Ive been having trouble with my truck decelerating correctly (see 'Stumble on Deceleration' thread). Ive replaced both the vacuum mod and the dash pot with no results. Advancing the timing seems to help, but not enough.

As I was messing with the timing again today, I noticed something. As I rev up the engine, the spark is advanced. As the engine decelerates, the timing slowly goes back down and then goes BELOW the idle setting. I was seeing a good 2-3 degrees of extra retardation! I pulled the cap off my dist and was able to twist the dist shaft a surprising amount. Putting 2 and 2 together, the gear must be worn down and so the timing gets thrown off on deceleration, making the truck stumble.

Before I go spending $50 on a new distributor, Id like you kind folks to confirm my theory/diagnosis. I also have 2 questions:

1. Any pointers/things I need to know about putting in a new dist? Ive never done this before.

2. Autozone lists 2 dists, one with single vacuum and one with double. How do I know which one I need? If ya'll can set me straight tonight, I can call over to Autozone and get the part tomorrow after school.

Thanks in Advance

 

You will be able to move the rotor a bit as you are turning it against the centrifugal advance springs. If you have 2 vacuum line going to the vacuum advance it's a dual port. I think Ford had a decel check valve years ago that solved this problem. But, not sure.

 

Just went out a messed with it some. If I push the rotor against the advance springs and let go, I can still push the rotor back(retard it) a little. Maybe the advance springs are just worn out?

 

Anyone?

 

Going over your 1st post again, if the timing get retarded when you let up on the gas, you probably have a dual port vacuum advance. One of those emissions deals. Try plugging the retard port - usually nearest the dist.

 

What gear are you talking about that you think is worn down ?
I've never seen a advance spring wear out.

 

Theres only one vacuum line going in

 

The distributor gear.

 

This is really beginning to bug me. A whole checklist gone through and still nothing.

 

Lemme throw my nickel on the decel problem:

The terms "rotational momentum" come to mind.

Since the dizzy is geared off the cam and the cam driven off the timing chain, and the timing chain driven off the crank - could the acceleration (timing and advance) be okay but timing is lagging upon deceleration? I mean, engines are synchonized in the direction of revolution. However upon deceleration, the crank speed is decreasing and the cam & dizzy also need to maintain that same deceleration rate but cannot due to slack in the timing chain?

I'm okay if you kill my theory but it just sounds to me that you're at your wits end on his problem. Whaddya think? Possible?

 

Ohh, big spender. HIO says screw $.02, im pitching in $.05


Thats sorta the way I was thinkin', but I turned the crank back and fourth by hand while watching the rotor and there didnt appear to be any play.

BTW, the PO said he put in a new chain 4000 miles before I bought it, so 5,000 miles ago. All new bottom end too If this problem lasts much longer, I may call him up and see what his thoughts are since he did spend 14 years with the thing.

If its any correlation, the PO had removed the EGR and retimed the engine. I drove it for a while and then put a new one on a reset the timing. It ran good for a few months like that though.

 

It sounds like it's doing it when the vacuum advance let's off. The advance unit is adjustable with a small Allen wrench in the vacuum line hole. Be sure to count your turns either direction so you can put it back were it's at, if it doesn't help. Does it drop below if the vacuum is unhooked??

 

Doesnt the vacuum advance let off on acceleration? Ill try it tomorrow morning though. Yeah, the vacuum advance if definitely working.

EDIT: stuck an allen wrench in the vacuum line hole, went in without hitting anything.

 

Ask him if he re-curved the distributor's advance mechanism which is sometimes necessary to do when the EGR is removed to cure pinging (pre-detonation) on low-RPM engine load.

 

Will do. How would I go about resetting this if he did?