I found this site the other day and looked for an answer to my problem but i couldnt find one so i decided to join and ask.
I have a 77 ford f150 2wd with a 351m engine and it shakes when it idles.It only does it when its in gear after it kicks the rpms down and will shake in park or neutral if i adjust the throttle stop. Once i get going it stops shaking at about 15-20 mph .I replaced the igniton box, spark plugs, rebuilt the carb, adjusted the idle mixture, air filter, fuel filter ,and checked the timing and replaced the timing chain and it never changed.I dont have a vaccume leak for sure. I was messin with the distributer the other day and left 2 plug wires off and forgot and drove it and it idled the same but shook until about 45 mph. Once i put the 2 wires back on it went back to the same old thing.I tried manually shifting and it doesnt shake nearly as much only when its setting idling and when i first start goin.So it seems to me that i have cylinders that arent firing till i reach i certain rpm.What would cause this to happen and how can i fix it?

 

Are you sure you have all the spark plugs in the correct place?
I swapped mine a while back, and it had a rough idle like that, because 2 wires were in the wrong place on the distributor.

 

They're all in the right place.I checked them 2 or 3 times just to make sure.

 

Are all the plugs firing?? Firing order?? Rotor?? Mixture on the carb??

 

Pulled and checked all the plugs a little while ago and they all looked the same no signs of fouling but i didnt idle much 2 day unlike other days so if there was any fouling buildup it should have burned off. I've set and reset the idle mixure 3 or 4 times.The firing order is correct and the rotor appears to be fine. The truck set for several years before it was givin to me and i havent replaced any plug wires or anything on the distributer but it was all replaced a few weeks before it was parked. How many volts should i be geting on the distributer side of the coil wire? I checked it earlier and it was only reading 6.45 volts.

 

I'm not saying this will sovle the problem, but try running some sea foam through the carb and maybe some through the gas tank and it should decarbonize the engine, and smoothe out the idle a little bit.

 

6.45 Volts on the coil should be OK. You might give it a compression check. My 460 did the same thing. It sounded like it was missing at idle and ran fine after the RPM's increased. It turned out that the #7 intake valve was warped. The brake booster was leaking and pulling cold air straight into the #4 & #7 intakes and the cold air warped the darn valve. A compression check would verify the valve.

 

Could be the EGR or the plate under the carb. The plates are famous for burning through and creating a vacuum leak, especially if you are lucky enough to have one of the aluminum units.

 

I saw somewhere that someone had the same problem on the same model year etc.. as mine and it was something on the transmission causing it to idle bad.Anyone know what its called and where its located.Im planning on buying a 4wd frame and dropping my body and engine on it but im having 2nd thoughts.

 

You are thinking of the Vacuam shift modulator on the Transmission.
If it was leaking you'd be burning transmission oil directly through the manifold.
You can test that easily by plugging all the Vac lines while your at Idle.
If the Rough Idle goes away, you need to hook em' up one at a time till you get the rough idle back.

I think I'd do a compression test right after that.

Verify the timing with the Distributor vacuam plugged. If that's wrong it indicates other problems.

One more test: With the vacuam guage hooked up to the Port on the Carb (not the port for the Distributor Vacuam line) and all the other lines plugged off,
Short the plugs one at a time. Watching the Vacuam guage, see which cylinders affect the guage the least.
More than one cylinder? Put a new plug into each of those! Four of Eight might be a plugged Idle or Main jet in the Carb.

Go out in the very dark, and see if you have any cross fire or leaks in your plug wires.
Down the sides of the Distributor cap too.

One more tip. This is from following up hackers.
Leave the Carb alone until your sure it's the Carb.
It's possible you have one Idle air orfice jet plugged. But I doubt it.

To test that, short the plugs for each side. 1,4,6 n' 7 from one jet, and the rest for the other jet.
So short the plugs, keep track of which make the biggest impression. If it's Four of the Eight, you'll wind up cleaning the jets in the carb.

Is the Choke completly open at Idle?
Is the Four BBl plate completely closed?
Both those conditions can cause the rough Idle, and smooth running.
You should be getting 10-12 Mpg with that outfit, and not be burning any oil.