Regarding 1978 351M Idle

[jgavac]

I have a 1978 Ford F-250 Custom, with a Ford 351M ci engine that my dad gave to me.

In the 1990's, he had the engine rebuilt multiple times, and one time, there was a engine performance kit installed. Kit included a block boring to 400 ci, a higher comp ratio, and racing cams (there were other things too). My problem now is that my idle has gotten worse.

The truck had been sitting for 2 years, and the engine started right up with a little starter fluid in the carb. The idle was rough, and with some work, I got the idle to smooth out. But, now that I have focused my attention to the interior (and the engine being left alone), the idle has started getting rough again. I'm not sure if it is a vacuum leak, or if it is something else.

I have checked:
Plugs
Wires
Distributer
Distributer Cap

Would like some feedback on what might be wrong.

[danny]

welcome to the forum.

when you checked the items listed what did you do? did the items appear good or did you do any testing?

[jgavac]

All the items listed were checked for corrosion, rust, carbon buildup, and cracks. None of the cylinders are missing.

[olddog]

First you cannot bore a 351M to 400 cid, however it was likely stroked to get there. It is common to bore 0.030" over bore and then increase the stroke. It sounds like your father knew what he was doing.

You said it has a race cam. A high duration cam shaft with a fair amount of valve overlap is not going to idle smooth. It is going to have a rumptity rump rump sound. It may not want to idle below say 800 or maybe 900 rpm. It depends on what the cam is. So, there may not be anything wrong. Ask your father if he has the cam card or remembers the cam specs. Duration at 0.050" lift is the important number and lobe separation angle. The lift impacts flow and power, but not much impact on the idle characteristics.

Do you have any idea what the cam is? Is it hydraulic lifters or solid lifters? Are the lifters flat tappet or rollers? If it is solid roller lifters (which I doubt) the idle must be kept fairly high and/or idle time kept to a minimum by the driver or they will fail.

Fresh gasoline with some Sea Foam may clean up the carb for you. Cracked vacuum lines very possible. Pull them (at the carb or manifold) and plug them one at a time to see if it improves anything.

[olddog]

First you cannot bore a 351M to 400 cid, however it was likely stroked to get there. It is common to bore 0.030" over bore and then increase the stroke. It sounds like your father knew what he was doing.

You said it has a race cam. A high duration cam shaft with a fair amount of valve overlap is not going to idle smooth. It is going to have a rumptity rump rump sound. It may not want to idle below say 800 or maybe 900 rpm. It depends on what the cam is. So, there may not be anything wrong.

Do you have any idea what the cam is? Is it hydraulic lifters or solid lifters? Are the lifters flat tappet or rollers? If it is solid roller lifters (which I doubt) the idle must be kept fairly high and/or idle time kept to a minimum by the driver or they will fail.

[dougbfresh]

You need to check timing/dwell (see if it's wandering around) and get a vacuum guage on it. I would suspect the carb 1st but test a couple of things 1st.

[jgavac]

Not sure about the brand of cam. my dad is guessing that it has solid lifters, but from what you are saying, then they may be hydraulic.


I'm pretty sure its not the carb. The carb was brand new in '96, and i'm almost positive that it does not leak (But i guess anything is possible...)

[dougbfresh]

Any carb sitting for a few years is ALWAYS suspect. So it was new 14 years ago-BIG DEAL, the 1st thing I would suspect is carb or distributor in something this old.

[jgavac]

Distributer cap or whole thing...?

[dougbfresh]

Originally Posted by jgavac

Distributer cap or whole thing...?

Distrubutor with all the old goodies in them have a lot a wear parts-shims, springs, cams, vacuum canisters, vacuum advance,....

If your going to tinker with this you need a dwell/tach, vacuum guage and a timing light (hopefully adjustable so you can see if the distributor is working).

[jgavac]

Originally Posted by dougbfresh

Distrubutor with all the old goodies in them have a lot a wear parts-shims, springs, cams, vacuum canisters, vacuum advance,....

If your going to tinker with this you need a dwell/tach, vacuum guage and a timing light (hopefully adjustable so you can see if the distributor is working).

I don't have the vacuum gauge or the timing light... this might be more difficult then i was expecting..

[dougbfresh]

Originally Posted by jgavac

I don't have the vacuum gauge or the timing light... this might be more difficult then i was expecting..

Kind of how life is

[jgavac]

Originally Posted by dougbfresh

Kind of how life is

haha yeah, it happens

[dougbfresh]

Well, you have a lot of unknowns, your not even really sure what the engine is internally, it's been sitting for an extended period and you have no diagnostic tools.

[jgavac]

well every thing done to the truck (rebuilds, tuneups, and purchase) was before i was born, so.... :/