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Discussion Starter #1
Just rebuilt my real 302 and want to break it in but timing marks aren't making sense. 8" balancer but not sure if bolt on timing tab is the correct one as it came with the car. Engine at idle runs smoother when mark is lined up at +15 with the light on it.
 

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Ben - Delaware - 67 Camaro
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do you have your vacuum line connected when taking this reading? 15* isn't crazy. just need to keep an eye on the total timing and not let that get up over 38* or so. might need to adjust weights or change the bushing to limit total (with vacuum disconnected)

for reference, my 406 wants 18-19* at idle without the vacuum. once the vacuum line is connected i'm idling at around 24-26*. total mechanical advance is limited to 18* giving me a total of 36-38* at WOT. part throttle with vacuum advance can see over 40*. i have an aftermarket distributor and adjustable vacuum canister.

had the hardest time trying to get my new motor to idle with the standard 12* initial and 36* total.
 

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My 68 Z/28 runs best with 20 deg. initial timing and 18 in the distributor all in by 2800 rpm. I did drop the compression to 10:2-1 to run pump gas. The cam is a 30-30 blueprint. Not the best cam/compression combo but it runs great. It pulls approx. 9-10 inches vacuum at 950 RPM. The lower compression and late intake valve closing allow it to start with that much timing when hot. Try to get as close to 20 btdc and still be able to start. Also can use vacuum advance to achieve that timing spec. Good luck with your project
 

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To correctly use a vacuum advance to supplement a reasonable INITIAL timing, to make a suitable IDLE timing, a positive stop needs to be in place.

NO, not the way Crane does it, their scroll stop mounting position simply loads the vacuum advance spring tighter as degrees are taken out of the pin movement. On the OTHER side of the pull pin is the way to do it right. NOT hard to do, and, when done that way, the Crane plate is worth its weight in gold, as the degrees stop adjustment is completely separated from the vacuum pull on the diaphragm.

And, YES, most engines really like between 20 and 24 degrees if IDLE timing, but not that much to easily start. So, initial up to 14, then full manifold vacuum source the degrees stopped down vacuum advance, either stofk or adjustable.

Fior a totally FREE set of instructions, no strings, no ads, just how to do it the right way, with either a home made stop, or Crane 99619-1 scroll plate, with pictures for both stock and adjustable vacuum advance, and either points or HEI distributors, please send email asking for them:

[email protected]


Please note, this is especially important to do with any Chevrolet crate engine distributor, and the ZZ series engine HEI's (removes the emissions type settings from the distributor GMPP is forced to send them out with). .
 

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Ben - Delaware - 67 Camaro
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I highly recommend the OP email Dave Ray and also do a search for a post called "timing & vacuum advance 101" ....very good info there. a lot of great knowledge on this forum.

Dave,

what do you think of the Accel vacuum advance canister #31035? it claims to be adjustable not only in curve but also total advance allowed by vacuum. i installed it with your stop plate in place and followed the accel instructions on adjustment. as an experiment, i removed the stop plate to test the canisters ability to actually limit the amount of advance and found that it actually worked. at roughly 3 turns on the adjustment of the canister i'm getting a max of 10-12* advance from the vacuum.

i ask because you gave me a lot of great information when i was having similar issues as the OP and i actually learned a lot from it. but you mentioned the crane adjustable canister that was available and the fact that the adjustability was limited to the rate at which the advance came in and not the total degrees of advance. i looked around and found the accel can and it's claims so i took a shot on it. seems to work...
 

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Yes, all adjustable vacuum systems are adjustable for degrees through the vacuum rating screw. But, once you changed the adjustment on the diaphragm to get the right degrees, the vacuum rating went straight out the window. change the vacuum screw, degrees do finally stop down, but vacuum to operate the canister also goes to the moon.

Think of the adjuster adding tension to the diaphragm when it is turned into the canister. Now, if you turn the adjuster in, to stop down the degrees, you ADD diaphragm spring tension, changing the vacuum needed to operate the diaphragm agsint its now harder spring tension. the scroll plate allows for the adjustment of the vacuum level, separate from the point you get to changing degrees added by the diaphragm.

