[glhs#116]

I'm chasing a new problem. Car is running really well until you try full throttle up above 4000rpm or so. Engine breaks up and runs rough. Will sometimes do some backfires if you stay in it. At all other times it pulls very strongly and is responsive at any throttle opening.

I'm scratching my head on this one. For the most part it's running great.

1971 Trans Am 455HO. TH350 with 3.42 factory posi. 1971 455 auto (but not HO) QuadraJet. Harland Sharp roller rockers and Ram Air manifolds but mostly stock. Flowmaster 2.5" dual exhaust system.

[Cliff R]

I'd bet fuel delivery, or carb not staying full on hard runs.......Cliff

[steve25]

Sounds like a fuel delivery issue to me, but a easy quick distributor check would be to unhook the vacuum advance, plus off its line and take a blast in the car to check if the issue is still there.

[HoneyHush]

With the backfiring and roughness I'd take a look at your cam lobes. If you are losing a lobe or two, it will not be so noticable until you open it up and raise the rpms. I've had it happen to me.

[Boostedbird]

Could be the coil going bad also. Sometimes they'll work ok until you start getting a lot of heat into them. Just another thing to consider.

[HWYSTR455]

Originally Posted by Boostedbird

Could be the coil going bad also. Sometimes they'll work ok until you start getting a lot of heat into them. Just another thing to consider.

That's where I was going to go, and especially true with HEI with the coil in the cap.

Also agree that the fuel system should be checked.

Those are the two basic areas that are going to show issues at higher RPM under load, and should be checked first.

You say 'mostly stock', are you running an ignition box? Are you running the low impedance cap button? I've seen those cook out and cause breakup at higher RPM.

How old are your wires and plugs?

.

[455rebel]

Weak valve springs will cause upper rpm mis and pop.(ask me how I know!)
I went all through the ign system and fuel delivery, before a dirt track buddie swore it was the valve springs being too weak.and it Was!

[Blued and Painted]

1/ Point bounce

2/ carb lean out. But it will normally just noise over then pop.

[Scott65]

Low voltage to the coil will do this, as well as the coil itself being bad as was mentioned previously.

[crm318]

I posted something similar to this a while back. The float in my quadrajet was too low. I put it .25 inches from the top of the body, problem solved.

[glhs#116]

I will try to get a look today. Stuck running errands at the moment. Given the two other ingredients I will be very pleased if I get off lightly on this one.

The first is that I also have a bit of an engine shake at idle although it sounds like it is running smoothly.

The second is that for the last couple weeks I have been wanting to investigate a new "tick" sound that is a lifter sort of a sound. I can hear it at higher speeds not just idle.

I will post as soon as I can take a look.

[Boostedbird]

Sounds like you may have lost a lobe like Honey hush said. Pull the valve covers and disable the ignition. Then have someone crank the engine while you watch the rockers rise and fall. You should be able to see the ones that are wiped.

[pippintook]

GLHS- I had a similar issue. My engine was running very poorly at higher RPMs..usually around 4000 and on hard/full throttle runs - for my engine, it was almost like it was hitting a wall. In my case, im pretty sure it was a fuel issue.
.
I would get a fuel pressure tester, like
http://www.harborfreight.com/fuel-pu...ter-93547.html

I had a spare long fuel qjet inlet housing. I drilled and tapped this fuel inlet housing for 1/8 NPT...the Harbor Freight gauge comes with a 1/8 NPT fitting..so, I screwed this fitting in and monitored fuel pressure on full throttle runs....had gauge tapped to windshield.

For now, I have added this electric pump , back by gas tank
http://www.summitracing.com/oh/parts...4070/overview/

My pressures seem to be much better and I think it solved the issue...though, need to do some further testing...in my case, from what I have read the real answer is to sump my gas tank

[rexs73gto]

If you have any noise in the valve train at all I'd still pull the valve covers to check them 1st. With as many problems as I've had & others have had with cams going bye-bye I'd double check that very closely. Then check the fuel system. Not enough fuel going to the engine will cause that concern to.

[71HOT/A]

Sticky mechanical advance can cause this as well.

[glhs#116]

Here's the strange update.

I took off the valve covers and broke free all the lock screws on the HS roller rockers. I backed the lock screws out so I would be able to adjust lash. I haven't had the valve covers off since I got the engine back from having the broken crank replaced over a year ago.

I first cranked over the engine and videoed it. All the rockers seem to be moving about equally which was a releif to see. I then started the car and backed off the rocker nuts until each one started clacking and then in until it stopped. After doing all I stopped the engine, gave each another quarter turn, screwed in the set screws and gave a final tighten. This is my normal procedure for setting the rockers but as I said it hadn't been done since I got the car back from repair.

Anyway, after I started again the ticking "lifter" sound was almost completely gone. I should point out that none of the rockers was actually loose. In fact, most of them were quite a bit further down than I like. I like them near the top of their travel so they can't pump up much.

I drove the car and got a full throttle pull in. There was a bit of roughness at high rpm but nothing like before and no backfiring. Still seems a bit flat and I won't be surprised if there is a fuel or spark component to this but my tick seems gone and it seems fairly unlikely at this point that I have a cam problem. Which is a great relief.

I could also still buy some spring problem. There is definitely something that feels rough around 4000-45000rpm that gets a bit better above that and really isn't there below that. Feels like the engine out of balance.

I can also hear a little bit of an exhaust leak on driver's side that only happens under heavy acceleration. Probably my driver's side down pipe not tightly clamped enough.

Thanks for all the suggestions. I'll update when I know more.

Sam

[455rebel]

I don't think the springs would get better with more rpm,once the start bouncing they won't quit untill u back off of the rpm, at least that's how mine acted before I changed them.

[Navy Horn 16]

Had a car with a Holley doing that. It was a carb issue with the secondaries not fully opening. It would rev like crazy in neutral, under a load it would stumble at 4K. Got the carb cleaned out and the secondaries open, and it's like a new car.

[Mezzo]

Check the power piston spring tension. If its too weak, it can cause a lean condition. This is apparent at high RPM. You will also get backfire coming off the QJet idle circuit.

[glhs#116]

So today I got under the car and replaced the one short piece of rubber hose on the inlet side that I hadn't replaced yet. It didn't actually look too bad but I'd replaced all the others. Took it out and opened it up for a full throttle run through second and third up to nearly 6000. Seems to be pulling strong again with no signs of hesitation. Fuel supply issues, I guess.

Thanks for all the help!

Sam