Rockn-Roll

My idle started getting rough and I was getting a lot of backfire even though timing seemed to be correct. I finally got use of the garage (my parents are on vacation). I started by checking plug wires and then pulled cap. I was able to move the rotor easily about 10%! There was no resistance like with mechanical advance! I pulled the distributor and found no play in the shaft or distributor gear...in fact the mechanical advance was rusted and not moving at all. The play must be from either wear in the meshing of the dist gear with the gear on the cam, or the gear on the cam is loose from maybe a bent key or other gear retainer.

I don't have my repair manuals with me...how does the gear fit onto the cam? Does it slip onto the cam in a keyway with a soft metal key, or is it splined, or maybe a pin. Any ideas before I pull the intake manifold to check it out?

Thanks

1979toy

The gear is cast with the cam and the gears are broached in. Nothing there to move. Go back and check the distributor again. Look down the hole from removing the dist. at the cam gear to see if the teeth are worn off. It has to be one of those 2. If the timing chain were worn it would not turn so easy that you thought that it was disconnected.

Tom454

It is normal for the rotor to rotate a few degrees. This is the correct relationship between the parts. This is controlled by the the mechanical advance limit slot & bushing. When you rotate the rotor these few degrees, the distributor gear does not turn, and it springs back. This is the action of the mechanical advance springs. The distributor gear is retained on the distributor shaft via a "roll pin". The cam gear is cast and machined with the cam. Excessive distributor shaft end-play allows the distributor shaft to rise up and fall down at will as it is being driven by the cam gear. This will cause unwanted timing variation. As the distributor moves up/down, the relationship/position between the gears changes and this changes the timing. .005 to .008 end-play is acceptable. Less than .005, and you risk galling due to heat expansion. More than .008, and you start noticing significant timing variation.

Check your timing chain & gears- This test is much easier if you remove the spark plugs first.

Remove the distributor cap... used a 1/2" ratchet or breaker bar and a socket.. rotate the engine in one direction until the rotor just begins to move. Now, take up the slack in the ratchet/breaker bar/socket, and rotate the engine in the other direction SLOWLY and observe how much bar movement there is before the rotor begins to turn back the other way. If there is a lot of bar movement before the rotor begins to turn, the timing gears & chain are toast. The weight of the distributor main shaft etc keeps the dist gear & cam gear in mesh while doing this test, so any observed "slop" is directly caused by timing gears & chain.

If necessary, replace them with a "true double roller" set which will last longer and stretch less than a standard set.