The stop plate allows degrees stopping, and completely independent from the vacuum rating setting in the diaphragm.

The best adjustable vacuum advance was the Crane, which is no longer made. Crane went out of business, and FAST EFI bought them, and FAST is owned by Comp Cams. NONE of those people feel the Crane kits and parts are worth producing, so, if I can swing it, I will do what they insist is not needed.

The Crane had the widest adjustment for vacuum, down to 4 in/hg, the others stop around 6 to 7 in/hg vacuum, and that is just too high a vacuum rate.
 

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Discussion Starter #7
Thanks for all the advice everyone. Bought a better digital timing light and changed the coil. Distributor still has points and condenser which I cleaned up. Right away mark on tab was at +10 degrees and car was idling smoothly. Revved it up slowly and timing mark moved as it should. Drove car and couldn't accelerate hard or would backfire through carb plus idle moved up and stayed there. Re-set idle and checked timing mark, looked good so out for another drive and same result.
 

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Just rebuilt my real 302 and want to break it in but timing marks aren't making sense. 8" balancer but not sure if bolt on timing tab is the correct one as it came with the car. Engine at idle runs smoother when mark is lined up at +15 with the light on it.
Since you just rebuilt the engine you've probably already checked this but I thought I'd ask anyway. Have you confirmed that the timing tab correctly marks TDC by using a positive piston stop to confirm TDC is where the tab says it is. If it's off, this process will be a whole lot more difficult.

I too have a 302 and I've stopped down my vacuum advance can per Dave Ray's method using a positive stop at ~ 10-11 deg and use full manifold vacuum - skip the ported vacuum. My base timing is 15 deg giving me a total advance at idle of 25-26 deg. I reset the max centrifugal advance on my MSD distributor to ~ 21 deg yielding a total advance of 36 all in by ~ 2800 rpm. You could try these settings with your 302 just for grins and see how it works out.
 

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Discussion Starter #9
Had a super chevy guy do a positive stop and marks are correct. Thanks will try yours. Am running JE Pistons at 10.4-1 and solid lifters on a high lift Crane.
 

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max, do you have EFI in that car? If not, LEAVE THE DIAL BACK FEATURE OFF. Mark the dampener, and read actual degrees. I don't care whom says to use the dial back feature on an analog ignition system, it is not for anything but computer regulated ignition timing curves, NOT for points, non-feedback electronic, mechanical, vacuum advance ignition systems. Why? Well, ignition systems that are NOT computer timing controlled don't have "computer lag" when determining timing, mechanical advance is right now, not delayed to read inputs to compute tming. Dial back lights have a delay built into them to allow the timing computer to compute and make the spark.

NON-COMPUTERIZED IGNITION TIMING FUNCTIONS DO NOT HAVE A COMPUTER LAG, so, using a feature on a timing light that reads a computer lag, just is not accurate, period, plain and simple, and not one lag time timing light can read actual timing, UNLESS the dial back computer timing feature REMAINS TURNED OFF.

I have within the vacuum advance info package, pics of how to mark the dampener to read advance with the regular timing lights, or, dial back left off. This can be done with white chalk, then wiped off if so desired, or, permanently added, your choice. Package is free, no ads, no cost, all it costs is your time to ask for it, and read it. email address for package is above in my previous post.

As for the acceleration issues, please mark the dampener, and then, leave the dial back off, and plot the timing curve, make sure the mechanical advance is coming back down at idle. Sounds like either the mechanical advance is binding and staying advanced a bit when the engine idles down, and/or there is a serious vacuum leak/carb issue. Let us know what you come up with, please.

Lets make sure the timing is working correctly, then, we can attack the carb and see if it has issues.
 

