'72 Olds 455 engine miss from 35 to 45 mph

[wolfman98]

My 455 was rebuilt several years ago. It starts fine, runs fine at low speed. The minute it shifts at about 35 mph it starts missing big time until 45 mph then gets better. I think it is vacuum related because I had the power steering unit rebuilt, the car ran incredible for a day or two, then back to it's old ways. All the normal tune-up things have been done. The car was changed over to electronic ignition before I bought it. If it is a vacuum problem, where is a likely place to find the problem or what specific tests can I do to hep the diagnosis. All ideas appreciated.

[92 Lumina]

We would need a little more information about when this problem happens, and what other things have been done to the engine ( is the engine hot/cold ,is it raining etc.) A power steering problem will not give these symtoms, so it didnt fix it. It sounds like you may have a stumble and not a miss. A miss is much more likely to happen under a high load condition( leaving a stop light ,going uphill)
Although they feel very similar the cause can be very different. A stumble could be caused by secondaries opening to quickly or not at all, vacuum advance not working or incorrectly routed hoses to vacuum advance.

Mark

[wolfman98]

Thanks for the reply. Here is some additional information. The situation occurs when the car is cold or warm and is not effected by the weather conditions, i.e. rainy, dry ,etc. It doesn't run perfectly at 0-35 but is quite smooth. But when 35 mph is reached, give or or take a couple of mph, it runs as if someone just removed 2 or 3 spark plug wires. Plugs, wires rotor, cap, etc have all been replaced as it behaves like a car with an electrical problem. No effect. I'm leaning more and more to a vaccum issue for two reasons. When I replaced the power brake booster, the car ran the best it ever has for a few hours, then symptons returned. Also, the vaccum actuated climate control isn't working properly either. The car does behave like the distributor is not advancing when it should. I'm going to check for leaks around the engine with the old propane test as well as under the dash with a 'stethoscope'. I've done the latter before but to no avail. I also have A/C but it hasn't worked for years. I didn't realize there was vaccum associated with that so I will plug all lines to take that variable away. Any other ideas or diagnosis is most appreciated. Thnask, David

[auto trainy]

check all the small vacuum hoses toward the front of the engine,they had a tendancy to crack and have intermittant problems.You might also try hand choking the carb at 1500 rpms briefly to see if there is dirt in the jets,good luck

[auto trainy]

Sorry I forgot that if you remove the heat riser oven assy (the metal around the exhaust manifold and the pipe going to the air cleaner) the engine will sometimes bog out at 35-45 at times mostly in cold weather.

[92 Lumina]

What kind of distributor was installed?

[wolfman98]

Please provide more info re the piping to the air cleaner from the exhaust manifold. Mine is missing. I bought some flexible piping some time ago but never installed it. Exactly what is suppose to be there? Can I just run the flexible hose (aluminum covered metal coil type) from the manifold to the air cleaner. What is the function, simple to provide warm air to the carb? Thanks for the tip and info.

P.S> Not sure what type of electronic ignition was installed, no manufacturer shown.

[auto trainy]

If the air cleaner is original the air going to the carb was heated from the exaust manifold temperature and controlled by a vacuum/temp operated portion of the air cleaner housing,it consists of an oven(sheet metal mounted on the manifold) and your flex tubing connecting the two,whenever the temperature is below what it should be the there is a damper door that is controlled by vacuum to keep the air temperature correct for the proper fuel mixture while the choke is being called off. If the system is working correctly the snorkle portion of the air cleaner should feel warm,good luck.

[wolfman98]

Thanks for the inof. The aprts are all there but the pipe has deteriorated and I took it off. I bought a new a while back but never installed. This will be done immediately to take another variable away in my engine miss. Thanks.