Distributor off a tooth

[Lawrence S.]

So I have been dealing with my 67 L79 chugging and jerking while in higher gears and low rpms for awhile now. Had no idea how to fix it. So I decided to take Duke's Tuning Guide and follow it to try and get this fixed. i followed the guide which was very helpful and a lot of fun. While in the shop came to realize that I could not get my initial timing lower than 16*. My freshly installed 12" VAC was hitting the coil stand like the WRONG B28 can that I took out. I was so bummed because I thought I had found most of the culprit when I discovered the wrong VAC. I need to move the distributor over a "tooth" I guess so I can further retard the initial timing so I can get the spark map dialed in. How do I do this??

FYI, the way the car sits now, I have 39* of timing all in at 2500 RPMS. I realize I should only have 36*. The VAC can is a MS 355, I have one black spring and one silver spring in the distributor from the Mr. Gasket 928 G kit, and I disconnected the VAC. With the VAC can disconnected almost all of the chug and jerk is gone. So much more fun to drive the car. I can put the car in high gear going up a grade at 1000 rpms and will not detonate under acceleration. I read on the web that if your total timing is over 52* (initial, centrifugal and vacuum advance) the car will chug and jerk like mine is doing. Should I be doing something else to my spark map to get this out? My Camshaft is the Comp Cam L79 not a true L79 cam. I had this engine rebuilt 11 years ago. If I knew then what I know now that would not had happened. Here is the spec.

Thanks

Lawrence

[Brian M.]

Re: Distributor off a tooth

I think you need to flip the gear over on the bottom of the distributor.

[Duke W.]

Re: Distributor off a tooth

If you read through my complete tuning seminar you will know that the optimum range for total WOT spark advance is 36-40, and I recommend setting it as high in that range as possible as long as there is no detonation. I usually shoot for 39.

What ever Comp Cams calls that PoS you installed is nowhere near the L-79 cam... not even close! It has more duration and a lower LSA, so effective overlap is much greater, which means the 12" B26 VAC likely does not pass the Two-Inch Rule, so you need to measure idle vacuum and apply the Rule, which will likely call for a 8" B28.

With an OE/OE equivalent camshaft the dimple in the dist. gear should be inline with the rotor tip, but that doesn't apply to some aftermarket cams. Pull the dist. and inspect the drive gear indexing. Whatever it is, remove the pin, rotate the gear 180 degrees and install the pin.

The OE '67 distributor has a total of 30 degrees centrifugal, so if initial is 16, total is 46, which is way too much. I expect you did not rev the engine high enough to achieve max centrifugal, so keep revving it higher until you are absolutely certain that it stops advancing.

Search for a thread started by me in Dec. 2012 - L-79 dist. blueprint overhaul. We used the two lightest springs, which I think are gold, and full centrifugal was not all in until 3500. The black spring is very heavy, so either you did not measure total WOT timing correctly or it's not an OE L-79 distributor.

Duke

[Lawrence S.]

Re: Distributor off a tooth

If you read through my complete tuning seminar you will know that the optimum range for total WOT spark advance is 36-40, and I recommend setting it as high in that range as possible as long as there is no detonation. I usually shoot for 39.

What ever Comp Cams calls that PoS you installed is nowhere near the L-79 cam... not even close! It has more duration and a lower LSA, so effective overlap is much greater, which means the 12" B26 VAC likely does not pass the Two-Inch Rule, so you need to measure idle vacuum and apply the Rule, which will likely call for a 8" B28.

With an OE/OE equivalent camshaft the dimple in the dist. gear should be inline with the rotor tip, but that doesn't apply to some aftermarket cams. Pull the dist. and inspect the drive gear indexing. Whatever it is, remove the pin, rotate the gear 180 degrees and install the pin.

The OE '67 distributor has a total of 30 degrees centrifugal, so if initial is 16, total is 46, which is way too much. I expect you did not rev the engine high enough to achieve max centrifugal, so keep revving it higher until you are absolutely certain that it stops advancing.

Search for a thread started by me in Dec. 2012 - L-79 dist. blueprint overhaul. We used the two lightest springs, which I think are gold, and full centrifugal was not all in until 3500. The black spring is very heavy, so either you did not measure total WOT timing correctly or it's not an OE L-79 distributor.

