72 Distribuot installation issues

[Gary S.]

I have followed the advice of Duke and others during the installation of my Jerry MacNeish rebuilt distributor. I have
1. pulled the #1 plug & rotated the engine until I felt compression, noting that the timing mark was close to the O index and both rockers were loose.
2. rotated the engine to 4 degrees BTDC
3. marked the spot on the distributor where the middle of the #1 tower/wire is (just to the right of the dwell window when standing at the front of the car) and then rotated the rotor backwards about 1.7" (which accounts for the curve of the gear)
4. positioned the distributor in the hole, after aligning the shaft with the oil pump, with the can nipple pointing at the middle of the wiper vacuum can, right at the seam, and it dropped right in.

It wanted to fire but wouldn't so I kept rotating the distributor CCW and now it runs acceptably but the timing mark is nowhere in sight. In addition, the nipple is now pointed at the vacuum can nipple. (Hmmm, wonder what Freud would say about my descriptions and directions here? )

I am assuming I am off one tooth? I have replaced all of my plug wires with new Delco wires and spent considerable time confirming and tagging each wire so that the installation would be easy.

Is there something else I might have screwed up?

Thanks for the help as always.
Gary
ps, trying to get the car on the road before the weather gets ugly

[William V.]

Re: 72 Distribuot installation issues

Gary

Sounds like you are off one tooth and that the timing is too early? Try moving the distributor gear one tooth CCW. (same direction you rotated the distributor to get it to run.) If after doing this, it runs too late and can't time the other way, then the distributor gear is 180 deg out of phase with the rotor. There is a dimple on the gear that should point in the same direction as the rotor contact. This is per Chevy specifications. However, with some aftermarket cams the gear dimple needs to be set 180 deg off of the rotor contact.

I have a COMP CAM in my 64 and could not get it to time when the distributor gear dimple pointed in the same direction as the rotor contact. Once the gear was set 180 deg from the rotor contact I was able to set the correct timing. (the gear is now 180 off specifications) Depending on your cam you may need to rotate the gear 180 deg. The distributor drive gear on the back of the cam has nothing to do with valve timing so setting it 180 deg from specs only affects the spark timing.

good Luck

[Duke W.]

Re: 72 Distribuot installation issues

IIRC the distributor and wire indexing on C3 is DIFFERENT than C2s, so the first thing you want to look at is the '72 CSM. Also, I think there was some information about C2 and C3 dist. indexing in recent issue of The Corvette Restorer.

Assuming you install the dist. with the proper indexing for your engine you MUST rotate the dist. housing until the points just open. After the dist. seats, you have to rotate the housing until the points just begin to open to get the timing set close to where the balancer index is on the timing tab. If you are careful you should be within about 2 degrees of whereever you set the balancer index line on the timing tab, but I would recommend that you use about 8-10 degrees. If your intial timing spec is 4 BTDC you can consider this to be an emission setting. If you're off by 4 degrees it could only be zero and the engine may be reluctant to start.

Duke

[Terry M.]

It is in the Summer 2004 issue *NM*

[Patrick H.]

Re: 72 Distributor installation issues

To add,

1. Your vacuum can should end up behind your right spark plug wire support.
2. Did you note if the dimple in the lower dist gear matched up with the rotor tip when you removed the distributor? It "should" have been oriented in the same direction. Make sure that dimple/dot lines up with the rotor tip.
3. I know your distributor is original to your car. Look REALLY close on the distributor and on the intake. You'll see a small stake mark on each (looks like a very small chisel mark) where the distributor and manifold were indexed to each other when the motor was built. Line them up and you will be perfect at the 8 deg BTDC that you need. This will also put your vacuum can in the correct direction. In fact, if your motor is at TDC, using this mark will put you EXACTLY where you need to be, and you can ignore virtually everything else said here, assuming you aren't off by a tooth on the lower gear when installing (which will be obvious when the marks then don't line up).

Patrick

[Gary S.]

Drs. DO make house calls

Our resident doctor, Patrick Hulst, called me and we talked about the options of this distributor and timing issue. He called me right as I tried to start it up and heard a large back fire. We discussed the probability of the dist. being one tooth off, the wires mismarked or the firing order off. We even discussed pulling the gear off of the dist. and rotating it 180. I was able to get the car to run semi-ok with the vacuum can pointing to the #4 or 6 plug, just like a mid-year.

I decided the easiest thing was to partially pull the distributor and rotate it one tooth CW. I reinstalled it and it started like in the first second of ignition and she purrs like a kitten. Thank you Patrick, Bill V., Duke, and others. Now for a complete tune-up. Dwell, timing, and carb adjust.

