Timing can only be set correctly with distributor gear 180* from correctly positioned

[John M.]

The ignition timing on our 66 327, 350 HP corvette can only be set at the correct 10* DTDC if the distributor gear is clocked with the dimple facing opposite of the rotor tip. There is not room to set the timing correctly with the distributor gear correctly clocked with the dimple and rotor tip facing the same direction. If the VAC is pushed absolutely against the intake runner the timing can only be set to 6 or 7 seven degrees BTDC. This is driving me nuts, and have spent an unusually large amount of time trying to correct - but nothing so far has provided a solution. When the dimple and the rotor tip face in opposite directions the timing can easily be set correctly and the VAC is sitting pretty much in the center of the available travel between the coil and the intake runner. Is there something that a person can do differently? Are the teeth on the camshaft out of phase with the timing gears or has there been a change in distributor gears or a known deviation of manufacturing that can be the root cause? Have others experienced this craziness and figured out what correction can be applied? Any opinions as to how or if this can cause any problems with performance. Lots of time consumed with no solution found and am very frustrated.

[Edward J.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

John. I had the same problem with my 63 327/340 and will not have any affect on the engine, I did find that Duke and others here have found this before. I have been driving my car for 14 years. As long as you remember if you remove the dust. To set it up as you did. the intake manifold runner on the 350 cu. In. Engine does not seem to interfere, the 327 hi horse engines have higher intake runners which interfere when setting timing.

[Duke W.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

There have been reports of this in the past. It may be that the gear on the cam is not indexed to spec or the distributor gear is mis- marked. Do you know the manufacturer of the installed camshaft?. A REAL L-79 cam idle behavior is 750 @ 14-15" How does yours compare?

Another issue is plug wire indexing. Be sure it is to AIM and service literature spec; #1 should be immediately to the right (pass. side) of the cap window and then clockwise in the firing order that's molded into the inlet manifold.

Let us know what you find.

Duke

[Mark E.]

[John M.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

In answer to your questions: The distributor cap has the wires in the correct position. As to the brand or model number of the camshaft I do not know and probably will never know as the former owner of this engine has passed on a couple of years ago.
and, as far as changing the distributor one tooth, that has been attempted already, the results being that to get the timing close enough to let the engine run the VAC must be pushed tight against the coil, and the timing was advanced to the point of "push back" from the starter.
the darn thing will just NOT let me set the timing properly with the distributor gear indexed as it should be with the dimple on the gear aligned with the tip of the rotor. Even as the math indicated NO GO, I tried it anyway, and there simply is not enough room for rotation of the distributor in the space between the coil and the intake runner. The car will run well and with correct timing IF the ignition wires are all moved counter clockwise one position on the cap and with the gear and rotor in prescribed alignment, and trying to correct this seemingly minor problem brought this quandary to light. A simple task, I said to myself! Well weeks have past, hours spent, and no positive results! Must have had the distributor in and out ( no exaggeration) maybe a dozen times and tried every way to get this correctly fixed. I can find some comfort knowing others have been up against this curiosity before. Any known remedy or cause?

[Mark E.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

Another issue is plug wire indexing. Be sure it is to AIM and service literature spec; #1 should be immediately to the right (pass. side) of the cap window and then clockwise in the firing order that's molded into the inlet manifold.
Duke

To Duke's point, wire indexing is the third variable affecting the range that timing can be adjusted with the available adjustment arc of the distributor housing. Those three things being: which way the gear is oriented, how the distributor gear is oriented with the cam gear, and wire indexing in the cap. See my prior post regarding the first two.

[Mark E.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

In answer to your questions: The distributor cap has the wires in the correct position. As to the brand or model number of the camshaft I do not know and probably will never know as the former owner of this engine has passed on a couple of years ago.
and, as far as changing the distributor one tooth, that has been attempted already, the results being that to get the timing close enough to let the engine run the VAC must be pushed tight against the coil, and the timing was advanced to the point of "push back" from the starter.
the darn thing will just NOT let me set the timing properly with the distributor gear indexed as it should be with the dimple on the gear aligned with the tip of the rotor. Even as the math indicated NO GO, I tried it anyway, and there simply is not enough room for rotation of the distributor in the space between the coil and the intake runner. The car will run well and with correct timing IF the ignition wires are all moved counter clockwise one position on the cap and with the gear and rotor in prescribed alignment, and trying to correct this seemingly minor problem brought this quandary to light. A simple task, I said to myself! Well weeks have past, hours spent, and no positive results! Must have had the distributor in and out ( no exaggeration) maybe a dozen times and tried every way to get this correctly fixed. I can find some comfort knowing others have been up against this curiosity before. Any known remedy or cause?

John,

From a functional stand point, if the distributor housing is oriented in a way that allows proper timing, everything is fine.

If you want to make sure it's installed as per the factory, as Duke suggested, begin with making sure the spark plug cables are oriented in the cap as per AIM. AIM also shows the expected orientation of the distributor housing in relation to the engine (this will vary a bit due to timing setting). To achieve this, you have two adjustments: the orientation of the distributor gear on its shaft, and the orientation of the distributor gear with the cam gear.

Again, if achieving this requires installing the dimple 180 degrees from the rotor tip, that's fine.

I have a hypothesis that the parts gurus may confirm/debunk: Chevy makes distributor gears with dimples on either side of the gear, depending on the application. If true, perhaps you have a gear with the dimple on the wrong side for your application.

[Duke W.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

I've never heard of OE gears with dimples on the "wrong side", but I have seen aftermarket gears with NO dimple.

If the wires are properly indexed, and with the initial timing in the range or 6-10 degrees the VAC is about halfway between the manifold and coil bracket interference points, and the plane of the cap window is approximately normal to engine centerline, leave it alone.

