worn cam

[390owner]

My 390 has 105000 miles on a professional rebuild. It does not have as much power here lately when the rpms are low usually 2000 or less while  pulling in 4th gear like climbing a hill. Once I get the rpm up past that is runs and pulls like it always has. This all stated a couple of years ago when we made a 2500 mile trip to Colorado. About 3 hours into the trip it just would not pull the hills good in high gear. If I went to third and got the rpms up it was fine. This thing always pulled hard in 4th with the camper behind it even at lower rpms. I have replaced just about everything I could think of under the hood with no results. I am wondering could my cam be wearing out or something. I am stating to think it is something in the engine. It has good compression and vacuum. I have done everything I can think of.

[KMcCullah]

It sounds like the ol' 390 has been a good workhorse for you. At 100k+ miles I'd be seriously considering a rebuild and adding a generous amount of stroke. Especially if you're planning on more tow duty in the Rockies. The lack of o2 and 8% grades will absolutely exploit any weakness in a towing package.

[machoneman]

What were btw the compression test results? With that many miles, lots of reasons for lost hp. Loose timing chain, leaky valve seats, worn rings or worn bore or likely both.

[390owner]

one 146, two 136, three 149, four 134, five 144, six 136, seven 126, eight 144. I checked it with all the plugs out wot each time I checked a cylinder.

[machoneman]

Some are kinda' low. One way to check is to repeat the compression test but this time after injecting about 1/2 to 1 oz. or so of oil into each spark plug hole. If the compression readings come up, it's bad rings. Virtually no change means perhaps a valve job is in order.

http://www.mhnetwork.com/news/properly-diagnose-your-customer-s-engine-problems/

[Joe-JDC]

With that many miles on the rebuild, you may have exhaust seats recessing/wearing in, and the compression is going down, losing power on climbs.  If you did have hardened seats installed back then, then the valve job may be going away, or valves tuliping from heat, lowering the compression.  Compression equal power.  Joe-JDC

[390owner]

The funny thing on the trip was I was getting 7.5 mpg for the first few tanks then it dropped to 4-5 the rest of the trip about the time it stopped pulling good on long hills.

[machoneman]

With that information, I'll venture the exhaust seats and/or exhaust valves are gone. By not sealing, when the plugs fire a good chunk of that compressed and now ignited gasoline is exiting the exhaust side rather than pushing down the piston as it should. With some more testing, you might get lucky and only need head work rather than a complete engine rebuild. Still, you've got a lot of miles on that engine and unless your bucks down, maybe that complete overhaul is called for. 

[390owner]

The block was bored 60 over when rebuilt several years ago. It still has good oil pressure, so if I had the heads worked and just replaced the rings and bearings with new cam that should get me back going right? One more thing is during a hard pull with low rpms it cuts out or more of a surging happens. Like mentioned above I have replaced everything from plug, wires, pick up, coil, wire harness, control modular so I know it is not these causing the surging. I even put another carb on it just to make sure it was not the problem.

[machoneman]

Surging even after the 2nd carb try could be a weak fuel pump (easy to test) some bad rubber hose in the line from the tank to the pump (where even a small pinhole kills a lot of the vacuum needed to pull that fuel from way back there) a blocked up 'sock' inside the fuel tank (take off the cap, blow some low pressure air through the line from the fuel pump back....careful as some gas will blow out). Also, the wrong gas cap or even the right one with a blocked vent hole can do the same. Pop the cap after a run and if it goes whoosh, your creating a vacuum in the tank, hurting fuel 'pull' up to the carb.

Eliminate all the above as potential issues to see if one of these fixes your surge not. All are cheap btw, a good thing!

[390owner]

I have replaced the fuel lines, fuel pump twice,new vacuum lines, I have also change the gas cap and the sock in the tank is not blocked. Like I said I think I have tried everything I could think of. Thanks for all the replies.

[machoneman]

Here's a wild thought. Had this happen on my old '63 Galaxie. The ignition switch was very loose and with  the key ring jangling off the actual ignition key, on hard bumps the car would stutter. A new switch fixed that in a hurry. Btw, if you can wiggle the key's cylinder a lot with the key itself, no kidding this can kill spark to the engine on takeoff. Try playing with the key by wiggling it side-to-side and up and down  while driving to see if the engine cuts out or stumbles at all.   

[390owner]

replaced it a while back. A friend of mine said to try changing it but did not help.

[machoneman]

Well, seems like we're both out if ideas now! 

[390owner]

That is why I think it is something inside the engine. Sounds like it could be the exhaust valves.