HEI Problem ZZ3 68 Camaro - Team Camaro Tech
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post #1 of 8 (permalink) Old Dec 18th, 00, 07:04 PM
Dave RG
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I have a 68 Camaro with a ZZ3 350 engine. It has a Carter carb. I removed the Mallory dual point distributor and put in a Hi-performance HEI distributor. I used the hot wire of the firewall which looks to be 12 gauge. I set the timing to 8 degrees BTDC. It idles great but lags and backfires on hard acceleration. I advanced the timing to 2 degrees BTDC and it improved but still won't take hard acceleration. Do I dare advance the timing? There is no pinging. With the timing light connected I can see the vaccuum advance working. Any ideas?
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post #2 of 8 (permalink) Old Dec 18th, 00, 07:43 PM
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Location: Knox. Tn. U.S.A
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Are you shure you blocked of the vaccum advance. I know I've done that before. You might look into an advance curve kit. I put lighter spring on mine to get more advance during high rpm's.
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post #3 of 8 (permalink) Old Dec 19th, 00, 04:09 AM
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Make sure the power wire you're using has 12 volts with the key on. The factory wire for the point system has a resistor built in to lower the voltage to avoid burning the points. It sounds like that's the wire you're using. The HEI MUST have 12 volts, no less. If true, run a separate wire from the accessory terminal of the fuse box to power the HEI.

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post #4 of 8 (permalink) Old Dec 19th, 00, 07:48 AM
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Make sure you tap 12 volts from a source that isn't shut off with the key in "start" position - some accessory circuits are hot in "run" and dead in "start".

'69 Z28 Fathom Green
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post #5 of 8 (permalink) Old Dec 19th, 00, 12:59 PM
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You need to determine what the total advance is. Do this with the engine at about 3000 RPMs. You need to go for 36-38 degrees advance with the vacuum unhooked and around 42 degrees with the vacuum hooked up. Then check to see what the advance is at idle. Depending on what cam and other engine components you have you may need as much as 20 degrees or more of advance at idle. Sounds like you need to do some distributor tuning to get the advance curve optimized. The advance weights and springs may need to be changed and the vacuum diaphram may need to be adjusted to get the engine to respond as quickly as possible. Try setting the advance at 16 degrees at idle and see if it responds better. If it does then change the springs so that it doesn't advance more than 36-68 degrees total at 3000-3500 RPMs without the vacuum. Then tune the vacuum diaphram for about 42 degrees with the vacuum. These numbers are just a starting point, every engine is different and requires different tuning.

'69 RS/SS396 pro street
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post #6 of 8 (permalink) Old Dec 20th, 00, 04:39 PM
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Hey Big Gearhead,

IMHO I have never ran more than 38 degrees total timing including the vacuum advance on any small block, unless it is a stock 305. Just a thought.

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post #7 of 8 (permalink) Old Dec 20th, 00, 10:14 PM
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Like I said, all engines are different and that is just a place to start. I built a 468 with closed chamber heads that had to have 45 degrees to make good power and I've built small blocks with open chambers that wouldn't run with more than 30 degrees. The vacuum advance is just for economy and emissions anyway and usualy cancels itself out under power if hooked up right.
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post #8 of 8 (permalink) Old Dec 21st, 00, 01:56 PM
Dave RG
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Thanks for the advice. I opened up the distributor and found the centrifical advance not seated properly. I ran 12 guage from the fuse bock to the distributor as I did find it only had 11 volts off the firewall. My timing light does not adjust but by setting the timing to TDC I get great acceleration to 5000 RPM. I haven't taken above that but it feels strong. The idle is fine and I will now adjust the carburetor. I don't get any advance when the vaccum is hooked up at idle. Is it okay to run at TDC? Any more advice?
And by the way, I did hook up to a dead circuit at start but have now corrected. Thanks for the heads up on that.
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