302 DZ tuning...just can't seem to get it right... - Team Camaro Tech
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post #1 of 20 (permalink) Old Jun 11th, 19, 02:28 PM Thread Starter
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302 DZ tuning...just can't seem to get it right...

Ok folks, a stock spec DZ motor with the Crane blueprint cam, stock exhaust, stock original carb, clean and with 70/76 jets, 6.5 Power Valves...refuses to idle when hot. Car starts right up,and idles ok, then when hot, it just dies. Even trying to back up and get it in the garage, just a little power steering load and it stumbles down to 5" of vacuum, I lose my advance, and it dies. I have to give it way too much gas to leave a light. I am ready to abandon it. I have had the distributor curved, I run up to 18* initial, with the manifold vacuum (not ported), and the vacuum can is the one suggested by JohnZ. Total timing at idle is 34*. Under full throttle the advance drops out. I use AC-R43 plugs, and stock ignition. I have found no vacuum leaks. Above 2000RPM, it's fine and cranks beyond 6000RPM, no issues, no miss....but when I come to a light, it wants to die, then required too much clutch feathering to move. (4:10 gear). Initial idle is 950-1000RPM........then 0. I'm blaming the cam. It just seems too much for this engine. I do not have any pinging or detonation, 11:1 pistons, 93 BP Amoco gas. I also just installed a NEW Holley 650 vacuum secondary carb. No difference! I'm ranting....

I need some expert advice on tuning this rascal, or I'm going to drop in an LT-1 cam '70 Z cam. I had one in a 70 LT-1 motor, and it ran great and idles fine.

Save me from myself!

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post #2 of 20 (permalink) Old Jun 11th, 19, 04:38 PM
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Re: 302 DZ tuning...just can't seem to get it right...

Black smoke out the exhaust ?
Exhaust burn your eyes rich ?
@ 5" the power valve are opening.

IF you want to abandon it, I'll PM my address …. on second thought, I'll come get it

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post #3 of 20 (permalink) Old Jun 11th, 19, 04:53 PM
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Re: 302 DZ tuning...just can't seem to get it right...

According to John Z, even the DZ 302's had the vacuum advance hooked up to manifold vacuum.

That's probably your problem. The vacuum canister should be a DV-1810 (autozone) that deploys on only 5.5" to 8" manifold vacuum.

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post #4 of 20 (permalink) Old Jun 11th, 19, 06:08 PM
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Exclamation Re: 302 DZ tuning...just can't seem to get it right...

Chuck is this new behavior or been a long term issue you are finally dealing with?
If new issue, not to sound crass but what did you change or touch? Often a seemingly unconnected change has ways of monkey wrenching the works.

I always do a bit of detective work starting at the base of the issue. You say:
Quote:
refuses to idle when hot. Car starts right up,and idles ok, then when hot, it just dies.
As the engine warms up things loosen up, have you checked for a vacuum leak carb base to manifold? You can use starter fluid or other substance and see if you get a reaction.

As Doug questioned you above, any change in exhaust smell or emission? Have you pulled a spark-plug yet and looked for evidence?

Lastly is heat vaporizing your fuel supply and the stumble you describe is the bowl refilling. Today's fuel boil off so rapidly compared to leaded 1960's formulas.

I'm sure it is something stupid simple so I would not abandon her just yet. At least until I get there with the trailer.

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post #5 of 20 (permalink) Old Jun 11th, 19, 08:41 PM
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Re: 302 DZ tuning...just can't seem to get it right...

Whenever replacing a vacuum advance, the stock replacements do not come with a section of vacuum hose over the pull pin that passes through the advance mount and into the points plate. No stop bumper, way too much vacuum advance degrees are given, and idle is usually way off kilter.

If you want a full FREE package on how to install the rubber stop, or, make a home made stop, or, best, install a Crane/FAST 99619-1 adjustable degrees stop, send an email and ask for them, NO STRINGS, NO ADS, ABSOLUTELY FREE. None of the methods cost more than a couple of bucks to do, and, YOU do them.

