I have recently noticed that I can no longer hold a speed going over my mountain pass (3000'). I have a 77 F100 three-on-the-tree. In the past I could hold a speed with ease, but now I struggle to hold 60mph. I have newer plugs, wires, and coil (these were all done without notice to power loss). The timing has been checked and is set to the emissions decal. It is winter now and the pass temp is 31 or below all the time (usually way below). This summer I did take the stock air filter enclosure off (more like ripped off in a fit of rage) and installed a chrome with element air filter. This worked great in the summer, but it may be not so great in the cold winter. I do not have any oil burning issues. Any suggestions?

 

Check the carb....

 

I was figuring it was in the carb, but are there any suggestions on what to check or look for. I have rebuilt the carb about a year ago and haven't had any issues until now.

 

When was the last time you did a compression check on the engine?

The air filter you installed is going to remove the engine's ability to warm up the air before it goes into the intake. It'll have a little affect on performance, but it shouldn't be that much.

 

I will do one right now. Do you know the numbers where they should be?

 

From what I've gathered:

< 120 - Engine's tired and will most likely need to be rebuild.
120 - 130 - Low on lifespan, some power loss.
130 - 140 - Not the best, but shouldn't exhibit too much power loss. Just a sign its getting long in the tooth.
140 - 150 - Not new, but plenty strong.
150 - 160 - Healthy and strong.
160+ - Great condition.

Then, on top of that, there's variation between cylinders. If you have a variation of 10% or more, something's wrong. Such as, if one cylinder is 140 and the rest are 160. That cylinder has a 12.5% variation from the rest.

Try this page: Engine Compression Testing



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Well - CRAP! Here are the numbers (from front to back) 114,115,115,115,120,115. NOT good. I thought the motor had 154,000 miles on it, but maybe it is more like 254,000! Any good recommendations for crate motor / rebuilt kits?

 

How did you do you compression test? (ie. how many cranks did you turn it over for each test?)

If that's the case, it sounds like it's completely worn out. Sorry to hear it.

I've never rebuilt/replaced mine, but there is a lot of good advice on these boards for it.

 

I cranked for 5 secs. Is that not enough?

 

From what I've heard, it's not necessarily how many seconds, but how many times it cranks. Be sure the throttle's fully open too.
When I did mine, I found that 5 - 8 cranks was enough to fill it up. 5 seconds seems like it would be long enough to do that though.

But when it comes to a full rebuild in front of you, a second test never hurts.

 

Here are the compression specs I found:

Ford 300 Inline 6 Cylinder Specs, Ford 300 Engine Information, Firing Order : Engine Facts.com

According to these numbers, my truck has the compression of a 54,000 miles vehicle. Check it out and let me know what you think.

(8.0 kg/cm2 is roughly 115 psi)

 

Hrmm.... I don't know if that works the same way or not

The compression test on my '81 was between 148 and 155 depending on the cylinder.

 

My apologies, I was hoping it was talking about the metric equivalent, but I think it was the static and not cranking compression. I will give it another go (longer cranks). I was just grasping at straws.

 

It's always worth asking.
Hopefully your second test is more promising.

 

Well - the numbers are better (I cranked longer) 140 on all cylinders. What now?