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Correct ignition timing

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Timgoheen

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I have an 85 Blazer that I recently purchased and just got running. Originally it had a 305 but has a 350 now. It has a holley carb and msd hei ignition. I set the initial at 0°, at 2500 to 3000rpm it only achieves20° total.
I set it at zero because it has the original type of timing marker, and I believe that it was supposed to be set at 0°.
Is that correct ? What should my total timing be ?
 
Timgoheen said:
I have an 85 Blazer that I recently purchased and just got running. Originally it had a 305 but has a 350 now. It has a holley carb and msd hei ignition. I set the initial at 0°, at 2500 to 3000rpm it only achieves20° total.
I set it at zero because it has the original type of timing marker, and I believe that it was supposed to be set at 0°.
Is that correct ? What should my total timing be ?
Click to expand...
The only ones I know you you see set at zero are computer controlled like tbi, and those are set at zero with the wire unplugged.
The HEI you set at 8-12 depending on how your engine works best.
Most of mine liked 10, but I had one that liked 12 better, maybe the timing chain was a little loose.
@Fastereddie beat me to it.
 
There's a lot of vacuum switches and such on an '85. This was about the pinnacle of complicated before they went to TBI. You should have some vacuum to the vacuum advance can at idle when everything is warmed up. Full/ported is a whole debate topic (search away!), but if your total vacuum is only 20 degrees, something isn't advanced enough. The mechanical advance loves to stick, so examine that and make sure it moves freely. See if you get a big timing change as the engine comes off idle. You can test the vacuum advance can with a Mighty Vac or something else (get creative, you have a 5.7L vacuum pump nearby). Some of them are adjustable.
 
I’m assuming it’s an automatic transmission?
If so, loosen the hold down bolt just a bit till you can only turn the distributor by grabbing it with both hands.
Then have a buddy fire it up, put it in gear, hold the brake, and run the throttle up to just before it starts trying to over-run the brakes and move the truck. At that point, advance the distributor until it starts pinging, then back it of just a kitty fur. Tighten it down. Good to go…
 
Manifold
vac advance is oem for EGR engines. Vacuum cans can have different amounts of advance most are @20°.
Accel makes an adjustable one.
Using the vacuum source can give you options for setting initial timing, and carb idle settings.
What carb are you running? The 10° in last post is measured under what conditions? Have timing marks to #1 tdc been checked and proven accurate ?
 
Hmm 10° and 750 idle rpm. This in drive or neutral ?
Right off I suspect a vacuum leak. Give the bowls a quick tightening and see if any change.
 
Isn't 750 normal :dunno:
Seems like about where all my old ones idled.
My LS burb idles at 950 ish. Different animal, but..?
 
Mine is around 750 in neutral, 500-550 in drive. Iirc correctly am running over 12° advance. Manifold vacuum. Advace can total movement of 10°. The rest is mechanical.


And I miss rembered in previouspost. Ported vacuum is the emissions EGR precomputer strategy the OEMs used.
 
Any of those numbers like 0-8 BTDC, that's with the vacuum can disconnected. Once you connect the hose to it again, you should see some initial vacuum. The no advance at idle thing is an attempt to reduce emissions, not make it idle nice. It's probably coupled with how they made them all too lean at idle. Here's an old table of mine I found - I've got 20-30 degrees sitting at idle. (bottom of the table is tons of vacuum, top is WOT).

1772741949169.png
 
Ok, so now that I have the fuel at a decent pressure, I'm back to the timing. I have tried 12°,10°, 8°, 6° and now it's at 5° with still a hint of the detonation sound, and what I really don't understand is that the idle rpm is 980 with the throttle screw backed all the way out. Anyone have any thoughts ?
 
my first thought is vacuum leak. Lean mixture can cause a ping, thou Holley's tend to run rich. Then there combustion chamber quench area. Never rule out an exhaust leak at the manifold, can at time sound like detonation.

Is the total advance still limited to 20°. I am not that knowledgeable on MSD dist systems, what controller is this one using. Is there vacuum advance ?
 
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