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Also, the correct distributor has the ident of 1111480, and IS the most prized, and expensive used distributor Chevrolet ever produced. Same curve for either as delivered single carb, and dealer installed 2x4 carb cross ram. These engines were tuned for road racing, not for the drags.
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post #6 of 20 (permalink) Old Jun 11th, 19, 08:45 PM
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Re: 302 DZ tuning...just can't seem to get it right...

Just a wild guess... but when was the last time you put fresh gas in the tank?
A few weeks ago, I took my car for a drive and it exhibited similar characteristics to what you are describing. The last time I had filled my gas tank was last winter (I also added some Stabil Fuel Storage). I drove my car a couple of time during the winter... but not enough to use that much gas.

When I recently drove my car with the gas from the winter fill-up, my engine ran like crap... would not hold and idle... and even died on me a couple times while I was driving. I read that most gas that is available in the winter contains some ethanol... and ethanol will start to degrade over time.

So, I drove my car to use most of the old fuel and filled up with fresh 93 octane and then my engine ran great as soon as I put the new fuel in the tank.
Something to consider.
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post #7 of 20 (permalink) Old Jun 12th, 19, 12:27 AM
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Re: 302 DZ tuning...just can't seem to get it right...

I think this may be your problem :

You said the vacuum advance can is the one John Z recommends but it is connected to ported vacuum. So the can is not the problem (unless the diaphragm is ruptured).

You can idle when the air-fuel ratio is rich - the choke may be engaged.
But when it heats up it leans out and you don't have enough timing to keep it running.

Plug your vacuum advance line into a place on your carb that has suction at idle (manifold vacuum) and see if it will idle when hot.

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post #8 of 20 (permalink) Old Jun 12th, 19, 06:13 AM
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Re: 302 DZ tuning...just can't seem to get it right...

Switch plugs to 45, 43’s are not good with today’s fuel. May not be the complete issue but will help.

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post #9 of 20 (permalink) Old Jun 12th, 19, 06:26 AM
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I run 45's in mine. That's good advice. 43 is cold. Crack the gas cap and see if that helps. I have to give my Z slot of clutch all the time. Its just the nature. The idle is set about right, and you have a good gear. I think it is fuel related. Pump? Vaper locking? You changed the carb with no change. So that eliminates the carb, but not the pump, old rubber lines collapsing, fuel filter, or non venting. Just some things to kick around.

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post #10 of 20 (permalink) Old Jun 12th, 19, 06:57 AM
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Re: 302 DZ tuning...just can't seem to get it right...

Possible fuel pressure problem, i.e., too high or too low? Are your carb floats adjusted properly and is it possible they are sticking and allowing the carb to flood at low rpm? Good you are running your vac advance off full manifold vacuum, but do get Dave Ray's info on setting up your vac advance. How do the plugs look?

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post #11 of 20 (permalink) Old Jun 12th, 19, 07:45 AM Thread Starter
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Re: 302 DZ tuning...just can't seem to get it right...

OK Folks, thanks for all the advise...duly noted. This has been a problem for over 5 years+. The carb is new last week, no difference. The -480 distributor is the original for 1969, had it rebuilt and curved. Gas is a couple of weeks old, plugs are new. ( Will switch to 45's...) Vacuum can has always been connected to manifold side, not ported. Fuel line is solid steel all the way. filters are new. Yes indeed, it smells way rich. Idle mixture screws did not have any effect on one side with the original DZ Holley. They do work both sides on the new Holley 650. Idle vacuum is 11"-11.5", full open choke, but when full hot, it drops to 10-10.5". I installed an Edelbrock isolator with gasket, no change....so now that's why I am reaching out. I have already looked into most if not all the stated suggestions which is why I'm so mystified by all of this. I'm sure I missed something, so keep hitting me up with your ideas. I will check again for vacuum leaks, change plugs and advise. Thanks ALL!

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post #12 of 20 (permalink) Old Jun 12th, 19, 01:36 PM
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Re: 302 DZ tuning...just can't seem to get it right...

