General Tech TR4A Runs Terrific

[KVH]

It's a joy to drive, and I'm so lucky. We all are. But, while the plugs look good, I puff black smoke on shifting at 3000+ rpms. Perfection eludes me, but I'll keep looking. I could replace my jets, but that just seems like such a shot in the dark. I could conceive of a situation where I got off by one link on the timing chain on rebuild, but I don't think I did. However, two points: a) my distributor vacuum unit sits parallel to the block whereas most point several degrees left; and b) even if I were one link off, the timing is dead on, and I think you can compensate for being a link off, given the flexibility to adjust timing at the distributor. If anyone has a thought (I've troubled you all about enough) I'd like to hear it.

 

[TR3driver]

The timing chain also controls cam (valve) timing; while twisting the distributor only adjusts ignition timing. Having the cam timing off can really mess up the mixture, due to an effect known as reversion. Basically it's a pulsation of air through the venturis, such that the air picks up a triple load of fuel (one on the first time going in, another coming back out, and a third when going in the second time).

Checking valve timing is pretty easy (with a stock cam). Basically you set the valve lash to .040" or .050" (doesn't matter as long as both are the same) on either cylinder #4 or #1, then turn the crank to TDC between intake and compression stroke on that cylinder and measure the resulting gap at each valve. If it's the same, the timing is right on. Or turn until the gaps are the same, and check how far off the timing mark is. (Assuming of course that your timing mark is accurate. Verifying that is another task.)

PS, I forget offhand how many teeth it has, but how the drive gear is meshed with the camshaft determines where your vacuum advance winds up with the ignition timing properly set. IOW having that right is no proof that the cam timing is right.

 

[Geo Hahn]

...a) my distributor vacuum unit sits parallel to the block whereas most point several degrees left;...

As Randall noted, that is a function of gear engagement. Mine pointed to far 'to the left' to suit me:

So I pulled the shaft and re-engaged moving it by one tooth:

Now it more or less matches the Practical Hints photo:

Good to know the technique for verifying timing gears - worth doing for your piece of mind.