get on U tube and Look up Jonatan W and his first starts of setting and rotting vehicles. And you will see the backfire that occurs when the fuel becomes too lean. When you rum out your tank is dry. theres not any gas available to backfire. When your engine is starving your still getting fuel just not enough so it leans out and will backfire thru the carb.
What happens to the fuel pressure when the engine lays over. Could be a restriction in the line, partially plugged filter, or a weak pump. It would run ok at lower speeds and power levels.
Yes that's why I stated its not a weak fuel delivery or a needle valve float issue. His engine isn't sutting down. It just sets there not gaining any more RPM. Maybe he has a clogged air filter?
Does the exhaust sound like a vacuum cleaner when you floor it. Put a free flowing dual exhaust on it.
I don't know, dudes. I think I'd want to road test this one. I'm not getting a clear picture over here. In any case, it doesn't sound like you're ready for any after - market or later model parts yet. You did mention a cam in the first post..So maybe a Crane or Melling 068 blueprint, some good quality lifters, poly-locks, and a good link chain and gears, plus a 2 or 4 deg. advance key. But then , as mentioned , you'd be lucky to get it to 5 grand ,without pulling the rocker studs out.
Don't think anybody has mentioned valve springs.........IF they are stock they could just be "flat-worn-out"! That would certainly cause it to nose over at lower RPM. I don't own a Pontiac but I have read....."do NOT cam them like a Chevrolet"! Even though they are made by the same company. Many cam grinders like to grind everything on Chevy specs. Ford/Dodge/Pontiac....stick a cam in it made for Chevy!! Do your homework and pick someone that knows what 389's like. Custom grinds are WELL worth the effort and do NOT cost more. (Check with Delta Cams and talk to Ken) 6sally6
IMHO, the cam in the rebuild may be a low performance version from a who-knows-what. Might be a regrind out of a 326. do the basic diagnostics for your current issue. after that, refer to the Pontiac thread on here for TONS of information. Pontiacs are TORQUE motors and you build them as such. If you try to treat them like a SBC, results will be poor. get the Jim Hand book. Tidbit, Pontiacs like dual pattern cams. The recommendation for a factory type 068 pattern cam are spot on. Good all-around performer and good street manners. https://www.jalopyjournal.com/forum/threads/tech-pontiac-engines.224124/
X2 hook up a vacuum gauge run up the rpm if the gauge keeps dropping when the flat spot shows up you found the problem.
If it still has the original exhaust pipe from the manifold to the muffler on it he may have hit on something. About that time frame some GM cars including Pontiacs had a laminated exhaust pipe that was actually one tube tightly fitted inside the other before they were bent into shape. Back in the day you could be driving and getting them hot and hit a puddle and the cold water hitting the outside would cause it to contract just enough to cause the inside tube to collapse. If that happened it acts just about the way you describe, idle's nice and low speeds are ok but it won't get up and go. If you stand behind it while someone guns the engine a time or two you can hear the exhaust whistle more than roar. If I had plans to dual it out I think I would go ahead and dual it out before doing a lot of other things.
Are you certain on the transmission? It may have been swapped in at some point but a TH400 wasn't stock for that car - they originally had the old style four speed Hydramatic.
Actually, the 303 hp 389 should have factory dual exhausts. The 230 hp low-comp, 2bbl was available in the Bonneville but I never saw one. If you have that one, with an AFB stuck on it, that could be a real problem.
Pontiacs have inner and outer springs. If someone has a set that won't allow over 3600 rpm's, then you could probably compress them by hand ! Speaking of...on the other hand, you wouldn't much more than stock replacements. Otherwise, you'll definitely pull the studs.
Thanks very much for your responses guys , I’ve got some checking to do to make sure I got the advance correct and just replace the exhaust .... I certainly wish those head codes hadn’t gone .... but hey ho ...some more studying for me ! Once again gents your a font of knowledge and I’m grateful to you all.... the beer is on me if/when we meet !! Once again thankyou! Griff (UK)
Have you replaced the fuel filter? Sometimes they will make a car run like this if the filter is half clogged.
I am kind of leaning to the need for better flowing exhaust and also for checking the cam that was put into the engine when it was rebuilt. Did I miss if the engine has single exhaust or dual now? Stock exhaust manifolds will pull past 3600, that is not the restriction. Even stock 389 cam will pull to 5000 or little more. BTW, Pontiac engine is not generally a high revving like some others, and with the relatively low strength cast rods (few exceptions of factory forged rods, most are cast), it is just not needed to turn it past 5500-6000 max. Your heads aren't the bigger valve size of the more perf Pontiac heads, but I really do not think that is your problem. Assuming that you have enough fuel, the carb is opening fully, so that should not be the restriction either. I have a 68 GTO with a stock 400/350 hp engine. I rebuilt it, and used the 068 cam if my memory is right, all else stock. It will pull to 5500 strong, through stock exhaust manifolds and dual exhaust, and a Quadrajet carb. My engine does have the bigger valve heads though.
Lunati voodoo cam, keep the qjet,buy cliff ruggles book to build it, These engines make tq, don't spin it over 5500, gear in the 323 to 355 range. I have this combo in a Pontiac 455 powered amc 4door rebel(sleeper). Tried a Holley on it, qjet way better mpg, and dialed in right equal power to Holley. Still have log manifolds, the factory 4b cast intake, is hard to beat.
Nice car...I can’t remember if 64 was one of the years that had a nylon cam gear. If it is change it ASAP and while your at it, upgrade the cam, manifold, carb. Sent from my iPhone using The H.A.M.B. mobile app
Since it sounds like you have checked most of the tune-up related stuff, I would look at the valve springs before making any significant changes. Also look or probe into the exhaust at the head pipes and make sure they are clear. I've seen obstructions from rust and previous exhaust work closing off the flow and causing limited power and RPM.
A partially plugged gas tank vent would limit power. Try running it with the gas cap loose to see if that is the problem.
Was the car sitting a long time before purchasing? Sometimes the mouse stuff never completely clears the exhaust, leaving it half blocked. Sent from my iPhone using The H.A.M.B. mobile app
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