HELP! 350 Chevy popping out tailpipe at high RPM

My 350 Chevy pops out the tailpipe at wide open throttle. Sounds like a machine gun. I'm thinking it's a valve out of adjustment. The motor's stock except for a Holley Street dominator manifold, and a Holley 600 vacuum secondary carb. The carb is a rebuild that I bought from a guy at work. It was rebuilt, never run, and then sat on his self for a few years. The distributer is the stock single point out of my 66 396 big block. The heads just had the valves done. New timing chain (gear drive, stupid me) Any advise? Thanks, Dean

 

This has nothing to do with an out of adjust valve or it would do it all the time. The culprit is the carb, holley carbs have a power valve in them and when they blow they cause this to happen at WOT because the powervalve then supplys exta fuel, and when they blow out it causes them to go extremely lean wide open, had this happen to me with a holley 600. You can cure this two ways 1.Power Valve Plug and bigger main jets. 2. New power valve with blow out protection. These carbs are notorious for this, rebuilt or not, even with a brand new power valve. One pop back through carb because of timing issues will do it or sometimes they just plain go bad.

 

Rule #1: always rule out the electrical first! Points/condenser are cheap, etc. Then there is the fuel system, but in my mind I'm thinking valves. Pull the covers one at a time, put some cardboard inside the lower edge of the head and lay rags above the pushrods to stem the flood, and start er up to make sure no lobes are flat. Look closely at the valve springs, could be a weak one(you can pull the rocker arms off and push on the top of the keepers with the palm of your hand) or not shimmed right, or even a broken one.

I certainly hope this helps, UNCLEE!

 

I knew there'd be lots of good advise, as a side note; you can drill a p***age that goes to the power valve(its a va***n port, not sure right now but easy to figure out) just big enough for a BB to rattle around in, and if there's a backfire it pops up and blocks the p***age. Read this in a car mag many times back then, never had a problem with them myself, ran many a Holley too, but they were always bigger, never saw a motor that needed a little ole 600!

 

gonna have to call "********" on this one.
power valve goes bad, makes eng go pig rich at anything besides WFO.....
that's how power valves work, low vac,carb thinks it's under load, richens mixture.
bad power valve , ****s extra fuel ALL the time, just doesn't notice under WFO...

still sounds like a bad valve spring (weak/broke) to me but i ain't there...
possibly tight valves though...

 

Dean, I just went through this with my truck. I put block offs on the power valves, checked time and then found 3 burnt plug wires. The plug wire were shorting out when getting to a higher rpm range. I replaced plug wires and fixed the problem. most of the time simple stuff is over looked. Good luck on your problem, Keith.

 

HMMM, SIX years!

 

long after they quit putting carbs on cars

first thing to look at is the ignition system, ie points, condenser, plugs, wires, cap, etc.

Used to be when we had leaded gas that would happen every time I went to the dragstrip...so I kept a fresh set of plugs in the truck to swap in just for racing.

 

Gonna hafta agree with the mob, check ignition stuff.
mainly plug wires. I have a SBC in my coupe that did that coming back from Pueblo NSRA last year. had 2 burnt wires. replaced 'em both, has run great ever since.

just sumpthin' to think about

Vance

 

I agree, look at ignition then valves/springs.

if youre running a 1966 396 points distributor and the #'s are valuable, Id sell that distributor, either go with a fresh points with pertronix or a malory unilite with a painted cap, just my opinion.

holley sells the power valve blow out kit with a diagram and a bb, Ive done several of them, didnt think to save the diagrams for future reference though.

 

Misfire.

The machine gun is the sound of combustion happening in the exhaust, because it's not happening in the cylinder.

Probably a spark plug fouled with oily carbon, bad wire(s), etc.

 

Check the charging system in addition to the other advice given. Does it get worse with the lights on?

 

Make sure the spark plugs are the correct type for the cyl head being used,not gasketed plugs in a tapered seat head or vice versa.I fought this problem for half a racing season before I woke up in the middle of the night realizing what was wrong!

 

Replace your distributor. I went through the same problem and had the heads rebuilt, new cam and lifters. Still had the same problem popping through the exhaust under acceleration. Replaced distributor and problem went away. Checked the old distributor and the shaft had some play in it.

 

If it's stock, I will have to go with it floatin on the high end and weak *** stock springs. Of course I have seen cross sparking at high end with hei units too .Ionization and all that. that was 2 cents worth. swaZZie

 

gonna add my vote for plug wires.. on my car I could only feel and hear evidence of a problem at VHS.. (very high speeds)


coming home from louisville (via michigan) I actually burn up the module in my hei.. luckily I had another with me and swapped it out roadside.. apparently the bad wire had been on he car for quite some time.. only after returning home did I actually figure out the wire was bad..

check the wires, and pull the plugs while your at it.. it'll be the cheapest way to check

 

Popping out the exhaust usually means advanced timing of some sort. I have seen this with points floating and changing the dwell, but only at high rpms. Hence the reason they went to dual points and then later transisterized and then to HEI and now MSD.

