Guys,

I'll apologize in advance if this has been asked a thousand times, but I am new to the site...

I have a bone stock 69 Z/28 with the 302DZ. Engine was last rebuilt in mid 80's and has seen little mileage since that time. The car generally starts right up and runs well.

That being said, I cannot get the car to idle @ 900 rpm without it dying at stoplights or when you turn the steering wheel hard. The solution to this was to turn up the idle speed to about 2000rpm...not ideal. I have checked the timing, which is supposed to be 4btdc @ "proper idle" and it seems close. I have also played around with the mixture screws, but lacked a vacuum guage and was not able to get an accurate read off the dash tach...ended leaving them at the approximate setting of three turns out from full in. I haven't found any vacuum leaks, although I am not 100% certain.

If I don't figure this out, I am going to just start taking stuff apart and soon I'll be in for a full rebuild and spending money I shouldn't be. What do you think????

take the air cleaner off and look down the top of it into the primary side of carb
in the front you will see 4 little holes 2 on left 2 on the right take a fine piece of wire and clean these out they are thye air passage for a holley carb at idle if this dont work try changing the power valve you may have blown it if car has backfired

You need to get a vacuum gauge so you can properly adjust the idle mixture for highest steady vacuum; on my bone-stock '69Z it's usually about 1-1/2 to 2 turns from lightly seated.

If it has sat for long periods of time, you probably have some gum and varnish that's formed in the idle system as well, which would explain having to go to three turns out to get it to idle - probably time for a thorough carb rebuild and cleaning. Get the correct genuine Holley rebuild kit - don't even bother with any of the generic parts-store rebuild kits.

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JohnZ
CRG
'69 Z28 Fathom Green

John,

What kind of vacuum readings do you see on yours at idle? On mine, with the idle screws at 1.5 turns from seat, I get about 5.5 inches of vacuum. If I go leaner (about 3/4 turn from seat) and raise the idle speed, I get about 8 inches of vacuum at 1,000 rpm. I also have a slight off-idle stumble with this setting. I am into the transition circuit?

I have to say I have the stumble at tip in as well on my new 302. I am sure it can be tuned out and when I do figure it out, I will post my findings. I suspect the vacuum advance is backing off as the throttle is cracked and before the RPM's come up, it tries to die. This is leading me back to try setting it up with ported advance. That is not the best setup for cool running at idle but it would be nice for the engine to actualy take off when the throttle is gently opened as it does using ported advance. It might also be idling on the curve but I have not had time in a few weeks to look into it. Maybe the parts will arrive tomorrow I hope.

-Mark.

As for an update, my distributor IS in fact idling "on the curve" so the first thing I will do is braze the advance slot up so I have about 24 degrees in the centrifugal advance. I have too much right now and the advance starts too early compounding the problem. The throttle gets cracked, the manifold vacuum drops a bit causing the vacuum advance to back off which lowers the engine speed which reduces the advance some more which basicaly, is like a house of cards. The RPM drops and can not come back on its own and within a few moments, it dies.

Jdawg, if you have a totaly stock spec DZ 302 distributor hooked up to ported vaccum, you need to read some of my other posts on the subject. The jury may be still out on the ported vs manifold advance but nobody will tell you the stock DZ 302 curve is the way to go. The emissions calibration will make the car run much worse than it would with a proper curve. If you look at some of the non-emission engines built with almost all the same parts, cam and specs, the 64-65 Corvette SHP 327/365 HP motors are almost exactly the same as a DZ 302 with .25 longer stroke. The ignition setup for those engines is WAY different because they did not have to worry about emissions back then. The distributor setup and curve is WAY better IMHO as compared to the stock DZ setup. I hope to be able to modify my advance today or tomorrow and test out the new curve and report back as time allows. I am working on preparing for a few job interviews next week so garage time comes second.

-Mark.