Trying to help my friend with his Z28. He had the engine rebuilt, and the carburetor rebuilt by holley. I set the points to 30 Degrees, The air mixture screws were closed opened them 1 1/2 turns . Car started better. Checked the timing it was way advanced, brought it down to 4 degrees. Idled good but as soon as you gave it gas it would back fire like crazy. I had to advance it to about 16 degrees for it to stop backfiring but still does Very little Idle and mixture are set good and it starts right up. Has me baffled that It can't be set at 4 degrees. I'm wondering if the valves might have to be readjusted. Please help me. Thanks

 

Do a search in the sticky section for Lars tuning tips.
You don’t say if it’s a 302 or a 350 but 4 degrees initial timing is way too low !
It won’t idle, it’ll overheat and perform poorly.
Forget what the factory settings are, they no longer apply.

 

It may be a botched distributor assembly/installation... common problem. Post a photo showing the distributor, particularly the orientation of the cap window.

Duke

 

Actually, GM specified 4 degrees BTC for the 1969 Z-28 with a 302 and the distributor had 32 degrees mechanical advance built in for a total of 36 degrees mechanical and a 15 degree vacuum can. Limiting the initial timing to 4 degrees BTC facilitates starting under hot conditions. Thus, if the OP is talking about a 302 rebuilt to factory specifications, he is on target for factory specifications. I ran a 302 with 12.5 / 1.0 compression and the solid Z-28 cam and there was no way to get it started when hot if the timing was advanced much past 6 degrees BTC unless I used the kill switch to allow the engine to get rolling before hitting it with spark. Of course, I was no expert 40 years ago nor am I now.
If any other camshaft, pistons, carburetor, distributor, or other components have been changed, it may require adjustment of mechanical advance.
First, more information is needed about the engine build, carburetor, and distributor. The OP states this is a new build and thus
I would ask that the OP verify TDC on #1 with a piston stop and thus verify the initial timing is correct. It is not unusual to find a substitute harmonic balancer of a later year with the wrong timing mark.
Then verify the carburetor is giving the engine a good shot of fuel upon acceleration. Vacuum readings and dwell readings will help diagnose the problem.
Hope this helps.
https://www.gm.com/content/dam/compa...let-Camaro.pdf

 

The '69 Z-28 spark advance map is set up for emission control, not performance or fuel economy. Understand that it's a short stroke version of the 327/365 with a more efficient inlet manifold and bigger carb. The 327/365 OE spark advance map has max centrifugal of 24 degrees at only 2350 engine RPM, which requires 12-16 degrees initial to get the total WOT advance into the optimum range 36-40 range, The VAC adds 16 degrees at only 8" and was signaled by full manifold vacuum. Idle behavior is 10" @ 900 with a noticeable lope, and they aren't any harder to start than a 327/300, but all vintage cars are subject to hard hot starting due to percolation of modern gasoline. If such is the case you use the "flooded engine" starting procedure specified in the owner's manual.

Chances are the spark advance map was modified sometime in the past, so you need to measure the centrifugal start and stop points and the maximum allowed. Also use a vacuum pump to test the start and stop points of the VAC along with whatever data is stamped on the VAC bracket and whether it is driven by ported or full time vacuum advance.

If the carb is the OE type it will have ported vacuum advance like all other emission controlled engines unless it's been modified, but verify the current status.

The ideal spark advance map is the same as OE for the 327/365 discussed above ASSUMING it still has the OE type "30-30" cam. Measure and report idle behavior like what I did, above. That will give a clue whether the cam is OE type or something close.

Duke