Have a '78 f150 stock with 351m and motorcraft 2150 carb. Engine idles great and has good throttle response when parked. Under load is another issue. Truck will not turn the tires when brake torqed and engine bogs down to near idle. I get the occassional blow back through the carb when it is stumbling too.
What i've checked, Ignition system, coil, wires, rotor & cap, and control module have been replaced along with the dizzy pickup. Plugged all of the vacuum lines and check the rest for leaks. Timing checks fine as does the vac. advance. Checked with manual vac. and with none. Rebuilt the carb, cleaned it out twice since then and replaced the fuel filter.
I'm just curious if I need to keep fiddling with the carb or if I'm missing something here. One curious thing that did happen, which keeps me thinking carb, is that twice the fuel bowl has been empty when I try and start it in the morning. The float is sitting down and a little touch to it, turn the motor over and fuel sprays like gyser. Since then I've checked the float and it is within specs.
Buddy of mine thinks that it may be the centrifical advance sticking. I'm not sure how that really works or kicks in with it having the vacuum advance. I also dont know if it would kick in considering that the RPMs are usually low when it stumbles, estimating 2-3,000rpm.

Any advise???????

 

First of all, you're not going to be doing much brake-torquing or parking lot burn-outs with a '78 351M and 2-barrel. 335-series motors of that vintage had relatively low compression and an offset timing set to meet emissions standards at the time. They're good motors, but don't expect it to throw you against the seat.

What do you mean you've checked it with and without manual vacuum? What vacuum lines have been plugged? When you rebuilt the carburetor, did you soak it in Chem-Dip to actually clear out all the internal passages? Spraying it down with a can of Gum-Out, as is often done, won't do.

What do your plugs look like? You've checked a lot of the basics, so at this point a compression check is probably in order.

Is the problem temperature-related? Does it happen only when it's first started, or once it has been running a while?

 

drerod-

we may have the exact same issue. my post is under SOS Stranded in Arizona

 

I have no dillusions of what the truck can do but the motor shouldn't bog down to an idle when brake torqing. I only do it because it's easier to check how it does under load rather than driving 1/2 a mile to a hill, hoping to get back up it.

I checked it with regular vacuum and with a mechanical vacuum hand pump, so that I could control the vacuum to the advance. I plugged all accessory vacuum lines outside of the advance, and booster. Prior to that I sprayed carb cleaner all around the engine compartment with no idle change. Just started plugging lines cause I didn't know where else to go, figured it would at leat illiminate something.


The carb soaked overnight in cleaner, used carb cleaner cans on it to blow the passages and slapped it together just the other night. I put the rebuild kit in about 4 months ago and it ran fine afterwards.

Temp doesn't seem to be an issue, does it hot or cold.

RayGant,
You may be correct. I dont seem to have an issue on level ground out on the Hwy. I did at one point so I started fiddling with the mixture screws. They are out about 2 1/2 turns now and it took care of the Hwy stumbles, I know that it is just masking my problem but I have to get to work somehow. If you figure it out please post the solution, I'm getting tired of chasing this gremlin.

 

How does it accelerate out on the road, under normal driving?
Thinking maybe you have a really stretched timing chain or possibly a camshaft going flat.