I am assuming that you have what is essentially an OE Delco distributor with breaker points, if not...

Duke

[John M.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

Duke, the distributor is a Delco, the dwell is 30*, the VAC new and meets your two inch rule. The window is perpendicular to the engine center line (within 2* probably) and with the ignition timing set to the specified 10* with the VAC disconnected the distributor is within the mid 30* degrees of possible rotation. I have not been to the shop to get the vacuum reading at idle but in the past it has been well within specs, Tomorrow I will install the gear assbackwards and set the timing and report the vacuum to you. You ended your reply by writing (if not...) Is there more?

[John M.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

Duke, just checked vacuum. Timing is 9*static, VAC is hooked up and deployed, run long enough that fast idle cam is not in use, idle speed is 775 (4 speed) so in neutral, and idle vacuum is very steady at 15.75 in. to 16 in. The ignition wiring is as shown in the AIM, and on the data card for my Sun 504 distributor tester with the number one cylinder ignition wire to the passenger side of the point adjustment window. Will be happy to provide any other info anyone desires.

[David H.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

John

Has distributor been rebuilt? If so, possible distributor gear was installed 180 degrees off.

Gear has odd number of teeth. With rotor on same side as dimple, gear pin should align with gear "gap" (not tooth).

(Aftermarket with no dimple - align gear with gear tooth "gap")

Dave

[Richard M.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

Thinking outside the box.......

Has the harmonic balancer and timing tab been inspected and verified at true 0* TDC with the #1 cylinder at TDC, to ensure the outer ring has not rotated, thus creating false timing readings?


Also, another thought on the distributor itself. I once had a distributor(TI, but I suppose could also apply to a points distributor), that had a very strange issue. Duke may remember this.....

The centrifugal "football" plate had been welded back onto the mainshaft end. These are normally press fit or spin welded on, I'm unsure. The re-weld must have been done as a repair before I got the car. The car used to run fine with this distributor. Then after engine rebuild and some new tune-up parts, it would run erratically. One of the tune up parts was a new K&B rotor, with extended tip.

The re-weld was done incorrectly and not repositioned to the proper rotational position on the shaft. This put the rotor tip in a offset position with respect to the TI stator plate tips, so that at high rpm, spark scatter would be introduced and cause the engine to run terribly.

Just check that your rotor tip is properly aligned to the proper cam lobe position(tip, assuming a points system), or is it K66 TI or a aftermarket ignition?

Rich

[Duke W.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

Duke, just checked vacuum. Timing is 9*static, VAC is hooked up and deployed, run long enough that fast idle cam is not in use, idle speed is 775 (4 speed) so in neutral, and idle vacuum is very steady at 15.75 in. to 16 in. The ignition wiring is as shown in the AIM, and on the data card for my Sun 504 distributor tester with the number one cylinder ignition wire to the passenger side of the point adjustment window. Will be happy to provide any other info anyone desires.

By my ending I meant that if the distributor was something other than an OE Delco it might be a different ballgame. Your idle speed/vacuum sounds like a L-79 cam or something close.

Like I said, since everything lines up properly just leave it alone. Swapping wires around as many suggest is just falling into a black hole. I had this same issue back circa 1965 when I removed the dist. in my 340 HP SWC to investigate high RPM ignition breakup. Upon installation I could not get the initial timing where I wanted it. Initially it would butt up against the inlet manifold and moving it one tooth yielded too much initial when the VAC hit the coil bracket. The dealer was clueless as were all the small block "experts" I talked to.

Upon the second removal following the first installation I noticed the dimple and it was pointing about 180 out from the rotor tip. It was a Eureka moment. I knocked out the pin far enough to rotate the gear 180 degrees, set the roll pin, dropped it in and Voila... got the initial timing dialed in.

There was NOTHING about indexing the drive gear in the distributor assembly/disassembly section of the 1963 Corvette Shop Manual. Years ago I asked John Hinckley if he had ever seen anything about proper distributor gear orientation in ANY Chevrolet service publication, and he replied he had not and had only seen it in distributor assembly drawings.

This is still an issue over half a century later, and I'm sure there are still plenty of vintage Corvettes running around with screwed up distributor assembly/installations.

The problem when you start reindexing wires is that with most wire indexing/gear orientation configurations there is enough rotor tip-cap tower misalignment that you get drivability and/or misfire issues.

Here's a thread discussing the most recent one I diagnosed and corrected three years ago.

https://www.forums.ncrs.org/showthread.php?127930-Another-botched-distributor-installation-corrected

Duke

[John M.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

Guess I will have to live with this oddity. The engine is now running with the gear 180* from where it normally should be. The dampener was rebuilt within the last few years and has always been accurate as far as running goes. The football has never been moved and rewelded from all evidence, etc. Will have to chalk it up to a camshaft with the gear indexing out of whack and go on with life. Thanks to all of you for your reply, the information available here is so great and so freely given.

[Leif A.]

Re: Timing can only be set correctly with distributor gear 180* from correctly positi

Guess I will have to live with this oddity. The engine is now running with the gear 180* from where it normally should be. The dampener was rebuilt within the last few years and has always been accurate as far as running goes. The football has never been moved and rewelded from all evidence, etc. Will have to chalk it up to a camshaft with the gear indexing out of whack and go on with life. Thanks to all of you for your reply, the information available here is so great and so freely given.

Dollars to donuts you've got an aftermarket camshaft in your engine. When I replaced my camshaft in my '67 L79 five years ago it required rotating the cam gear on the distributor shaft 180* to obtain proper/correct timing. No so uncommon with an aftermarket camshaft. As others' have stated...leave well enough alone.