289 misfiring under acceleration

67pony · Jul 10, 2009

Short history... 289 with a motorcraft 2100 carb. It had a stall problem after 30 min of driving. We replaced the fuel pump (old one was well worn) ignition coil to a flamethrower, the car has pertronix instead of points and inline fuel filter to the carb. Took off the carb and put a new gasket on. Now the car has a misfire under acceleration... Ok at idle and if you feather the throttle it is smooth. But as soon as you give it a little gas and accelerate (not even that hard) the engine sounds rough and shakes.. it still accelerates but not as fast as it should... it eventually smooths out once you stop accelerating.

Is it a timing issue... could it be the carb... not producing enough vacumn advance with the new gasket? It didnt have this problem before we changed the above parts... oh yea, we changed the PCV valve too.

Thoughts?

zookeeper · Jul 11, 2009

If I didn't know any of your history on the car, I'd say it was an accel pump problem. It simply sounds like there isn't a big enough accel pump shot to allow the engine to accelerate. But you've changed so much stuff, and you say it didn't do that before, so perhaps it is something you did. Try checking the plugs. Look at color, wetness, overall condition and gaps. Check timing as well. I have a freind who swears his car won't run with more than 10 degrees initial advance, yet I have the same combo and mine is a dog with only 10 degrees and runs best at 16 degrees. Check total advance as well. If you have good ignition, then it has to be the carb. I'm not familiar with the Autolite carbs, but carbs are carbs and it sure sounds like an accel pump problem to me.

JonK · Jul 11, 2009

Sounds like an ignition misfire to me since it happens under load. I'm sure you removed the spark plug wires and/or ign. cap wires. recheck your wires vs. firing order and have someone recheck it. If you are sure it's a 289 the firing order should be 1-5-4-2-6-3-7-8. The block numbers run from #1 being the front right cylinder (passengers side) to rear 4 and LF drivers 5 back to 8.

As you can see the last 2 cylinders fire in succession, many times you'll get cross fire between these 2 wires so be sure to route them from the cap back to the cylinders so they're not next to each other. Even with good wires you can get crossfire if they're next to each other in a wire clip or parallel from what I've read.
An early 351W cam will slip right into an early 289/302 engine and run fine but you'd have to use the 351W firing order.
Jon

jcode68 · Jul 11, 2009

+1 on checking the firing order. If 2 wires are wrong, it will seem to idle and run ok in park, but under load it is obvious there is a problem.

67pony · Jul 11, 2009

Thanks for the advice.. I knew I should have test drove it before I changed the fuel pump... In any case, I put the old coil back on and it ran fine for about 5 minutes... accelation was back, but when I almost got home the problem reoccured agaiin. Im going to wait till the car cools down and check the plugs... actually probably replace them. I will also check the plug wires, they are about 7 years old so maybe its time for a new set. If that all fails then I guess it might be the carb.

5280/4 · Jul 11, 2009

67pony said:
Thanks for the advice.. I knew I should have test drove it before I changed the fuel pump... In any case, I put the old coil back on and it ran fine for about 5 minutes... accelation was back, but when I almost got home the problem reoccured agaiin. Im going to wait till the car cools down and check the plugs... actually probably replace them. I will also check the plug wires, they are about 7 years old so maybe its time for a new set. If that all fails then I guess it might be the carb.

also, could you have an issue with the timing chain. Like it might be stretching under load. Also look into the arc jumping around inside the cap. If you are running a small cap distributor.

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67pony · Jul 11, 2009

Thanks for your advice... i located the problem, it was spark plug #8... it had eroded down (along with the insulator) till the gap was literally 1/8" of a inch. it was strange because the plug looked absolutely normal except it looked like someone pushed the electrode and the insulator down... I guess the gap was too far to bridge and it was only sparking intermittely. The boot of the ignition wire to #8 was soaked in oil... its super soft now... I am not sure if that is the cause of the problem or how oil even got there.
I am guessing that it could happen again, all the other plugs seemed fine but I replaced them anyways.

JonK · Jul 13, 2009

I hope you replaced the old wires, pick up a set of BWD (BorgWarner) at most parts stores for ~$25. The oil is probably from a leaky/old/dried valve cover gasket, good to change when you have the plug wires off, I use the FelPro rubber ones.
Jon

67pony · Jul 13, 2009

Thats my next step... any plug wires to stay away from?

289 misfiring under acceleration

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