This Mustang is driving me nuts

[RagTop]

Like the pirate with the steering wheel sticking out of his pants said. The damn thing has developed an increasingly serious miss and, when on the throttle, makes a noise like pinging. I'm running good old CA premium (91 octane), but I don't think this is about gasoline. I'm thinking valve train. The clackety clackety sound on hard acceleration, along with a noticeable miss under load has me fearing the worst with regard to rockers, valve springs, etc. The car backfired out of the carb a few weeks back and never repeated that act, but this miss began showing up after that incident. Any clairvoyants out there who do psychic engine analysis?

 

[huskinhano]

Could be a valve not sealing well. have a compression test done.

 

[B67FSTB]

backfire ??
maybe a intake seal is blown.??

 

[JeffTepper]

Ken:
If you are running an older Holley, the backfire could have blown the power valve in the carburetor. Modern Holleys have a different design that is supposed to protect the power valve in the event of a backfire. However, not sure if your engine's rough running symptoms are entirely attributable to the backfire.

Regards, Jeff

 

[tarafied1]

Like the pirate with the steering wheel sticking out of his pants said. The damn thing has developed an increasingly serious miss and, when on the throttle, makes a noise like pinging. I'm running good old CA premium (91 octane), but I don't think this is about gasoline. I'm thinking valve train. The clackety clackety sound on hard acceleration, along with a noticeable miss under load has me fearing the worst with regard to rockers, valve springs, etc. The car backfired out of the carb a few weeks back and never repeated that act, but this miss began showing up after that incident. Any clairvoyants out there who do psychic engine analysis?

If the issues all started around the time of the backfire it could be carb related. A backfire thru the carb is often related to a lean condition. Lean engines have higher cylinder temps. Higher cylinder temps can ignite the incoming air/fuel and cause the backfire. Higher cylinder temps can also lead to preignition in the cylinder making the pinging or spark knock you describe.
Pull a plug and look at them. If they are white or even chalky it would confirm hot cylinder temps and lean burning.
If you confirm it's lean, you could have a dirty carb. Possible rust or crud sucked up from the bottom of the tank? Anyway, that's one possibility.
A compression test is not a bad idea. You will be looking at the plugs anyway. It is true a backfire can blow an older Holley power valve as well. Typically a blown power valve will cause the engine to run very rich because once blown it will dump fuel all the time instead of when there is enough vacuum to open it. If it's blown and leaking fuel the plugs will be black and probably wet.
Plugs can tell you a lot and unless you have a 429 stuffed into a 67 with the shock towers in it still the plugs aren't too hard to pull out and examine.
Let us know what you find.



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[Starfury]

I agree with the possible lean condition. Lean mix would explain the miss, pinging, and possibly the backfire.

 

[Mach1Mark]

How many miles on the rocker studs? Ive experienced (a long time ago) stud deflection after extended high rpm operation (racing). At the end of the racing season I was adjusting the lash and the top of several of the screw-in studs just came off. Then I noticed the studs where bent away from the valves which increases the lash. I never experienced any clatter or reduced performance though.

 

[bartl]

Inductive crossfire? How good are the plug wires? Are numbers 7 & 8 running next to each other?

 

[rbohm]

when engines backfire through the carb, it usually indicates one of two things, a lean fuel mixture, or an intake valve that has hung open for an instant at the wrong time. depending on the age of this engine, i suspect an intake hung open on you, and with a holley carb that often means the power valve have been blown, even with the upgraded newer ones(sometimes the check valve they installed doesnt react like it should or doesnt deal properly).

i would start with a compression check and see if any cylinders are down. another possibility that occurs to me is that you might have a head gasket that is leaking between two cylinders, and that can cause a backfire as well. a compression check will show that as well.