It's all about the Timing

I have a 78 roadster that first and for most runs great right know. It gets 30+ mpg on the highway and idles for ever around 800 rpm. I looked at all the plugs and they are in great shaped, gapped perfectly and a light bownish color. I am using a Pertronix LU-141 ignition.

I got the brilliant idea over the weeked to look at the timing more out of curiousity than anything. What I found was the marks on the crankshaft pulling lining up with nothing. The attached picture shows my car and the timing marks on the timing cover. The little silver line on the cover shows where the car actually times to, and again, runs great.

Just for the sake of it, I retarded the timing to the manual recommended 10 dgrees BTDC @ 1500 rpm and she ran awful! I put it back where I found it and all is well again.

Can anyone explain how this can be? Did a PO do something I should at least be aware of? Are there issues?

Shocked I haven't gotten even one opinion on this. thought I would get a bunch.

did you check(and set) the timming with the vaccum hose connected to or disconnnected from the dizzy?

Both. did it as instructed in the manual. Made no real difference as on a 78 the vacuum advance only comes into play in 4th gear.

Jon, you have a pm

Mine is off also. I manually brought #1 piston to TDC on the compression stroke and then marked the pully at the 0 degree mark. I now time the car from the new mark on the pully.

The pulley can "shift" with age. Pretty common - check prior posts. I think there is a rubber center on the harmonic balancer that can age and slip. Also, your timing cover may be installed slightly off. Mine runs best at 18 degrees BTDC. PO had it at 23.

See the following:

http://www.mgexperience.net/phorum/read.php?1,1211131,1211131#msg-1211131

James has noted the proper method of checking for outer-pulley slippage.

That pulley is becoming notorious for slipping. The earlier style is not as bad.

Ditto on al the notes concerning shifting.

There is at least one dependable outfit out in California that rebuilds the dampers. I can't remember who it is, but Hap will know. PM him for contact information.

The outer ring of the damper represents a rather large mass and it is, as Dave pointed out, sitting on some fairly old rubber. The combination of the steel mass, the natural vibrations of the engine, the aged rubber, having the belt tight over a long period of time, and perhaps the driver's tendency to change revs abruptly will cause the outer ring to shift.

It is unlikely that the thing will fly apart under normal use, but no one has a crystal ball.

If you send it off and have it fixed, take the time to check the positioning of the mark using a degree wheel when you install it, just as you would if you were degreeing in a cam. It would be a shame to go to all of that effort and expense and end up with the mark being in the wrong realtionship with the pointer.

One of Jeff's degree/timing tapes would be of great benefit in doing this.

Jack

The downfall is that the timing tape for the late model dampers has to be applied to the face of the pulley, so its difficult to see and not very effective.

This guy is fast, cheap, and great at rebuilding dampers:
http://www.dalemfg.com/harmonicbalancer_018.htm

Or:
Damper doctor
damper dudes

Thanks for the comments. I am a real newbie when it comes to this part of the car.

I can see the idea of the pulley shifting, it does not appear to be keyed in place? Looking at a blow up of the area, the bottom timing chain sproket looks like it is keyed in, but not the pulley. I can see how this will shift.

I am not familiar with the harmonic balancer. Where is this located? The bottom pulley that the timing mark is on is what appears to be steel? What am I missing.

I really like my Moss supercharger aluminum flat belt pulleys. [w/o supercharger]
No slip!

There's a rubber layer sandwiched between 2 steel layers. That layer will allow the outer ring to slip.

Crankshaft pulley is keyed to crankshaft. Think of the pulley as two doughnuts of different diameters outer one larger than inner one.

They are connected together with the aforementioned rubber that detiorates over time...

I found the rubber part and can now see what you all are talking about. It makes perfect sense the the outer has slipped on the inner. Probably will leave that until I need to take it off for any reason. Not an issue at this time. Thanks for the comments.