Timing Cam is Moving(?)

[TheFargoA]

Hello,
My A drives great for 10 miles or so, then it begins to run rough and slowly heats up. Always, I shut engine off, crank it to #1 TDC, then find that the timing cam has moved a degree or so clockwise (allowing .018 gap between points). This tells me that the timing cam is slowly I moving while I drive, thus advancing my timing. Here are some facts (which add to frustration): (1) ignition, switch, ammeter, circuit, reducer (12 volt conversation); coil (new); popout cable; distributor plates, consdenser, and points come out perfect during tests (engine off and running tests). Distributor cap is clear, and the visible sparks inside are blue and strong, and so is the spark to the plugs. Plugs remain clean. The visible sparks in the distributor appear strong, but they do run back and forth (right to left) along the contact points. Symptoms described are not associated with any type of driving (highway, in town, climbing hills, etc., are all great for 10 or so miles, then it creep warm and runs bad. When engine acts this way, I pull over, reset timing (bump camp up 1 degree), and in less than 3 minutes the engine is 100 percent well performing and running cool - for another 10 or so miles. I tried 40 pounds on the cam screw and with lock tight. I tried new cams and screws, and various types of lock washers too. Is anyone else seeing something that I may be missing? Thank you for your help!

[Sunnybrook Farm]

I had a gear that was slipping on the shaft. Put an old original in and no problem.

[TheFargoA]

Thanks for the suggestion. I went ahead and did that too. I tried one an original and a .001 oversized to mesh better with the old cam. Neither did anything different. Someone on another post said that Bill Stipe grinds higher quality distributor cams. Perhaps the half-dozen I purchased from Mac's are all bad (but that would not explain the 10 miles of great driving). Thanks again for your comment.

[SteveB31]

Years ago, upper distributor shafts were made with the ring below the cam pressed on the shaft, rather then as part of the shaft. This is likely your problem.

Remove the distributor, remove the pin that holds the shaft in the distributor, and pull out the shaft. Look then to see if that ring is a separate piece. If it is, buy a new shaft made with the integral ring and problem solved.

Steve @ Berts

[TheFargoA]

Thank you for this insight. Yes, mine from Macs appears to be pressed onto the shaft. My old one (not original though) appears that way too. Are you suggesting that the ring on which the cam rests is moving around? If I have trouble finding an integrated shaft, do you think it may be okay to make a spot weld of the ring to the shaft to solve this issue? Thank you again for this interesting idea.

[Bob C]

Sponsored Links (Register now to hide all advertisements)

This is one I had with the ring missing.

Bob

[SteveB31]

They are around $11 and readily available. We can mail you one anytime.

Welding it would be difficult

Steve @ Bert's
Modelastore.com

[TheFargoA]

This is certainly not what I have. I went to your site and searched the part but could not find it. I did find a USA cam and ordered a catalog. What would be a good search querie for the distributor shaft part? Also, I know have a one-piece distributor shaft. Is yours a one-piece? Thank you.

[katy]

A-12178-D Upper Ditributor Shaft- Drilled $11.50

http://parts.modelastore.com/show_Product.asp?ID=4215

[redmodelt]

You will also need some thrust washers, pin to rivet the collar on, the collar and the intermediate shaft.

[Mike V. Florida]

Originally Posted by TheFargoA

This is certainly not what I have. I went to your site and searched the part but could not find it. I did find a USA cam and ordered a catalog. What would be a good search querie for the distributor shaft part? Also, I know have a one-piece distributor shaft. Is yours a one-piece? Thank you.

Can you post pictures of what you have?

One piece is "modern", two piece is like original. Berts sells the original style upper and lower shaft not the one piece shaft.

[TheFargoA]

I should clarify. I now have the two piece shaft: the upper piece from the distributor through the head, and then the second pice from the head to the oil pump. The shaft shown on the the ModelA store website looks like the one I have now (the plate looks pressed on). I have not uploaded a photo yet, and it may take a week for that response. I am also going to try to rotate the existing plate with the shaft in a vice, and see what it does. If it moves, I will give Jerry's team a call. This sounds more plausible than anything else I have tried. Thank you!!!

[Jim/GA]

Having the ignition cam move while the engine is running is a known problem. The screw bottoms out before the cam is locked down tight. Add another washer or lockwasher under the screw head and the cam will be tight.

[Tom Wesenberg]

Originally Posted by Jim/TX/GA

Having the ignition cam move while the engine is running is a known problem. The screw bottoms out before the cam tight. Add another washer or lockwasher under the screw head and the cam will be tight.

That was my first thought also, but Steve from Bert's brought up a good idea that the ring may have been pushed down because it's a separate part and just pressed onto the shaft.

Best idea is to be sure you only buy the one piece shaft, and by "one piece" I mean the ring and shaft is one piece, but you still have a separate lower shaft to the cam drive gear assembly.

[tbirdtbird]

Steve at Berts has the correctly made part. ALL his stuff is good.
Not necessarily so from some other vendors

[George Miller]

Originally Posted by SteveB31

They are around $11 and readily available. We can mail you one anytime.

Welding it would be difficult

Steve @ Bert's
Modelastore.com

Maybe you could send the dist. to Steve and get it rebuilt. But for sure replace the shaft with a good one from Steve.

[Joe K]

Is anyone else seeing something that I may be missing? Thank you for your help!

It may be a rebuilt distributor? (given all the rest you have done this is likely)

The usual line reaming the two distributor bearings can leave "high spots" which can quickly wear down and leave the shaft with a little "wobble." (better than reaming is to have a machine shop use a Sunnen Hone on it for sizing.)

The wobble can cause problems with timing, particularly with the engine in operation as opposed to setting timing by always going "forward."

A dial indicator can be taken to the shaft/cam with the engine stationary to check again for accumulated looseness. (which doesn't go away once worn.)

Joe K

[larrys40]

Another thought is that maybe you have an old 2 piece distributor oil pump/ distributor drive gear assembly and the gear is spinning on the shaft slightly , which would account for your retarded cam movement .
There were lots of these poor repros back in the late 70's and 80's that were out there. The originals are one piece as are the new repros .

You have to pull the distributor , valve cover and return line to access . Not a big deal but would be my guess if try cam is tight on the distributor shaft
Larry Shepard

[TheFargoA]

Thank you all for your suggestions. This is the first substantial discussion on this topic I have seen. I will make some changes in the next week and let you all know how it goes. I will also look into the parts available with Steve. Thank you again.

[Jacksonlll]

Make sure your point gap is not changing. Sometimes that block will not lock on the adjustable point in the original design. Easy fix.