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83 Ford F-150 won't go faster than 45-mph

Its a good runner. It has lots of get-up-N-go until you get to 45-mph. Then it just loses interest.

It’s a 1983 Ford F-150 King Cab Short Box with a 302? V8, a 5 speed manual transmission, 2 wheel drive & a heavy towing package complete with low speed rear end, over-load springs & a 800 tongue Weight rear bumper. It has about 116,000 miles (long haul miles) on it.

The previous owner said it had had this problem before, and new spark plugs fixed it. I don’t know, but it seems more like a fuel or vacuum problem. It behaves just like one of those rental truck governors. Plenty of power up to a preset RPM, then blah.

A mechanic I know said it may be a RPM limit switch in the transmission that prevents over revving the engine. Is he talking straight? or is he full of it?

Any suggestions?

Jim

Vacuum problems tend to go away as RPM rises and your foot gets deeper into the throttle (low vacuum conditions).

Fuel delivery problems would be more dependent on throttle position than on RPM.

My vote is ignition - weak spark - probably due to corrosion in the primary circuit connections in a vehicle this old. Possibly due to a failing coil. .The fact that new spark plugs reportedly helped a similar problem before adds weight to this theory. New spark plugs with nice sharp edges require less voltage to fire reliably.

Pull a plug and ground the threads to the engine and crank/start the vehicle. Strong blue spark = I am wrong. Weak yellow/orange spark = I am correct.

If the engine will rev way up there in first and second gears it’s not the RPM limiter…A long shot could be a plugged up exhaust system but this would effect performance at lower speeds too…

So we are back to ignition breakdown or fuel starvation. When is the last time you changed the fuel filter? On Fords they usually screw into the carb inlet…

I had a 1985 Ford Bronco that did this. Replace the fuel filters and it fixed it. Check your fuel line from the tank to the motor to make sure how many filters you have. I miss the one over the axel on mine. While it ran better it was not fixed till I found and replaced that one.

In an old truck I’d suspect clogged fuel filters, a gunked up and restricted pick up screen in the fuel tank, and a weak fuel pump. You have fuel pressure which allows the motor to start and run OK until is starts demanding more fuel flow. I think low flow is your problem.

It is easier to change out the spark plugs, dist. cap, wires and this could be an ignition problem, but I’d still investigate low fuel flow.

Another thought - you replace the fuel filters and ignition parts and you still have the problem. Then, you might have a distributor problem. If it isn’t advancing the spark timing you’ll not get full power. Not sure on a 302 of that age how spark timing is controlled, via computer or the centrifical advance on the distributor. If you have a good timing light you can easily check to see if the timing advances properly.

The catalytic converter may be melted down.

Yeah I was thinking the same as Rod, but as someone else said that would cause RPM issues in all gears. I know when it went in my Jeep, I could not rev past about 3-4,000 RPM…

If this is a pre-fuel injection engine with a carburetor, take off the air cleaner, hold the choke open and, with the engine off, have someone floor the accelerator. The plates in the base of the carburetor should be at 90 degrees. If not, the linkage is out of adjustment. If your truck is fuel injected, ignore this possibility.

Thanks for all the ideas!. I have some experimentation to do. Yes it has a carburetor.

Jim

i would change the fuel filter first and see if theat makes a differnce then check the flow rate coming off the fuel pump also the pressure at the pump. maybe the pump is tired.

A slipping clutch will also show up under high gear first, If the engine revving wien the truck gets to the 45 mph?