Mechanical Tach malfunction?

[Larry T.]

I took the car out this morning and decided to open it up. I've had the car for about eight months and I have never taken it up to 6000 RPM and decided to this morning. It is an L72, M21 with 4.11 gears.

I was watching the tach closely since I did not want to go over 6000 RPM and just before it hit 6000 it jumped past 7000 and I assume bottomed out. I think I was in third gear. Instinctively I immediately let off the gas and then put it into neutral to take it all the way back down. Surely the engine did not pick up 1500+ RPM in that split second? It certainly did not seem to rev higher. I thought about taking it back up to 6000 slowly to see if it happened again, but I was a little skittish about it.

Is there anything mechanical that could have caused this. I have checked the accuracy of my tach up to about 4500 in the past using two different non mechanical tachs and it was right on. Thanks.

[Bob R.]

Re: Mechanical Tach malfunction?

The Tach in my 63 started to jump just before the cable broke. You may have the same problem. The cable usually breaks near the distributor. I think the sharp bend in the cable causes it to fail. I bought a speedo repair kit and spliced the cable together and thought I fixed it but that splice only held for about a year or 1000 miles. I ended up getting a reproduction cable. I saved the original cable but I have crawled under the dash enough times. I'll leave the replacement one in.
When I was a Carslile a couple of years ago I watched the dyno setup they had and was quite surprised to find out that these old mechanical tachs are not very accurate. Most on the dyno were off by 500 rpm.

[Jim T.]

Re: Mechanical Tach malfunction?

Lubricating the tach cable and the tach distributor gear is easy enough to do to eliminate these as a problem area. Keeping the tach gear lubricated will save a lot of money and work if it shears and takes out the gear source for imput on the distributor shaft. The specific distributor shaft for your engine is no longer available.

[Terry M.]

Re: Mechanical Tach malfunction?

Be certain the distributor is properly installed. Neither C2 nor C3 should have a sharp bend in the tach drive cable if the distributor is properly oriented.

If in doubt check your AIM for proper orientation. In those years where it was available K66 option will show the proper orientation. Of course if you have the original distributor and intake manifold, there is a stake mark made by the UAW worker who timed your engine at the plant where it was built. Even if the cam drive was replaced proper parts should still allow the use of this original timing indicator. I am not sure if a new cam upsets this orientation, but I know who will know -- and perhaps we will hear from them now that I have given out almost all of the secrets.

Don't kink your tach cables.

[Larry T.]

Re: Mechanical Tach malfunction?

Lubricating the tach cable and the tach distributor gear is easy enough to do to eliminate these as a problem area. Keeping the tach gear lubricated will save a lot of money and work if it shears and takes out the gear source for imput on the distributor shaft. The specific distributor shaft for your engine is no longer available.

Jim,

You bring up a point that is an aside from my original inquiry that I have wondered about. You mention "the specific distributor shaft" for my engine. I have tried to find out in the past what that part is and how to identify it. Can you help me with that? I know there are differnt numbers on the cams. I have one that is 45m and one that is 94. There are aftermarket shafts out there that supposedly will work, but I tend to appreciate original parts as I believe they are more likely to perform as they should.

Thanks,

Larry

[Jim T.]

Re: Mechanical Tach malfunction?

Jim,

You bring up a point that is an aside from my original inquiry that I have wondered about. You mention "the specific distributor shaft" for my engine. I have tried to find out in the past what that part is and how to identify it. Can you help me with that? I know there are differnt numbers on the cams. I have one that is 45m and one that is 94. There are aftermarket shafts out there that supposedly will work, but I tend to appreciate original parts as I believe they are more likely to perform as they should.

Thanks,

Larry

Larry taken from my May 1981 GM Corvette Parts Book the part number for a 66-67 427 distributor shaft is #1852764 (EXC. SP. H/PER. , H.D.). There are over 30 distributor shafts for all Corvettes. I did not see one for 427 SP. H/PER. , H.D.
The option you have if you do need a new distributor shaft is to order just the standard distributor shaft and have the top piece removed from your 66's shaft and remove the top piece on the new shaft. Have your shafts top piece attached to the new shaft. The top piece is what makes the difference on all of the GM shafts that were once available, the shaft is the same on all of them. The top piece is a controlling piece in the advancement of timing working with the centrifical advance.

[Larry T.]

Re: Mechanical Tach malfunction?

Larry taken from my May 1981 GM Corvette Parts Book the part number for a 66-67 427 distributor shaft is #1852764 (EXC. SP. H/PER. , H.D.). There are over 30 distributor shafts for all Corvettes. I did not see one for 427 SP. H/PER. , H.D.
The option you have if you do need a new distributor shaft is to order just the standard distributor shaft and have the top piece removed from your 66's shaft and remove the top piece on the new shaft. Have your shafts top piece attached to the new shaft. The top piece is what makes the difference on all of the GM shafts that were once available, the shaft is the same on all of them. The top piece is a controlling piece in the advancement of timing working with the centrifical advance.

I wish I knew how to identify whether or not either of my shafts are correct. And if not, how to identify a correct shaft.

[Jim T.]

Re: Mechanical Tach malfunction?

I wish I knew how to identify whether or not either of my shafts are correct. And if not, how to identify a correct shaft.

Larry a person who may be able to help is Dave Fiedler of www.tispecialty.com