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Jimm

Jimm 4/23/2011

1980 Ford Mustang Base 8 Cyl 4.20L

Engine

Actually for 1966 Mustang with 289-2V (the year selection range does go back that far!)

Have an Autolite 2100 carb that has been rebuilt by Pxxx Carburetors twice - still has same off-idle stumble and hesitation. The carb is installed on a mostly stock, rebuilt 289 engine with strong compression in all eight cylinders, and strong idle vacuum (20-21") with steady guage needle.
The carb has a new accelerator pump. The engine has the following new parts; carburetor base gaskets, new vacuum advance, new spark plugs, new ignition wires, distributor vacuum solid line to the carb, Pertronix ignitor module. I've timed the engine using the Pxxx method (old school) recommended seat-of-the pants (without a timing light); to select an advance setting beyond the factory 6 degrees BTDC just until the engine starts to stumble and exhibit hard starting - then back (retard) a little - lock down and test drive. The off-idle stumble still remains.
Any ideas or suggestions at this point?

4 Answers


mopar

mopar 4/23/2011

check float level..if good....go back to timing using a timing light.

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mopar

Jimm 4/23/2011

Float level checks OK. Used a timing light to set the timing - still have off-idle stumble / hesitation. Any more suggestions?

HouseCallAuto

HouseCallAuto 4/23/2011

Block off the vacuum hose to the brake booster to rule out a leaking booster. What is the vacuum reading with a vacuum gauge attached to manifold vacuum with and without the booster hose pinched off? Check all other vacuum hoses for leakage (I'm sure you did that already)

Should be:
Steady idle (800-1200rpm) Gauge steady, 17-22 Normal & healthy

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HouseCallAuto

Jimm 4/24/2011

Thanksd for the replies. The vacuum is strong and steady (20-21") at idle speed around 700 RPM. The off-idle stumble occurs at all different timing settings; seems to happen regardless of timing settings, wheter advanced or set at the recommended 6 degs BTDC. I'm certain that the timing issue can be mostly ruled out at this point. There no vacuum leaks; the engine condition is solid as evidenced by the steady and high vacuum reagding at idle.

HouseCallAuto

HouseCallAuto 4/24/2011

Remove the distributor cap, put a wrench on the crankshaft pulley and turn the engine clockwise (or counter-clockwise) and place the mark on TDC. Now turn the crankshaft the opposite direction while watching the ignition rotor and when it begins to move stop rotating the engine and see how many degrees of travel there was before the rotor moved. This is the freeplay in the timing chain that you are discovering. The freeplay should not be more than about 3 degrees. For you to have an off idle stumble the chain would have to have like more than 5 degrees of play. Also, verify that the vacuum advance is working. I know you replaced it but verify that with vacuum hose plugged and unplugged the timing advances or with apply a vacuum to it with a hand vacuum pump and verify that the distributor plate moves.

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HouseCallAuto

Jimm 4/26/2011

Thanks for the suggestions. The timing chain checks OK; little to no freeplay and also the vacuum guage is strong and steady at idle speed. I've already first replaced then re-checked the new vacuum advance on the distributor - by virtue of handheld vacuum pump - works OK with vacuum applied (holds over time without decreasing). The distributor plate does move with the vacuum advance connected to the plate and vacuum applied by hand.

HouseCallAuto

HouseCallAuto 4/26/2011

Try to recreate the stumble by manually goosing the throttle from the carb. Verify that you can see the stumble. Then, recreate it again but this time spray carb cleaner / 2+2 into the carb and see if the add'l fuel flattens the stumble. You have to spray it at just the right time but you understand what we are tryng to do.

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