Duke

Duke,
I did see that you wrote up to 40* full spark advance is the max and thought my 39* would be ok but I assumed since I was still chugging it was too high.
When I checked my vacuum at idle I had 12" so I thought I would be fine. But after driving the car there is not enough vacuum to engage the can to keep it idling at a stop sign. It dies. Yes, the B28 can has to go back in. I will check for WOT timing. I stopped at 2500 and measured 39*. I assumed it was all in.
Duke, I believe the black springs are the light ones and the Gold are the heavy ones for HEI?
Ok will pull the distributor and check the indexing of the gear, and will read your thread about the L-79 overhaul. My guess is that the distributor needs some help. I am going to assume its the original one since it had the correct tag on the distributor when I bought the car in 1993. The car was not restored just a driver back then.

Thanks for the help

Lawrence

[Lawrence S.]

Re: Distributor off a tooth

Duke,

39* WOT is confirmed. I revved the engine up above 3500 rpms and the advance did not get any higher. I am by myself so I don't know for sure how high the engine revved but my tach/dwell meter was pegged at 3K rpms and I know it was higher than that.
The B28 can is back in the car, and I have black springs on the distributor and VAC is attached to manifold vacuum.
What's next? I will read the blue printing of the distributor.

Lawrence

[Lawrence S.]

Re: Distributor off a tooth

Duke,

I have a breakerless point set up in the distributor. Would this be an issue in setting up the distributor?

[Jeff P.]

Re: Distributor off a tooth

Lawrence,PM sent.

[Duke W.]

Re: Distributor off a tooth

Referring to your earlier post, it's a common mistake for guys to rev the engine to some arbitrary number and set total WOT advance. Over on the CF it seems that everybody thinks it should be set at 3000, but if the centrifugal is not all in at that point, you end up overadvanced. That's why the first step in mapping the spark advance is determining the centrifugal start and stop points, and the OE specs are in the service manuals and AMA specs.

You mentioned HEI, but have what is apparently the OE single point distributor. Mr. Gasket makes spring kits for both single point and HEI. The single point kit part number is 928G. The springs in the HEI kit will probably work in a single point, but I don't know what kind of centrifugal curves they will yield.

Did you remove the dist. and inspect the gear orientation? What was the result? An electronic switch has no effect on the centrifugal or vacuum advance, but it may have an effect on distributor orientation for a given initial timing, and I don't know how to static time an electronic switch.

It's best to have a helper when you are checking centrifugal. After finding the start point I like to record advance in 500 RPM increments starting at 1000 and go at least 500 above the stop point, then blip the throttle to higher revs to make sure it doesn't over-advance at the top end.

Assuming you have correct gear orientation, your next step should be to tell us what spring kit you have. Select the lightest springs and see what they yield. Measuring the wire diameter with a mic will help as the lightest springs will usually have the smallest wire diameter. In addition the greater the number of coils the lighter the spring and the larger the OD the lighter the spring.

Also tee in your vacuum gage to the VAC signal line, record vacuum and RPM. Manifold vacuum data without RPM is meaningless because idle vacuum increases with increasing idle speed, so speed must always accompany the vacuum reading.

What's left is finding the lightest spring combination that gets the centrifugal in ASAP without detonation, and you should also go through the idle speed/mixture adjustment procedure. As far as idle speed is concerned, set it as low as possible commensurate with acceptable idle quality (and passing the Two-Inch Rule with the installed VAC), so that's your call.

Duke

[Lawrence S.]

Re: Distributor off a tooth

Finally got around to pulling the distributor from my L79. The drive gear is installed with the dimple off 180*. I will remove the gear tomorrow and re-install. I have to purchase a new drift and buy a roll pin. Will keep you posted on results of the installation.

Thanks

Lawrence

[Duke W.]

Re: Distributor off a tooth

The roll pin should be reuseable. If you don't think it's snug enough, use a tool like a screwdriver to spread it. A common 1/8" punch is the proper tool to remove or install it, but if it's old and slightly mushroomed, dress down the upset metal with a grinder or sharpening stone.

