390 Compression Test & Crankshaft Question

[TEL73]

Hello all,

I have a 65 Thunderbird with what is supposed to be a 390. I do know that the motor has been replaced at some point in time. The engine is extremely weak. Car struggles to spin the tires even on grass. I performed a dry compression test on the warmed motor with carb fully open. Readings were all really close but extremely low (84-91). The 6 month old spark plugs all look good, no sign of oil fouling. Car doesn't use any oil. Before I spend anymore time doing more testing I have a few questions. (I know very little about the FE engines)

1. Could their be any weird mismatched combination of parts that could result in compression being so low?

2. What web is the casting number located on the crankshaft?

I want to make sure this is actually a 390 before I go any further and I think the next step is to find out what the crankshaft is. Since I cant pull the pan without raising the motor I'm considering draining the oil and inserting my scope camera into drain hole to try and see the casting number on the crank. What piston would need to be at TDC for the web with the casting number to be at the bottom?

Thanks

[fastback 427]

With that low cranking number, sounds like it's worn out. Find TDC, take a welding rod, wooden dowel or the like, make a mark on it, then turn the engine over till it's at the bottom of the stroke and make another mark. Hopefully it comes in at 3.78.

[machoneman]

It's likely worn out.

http://www.gregsengine.com/cylinder-compression-testing.html

Odd though that an engine with badly worn rings should be pushing out lots of crankcase pressure and oil from the breathers. That and shot valve seals/worn guides and/or a worn out valve job should show lots of oil on the plugs and overall oil loss.

Don't any of these conditions exist?  Very odd if they don't.

One suggestion: badly worn cam lobes that minimally open the valves can also result in very low compression readings, even though it again would be odd if all 16 lobes, or at least all eight intake lobes, were equally worn down. A leak-down test would be my next step as it will determine if the rings are holding compression and/or the valves are seating well against the valve job.   

[Heo]

Wornout camchain maby

[jayb]

Heo may be on to something there; if the timing chain is worn out and jumped a tooth, that would help to explain the low compression test numbers.  I have never seen compression test numbers that low before.  Does the engine start OK, or does it need a lot of cranking before it fires?

+1 on using the welding rod to check the stroke; much easier than trying to look at the crank, and I don't think all FE cranks are marked anyway.  A 352 will have a 3.5" stroke, a 390 will have a 3.78" stroke, and a 428 will have a 3.98" stroke.  For the accuracy that you can mark on a welding rod, figure, 3-1/2", 3-3/4", or 4".  Doing this is easier than it seems, and a lot easier than trying to read the crank.

On the timing chain issue you could check the cam timing.  If you can put a timing tape (<$10 at Summit) on the harmonic balancer so that you have 360 degrees marked, and then pull the right side valve cover and put a dial indicator on the valvespring retainer for the #1 intake valve (second from the front on the right, or passenger, side), you can watch the dial indicator and tell at what crankshaft degree the intake valve starts to open, and at what degree it closes.  Post those numbers and we can probably figure out if the chain has jumped.

[TEL73]

Thanks guys for all the helpful suggestions. I'll check those this afternoon. As far as cranking; it cranks almost instantly. Idles a little rough though.

[machoneman]

On the timing chain, an easy check. Pull the fuel pump, get a flat blade screwdriver and a length of steel coat hanger. Cut a piece out of the hanger and bend one end into about a 1/2-1" "L" shape. Push the screwdriver into the timing chain links through the fuel pump hole. The chain if good should only move in a tad. Then, hook the backside of the chain with the coat hanger. Pull and see if there is play....should again be just a tad. You may be able to do both at once as well...in/out to see if there is a lot of play, which of course is no good!. Careful when re-installing as the cam that drives the pump lever on the front of the upper timing chain may be on the high side. If so, crank the engine a tad to rotate it a bit to get to the cam's low side. Ford's did btw use the infamous nylon coated 'silent' camshaft gear in a lot of engines that is known to shed teeth (retarding the timing a bunch) with old age. Do you remember if it has a the brown colored teeth on the cam gear or is it all steel? See pic of a destroyed Chevy gear but note that Ford used the same crappy gear as well.

http://www.thirdgen.org/forums/tech-general-engine/422869-why-you-never-use.html

[ScotiaFE]

1. Could their be any weird mismatched combination of parts that could result in compression being so low?

Yes you could have a weird mismatch of parts that could result in compression being low.
I'd do the stroke check and get the head casting numbers.
If it runs the timing has to be close enough to get it to start. A tooth out and they tend to just cough and spit
fire out the carb. jmo

[jayb]

Agree with Howie, if it starts OK its probably not the timing chain.

[Heo]

Take of the dist.cap and rotate
The Engine one way so the rotor
Moves .Rotate the Engine the other
Direktion and the rotor should rotate
The other direktion almost emedeatly
IF you can rotate the Engine Much
Before the rotor change direktion
There is s slack on the chain

[chris401]

Timing chain was my first thought all so. Your static compression is too consistant to suspect worn engine first. What is your vacuum reading? My guess its around 9" or less.

[TEL73]

The compression test readings were incorrect. To make a long story short the quick connect fitting was the problem. Here are the new results from testing. Dry comp test all cylinders around 120. Leak down test was around 25% to 30%. Vacuum was 18. Fuel pressure 7.5lbs. Timming is @ 10. Power is just not there though.

[Nightmist66]

7.5psi is enough to hold the needle and seat open. I like to run around 5.5-6psi for street and strip. Which pump are you using, I'm guessing electric?

[TEL73]

No, this pump is mechanical. It's definitely not the correct one for the 65 thunderbird. Not sure what vehicle this motor came out of. Heads are 68 models.

Also checked the stroke and it is a 390.

[chris401]

No, this pump is mechanical. It's definitely not the correct one for the 65 thunderbird. Not sure what vehicle this motor came out of. Heads are 68 models.
As for non-performance stock compression: Your static compression looks like a truck 390. 352 is usually 100-110 360 ? How does it drive? If you can't drive it on the highway you may have a trash or rust restriction. The pump should maintain pressure while driving.
Also checked the stroke and it is a 390.