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I have a 289 that was just rebuilt last year the it was running perfectly. Until it recently started to run rough and back firing though the carb.also when I got on it and kicked in the 4 barrel it seems like when I backed off it and seem like a stuck thottel but the linkage is not hung up any where.
Thanks for any help
 

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I have a 289 that was just rebuilt last year the it was running perfectly. Until it recently started to run rough and back firing though the carb.also when I got on it and kicked in the 4 barrel it seems like when I backed off it and seem like a stuck thottel but the linkage is not hung up any where.
Thanks for any help
Hello MJH,

Are you by chance running a points distributor? Granted it could many things, including fuel related. But from the brief description of your troubles and the somewhat sudden onset of problems it sounds like a bad condenser. However if you do not have a points distributor, it's either running lean (back fire through carb) or the ignition timing went wonky which could be a failing module, carbon tracking, arcing wires, etc.

If you could provide more information that would be helpful.

Cheers
 

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Discussion Starter #3
It does not have points and a condsenor. Also when I am in third gear going uphill it does back fire the carb it never did it before it was always real strong.
 

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Lean fuel mixture, as mentioned, is a possibility, so investigate that for sure. It is also possible that your timing has changed for some reason or another. Maybe the distributor hold-down has worked loose and allowed the housing to rotate. Maybe it was somewhat retarded to begin with and worsened by a bad vac-advance unit or vacuum leak. The vacuum advance plays a bigger role in cruise situations like you mentioned in 3rd gear. If it were not working adequately, your timing may be retarded compared to what it was previously. Recheck your static timing and manifold vacuum at idle. I strongly recommend setting your base timing at 12* BTDC for optimal performance with today’s fuels. The issue of whether your vacuum advance should be connected to a ported or manifold source is a whole other debate, but there are advantages to both.

Make sure the breaker plate in the distributor is freely moving. These can become seized due to dirt and corrosion or something blocking it. This can have the effect of retarded timing. I had a late timing and backfire issue once from my condenser wire accidentally getting trapped under the distributor cap, so the breaker plate couldn't move, so my vacuum advance was totally out of the picture. I scratched my head a while before discovering that's all it was.

The stuck throttle feeling could be a ruptured power valve from the backfires you’ve experienced. That is the main reason those fail, and it will cause very bad performance if it’s blown. The good news is they are cheap. I prefer to get a power valve that is correctly valued for my engine. The best functioning power valve will be of a value that’s half your manifold vacuum inches at idle and then add a point or two. The ones shipped with standard carb rebuild kits are pretty much all around 7”. The trouble is if you are running 21” of Hg at idle, that takes a relatively long time to open and enrich the fuel, and many cars have a temporary lean condition just off idle that results in stumble or hesitation. An 11 or 12 power valve would be better. So, this gives you a chance to perhaps fix your problem and make a tuning improvement at the same time.
 

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I was kind of leaning toward power valve as well in reading your description. But as mentioned, it could be various things.

Pat
 
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