Duke

[Lawrence S.]

Re: Distributor off a tooth

Flipped the distributor gear and got the initial timing back to 10*, but could not get more than 32* total timing. I went back and moved the initial to 14* but could not seem to get the total timing up to 36*? So what I did was increased the idle to 3200 rpm with total timing "in" and set the total timing at 36* with my dial back light. I took the car for a ride and it ran fantastic as compared to how it did run. So much more low end torque, much better throttle response. Did not spark knock in 4th gear chugging up a grade. Car did not chug and buck like it use to. Seems like I may be able to add more timing still yet?
I am having an issue of trying to get the idle down to a nice steady 800 rpm. Seems like when I set the curb idle down low to around 750 - 800 rpm or so it stalls. When I increase the curb idle up seems like the centrifugal timing kicks in and the idle goes up too high. Right now I have the very light springs in the distributor, full manifold vacuum. Seems like I still have tweaking to do, but so glad that my VAC is not pinned on the coil support and can't get the initial any lower than 16*. Any further comments please let me know.

Lawrence

[Timothy B.]

Re: Distributor off a tooth

Lawrence,

With the timing set at 10* where is the vacuum advance control, did turning the lower gear 180* move it too close to the intake manifold, I am just curious to know.

Are you using a 15* vacuum advance control that's pulled full at 8" vacuum. I would shoot for 10-14* initial timing with the full manifold vacuum advance and get the engine warmed good, set the idle at a speed that pulls 8-10" vacuum (probably around 850-900rpm) then adjust the curb idle screws on the carburetor.

After you are sure these setting are correct (approx 25-28* initial timing with vacuum advance and stable idle speed, no centrifugal timing) if you are still having stalling issues I suggest it's time to learn about some carburetor tuning. This involves correct throttle blade position at idle and not over exposing the transfer slots then see if the emulsion screws are more than 1 1/2 turns out from the seat. It's possible that with the camshaft the idle circuit is to lean AFTER you set the throttle blades correctly.

[Lawrence S.]

Re: Distributor off a tooth

I have a B28 VAC due to the low vacuum the camshaft produces. I tried the correct VAC for the L79 but it would not work well with this cam.

Moving the lower gear 180* moved the VAC FURTHER from the intake manifold as it should be. Previously it was adjacent to the coil stand.

I need to confirm that I had the vacuum advance plugged back up when I was solving for total timing. I many have had it plugged. I just need to go back through everything and write it all down. I am on the right path now with the gear installed in the correct phase. Car runs so much better! I was amazed how much power it had with the timing not so out of whack! So much more fun to drive. Can't wait to spend some detailed time working on it.

[Terry D.]

Re: Distributor off a tooth

Did you adjust the air/fuel mixture on the carb? Changing the timing can effect this.

[Duke W.]

Re: Distributor off a tooth

OE L-79 distributors have 30 degrees total centrifugal at 5100, but your data indicates less total centrifugal. Lighter springs will bring the advance in quicker, but will not change total centrifugal. Either there is something wrong with the dist. or it has been modified. You need to determine both the centrifugal advance START and STOP points. With light springs it may start at 700 or less, so there will be some centrifugal added at idle speed. Once you THINK centrifugal stops, blip the throttle to about 6000 and verify that it doesn't advance further.

Being as how your installed cam has considerably more overlap than a real L-79 cam - probably close to the 30-30 cam overlap - you will never be able to achieve typical L-79 cam idle behavior of 750 @ 14-15". Set idle speed "as low as possible commensurate with acceptable idle quality." That will probably be something on the order of 900, AND REPORT IDLE VACUUM AT WHATEVER IDLE SPEED YOU CHOOSE.

Once the spark advance map is dialed in and idle speed target is established the final process is to go through the idle speed/mixture adjustment procedure in the CSM. I believe the initial idle mixture screw setting is one turn out from the seat, but with your higher overlap cam it will likely need more. Turn both screws in and out in quarter turn increments until you get the best idle quality at the target speed, which is indicated by the highest vacuum reading. The final settings should have both screws out an equal number of turns from the seat.

Of course, you should document EVERYTHING, and put it in your Corvette maintenance file for future reference.

Duke