Duke, or anyone else, feel free to jump in and tell me your suggestions on timing. My factory spec is 4 degrees BTDC at 900 rpm. I think it was Duke who suggested that this was an emission thing. Does anyone have a good idea on an optimum setting for performance? Right now it is purring at 8 BTDC. I actually like the idle in the 700-800 range because 900 sounds to high when it is idling. Comments anyone?

Btw, we talked about the two marks, one on the back of the distributor and the corresponding mark on the intake. She started right up (once I got the tooth thing fixed) when I lined up the two marks and the timing mark at 4 BTDC

Thank you all for your help.

Gary
72 LT1

[Patrick H.]

Re: Drs. DO make house calls

You're welcome.

I was thinking about it after we got off the phone, and the tooth thing made more sense as time went on. Glad it was the correct thought.

If you have the dist curved as per 1972 specs with the TCS canister active, then stay with 4 BTDC. If you recurved to pre-emissions specs then use the 8 and run direct vacuum from the carb to the distributor (except at NCRS meets ). I've tried a mixture of the two on my cars and never found that to work as well as "all one" or "all the other" in day-to-day driving.

Patrick

[Gary S.]

Re: Drs. DO make house calls

Re-curved to run better with pre-emissions settings. I am running straight vacuum with the TCS hose plugged and secured. It runs quite well now. No surging and accelerates like, well, like an LT1.

Thnaks again,
Gary

[Duke W.]

Re: Drs. DO make house calls

Tell me the centrifugal and vacuum advance specs, and I can make some suggestions. I don't have that data for a '72 LT-1. It will be in the CSM and AMA specs.

Also verify whether or not the engine has ported vacuum advance by checking timing with and without the vacuum can connected. If it changes little or none it is probably ported. You can also check by connecting a vacuum gage to the vacuum can signal line at idle and comparing to idle vacuum from a full manifold vacuum source like the choke vacuum break.

Also, does the engine have TCS?

Duke

[Gary S.]

Re: Drs. DO make house calls

Duke, the more I read, the more I understand. (Amazing how that works )

Reading Jerry MacNeish's distributor calibration report tells me:
1. Centrifugal advance curve total degrees: 26
2. Total advance in at: 3000 rpm
3. Set total timing at 36 with vacuum line disconected at 3000 rpm.

I will make an assumption here that my inital timing should be 10 degrees and, if I set my dial back timing light at 36, I will read 0 at 3000 rpm. Therefore, I can check my total timing at 36 degrees and work backward and it should read 10 degrees BTDC or I can set my initial timing at 10 and it should read 36 at 3000 rpm, all with the vacuum line plugged

Am I understanding this correctly? This is on a 99% stock and unmodified 72 LT1 with an original cam. Btw, he did remove the originla vacuum can and put a GM can on it that performs the same.

Thanks to all who have helped me.
Gary

[Patrick H.]

Re: Drs. DO make house calls

Gary,

Did he send you back the original vacuum can and advance springs? If so, should you ever wish it will be a short job to swap them back into place. Not that you intend to know, but as one of those anal-retentive keep-every-part Corvette owners I had to ask.

Patrick

[Duke W.]

Re: Drs. DO make house calls

Thanks for the information, but you did not answer my questions. The answers would be useful to me in order to make any recommendations.

1. What is the OE centrifugal curve.

2, What is the OE vacuum advance curve.

3. Is the vacuum signal line "ported" or full time (including idle)

4. Does the engine have TCS.

If your distributor was measured to have maximum centrifugal of 26 degrees at 3000 then (with the vacuum can disconnected) if you set the initial at 10 (and assuming you set it at a speed below the beginning speed of the centrifugal curve) the total timing at 3000 or above should read about 36.

Likewise, with the vacuum can disconnected, if you set the total timing at 36 degrees at above 3000, the initial timing should read 10 assuming you read it at a speed below the beginning of the centrifugal curve.

Duke

[Joe L.]

Re: Drs. DO make house calls

Gary-----

NOT Dr. Rebuild. He draws the line at house calls. He doesn't do windows, either!

[Patrick H.]

Re: Drs. DO make house calls

But he makes great vacuum hoses for 1970 Cutlasses. One time only.

Patrick

[Geoff C.]

I now make house calls in my NEW C6

Cummings 6 speed Allison Kountry Star that is

Geoffrey Coenen

PS Yesterday while on a house call, I even took a dump in a Home Depot parking lot.