chad89ta

my holley 750 is backfiring though the carb,the timming is set at 38*floats are fine fuel mixture srews are set right,all the plugs and wires are good,cap rotor ect,what else could cause this, the carb was fine till i added the victor jr intake and proform main bodys,now i cant get it to stop

five7kid

iTrader: (14)

What model of carb?

When is it doing this, at cruise? Off-idle? When you floor it?

Did you jet it according to the Proform instructions?

chad89ta

it is the holley 4150 650 cfm dp,it was jeted at 71 84 with the 650 then when i got the proform i jeted it at 72 84,does it at low rpm,after 3000 rpm it stops

five7kid

iTrader: (14)

I don't think I've ever seen a carb cause backfiring that was RPM-dependant. Usually it's condition-dependant (like when floored).

What distributor are you using, and how did you set the timing at 38 degrees?

chad89ta

im using the msd probillet dis,hei with msd 6al and blaster 2 coil,car has good power after 3500 rpm but if your in 2nd gear and the rpm is about 2500 then give it a slow increase in thottle,it will back fire out the carb,shots flames about 2 feet hi

Chickenman35

Hmmm...could be a vacuum leak somewhere. What sort of vacuum are you pulling at idle? What are the cam specs?

One thing to check is that yoy installed the correct throttle base to main body gasket when you changed the to the Proform body. There are several different types, with subtle differences. You could have a vacuum leak there....or somewhere else.

A Vacuum leak could cause the backfiring condition that you describe.

Nothing silly like crossing #5 and #7 wires is there? Done it myself when burning the midnite oil

five7kid

iTrader: (14)

Still think it sounds weird that it is RPM-dependant, not load-dependant.

At the risk of repeating myself, how did you go about setting the timing to 38 degrees?

chad89ta

cam specs are 250 256 @50 and 519 524 lift,i set the timming with a timming light,and the proform main body came with the gasgets,they line up the same as the old ones the throttle plates have 172 173 stamped on them and that is what the proform calls for,like i said before the only thing i did to the car was add the proform and victor jr intake,i sprayed brake parts cleaner around the intake to see if there was a leak but nothing,im stumped,and cant drive the car like this,and all the wires are in place

chad89ta

also added all new plugs and wires new cap and rotor,i even put the old coil in to see if it was the msd box,and it still does it

1FastZ

Guys this may sound off the wall but mine was doing the same thing and it turned out to be a bad fuel pump. I guess since it was running so lean it was causing it to back fire.

chad89ta

yeah but the fuel psi on my gauge says 6psi

five7kid

iTrader: (14)

I'm assuming when you set the timing you ran the RPMs up to max advance, and then adjusted it to 38 degrees, correct?

Did you disconnect the vacuum advance when you did this?

chad89ta

i have never used the vacuum advance,ive drove the car for a year without it

five7kid

iTrader: (14)

Okay, finally got that established.

RPM-dependant miss/back-fire doesn't fit a carb problem. Load, yes. RPM, no.

The most likely candidates are things that would be causing a particular cylinder to mis-fire back into the intake manifold, which is overcome when RPMs get high enough. Mis-routed sparkplug wires really sounds like a good candidate. Re-check your firing order. As stated, it isn't hard to get a couple of adjacent wires crossed.

Next would be a vacuum leak. You covered the outside, apparently, now you've got to consider inside. It's possible for a gasket to be misaligned on the underside but not the upper side. Or, an accessory hooked up to a runner source that is leaking (power brakes, PCV, etc.).

After that is an intake valve. Perhaps a piece of old gasket got into the port and is trapped in the seat now.

As already asked, what's your vacuum look like? If it is low (and I know you've got a lot of cam), that could be a leak. Does it bounce around alot at idle? Vacuum or valve leak.

Chickenman35

If you've established that you have absolutely no vacuum leaks ( And it looks like you have...but triple check ). Then I would suggest the following.

The Victor JR probably requires more pump shot. What combination of pump shooters and pump cam do you have? For baseline, I would recommend a Blue cam in the #2 hole and a #37 shooter in the Primary side. Use a hollow shooter screw.

BTW, what gears and what tranny\convertor do you have? What is your cruise RPM?

Next timing curve. Hook up that vacuum advance. You need the extra advance for cruise and light load, especially with that big cam. Hook it up to manifold vacuum.....again this will help the idle and low speed driveabilty with your cam. Big cams really need a ton of advance down low for street driveability. Only a properly functioning vacuum advance can provide this.

The tension on the vacuum advance spring ( set with the 3\32" allen key ) should be backed off all the way. If you have a Mighty Vac....adjust the vac can so that full advance occurrs with about 5" of vacuum. This usually means backing the setting out all the way.

Set up your initial so that you have 16 deg BTDC at idle ( vacuum advance disconnected of course ). Total mechanical advance of 38 deg sounds about right. Make sure that the mechanical advance does not start before 1,000 rpm and is all in between 3.200 and 3,500 rpm. Pay attention to that minimum figure of 3,200 rpm . Bring it in too fast and you will get detonation.

Try out these suggestions and let us know what happens.

One last thought... are you running a flat tappet cam? ( You don't want to know where I'm going with this.... )