oamhmad

iTrader: (1)

I have a Holley 4160 4 barrel, Edelbrock Performer intake and Goodwrench 350 with stock cam.

The car runs fine around town, good idle, strong acceleration, no lean spots that I can tell, no bogs, responds pretty good to WOT.

I had a really bad backfire the other day while on an exit ramp getting off the interstate. It blew the power valve out.

The last time the car backfired was also on the interstate at steady cruise. Normally I don't travel very far with this car and almost never cruise that fast for any length of time.

So I'm trying to figure out what the problem is, and the common denominator seems to be driving at 70-80 mph for more than 20 minutes. Why does this cause back fires?

I've tuned the carb with a wide-band AFR and I'm getting 13-13.7 AFR at steady cruise 70-80 mph.

I've also noticed that at idle opening the throttle ever so slight so that there really isn't enough to increase the rpm causes fuel to dribble out the accelerator pump discharge nozzles. Seems to be a terrible waste of gas. Not sure if it's related to the back fires.

oamhmad

iTrader: (1)

Found out what the problem was. The 6.5 power valve was opening too late creating a lean condition.

I have 21" of vacuum at idle and around 12 t0 13 while cruising, so I installed a 10.5 power valve. No more back fires.

Now if only I could cure the lean condition at WOT.

92z28outlaw

Glad you found the solution and thanks for sharing it,

bestracing

What size 4160 do you have 600?
What RPM are you leaning out at?
Have you decreased the secondary spring pressure so that it opens sooner?
Have you jumped up in jet size by 2 to 4 numbers and tried that?

oamhmad

iTrader: (1)

Yes its a 600 CFM. It does not matter what the rpm. It goes lean and stays lean when i go WOT. I have tried every single spring from lightest to heaviest, and yes I have confirmed the secondaries are opening and all the passageways are clear. I'm using 63 jets which give me about 13.3 AFR while cruising. I switched to 65 jets and that did not help at WOT.

pancherj

Sounds like it is time to open up the power valve channel restriction to allow more gas in when under full throttle. Or, perhaps, the secondary side just isn't getting gas through the boosters?

I don't like using the main jets to tune in the WOT fuel. That's what the PVCR is for. I like the jets to set the cruise condition. As a test, I would try jumping a lot more than 2 jet sizes. Go 6 or 8 just to see if it makes a difference. And, as I said in your other thread, I just don't like the vacuum actuated secondaries. I have had zero luck getting them to perform as good as a double pumper.

oamhmad

iTrader: (1)

Yeah, I'm hearing the same thing from other people as well. Sigh...$400 toward a double pumper or save towards an eventual LS1 swap...

pancherj

Try the 6 size jet jump first (on the primary side) and verify that fuel is coming out of the secondary side. If you are getting fuel and the secondary side is discharging fuel, then you need to open the "jet" on the secondary plate (or do the conversion to a 4150 style block).

bestracing

I haven't had too many problems with vacuum secondary carbs on mild motors.

Not all 600 are the same. Some have larger secondary plates. You might need to increase this as well as try some larger jets on the primary.

bestracing

There are only two things you can adjust for WOT tuning.

1) At what Vaccum the power valve opens, in your case that is 10.5 or lower

2) The jets.

Secondary jet tuning for WOT is one of the advantages of a dual metering block set-up. There are kits out there to convert the secondary metering plate into using jets instead and not have to install a metering block.

Also a 1850 carbs I once had from the 80's came with 66 jets to start with and I want to say I use to use 68 jets in a 72 Camaro/350 that had headers and an Edelbrock Performer with stock long block.