If it was a carb problem it would show up at low rpms under a load. If it was a valve issue it would happen from idle.
Plug wires possible, get the engine hot and spray with water from a spray bottle at night and see if there are any "fire works". Always works for me. But what do I know......

As far as ASE certs are concerned, I am a master tech, meaning I have them all, and they don't mean $#!%. You can't learn how 80 ftlbs feels from a book, or what a good engine combo runs like with out hearing, and feeling it. All those tests mean is that you can take a test.
Most of these older rodders will forget more than you'll ever know.
6 years don't equal braggin rights or even equal experience.
I have 17 as a diagnostic spe******t and I learn something new everyday. I own my own business where I wire hotrods and customs. During the day, all day I fix electrical nightmares that the other shops in town send over.
Best advice I ever heard was to shut up, and learn something.

 

let's not get in a pissin' contest.
"only" 6 years ain't even practice for how long i've been an ase master...

 

I had a similar prob with my SS back in the 80's...

turned out to be two things. First one I found was timing chain stretch; allowed the cam timing to 'float', especially at high revs.

Second? This was wacky...

there was a wiring ferrule which ran down to the starter; at some point in the car's life someone shorted out the wires and created a bare spot on the solenoid lead. During hard acceleration, the solenoid wire would contact the ferrule and the motor'd get a dead short to ground; momentary kill of the ignition system. When the car'd nose over, the circuit would open again and the raw gas dumped in the pipe would ignite. Took me a damned YEAR to find that prob, as there was no way to replicate it in the shop - and npo one ever thinks to look at starter wiring for an ignition problem.

As far as experience goes...

Your past life don't count for much here. There are enough folks on this board that there will always be someone smarter/faster/stronger/older than you.

You're better off demonstrating your expertise as opposed to talking about it.

 

hey, i had a 70 blazer did it to me.
had the coil mounted in such a way that when you went WFO, the throttle linkage ( before cables)would hit one of the terminals would do all kinds of popping and banging....

ase tests won't tell ya about THAT one

 

They why do you have the workings of a holley power valve backwards?

When it ruptures it just leaks fuel in - and an open powervalve richens the mixture the equivilant of about 10 jet sizes. It runs the worst just at idle and off idle as the idle slot starts richening things even more, along with the pump shot. It usually runs ok at WOT with a blown power valves a and makes your eyes water everywhere else.

I personally would look at the ignition and valve springs.

 

Based on what has been done allready, and ***uming it was done correctly, the MOST LIKELY cause is point bounce or a lean mixture (or both). A good quality HEI setup should be considered anyway. If you're not good at reading plugs, then I would suggest installing an air/fuel ratio gage or an exhaust gas ****yzer which will tell you where you're at mixture-wise.

 

OK, this gives me all kinds of fun things to check. The valves need adjusted anyway, the power valve is easy to check, but after reading all the responses I'm leaning toward the distributer. I have the factory HEI that came with the motor, but I went with the points distributer because it is small. I wanted the engine to sit back as far as possible. Thanks, a lot. I'll start checking things out this weekend. Dean

 

A stock point distributor that is working properly, connected to wires and plugs that are working properly, will not give any problems. You'll need to do a tune up every 10,000 miles, which should not be a big deal.

 

Sorry everyone, I was just trying to state my opinion, didn't meen to light a fire under everybody ***, but It happened anyway.

 

I'm still having the same problem with this thing. Here's what I did. I set the dwell, and I adjusted the valves. I drove it, and it didn't change. So today I swapped the carb for a brand new one, (I borrowed it) and it's the same, only difference is that the secondaries are opening on this one. I was sure they weren't opening on the old one. It gets to high RPM (4500-5000, just a guess) and starts popping out the tail pipes, and falls flat on it's face. I'm leaning toward this being ignition. I think it might be the coil. It's one of the chrome ones from Summit, with a ballast resistor, it's brand new. If I change that and it still does it, I'll change the distributer. It has the stock distributer out of my 66 big block in it. I have a Mallory dual point I could swap into it. If I keep my foot out of it, it runs great.

 

Did you change the plug leads?

 

No, everything's new. Only been running about 2 months. I bet it doesn't have 200 miles on it.

 

Try the simple stuff first,plugs and leads can fail no matter how old they are.