I have a 1968 ford F250 with the 300 cu.in. six in it and it has the stock 240 cu. in . carb and intake/exhaust manifold on it. It may in deed have the 240 head on it also as it doesn't seem to have all the power it should. It runs good and smooth but will not go up hills like it should having 4:10 gears in the rear end. I haven't checked the timing yet and the advance. Was there a big difference between the 300 head and the 240 head? Also, someone here said the Chevy 250 six rocker arms would add a little more lift for the 300 cam, is that true?

 

Hi portagoosey,

The 240 head has smaller combustion chambers than the 300 head, which actually is a performance increase since it bumps up the compression by about a half a point. Roughly from around 8.0 to 8.5, depending on the year of the 300. So, using a 240 head would not cause a loss in power.

The Chevy 250 rockers have a 1.7:1 ratio as opposed to the 300's 1.6:1 ratio, so they'll open up the valves a bit further and simulate a mild performance cam.

For performance issues, you might look into other things.
Maybe brakes are dragging? A clogged exhaust? (Although I don't imagine you have a catalytic converter on that year).
Engine-wise, definitely check the timing and be sure the vacuum advance (if equipped) is working properly. Also be sure it's disconnected when setting timing.

When was the last time the engine had an ignition tuneup? New plugs, wires, cap, rotor, etc.

When was the last time the fluid was changed in the differential(s)?

There's lots of things that can rob power.

Good luck!

 

Do a compression test. Might just be worn out. How many miles are supposed to be on it?

 

What age is the 300?

The 240 carb may be smaller then a 300 carb too. It may be restricting things.

Confirm the timing mark on the dampner is correct. The outer rings are known to spin causing timing mark to be off. So confirm the timing mark is right and then check the ignition timing.

What's it got for a distributor? If it's a newer engine ( 83 or 84 or newer ) it will have a TFI distributor. Those don't have any mechanical or vacuum advance. It's all computer controlled. So it won't be getting any advance without the computer. Which means it will be a dog. So can you either take a picture of the distributor or try to describe it?

If it has a older distributor, if you can check and make sure you are getting the ignition advance. If it's not advancing the correct amount at the correct time the power is going to be way off.

Also hook up a vacuum gauge to the intake manifold vacuum port and see what it's doing. And a compression test isn't a bad idea either.