I just got my mild 350 w the 600cfm Holley Vac Sec going again. I brought the engine back from Colorado Springs (6000ft elv) to Florida (like 9ft elv) and I can get the ting to stop backfiring thought the carb. It will make a quick pop every couple of seconds. The original front jets were 69's and I replaced w 71's. This should be enough right? I have set the timing and played around with idle mixture screws to no avail. Anyone have any suggestions on what to try next?

------------------
94 M6 Vert
http://www.geocities.com/flyinz1/

 

Fisrt of all 69's to 71's is that square jetting?

I would check your power valve as a start.
I need more info. Does it do it during light acceleration or heavy? when was the last time you adjusted your valves? any chance of one hanging open? Normally a pop through the carb on WOT is a lack of fuel. what squirters are you running?

------------------
Arron Spitzer
Musclecar Technologies

Honest answers........LOW PRICES!

 

Square jetting???dunno what that is...The jets are physically round w the slot in center...the rear jets are actually of the non adjustable plate type..It is a "clean air" style carb. The vehicle is not being driven yet..it is a fresh transplant from my dead 69 Camaro "see pics" into an 89 Jeep Wrangler..until I can sell for another 69..lol.

I did have the heads off and put new gaskets on and did teh adjusteing without it running. The squirters are stock..havent messed with anything else but jets. This popping is happening at idle and at all other throttle positions.

Could one of the valves be causing this?? I'll adjust if that's the case.

 

You probably have a exhaust rocker that kicked the pushrod out or has a flat lobe. It will pop out the carb because it can't exhaust the air and when the intake opens it trys the go out the intake. If the engine set for a long time without the valves backed off of in a lot a moisture the valve could be sticking or has the intake seat rusted and the intake valve isn't sealing when it fires.
Generally when we go from around 500 to 1000 feet above sea level to denver altitude. we have to increase jet 4 sizes front and back and add 2 degrees of timing. this is on 15 to 1 circle track engines and may not be the same as a mild street engine.
what is meant by square is the primary jets and the secondary jets are the same size. This is true if both metering blocks have power valves. if the front only has the power valve the front jets should be 5 to 6 sizes smaller the the back. since you say you have a 600 vac. it probably has jet in front but a metering plate(instead of a metering block) in the back which doesn't have removable jets you change the metering plate or drill the orfices. I don't think the carb is the problem (it may not be right but probably isn't you problem) I'd look at the valves. adjust--check for flat lobe--check for severly leaking valve. Make sure you don't have #5 and #7 wires crossed. (18436572)
Hope that helps!

 

I know what I'm doing this weekend. That makes a lot of sense. The heads were taken aprt and reassembled and valve lash set about 1 year ago so maybe the problem is in the valve...Man I hope so. I'll let ya know. Thanks....I can always count on this site for great info...

 

It took a while to get to it but we adjusted the valves last night and what'd ya know...Runs awesome now. Thanks guys...