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I too have a 302 and I've stopped down my vacuum advance can per Dave Ray's method using a positive stop at ~ 10-11 deg and use full manifold vacuum - skip the ported vacuum. My base timing is 15 deg giving me a total advance at idle of 25-26 deg. I reset the max centrifugal advance on my MSD distributor to ~ 21 deg yielding a total advance of 36 all in by ~ 2800 rpm. You could try these settings with your 302 just for grins and see how it works out.[/QUOTE]

Which MSD dizzy do you have? I have a MSD without vacuum advance and want to swap it for one that has vacuum advance. The one I’ve looked at their website is 10 deg max advance. I sent an email for Dave Ray’s info so I’m sure that will give me some insight. Thanks!
 

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Which MSD dizzy do you have? I have a MSD without vacuum advance and want to swap it for one that has vacuum advance. The one I’ve looked at their website is 10 deg max advance. I sent an email for Dave Ray’s info so I’m sure that will give me some insight. Thanks!
I have an older Pro-Billet Ready to Run #8360. I believe the vac advance can that comes with it provides 10 deg advance at 10" vacuum so you'll need to determine how much vac advance you want, at what manifold vacuum in the context of how much manifold vacuum your car provides at idle. Definitely read the Timing 101 article referenced above if you haven't already done so. I don't think that Dave Ray is a fan of the MSD RTR distributors and may have a better recommendation for a small cap distributor with a vac advance.
 

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Thanks!
 

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The 8360msd vacuum can gives you 18 degrees. I just switched from full vacuum to ported to help with the drop in rpm from neutral to drive, as the vacuum dropped going from neutral to drive and pulls the advance out, I was dropping over 400 rpm, I miss my 4 speed!
 

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On average, for the GM point and HEI, along with other GM design distributors for just about everything, like an MSD, GM top made to fit whatever engine, the crankshaft degrees vs points/pickup plate pull pin travel is:

8 crankshaft degrees = .086 to .090 inch
10 crankshaft degrees = .110 to .112 inch
12 crankshaft degrees = .132 to .135 inch

The way I measure the distance is to use a drill bit the right size for the degrees I want, between the pull pin and its rubber bumper, to the edge of the pin travel slot, and "adjust" the slot stop to get the correct pin travel, this on stock and other NON-adjustable vacuum advance.

The much easier method is to make either a home made positive metal stop for the degrees you want, or, the best way, Crane adjustable stand alone scroll stop plate, mounted NOT as Crane says off the end mounting screw, but, by the way I modified it, plate mounted on the diaphragm side of the pin, to stop the pin travel, NOT push the pin into the diaphragm as degrees are stopped down.

If anyone doesn't understand any of that, ask for the vacuum advance modifications package, I made it all very easy to understand, with pictures.
 

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Discussion Starter #17
Dave; Yesterday I changed out the points for a Pertronix iii electronic ignition and left the rev limiter off. Timed car to 10* which seemed to run okay. When tightening down the distributor harder the rpm's increased so loosened a little. Driving resulted in worse carb backfire so today am advancing timing back to +15*. Holley settings all seem good but I have a rebuilt double pumper jetted for the 302 I may put on. Whenever I put the air cleaner on and drive the car idle speed increases. Running Chevron 94 which is the highest octane available here.
 

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Double check to see you haven't crossed a couple of plug wires. Could be a lean backfire. Check the operation of your accelerator pump. Does the pump squirt fuel instantly when you move the throttle, you don't want any lag. If this is a lean condition, you may need to adjust the pump shot.


Alternatively, is it possible you have a rich condition causing the backfire? If running a electric fuel pump is the fuel pressure 7psig or less? What carb are you running and what are its settings you reference?
 

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Discussion Starter #19
Running stock 4053 Holley I purchased rebuilt. Mechanical fuel pump. Gas squirts right away so will recheck float levels and change brass fuel filters. Accelerator pump seems to have proper setting. Think I should just throw the double pumper on? I had it rebuilt and jetted for this engine; also it's the carb the car came with 13 years ago
 

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I'd definitely throw on the DP, what do you have to lose, it's worth a try and quick to do.
 
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