'68 Mustang dwell keeps decreasing

[29 Chandler]

My 1968 289-2v Mustang with a functioning dual-diaphragm distributor lowers its dwell as the engine warms up. As the dwell approaches 14 degrees (normal is 24-29)the engine starts to hesitate. If you are unfortunate to be driving at this point the engine dies and will not re-start. If I let the engine cool and not try to re-start until she is cool the engine will fire right up.

Possibly those with more experience could offer me some trouble shooting advice.

Thanks!! <img src="http://www.aaca.org/ubbthreads/images/graemlins/confused.gif" alt="" />

[oldford]

How do you know the dwell angle changes? If you use the timing light, does the spark advance as you rev the engine? I have seen cases where the crank shaft damper causes a mis-reading on the timing mark. Let me explain... The damper is a two-piece iron pulley that is joined by a rubber inner core. This inner core can separate from the iron halves and cause one iron half to 'spin' slightly from its correct position. This will cause the timing mark on the pulley to move from its correct position. If you are seeing the timing mark move and give you the impression the dwell is changing, it may move when hot and then go back when cold. If your timing is off to begin with due to the incorrect mark position, all bets are off. Try to time it by ear and see where the mark is. Your ears are better than you think..... Of course, you could get a new damper and see how it effect your timing.....

Frank

[29 Chandler]

Thanks for the tips Frank. I had my dwell meter on the car as it was warming up. When it hit 14 degrees it shut off. The harmonic balancer should be good it does not have many miles on it. You are right about my ear I always check the timming by ear. I tried to adjust the timing to see if it made a difference by it still stalled out. I did find the arm that advances the timming was a bit askew. I fixed that and now the dwell if steady and whithin spec. BUT the engine still will not stay running for extended periods. A few minutes is the max! I was concerened about the carb so I rebuilt it. No change in the behavior. What next?? <img src="http://www.aaca.org/ubbthreads/images/graemlins/confused.gif" alt="" />

[Dave@Moon]

A decreasing dwell would normally indicate that something is increasing the point gap. What kind of points are you using? Are you sure they're what the box said they were, and not counterfeit? Cheap points with either a rubbing block that flexes or a mounting plate that warps when warm might be the problem.

Is there enough dielectric grease on the lobes for the rubbing block to prevent heat from being built up there? If that block is getting hot, you'll have this problem even with good points.

Another possibility is that the points aren't secured tightly enough to the distributer plate. With heat the screw could allow the point gap to slip due to the tension of the spring, and (for now) it's retightening itself as it cools. It's weird, but maybe it could happen!

I'd try switching to a different set of points, especially a different brand. My Falcon always seemed to do better on Borg Warner brand points than any other I tried, including the Motorcraft.

If there's no change then maybe you have a structural probelem in the distributer, like a worn bearing, that's allowing the shaft to move when warm. If that's the case the spring/rubbing block on the points would be pushng the shaft away, effectively increasing the point gap.

Good Luck!

[oldford]

If the dwell angle is stable after you adjusted the timing rod, I'd stay with troubleshooting the distributor. My old distributor was shot so I replaced it with a factory rebuilt unit. Yes, you can still get them at auto parts stores that are not the 'fast food' chain stores. I paid $50 for mine and the problem went away. Now that I think about it, I bought two. I ordered them from two different stores, thinking that one would not come through, and they both did. I didn't have the heart to ask one to send it back. Send me your address via private email and I'll send it to you. If it works, pay me the 50 bucks and if it doesn't, send it back. It still sounds like the distributor.

Frank

oldford@frontiernet.net

email editied by FAI

[critterpainter]

check your breaker plate, on fords when they get worn they sort of scoot sideways thus changing the dwell before rotating as the vacumn advance starts rotating the plate. This can make a noticeable "bog" in acceleration or i guess cause the problem you are haviing.

One way to check is to use a vacumn pump to advance the dist while holdiing the engine speed constant. if the dwell shifts before the timing changes check (or chuck) the breaker plate.

Bill