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Old 11-26-2012, 12:58 PM
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305/T-350 drops RPM when in gear


I'm a novice to this so please bear with me..

Short question:
What would cause a carbed mild/stock Chevy 305 coupled to a TH350 trans (manual valve body, might have other mods such as a mild shift kit) to drop RPMs excessively and jerk when put in gear? This is independent of engine/trans being cold or at operating temp.

Long version if needed:

I have a '79 Pontiac Grand Prix with a carbed Chevy 305 engine and a TH350 trans. The carb is a 4 barrel Edelbrock (4487 I think it said), about 600 cfm according to previous owner. Also has HEI distributor. The trans has a manual valve body.

There are a few problems that appear to be related:
  1. Car doesn't start very willingly when cold, often takes a few tries. Will then appear to start, but die immediately (fail to keep RPMs up). After doing this a few times it will start and run relatively okay in park/neutral. Starts immediately when warm.
  2. When put in gear (reverse or any forward gear) RPMs drop massively (independent of engine hot or cold). I unfortunately don't have a tachometer so I don't know how much, but it's in the 500-1000 RPM range. The car will also jerk when this happens (backwards in reverse and forwards when in a forwards gear).
  3. When put in gear when engine is cold, the engine will die down. The amount of RPMs dropped seems to be the same as when hot (cannot verify without tacho), but the engine appears unable to idle at that speed when cold and just stumbles to a halt. Note that this appears to be "solvable" by adjusting idle speed high enough (currently about 2000 RPM judging by engine note, don't want to go higher).
I'm guessing that problem 3 is a natural effect of problem 2 + problem 1.

In addition to the above (don't know if it is related), engine uses some coolant and smokes white/grey a bit more than a "healthy" car. I figure some of the extra smoke is due to high idle, but I guess any smoke is bad. Smokes independent of engine temperature (same amount cold or hot). Does not use engine or trans oil (as far as I can see so far). Suspect head gasket(s).

When driving car shifts okay. A bit jerky, but not too bad. I figure an earlier owner might have installed a shift kit since it has a manual valve body. The engine does not appear to be way down on power compared to what it should have. Have not revved it up high since I have no tacho and am careful not to break my new toy

Any input from more experienced car enthusiasts is greatly appreciated! (Did I mention that I have no tacho, by the way?)

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Old 11-26-2012, 01:39 PM
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Hi, thanks for replying, I'll do my best to answer.

When I idle it up more I assume it would try to pull away, since currently if I take my foot off the brake it will go (and rather quickly too).

Don't know if I have vacuum advance hooked up, but will read up on it and check it (sorry, first car with a carb). Same with vacuum leaks.

The choke is automatic, don't know if it works like it should since I am just getting into this carb-thing.

I did not change the converter, just bought the car. Don't know if it's lock-up, but doesn't feel that way when driving it (first time with auto trans as well, so might be wrong here). Anyway I doubt it is lock-up causing it, the engine does not "bang" to a stop, it is more of a struggle and slow die (1-2 full seconds).

The kickdown cable is not connected (manual valve body), that's really all I know. At the moment it's a bit hard to get under the car to check for more, but I'll see what I can do. As for the linkage, all I know is that the arrow to indicate which gear it's in is a bit off compared to the letters on the gauge. Probably useless info, but what do I know... I'll read up on it.
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Old 11-26-2012, 02:20 PM
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Thanks for info. I need a vac gauge and a timing light before I can check those things. Soon Christmas, luckily!
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305, drop, gear, rpm, th350

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