"rebuilt and curved" WHY on the curving? They worked excellent as they were.

Also, does whatever vacuum advance in the thing have the rubber vacuum hose section stop over the pull pin, or does it over advance because the hose isn't there?

Correct GM p/n vacuum advances for distributor 1111480,
1116289, first series 1969 new OEM, thru 2/1972
1973437, replacement, after Feb/1972
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post #13 of 20 (permalink) Old Jun 12th, 19, 05:13 PM
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Re: 302 DZ tuning...just can't seem to get it right...

Sounds to me like a bad power valve.

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post #14 of 20 (permalink) Old Jun 13th, 19, 05:33 AM
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Re: 302 DZ tuning...just can't seem to get it right...

Chuck,
With the new 650 carb, how much transfer slot is exposed when you have the curb idle screw set so the engine will idle at its normal rpm? I suspect there is way more than the 0.02" square. If this is the case then a lot of excess fuel is being dumped from the overexposed slots into your engine and causing the overrich condition. The engine may well tolerate this extra fuel when cold, but once hot it's way too much causing the symptoms you've described. Pull the carb and measure the slot exposed below the throttle blades and let us know what you find. If the idle mixture screws on both sides of the carb work well and aren't wide open then perhaps the amount of transfer slot exposed is not excessive, but let's eliminate this issue by checking what they are. Dave's suggestion on the rubber stop for the vac advance is worth checking too unless you've fashioned or installed a positive stop of some sort. You mentioned you are using the same vac advance can that John Z recommended. That would be the Eichlin #VC-1810 or equivalent. That vac advance provides 15 deg of advance all in by 8" of vacuum so if your base timing is 18 deg and the vac advance adds the full 15 deg at your idle vacuum, that's 33 deg of advance at idle which is too much. At idle you want to be around 24 deg max I'd think. If you need 33 deg of idle advance then perhaps the cam is the issue. You mentioned the cam may just be wrong for the engine. Post your cam specs and maybe one of our cam experts here can offer suggestions on the cam being the root problem. (For what it's worth, I swapped my Holley 780 for a 670 and ultimately had to drill my throttle plates to fix the overexposed transfer slots, fat idle, run-on and other issues and this was after getting all the timing issues sorted including a positive stop for the vac advance so I feel your pain.)
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post #15 of 20 (permalink) Old Jun 13th, 19, 08:09 AM
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Re: 302 DZ tuning...just can't seem to get it right...

Quote:
Originally Posted by ctcz28 View Post
Dave's suggestion on the rubber stop for the vac advance is worth checking too unless you've fashioned or installed a positive stop of some sort. You mentioned you are using the same vac advance can that John Z recommended. That would be the Eichlin #VC-1810 or equivalent. That vac advance provides 15 deg of advance all in by 8" of vacuum so if your base timing is 18 deg and the vac advance adds the full 15 deg at your idle vacuum, that's 33 deg of advance at idle which is too much.
The OP stated in post #1 that they set their initial timing (no vacuum advance) to 18°... and when the vacuum advance was connected to a full time vacuum source, the initial timing went up to 34°. I agree that 34° of initial timing seems like too much. I have read that the Eichlin #VC-1810 provides 8 distributor degrees of advance (16° @ crankshaft).... 18° Initial (Mechanical Only) + 16° VA = 34° Initial with VA connected. This leads me to believe that the vacuum advance canister on the OP's distributor does not have any bushing on the VA pull pin located in the advance slot.

I also recommend that you email Dave Ray (as shown in post #5) to get a copy of the detailed information for installing a vacuum advance limiter plate.

P.S. I also think that Keith's suggestion (post #9) to check for proper function of a vented gas tank cap is a good idea. If your vented gas tank cap is clogged, I would cause a vacuum to form in your gas tank as you are using fuel... and at low idle, your fuel pump might not be able to pull enough gas thru the fuel lines due to this vacuum. The next time you experience this problem, go to the back of your car and slowly turn your gas tank cap to remove > does it sound like the gas tank filler neck sucked some air in?
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