351w bucking under load

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Oct 16, 2002 | 04:51 PM

Ferrite

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Joined: Dec 2001

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351w bucking under load

1983 351w, 2 bbl carb

Mechanic's Special

Idles "okay", not too rough, fairly smooth when revving
the throttle in neutral. Can accelerate and drive down the
road fine, until you come to a hill or just about any incline.

Going up a hill (or under load) the truck will hesitate, buck
and what sounds like a carb backfire hiccuping can be heard.

I did some basic stuff. I'm pretty mechanically inclined and
have successfully rebuilt carbs before. Been awhile, though.
Took the carb off, cleaned it very well.
Removed the top and cleaned the float bowl.
Reset the float to spec, replaced accelerator pump diaphragm.
New fuel filter, too. New spacer and gasket.

No change at all.
Still can't get up a hill faster than 20mph.
I am leaning towards getting a replacement carb from junkyard,
rebuilding it if needed and trying that.

Looking for thoughts/suggestions other than carbeurator.

Any help would be appreciated.

Thanks,

-Ferrite

Oct 17, 2002 | 09:21 AM

airharley

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Joined: Nov 2001

Posts: 3,351

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From: Escondido, CA

351w bucking under load

You could be suffering from a few things all combined to make it wuss out like it is. Do a timing check to see where you timing is set. If that is good, pull a valve cover and watch the rocker arms as you turn the crank. If they hesitate a little before moving your timing chain has slack in it. Which is cuasing the valves to open to late. Hence the back fire. Last do a compression check on all the cylinders. This shouldn't be the problem but it will let you know if you have a leaking intake valve.

Oct 17, 2002 | 09:32 AM

airharley

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Joined: Nov 2001

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From: Escondido, CA

351w bucking under load

Apr 16, 2004 | 11:25 PM

tec782d

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Joined: Dec 2003

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i'm having the same problem with my 1979 E-150 351w. put on rebuilt carb,new intake gaskets.checked for vacuum leaks,new dist,ign module,then i put a new coil on and it ran GREAT for one week now it is doing the same thing,Backfire like crazy especially when under load, tried a new coil again and still the same!! Any one have any new things i can try(local mecanic is stumped too he doesn't think it' the timing chain) any help would be appreciated as i'm going nuts over this! Thanks

Apr 17, 2004 | 09:33 AM

91f250460

Senior User

Joined: Sep 2002

Posts: 431

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From: Midwest

If you have cats on your truck I would check to make sure your exhaust is not plugged or somewhat constricted. This would cause the truck to run okay under a low load situation but when trying to get going or a high load situation the exhaust can't be expelled fast enough.

Apr 17, 2004 | 12:25 PM

tec782d

New User

Joined: Dec 2003

Posts: 6

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no cat , i have true dual exhaust, checked the + side of the coil and it is 12 volts,the thing i don't understand is after putting on a new coil the first time it run great for a week i tried everything to make it backfire but it wouldn't- then it just started backfiring like before,tried a new coil and ign module but it still backfires!!