Performer RPM timing and vacuum - Team Camaro Tech
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post #1 of 26 (permalink) Old Jun 9th, 01, 06:29 PM Thread Starter
 
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Just finished building SB 350 with Performer RPM cam, heads, HEI, 600cfm Holley. Edelbrock recommends about 12 degrees initial timing. I can't get it to run at 12 degrees. Even at 18 degrees it is pretty rough. Also, it comes really close to dying if I set the idle to less than 1000RPM. Additionally, the best vacuum I can get is 6 inches. I've heard mixed reviews of this cam, but I know some of you are idling at 800 rpm and running power brakes with this set up. How!? This is my first engine build. Could there be something else wrong, like a vacuum leak? Thanks
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post #2 of 26 (permalink) Old Jun 9th, 01, 08:10 PM
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I have almost the same setup, I have to run it off the scale. I would guess around 32 degrees. 1100 rpm. I can run it at idle around 700 but it seems to like the higher RPM going from stop to go. I also have the Edelbrock Accu-drive gear drive. I have around 16 inhg. I am running the 750 edelbrock carb.

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post #3 of 26 (permalink) Old Jun 9th, 01, 09:03 PM Thread Starter
 
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Tim,
Just clarifying: you have the initial timing at 16 degrees and end up with only 32 degrees total timing? At 18 degrees initial timing, the idle is still pretty sketchy; sounds like it wants to die. I end up with over 40 degrees total which sounds like too much to me. I guess I have too much mechanical advance. Sound right?
I would like to idle at 800-900, not 1000+; hope I get this sorted out.
BTW, any idea how much vacuum you have. When I got this cam, I was under the impression that it was fairly streetable. I need A LOT more vacuum than I have in order to use the power brakes. I get the impression even a vacuum reservoir won't be enough (hope I don't have to dish out the 200-300 bucks for a pump).
I have read of some people on this site idling at 800, 12 degees timing, and running power brakes, so I still have hope. Any help is GREATLY appreciated.
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post #4 of 26 (permalink) Old Jun 9th, 01, 09:12 PM Thread Starter
 
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Sorry, I just reread your post. You have 16in vacuum? What am I doing wrong? I only have 6in.
Also, just clarifying, 32 degrees is your total timing? That sounds about right to me and is what Edelbrock recommends. If I set 32 degrees total, when I idle down to 1000 rpms or so, it dies. Also ,what do you mean that you have to "run it off the scale"?
Thanks for the reply and any advice is greatly appreciated.
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post #5 of 26 (permalink) Old Jun 10th, 01, 03:11 AM
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I have the RPM package and a 750 carb. I had 11" vacuum at idle of 800 rpm, and that was with a small oil leak at the front of the intake seal. I just finished changing the intake gaskets and resealing the other day and have not had a chance to fire it up yet.
I was running 12 degrees initial and 34 degrees total, but it took a lot of work to get the distributor to achieve this. I was shooting for 32, but was so happy when I finally got close that I stopped. It ran great even with the very small leak. It would only leak when driving with high rpm's for several miles.
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post #6 of 26 (permalink) Old Jun 10th, 01, 07:29 AM Thread Starter
 
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What kind of distibutor work did you have to do to achieve this? Thanks for the replies, please keep em coming.
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post #7 of 26 (permalink) Old Jun 10th, 01, 08:05 AM
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I changed the weights, springs, and vacuum canister because mine was shot. I purchased the Crane adjustable vacuum canister, but had to make a plate to stop the amount of advance that the canister gave me. The ratio was fine, but I only wanted roughly 22-24 degrees initial with the vacuum hooked up, and without a limiter, the canister gave me 36. Crane does sell a limiter plate for this purpose.
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post #8 of 26 (permalink) Old Jun 10th, 01, 08:23 AM
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Are you using a full manifold-vacuum source for your vacuum gauge? Only 6" is awfully low, as if your gauge is connected to a "ported" vacuum source and your throttle blades are open enough to expose it due to your problem with high idle speed.

