What is proper method to find out if timing chain has jumped a tooth or two? My 74-390 will not rev even with wot. It dies when gas pedal is released.

 

I'm not so sure it will run if it is a couple teeth off, but you need to bring the number one piston to top dead center on the compression stroke. When you remove the distributor cap, the rotor should be pointing where the number one terminal would be.

Is the ignition points or electronic?

 

Im in the same boat, truck bairly holds 1000RPM's with the petal to the floor, and I have to play with it just to keep it running... rotated it over to TDC and my roter is inbetween #1 and #5. When it warms up enough to melt the rest of the snow off the hood I am gunna put a dowel in my #1 cylinger and make shure that my dampner hasent slipped and is fooling me...
then I get to tera apart my engine in 30-40* weather, when it aint snowing that is!!!

 

had a high school musical I had to go to today. I'm back out to the garage in a few. While dreaming about this last night, I decided that piston had to be a top of comp. stroke. If timing is off, I hope its way off so I know for sure. I'll keep you posted. Oh--the points have been replaced with Pertornix. If timing is not off or appears to not be way off, I'm going to switch out the coil and/or the ignitor. I have another truck with same set up.

 

Now I'm really confused. #1 cyl. at tdc. Dizzy is pointing to #4 cyl. Timing marks align perfectly. Dist gear and cam gear both lock great. Compression ranges from 155 to 175 lbs. What do I ck next?????

 

could your distuributor have been loose and rotated around???

 

Dist cannot physically rotate far enough for dizzy to align with #1 terminal on cap. Wires and vacuum advance prevent that much movement. Orientation of dist when I first started diagnoses is same direction that it always had been. I did rotate dist clockwise and engine sounded like it was going to rev up but additional rotation made engine backfire. Rotating dist the other way caused no start condition. I know timing chain has a bunch of slack but something caused a "sudden" runability issue.

 

That is pretty good compression, I kinda doubt the valves are out of time.

Pull the disributor, pay close attention to the roll pin that holds the drive gear. I've read that they can shear and let the drive gear slip, never had the experience myself.

 

With the dist. cap off and using a ratchet and socket rock the crank back and forth. If the crank moves and the rotor on the dist dosen't you have a bad timing chain that may have slipped. If you have to replace the timing set be sure to use only one that has iron teeth and not the OEM type with plastic teeth on the cam gear.
One way to tell if the gear set is going bad is if, when you are driving at a cruising speed the engine won't maintain a steady speed and requires you to give more gas instead of staying at a steady speed your timing set is about to fail.

 

If it was off so much that it pointed at #4 when it should be at #1, I doubt it would run.

I'd get that #1 to TDC and check your balancer reads TDC against the point first. Also at TDC, with valve cover removed, I would think either #1 valves are going to be both tight (during valve overlap) or both loose (both valves closed). #6 will be the opposite of #1 if you want to double-check. If one valve is further compressed (noticeably) than the other, I'd look at the timing chain as this may indicate the cam is substantially out.

NOTE: I havn't tried that, so I can't confirm, but take a look and report back. There are people here a lot more expert than me that can interpret what you find.

I did have a friend who had his timing chain installed 2 teeth off. Engine ran fine, but wouldn't rev over 3000 rpm. Not sure what would happen if it off 2 teeth in the other direction

 

Dist gear pin shattered allowing gear to rotate. Hole in shaft is oblong on one side indicating gear had been walking back and forth for awhile. Who sells a good quaility reman dist for points? I will be putting Pertronix from present dist in it.

 

Unless the hole for the pin is seriously wallowed out you could just drill it to the next size and install the larger pin. Pins are available from Home Depot.

 

I had the pin in my Mallory dist shear. This was due to the combination of using a HV oil pump and the fact that the dist gear is not a tight fit to the dist shaft, a weak point with Mallorys.

I'm surprised yours kept running. It must have fetched up with broken pin parts jamming, and a full turn slip once in a while. Mine died. Lucky as with the resulting zero oil pressure, I had no igntion either, so engine was OK.

Here's the fix that I did and might work for you:

The problem with Mallory Distributors, at least the YL units, is that the drive gear 1/8" roll-pin can shear. I queried Mallory about it and was told the failure is normally associated with using a high volume oil pump.

What I did to resolve the problem was to re-drill the roll-pin hole through the drive gear and dist shaft with a #22 drill bit. I then pressed in a 5/32 roll-pin (the split kind, not the rolled up kind) with the split up (down also works) and then nested a 3/32" roll-pin inside the larger one, but with the split 180 Deg apart.

Despite the fact that a loose drive gear to shaft fit is also a problem with the Mallory (you get more bending of the pin), the mod has not failed me since, at least 5000 miles ago.

 

I would guess that the combination of the HV oil pump and heavy oil or cold oil could cause that problem. Yours is a good fix, I'll try to remember it. I have run 70W oil in the past and never had that problem but my last Mallory was in a 292 Y block. I did have a Mallory Uni-Lite in a 5.0 Mustang that I gave to my son though and never a problem with either.
Hmmm, double roll pins, that's good thinking.