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Author Topic:   stumbling on acceleration
skips69
Gearhead

Posts: 124
From: Catlin, IL USA
Registered: Mar 2001

posted 05-25-2002 08:01 PM     Click Here to See the Profile for skips69   Click Here to Email skips69     Edit/Delete Message   Reply w/Quote
Just curious before I throw parts and money at this problem: I have a 1979 F150 4x4 with the original 148,000 mile 351M engine. I bought it about 3 weeks ago. If you accelerate too fast, it stumbles and hesitates badly. If you take off at a low start, it eases on down the road ok. Once you get up to about 42 mph, you can mash the accelerator to the floor and it will respond with no problems. Before 42 mph, whenever you give too much gas, it hesitates and stumbles until that magic number, then it runs good. Any thoughts or past experiences with a similiar problem? The list of things to check and replace is quite extensive in the troubleshooting guide in my Chilton. A new Engine will be built this fall, so just trying to get it to run a little better until then without investing too much money on it now.

Thanks!

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MLariviere
Moderator

Posts: 2393
From: Biddeford,Me.USA
Registered: May 99

posted 05-25-2002 09:10 PM     Click Here to See the Profile for MLariviere   Click Here to Email MLariviere     Edit/Delete Message   Reply w/Quote
It sounds like you need a carb rebuild. It could be the accelerator pump is weak. Another thing to look at is ignition. I had an 85 with a 351W and it did that. It turned out to be the cap. It had some tracking in it and was crossfiring,with the same symptoms you describe.

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skips69
Gearhead

Posts: 124
From: Catlin, IL USA
Registered: Mar 2001

posted 05-25-2002 10:24 PM     Click Here to See the Profile for skips69   Click Here to Email skips69     Edit/Delete Message   Reply w/Quote
The cap and rotor would be the first thing I replaced too since it is the easiest and by far the cheapest thing to throw at it. You know how it rolls from there sometimes, so I'll probably start there.

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skips69
Gearhead

Posts: 124
From: Catlin, IL USA
Registered: Mar 2001

posted 05-29-2002 09:07 PM     Click Here to See the Profile for skips69   Click Here to Email skips69     Edit/Delete Message   Reply w/Quote
Replaced the cap and rotor finally today. Man, what a big difference. They were pitted pretty bad. Now it runs much better and smoother. Still some hesitation, but only up to about 25 MPH and not nearly as bad getting up to speed. I have aline on a 400 C.I. motor with only 5,000 miles on the rebuild. It is from another 79' truck. I hear the advantages over this motor as opposed to what I currently have (the 351M). What are the basic differences between the two - just cranks and connecting rods? Why is the 400 more popular? And, finally if using this engine, will all my brackets, mounts, etc from the 351M bolt on with no problem? I would think so.

Thanks as always!

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1MonsterEffie
Journeyman

Posts: 6
From: Gloucester, VA
Registered: May 2002

posted 05-29-2002 10:23 PM     Click Here to See the Profile for 1MonsterEffie   Click Here to Email 1MonsterEffie     Edit/Delete Message   Reply w/Quote
Do you still have the stock exhaust? It is strange, but I have heard of trucks that people thought were ignition/carb problems causing hesitation, but in fact it was the catalytic converter clogged up. Being a '79, you will have one unless the exhaust has been replaced, maybe you should look into this.

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skips69
Gearhead

Posts: 124
From: Catlin, IL USA
Registered: Mar 2001

posted 05-30-2002 07:30 AM     Click Here to See the Profile for skips69   Click Here to Email skips69     Edit/Delete Message   Reply w/Quote
It may, I'll check that out. It is very quiet, so it could be the original and still have the cat on it - definately single exhaust (for now). Thanks for the tip.

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MLariviere
Moderator

Posts: 2393
From: Biddeford,Me.USA
Registered: May 99

posted 05-30-2002 09:03 PM     Click Here to See the Profile for MLariviere   Click Here to Email MLariviere     Edit/Delete Message   Reply w/Quote
I'm leaning more to a worn out accelerator pump.

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4Zsnow
Journeyman

Posts: 25
From: Western Washington, USA
Registered: May 2002

posted 05-30-2002 11:57 PM     Click Here to See the Profile for 4Zsnow     Edit/Delete Message   Reply w/Quote
I second the pump theory.

2 pennies

------------------
God Bless Big Oil

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skips69
Gearhead

Posts: 124
From: Catlin, IL USA
Registered: Mar 2001

posted 06-02-2002 09:00 PM     Click Here to See the Profile for skips69   Click Here to Email skips69     Edit/Delete Message   Reply w/Quote
Went ahead and finished up the tune up today - New plugs, wires, air cleaner, oil breather and PCV Valve. Runs like a totally different truck now. Very responsive at any speed, great acceleration, etc, etc. The #4 plug was an autolite, all else were champions. That tells me the past maintenance practice for this truck. Either the previous owner or his mechanic intentionally left that plug because of the difficulty in getting to it due to the heater/A-C box being in the way. Everything was badly worn, filthy or pitted on this truck's basic tune up components, so it hadn't been done for quite some time I would imagine. Didn't even check the timing, but that will be next. Just glad to have it performing so much better for now.

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