Hey everybody, getting confused on this 351m in a 79 f150 4x4 i bought for 500$ ive rebuilt the carb, changed valve gaskets, and changed fluids
the previous owner changed alt, distributer and cap, tuned up etc. for some reason it idles fine (little pinging from driver side rear @valve cover) but soon as i put it in gear itll spit and stutter, itll drive down the road at no more than a light half throttle in 4lo loc, struggles in 4hi loc (np203) but will half throttle spin tires in dirt (converted to manual hubs for 2wd) and none of the vacuum lines make sense, most have been capped off or lead to nowhere.

 

I'm confused about your run-on sentence.

Are you asking about the "pinging" coming from the left rear valve cover? Pinging usually denotes an ignition/timing problem.

Are you questioning why she "spits and sputters" when put in gear? As in a carb problem?

Do you have a C6 trans? Is there a question about the trans only getting to half throttle in 4WD?

I don't think you have an NP 203, rather an NP 205 unless someone did a "downgrade." Nothing wrong with a 203, it's just that in '79 the 205 was king. And you shouldn't be driving in 4 wheel, Hi or Lo, with the hubs locked, at more than 15 mph or on dry, hard surfaces, for an extended period of time, let alone throwing up dirt.

What are manual hubs for 2WD?

Is your question one concerning vacuum line routing?

 

sorry for grammer. i do question the pinging.

yes i am guessing a carb issue,as it struggles under load going down the road.

i do have np203 with
4hi loc
4hi (does not stay in gear)
n
4lo
4lo loc

i was told i should not have factory manual hubs, so im assuming the previous owner installed them to take the power away from the front wheels? i leave the front hubs locked out rather than in.

im very confused with the most of the main vacuum lines, many are missing, others just run out to nothing under the frame, the previous owner used it on a farm and jerry rigged it over the years.

 

Pinging and carb problems under load suggest timing to me. And I like to troubleshoot one thing at a time. Doing a few at once leads one to not finding the original problem.

On the M Block, set the timing to 12* BTDC. Pull and plug the distributor's vacuum advance line first. I don't know if you're running manifold or ported so pull and plug it. Then, check the vacuum advance can to see if it's in good shape...apply suction to the line leading into the can. If there's resistance to vacuum (or sucking hard), it should be ok. If you feel no resistance it's time for a new one.

Take her for a spin after you set the timing, under various load conditions. If you still hear pinging, we'll play with timing/vacuum advance next.

I won't get into the 4WD aspect yet as I'm not sure what you're asking. Besides, the timing takes precedence.

I'll post up a vacuum diagram of the 351M lines to keep you busy....


This is from a 79 Bronco, loaded, 400, C6, 4WD

Vacuum lines are the same for the 351M/400.

 

vacuum leak possible from the EGR plate they are notorious for warping