25-2 at the most.

Like mentioned above, set the timing with a light. :yes: 34 total all in by 2,500rpm (2 light silver springs) should be a good starting point. I would also install the largest stop bushing for the mech advance. Should put you at 16 initial + 18 mech = 34 total.

The popping you mention is likely a lean condition under throttle. I had an issue similar with an old 3310 750cfm Holley a few years back that had a metering plate for the secondaries which was non adjustable. However yours should have a secondary metering block. Pull the carb and check what jets are in it. For reference my 950 HP runs 78 front and 86 rear.

What fuel pump are you running?

 

[Fummins]

Corrected Cam specs: .540 .540 lift and 238-248 duration.

 

[1224]

You stated that you had a bad carb leak at the base. Was that a vacuum leak? What intake manifold are you using?

 

[Fummins]

3 turns out on the air/fuel mix screws seems very excessive at idle. Typically 1.25-2 at the most.

Like mentioned above, set the timing with a light. :yes: 34 total all in by 2,500rpm (2 light silver springs) should be a good starting point. I would also install the largest stop bushing for the mech advance. Should put you at 16 initial + 18 mech = 34 total.

The popping you mention is likely a lean condition under throttle. I had an issue similar with an old 3310 750cfm Holley a few years back that had a metering plate for the secondaries which was non adjustable. However yours should have a secondary metering block. Pull the carb and check what jets are in it. For reference my 950 HP runs 78 front and 86 rear.

What fuel pump are you running?

. Thanks, good ideas and i will get the light on the car tomorrow morning and start that analysis

 

[Fummins]

You stated that you had a bad carb leak at the base. Was that a vacuum leak? What intake manifold are you using?

Yes it was a vacumn leak, sprayed carb cleaner around the base and the idle went up. Holley street dominator intake.

 

[Fummins]

Running a stock mechanical fuel pump, 5 to 7 psi at the carb fuel rail.

 

[jeff swisher]

crappy intake..sorry
but you needed a spacer to fix an air leak at the gasket base..
this makes me think the carb pad may be a little warped or not flat..
or possibly the carb is not flat...

I would double up the base gaskets and no spacer and see what that gets me.

I built a 396 for my buddy..revs clean to 6500rpm with the comp 280H
It has the STRIP dominator intake and 800 edelbrock (afb)

That sucker popped and spit...weak on the top end..
The final cure was 10 jet sizes larger front and back....still a tad lean..
plugs always looked the same.
but no more popping..
Also found out that the 2.19-1.88 valves and that cam want more seat pressure than 121 pounds.
Floats the valves at 6600rpm
i feel 135 seat would have been closer to optimum.

 

[Fummins]

Retimed car today, inital set at 16 degrees. Vacumn gauge showed a delay in vacumn when reving the engine, shouldnt it be instant? gauge connected to carb vac port. Also the timing mark literally disappeared a couple of times when i increased rpms. What the heck causes that?

 

[1224]

Delay is caused by hooking your vacuum advance canister to the "timed" port on the Holley carb.

 

[Fummins]

Delay is caused by hooking your vacuum advance canister to the "timed" port on the Holley carb.

So straighten me out on this Tino, shouldnt the carb port be plugged during timing to prevent a vacumn leak?

 

[Fummins]

Just wanna add im starting to think distributor/ coil now, msd #8060 distributor and blaster coil, both new. I did splice into stock harness wiring, maybe a loss of power from a resistor wire?

 

[1fast71elco]

1. That is a good timing number for idle, but do you know what you total timing is? Base timing is not a crucial as total...rev it to 2,500-3,000rpm and check it with the light. Shoot for 34-35 total.

2. Have you looked into what size jets are installed in the carb? This is a big one to verify, cuz carbs are sold with a generic tune that does not work on all engines. Like I mentioned I have had this happen before...I bought a 650 Demon for my original small block, which I assumed would be plenty of fuel delivery for a stock engine. It did just about the same as yours is doing. Started popping around 3,000rpm or so. Turned out to be lean pops and as soon as I installed larger jets it ran perfect.

Keep us posted.

 

[Fummins]

1. That is a good timing number for idle, but do you know what you total timing is? Base timing is not a crucial as total...rev it to 2,500-3,000rpm and check it with the light. Shoot for 34-35 total.

2. Have you looked into what size jets are installed in the carb? This is a big one to verify, cuz carbs are sold with a generic tune that does not work on all engines. Like I mentioned I have had this happen before...I bought a 650 Demon for my original small block, which I assumed would be plenty of fuel delivery for a stock engine. It did just about the same as yours is doing. Started popping around 3,000rpm or so. Turned out to be lean pops and as soon as I installed larger jets it ran perfect.

Keep us posted.

Total is right at 38, thing is the timing mark disappeared on me the other day....it crept up to the 12 o'clock position and i couldnt see it anymore, only did this once but still concerns me. Carb makes sense, but heres the deal, its the 4th carb ive tried. 2 each Eddy 750's one Holley 750 and the current Holley 870 on it now.,same symptoms on all carbs. Checked the ground strap from motor to firewall, installed bypass wire from + coil to + battery thinking maybe volt loss to no avail..... im friggin at my wits end here.