Bouncing timing mark

[JG1966]

The timing mark on my 340 just started bouncing when I try to check total timing. It holds perfect at 18* at idle but when I get to about 2500-3000 rpm it bounces so far, the timing mark goes out of view. Any suggestions? I recently put in a light spring in distributor in exchange for heavy green stock spring in stock Mopar distributor. I'm going to pull distributor and see if maybe spring came undone. What would cause bouncing timing mark only at higher RPMs? It's perfect at idle. Thanks.

 

[Daves69]

Wayward spring or a loose plate I would think could be possible causes.

 

[JG1966]

Well, I pulled distributor. Everything looks fine. I put the heavy green spring back in just to see if that did anything and it's still bouncing when I get to about 2500-3000 RPM. The engine is running fine but don't like the bouncing mark.

 

[Fran Blacker]

Check the distributor shaft for side to side looseness. If shaft is sloppy timing will bounce around.

 

[Daves69]

There is a "local" rebuilder on the forum if it come to that...........
http://www.forbbodiesonly.com/moparforum/threads/rebuilt-distributors-for-sale.131170/

 

[JG1966]

Check the distributor shaft for side to side looseness. If shaft is sloppy timing will bounce around.

But wouldn't it bounce at idle too? It doesn't act up at all until higher RPMs.

 

[Daves69]

Does it have vacuum advance also. Try disconnecting?

 

[JG1966]

Vacuum advance is always plugged. It just started doing this. Only thing I changed since I last timed it (and it wasn't bouncing around) was swap carbs from Edelbrock 750 to a AED 750 and changed out plugs and wires.

 

[Fran Blacker]

But wouldn't it bounce at idle too? It doesn't act up at all until higher RPMs.

It's an easy check. The faster it goes the more unstable it is. It's not like a spinning top.

 

[JG1966]

It's an easy check. The faster it goes the more unstable it is. It's not like a spinning top.

Ok. I'll look at it. I just pulled distributor to check springs and shaft didn't seem loose. I'll yank it again.

 

[Fran Blacker]

Did this just start or did you just discover it? No vacuum advance, any chance the plate is moving? In a Mopar Action article Rick Ehrenberg didn't like some of the Mopar Performance distributors the ones without the felt in the shaft if my memory is ok.

 

[JG1966]

Did this just start or did you just discover it? No vacuum advance, any chance the plate is moving? In a Mopar Action article Rick Ehrenberg didn't like some of the Mopar Performance distributors the ones without the felt in the shaft if my memory is ok.

It just started. I checked my timing a week or so ago after I put in lighter spring. FWIW, I have the stock distributor (with the oil felt in the shaft). Only changes since then is carb, plugs and wires (which wouldn't seem to cause timing mark bounce above idle).

 

[Fran Blacker]

I'm out of idea's.

 

[JG1966]

I'm out of idea's.

I appreciate your help. I'll figure it out and keep updated. It can only be so many things.

 

[Cranky]

A timing chain with a bit of slack will show this very same thing....even after the chain has not that many miles on it. I've run into this many times and the engine was still running excellent. If the engine is running good other wise and it's not misfiring or stumbling all over itself, enjoy it.

 

[Fran Blacker]

Check the gap on all 8 points on the reluctor ring and pickup. Don't know a spec. on how much they vary a couple thousandths? If more? This my last idea, I hope. What said make sense.

 

[JG1966]

A timing chain with a bit of slack will show this very same thing....even after the chain has not that many miles on it. I've run into this many times and the engine was still running excellent. If the engine is running good other wise and it's not misfiring or stumbling all over itself, enjoy it.

So a timing chain with slack can be steady at idle but maybe not at higher RPMs? Is that right? I can check for slack by turning engine by hand from 0 on timing mark and looking at rotor movement, correct?

 

[JG1966]

Check the gap on all 8 points on the reluctor ring and pickup. Don't know a spec. on how much they vary a couple thousandths? If more? This my last idea, I hope. What said make sense.

I have the reluctor ring and pickup gapped at .008.

 

[Cranky]

So a timing chain with slack can be steady at idle but maybe not at higher RPMs? Is that right? I can check for slack by turning engine by hand from 0 on timing mark and looking at rotor movement, correct?

Yup....but you will find some slack. How much is the question. Even a brand new engine is going to have some clearance on all of that.

 

[Daves69]

A little outside the box maybe. Any possibility your picking up spark scatter off another terminal/wire? Have you tried a different timing light?