Tune Up Tips 351
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Cargo Master
Joined: Jul 2003
Posts: 2,730
Likes: 20
From: Four Oaks, NC
Tune Up Tips 351
I have an off idle stumble in my 69 F100 that I just can't seem to overcome. The engine idles fine, smooth as glass. After it gets past the stumble it runs great and makes good power. The engine is a 1973 H code 351c (2 barrel, originally from a Cougar).
I do not know the real timing, I think the harmonic balancer has probably slipped over time. It shows my initial at about 20. When I had it set to 8 it would barely run, as I increase the timing it has improved, up to a point, from about 15 to 20 it has not made much (if any) difference.
My accelerator pump is working, I can see squirts in both barrels, and I can also overcome the off-idle stumble by patting the accelerator to put more gas in it.
Can anyone offer any tips or suggestions? This is driving me nuts (not far to go), I've been tinkering with this trying to get it "right" for a few months now.
I do not know the real timing, I think the harmonic balancer has probably slipped over time. It shows my initial at about 20. When I had it set to 8 it would barely run, as I increase the timing it has improved, up to a point, from about 15 to 20 it has not made much (if any) difference.
My accelerator pump is working, I can see squirts in both barrels, and I can also overcome the off-idle stumble by patting the accelerator to put more gas in it.
Can anyone offer any tips or suggestions? This is driving me nuts (not far to go), I've been tinkering with this trying to get it "right" for a few months now.
Fleet Owner
Joined: Dec 2009
Posts: 20,678
Likes: 82
From: NorCal
My first thought is a timing issue since yer unable to verify it. I'd address that first since it needs to be done anyway.
Second, is the vacuum plate advancing or is it stuck? Check to make sure it moves freely.
Third, it might need a bigger pump shot... I assume it's an Autolite 2100. Is the accelerator pump arm on the outer hole?
Check for trash in the booster pump circuits. Is it an instant jet or kinda lazy?
Second, is the vacuum plate advancing or is it stuck? Check to make sure it moves freely.
Third, it might need a bigger pump shot... I assume it's an Autolite 2100. Is the accelerator pump arm on the outer hole?
Check for trash in the booster pump circuits. Is it an instant jet or kinda lazy?
Thread Starter
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Cargo Master
Joined: Jul 2003
Posts: 2,730
Likes: 20
From: Four Oaks, NC
My first thought is a timing issue since yer unable to verify it. I'd address that first since it needs to be done anyway.
Second, is the vacuum plate advancing or is it stuck? Check to make sure it moves freely.
Third, it might need a bigger pump shot... I assume it's an Autolite 2100. Is the accelerator pump arm on the outer hole?
Check for trash in the booster pump circuits. Is it an instant jet or kinda lazy?
Second, is the vacuum plate advancing or is it stuck? Check to make sure it moves freely.
Third, it might need a bigger pump shot... I assume it's an Autolite 2100. Is the accelerator pump arm on the outer hole?
Check for trash in the booster pump circuits. Is it an instant jet or kinda lazy?
Accelerator pump is on the outer hole. On the throttle assy the pump rod is on the upper hole.
Thread Starter
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Cargo Master
Joined: Jul 2003
Posts: 2,730
Likes: 20
From: Four Oaks, NC
Well no matter what I did I could not get rid of my off idle stumble, I verified the accelerator pump had a good squirt. I even replaced the diaphragm to be sure that was in good shape. I set the idle mixture using a vacuum gauge and got that as close to prefect as I could. It ran fine cruising down the road, it idled smooth as glass, but the off idle stumble was still there. It beat me, I gave up, I put another carb on it and it runs perfectly now.
Fleet Mechanic
Joined: May 2001
Posts: 1,396
Likes: 4
From: Cupertino
Your problem might be caused by worn bushings on the distributor plate or a problem with the vacuum advance.
Pull the vacuum line at the distributor and plug it with a screw. That takes the vacuuum advance out of the picure. Only takes a second. Drive around the block and see what happens.
As far as a slipped damper goes, Hio put up and ingenious dowel method for measuring stroke that might word to feel out TDC and check your mark. Look back.
Something is wrong with your crank mark. I do not believe the motor would idle at all with 20 degrees advance.
Pull the vacuum line at the distributor and plug it with a screw. That takes the vacuuum advance out of the picure. Only takes a second. Drive around the block and see what happens.
As far as a slipped damper goes, Hio put up and ingenious dowel method for measuring stroke that might word to feel out TDC and check your mark. Look back.
Something is wrong with your crank mark. I do not believe the motor would idle at all with 20 degrees advance.
Thread Starter
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Cargo Master
Joined: Jul 2003
Posts: 2,730
Likes: 20
From: Four Oaks, NC
Your problem might be caused by worn bushings on the distributor plate or a problem with the vacuum advance.
Pull the vacuum line at the distributor and plug it with a screw. That takes the vacuuum advance out of the picure. Only takes a second. Drive around the block and see what happens.
As far as a slipped damper goes, Hio put up and ingenious dowel method for measuring stroke that might word to feel out TDC and check your mark. Look back.
Something is wrong with your crank mark. I do not believe the motor would idle at all with 20 degrees advance.
Pull the vacuum line at the distributor and plug it with a screw. That takes the vacuuum advance out of the picure. Only takes a second. Drive around the block and see what happens.
As far as a slipped damper goes, Hio put up and ingenious dowel method for measuring stroke that might word to feel out TDC and check your mark. Look back.
Something is wrong with your crank mark. I do not believe the motor would idle at all with 20 degrees advance.
I do think with the carb the problem is the in the "power circuit". I had a lot of backfires in the past while working on it, maybe I messed something up. At any rate, it's all good now
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