I have a few of these engines and all but one of them I can get a really smooth 'sewing machine" idle out of them.

The 351M in my 77 F250 is giving me a hard time. I can't seem to tune the shake out of the engine.

I have exhausted all vacuum leak possibilities, I have tried different ignition timing ranges, new plugs, wires, cap, rotor.

I have this engine tuned as good as i can make it run with the external influences I can adjust such as air/fuel/ignition.

It pulls good power under a load down the road.

But it shakes at idle. It will shape up, sort of stabilize if you leave it to idle for a bit. but a couple quick snaps of the throttle and the engine recovers with a pretty wicked shake which eventually smooths out some.


I run this engine side by side with my other 77 351M which is in a F150. Both engines are stock with the exception of the dual exhasut on the F150's engine. the F150's 351 idles smooth, so smooth you can't see the engine moving while it's running. You can snap the throttle open a couple times real quick and it settles immediately back into a smooth idle with no stumbling.

The F250's 351M has a mild shake at the same warm engine idle speed and when you snap that throttle open and let it snap shut to idle setting, it stumbles and shakes hard before settling back into it's normal somewhat shaky idle.



I'm suspecting timing chain/gears.


Does anybody else have any ideas?

 

Pull the plugs wires one at a time and see if you make a difference on that cylinder then run a compression check. It seems you have all the external things checked, then it would be time to go deeper. Sticking valve, worn rings, low compression on one or more cylinders. Good luck and keep us updated on what you find.

 

Did you throw a vacuum gauge on it? the needles behavior will tell you a lot.

http://www.classictruckshop.com/club...ts/vac/uum.htm

 

Follow Montana Highboys post, a vacuum gauge will tell alot plus its needed to dial your carb in properly and also pick up a couple of cans of Seafoam and use it as a fuel addative, use 16 oz can and fill tank. If you have the extra coin do it a few times. My 400 ran the same way and checking specs twice the Seafoam made a big improvement within a half tank and after three it is still perfect.
Just a few easy things not to overlook before you go in

 

The plug wire trick was one of the first things I tried, I've done it several times actually. no delineative findings there. Also I should have mentioned I have read the plugs and they all look fantastic, (even the autolite 25's that I replaced, they looked great and I felt silly about even changing them after seeing them all look so good)

I have not done a compression test and I have not done the vac gauge.

I may borrow the vac gauge out of my 1970 f250 to see what I'm pulling for manifold vacuum.

To this point I have tuned my carb with a tach on a dwell/tach meter.


I suppose I could maybe gain an idea of timing chain slack by pulling the cap off my distributor and observing dist shaft rotation in relation to crank rotation maybe while turning the crank by hand?



I've done a little of the farmer/mechanic tricks like running the engine RPM up to 3500 RPM or so then manually slapping the choke plate shut to pull hard through the idle circuits for a moment.


I haven't messed with any cleaners like seafoam yet.

 

montana highboy, thanks very much for that link. I had no idea the vac gauge could be that handy. I printed that off and I will likely pic up a vac gauge at the parts store on my way home tonight.

 

Hey no problem northerndave, they're a very handy diagnostic tool, i have one installed in the cab.

 

How hard is it to install a vac gauge? Do you just run a piece of tubing from the gauge to the intake manifold?

 

That's pretty much it, really simple, just be sure it's full/manifold vacuum and not timed/ported vacuum (any vacuum source below the throttle blades will suffice).

 

my 1970 F250 with a 360/4spd has a vac gauge on the dash. It was on there when i got the truck many years ago. I guess I never knew all the things it could tell me if I knew how to read it.

 

The vacuum gauge leads me to believe restricted exhaust seems to be my only irregular reading indicator.

 

This link below offers a more detailed interpretation of gauge readings, it even has a nifty "interactive" gauge to illustrate the various scenarios...

How to Use and Interpret a Vacuum Gauge

 

I never knew you would tun out to be such a big help and after being married all these years I too should have explained "Vacuum"

 

that one I will have too look at tomorrow at work as my home pc browser is quite crippled and will not install the flash viewer needed for that page.

Maybe I can get it on my smart phone.

Thanks.

 

I'm bumping this thread and thanking the guys that told me to try a vacuum gauge.

The gauge indicated restriction in the exhaust.

I hooked up my new dual exhaust tonight and wow, no more shakey 351M

She runs very, very nice.

And she got new wheels and tires tonight too.