I have a 1973 hi-boy with a 390 in it it..... just recently it started backfiring thru the carb... afraid it might be a valve, checked the timing, vacuum leaks, made sure firing order is all ok, replaced plug wires and dist. cap and made sure that #1 was at TDC with harmonic balancer.......

My question is, before I yank the heads, should I try a combustion cleaner like seafoam or berrymans? and if so, what are the pros and cons

 

Are you running points? They sound like they're bad. If not that, check your carb tuning and acclerator pump.

 

How about a compression test?

 

This would work well, but a vacuum gauge connected to a source of constant manifold vacuum at idle would tell the tale instantly. They are just way easier to use, and don't torture the starter/battery/solenoid.

 

I did a compression test and all cylinders are reading the same.... not familiar with checking with the vacuum gauge... do i just hook it to the vacuum advance? and what do I look for? I know it pulls about 14 when doing the timing.

 

How high in Arizona? 14" is pretty low, allowance has to be made for elevation though. Normally at sea level 18" to 20" indicates a healthy motor w/stock cam. That said backfiring through the carburetor is usually a timing/ignition problem. An excessive lean condition can do this too.

 

Anywhere with constant vacuum (not ported vacuum, that only acts as the throttle is opened) should suffice.

 

https://www.google.com/search?q=vacu...3ymmivPmJ9M%3A

 

To the OP,

Do you have a video of the backfiring condition? If you do, I can dig one of mine up and we can compare the two. If they match up, then there's a good chance you have the same problem I did.

 

Tedster9, I am on top of the Mogollon Rim, so about 7700 feet.

Mountaindewd, don't have a video(can make one... but I guess I can hook up thru the EGR port on the manifold and test there

 

Also, is the backfiring under a load, or when cruising, or constant no matter what?

 

it is constant... and regular like it is one cylinder popping.... just gets louder when you rev it a little

 

Ok, that's what I was curious about, 14" mercury is pretty good at 8000', would be a cause for concern at Sea level. When was it checked last, would be useful to see if that's still pulling 14 or changed or is no longer steady etc.

 

well, guess I shoulda done this first... pulled the valve covers and it seems I have a collapsed lifter...ugh.... just got back from my local parts store, well the next nearest cuz the nearest didnt have one (nearest one is 15 miles away, the next nearest was 52 miles away), now I get to see if I can finagle the old lifter out and the new one in with a magnet, or pull the whole durn intake..... everyone please pray to the magnet gods for me...lol

 

I'd pull the valve cover(s) and watch for a rocker that is not moving.
I'd put money on a wiped cam lobe, bent pushrod, or rocker problem.