I have a '65 with 327/365 with recently restored engine, including new 30/30 cam. We're having trouble setting the proper timing. We set the dwell at 30 -- new points, plugs, even a new NOS 202 coil. We followed Duke' s procedure published here for setting timing for a 365 using dial back timing gun. Initially, timing was too retarded, even with vacuum advance all the way against the intake manifold. So we walked the distributor over one tooth. Then it was too advanced, even with vacuum advance all the way against the ignition coil bracket. So further research here, and then we pulled distributor expecting to find that someone before had restored distributor and inadvertently installed lower gear 180 degrees backward. Nope, indentation on lower distributor shaft lined up with rotor. We then assumed maybe we had misteakenly walked the distributor over two teeth, instead of one. Apparently not, because after walking it back again, it's too far retarded again. Kind of at a loss as to what we are doing wrong. No after market hot rod stuff here-- we tried to restore using original parts. Would appreciate any help.
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Re: Timing too advanced, then too retarded
Some replacement cams are ground with the distributor drive gears indexed differently than OEM. Although the dimple in the distributor driven gear is indexed correctly for OEM, you will now need to rotate it 180 degrees so it points opposite the rotor tip. That should cure your problem.
Larry -
Re: Timing too advanced, then too retarded
Michael
I have a 327 with an LT-1 cam, and to get it timed properly I have to move the dimple to the "wrong" position. With the dimple in the "right position, I can only get 8 degrees max initial advance with the can against the manifold. But with the gear rotated 180 deg, I can get 11 or 12 with the can midway between the manifold and the coil. So, if nothing else works, you might try pulling the pin and rotating the gear. It doesn't take long, and although counter intuitive, it might work for you.
Edit_I had not seen Larry's post when I posted this. My cam is a Federal Mogul, not OEM, so the indexing must be the explanation for my experience with this cam.Comment
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Re: Timing too advanced, then too retarded
Michael
Did rotating the gear work for you?Comment
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Re: Timing too advanced, then too retarded
Steve, I haven't tried rotating the lower distributor gear as yet. I think my distributor (which is correct and original) is out of calibration, so I'm having someone take a look at it first. I realize that the distributor is not the likely culprit for the particular problem I raised above, but I thought I should at least wait to get it back and try it out before reversing the gear.
One thing that throws me if that if the repro cam has its gear indexed backwards, thus necessitating the reversal of the lower gear on the distributor, why isn't this something that would have become well known and better documented here among insiders? In other words, one would think that literally everybody who replaces their cam would have the same issue, yet I see little reference to it here or on the "other" Corvette discussion board. I know I can probably fix it by reversing the distributor gear, but the lack of such a problem among other enthusiasts make me wonder whether the actual cause for the problem stems from something other than the cam manufacturer's failure to index the cam gear properly. I would sure appreciate hearing whether Duke has any thoughts on the subject.Comment
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Re: Timing too advanced, then too retarded
Michael
You might call the service line of the company who made your cam and ask about the indexing issue for your specific cam.Comment
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Re: Timing too advanced, then too retarded
The first question I have is who manufactured the "repro" 30-30 cam?
Reports indicate that some cams may not have the distributor drive gear indexed per GM drawings, so the driven gear on the distributor must be rotated 180 degrees to achieve proper initial timing without the VAC interfering with the inlet manifold or coil bracket.
This gear indexing has been discussed here since the early days of the TDB. I discovered it in 1966 the first time I removed the dist. from my 340 HP SWC to determine why the ignition often broke up at 4500. I talked to all the so-called "experts" who told my to reindex the wires, etc, which just made the problem worse. It was probably the third time I removed it and counted 13 teeth on the gear and noticed the dimple that I had a Eureka moment!
And it's certainly no help that to the best of my knowledge, no GM service publication mentions proper indexing of the distributor gear. It's only noted on the assembly drawings. So we're left to figure it out for ourselves, and there are many including so called "professionals" that are clueless about this issue.
As far as your distributor calibration is concerned, you can easily check the centrifugal and vacuum advance against specs listed in the '65 Corvette Shop Manual Supplement and AMA specs with a dial back timing light and Mity-Vac. I don't recommend any changes to the OE spark advance map for 365/375 HP engines because it is essentially ideal as is - one of the few OE engines whose spark advance map can't really be improved from the original design.
It's also been pointed out that some replacement distributor gears have no dimple at all, so it's a matter of trial and error to get it properly indexed, but since yours had a dimple, it's probably an GM part.
So let's identify the manufacturer of your cam.
DukeComment
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Re: Timing too advanced, then too retarded
Thanks for responding, Duke. I knew about your discovery many years ago of the mis-indexing of the distributor drive gear from reading the Archives here -- that's how I knew to check for that as the possible culprit when we removed the distributor.
As for the 30-30 cam, we replaced the original with a Speed Pro CS 118R, which we understood to be a correct replacement.Comment
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Re: Timing too advanced, then too retarded
The CS-118R should be an exact duplicate of the OE ...346 - manufactured to the GM drawing. This is the first time I've ever heard that a Federal Mogul cam might have the distributor gear indexed incorrectly, and many have bought OE replacement cams from F-M.
In any event, reindexing the distributor gear 180 degrees should solve the problem.
DukeComment
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Re: Timing too advanced, then too retarded
One could also consider the shaft of the distributor if replaced. Consider the cross drilled hole is not set correctly as a few degrees off from setting up the dimple in line with the rotor. Also the gear tooth is equal to 27 degrees of distributor position. Remember the crank to distributor is a 2:1 ratio.Comment
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Re: Timing too advanced, then too retarded
Just a side note.. Here in canada to be politically correct..the article should be labeled " timing challenged ".Comment
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Re: Timing too advanced, then too retarded
Good reasoning if the hole in the shaft is in spec position...........!Originally posted by Dave Perry (19643)Comment
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