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post #9 of 26 (permalink) Old Jun 10th, 01, 10:01 AM Thread Starter
 
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I tried connecting the guage to both the carb and the small fixture on the intake that trans is connected to (located between the carb and distributor. There was no difference.
I also covered the carb with hands (and a towel) which made the engine falter. Spraying the edge of the intake with carb cleaner did nothing. Am I correct that these point against a vacuum leak?
Any other thoughts? Would a vacuum canister help enough to let me use the power brakes?
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post #10 of 26 (permalink) Old Jun 10th, 01, 11:00 AM
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Drop the distributor in with the balancer pointing at 36 degrees fully advanced then turn it back some then start it up and set the initial to 12 and you should be right there. Use the ported vacuum port on the carb and it should idle at 800 or 900 RPM and give you 10-12 inches of vacuum. This is how I did mine and it runs great.

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post #11 of 26 (permalink) Old Jun 10th, 01, 03:20 PM
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John,

You also might try forgetting about what your timing is at idle altogether. Just start it up and advance the timing until it idles on its own at say 800 RPM. This should get you in the ball park.

Then, using a dial back timing light rev the engine to around 3000 RPM (leave your vacuum advance line connected) and check your total timing - set it to around 36 degrees. Hopefully the car will still idle when you let off the gas.

Also - I prefer to use manifold vacuum for the advance rather than the ported source. It provides more advance at idle speeds and therefore a smoother idle. When I switched from ported to manifold vacuum, my vacuum at idle increased from a shaky 11" to a stable 14".

Keep in mind there are two different pointers and harmonic balancer marks (pre-69 and 69 and later) - if you mix them your timing reading will be way off - usually they will read much more advanced than reality causing you to inadvertently retard the timing.

Good luck!
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post #12 of 26 (permalink) Old Jun 10th, 01, 07:31 PM Thread Starter
 
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I'll try using a manifold vacuum source. (Although I put a gauge on it and the vacuum gauge still read a shaky 6in vacuum). Does the fact that the gauge's needle bounces between 4-6 indicate a vacuum leak?
Eric, I think I could get it to idle good with a bunch of initial timing. When I go to 3000rpm and set the total to about 36, the initial drops down to the low teens when I let off the gas. Then it won't run.
I thought about the timing tab thing too. I was pretty sure that I had a matching tab and balancer, but just to make sure, I used a piston stop (turned engine back and forth and split the difference) to find true TDC. (It was about 4 degrees off from the mark on the balancer, so I used MY tdc when I put on the timing tape).
Any other thoughts; I'm stumped. CA420, I'm not sure that I understand your recommendation (remember, I'm new to this and just a little dense). I put the engine at 36 degrees, then drop in the distributor? Won't the timing be WAY off?

Thanks again
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post #13 of 26 (permalink) Old Jun 10th, 01, 10:14 PM
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Have you tried re-adjusting the valves? You might have a few that are too tight which will cause a screwy idle. Also, did by chance the engine ever backfire while you was trying to start it or break it in? If the valves are adjusted correctly, it may be too rich...a blown pv from a backfire or too high float level or way misadjusted idle mixture screws can cause problems also. That cam will pull more than 6" of vaccuum in a 305...theres definately something wrong here.

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post #14 of 26 (permalink) Old Jun 11th, 01, 12:25 AM
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John, if it won't idle in the low teens then maybe there is something else wrong as Travis suggests. The valve adjustment is something worth checking - you could also do a compression test to verify this before pulling your valve covers. All the cylinders should be close. If compression tests out OK then . . .

The other possibility is that your mark slipped when you applied your timing tape or somehow got it in the wrong spot. You could just set the timing to least advanced setting where it will idle and make some slow speed runs up a hill in high gear (lug the engine in high gear). Advance the timing a couple degrees at a time until you hear pinging, then retard the timing a couple degrees below that. If the car cranks slowly when you start it then retard the timing a couple more degrees. This isn't the most accurate way to set your timing but it will at least get you on the road if timing is really your issue.

Does the vacuum advance add much timing when it is connected or take much out when disconnected? You should see about 12 degrees of change. Could the vac advance be sticking, leaking, or not functioning altogether?

[This message has been edited by Eric68 (edited 06-11-2001).]
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post #15 of 26 (permalink) Old Jun 11th, 01, 12:49 AM
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One more thought John,

You should have about:

12 degrees initial timing (vacuum advance disconnected idling)

24 degrees at 3000 RPM with vacuum adv disconnected (12 initial plus 12 mechanical = 24)

36 degrees total at 3000 RPM with vac adv connected (12 initial + 12 mechanical + 12 vacuum).

Most small blocks will run decent with these numbers, you can play with the advance curve a little once you're in the ball park.
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