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Old 10-29-2018, 07:47 PM   #38
SRT8-X
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Join Date: Feb 2017
Location: Cypress, Texas
Posts: 42
Re: '69 C10 327 SBC Off Idle Stumble / Misfire Help - I feel like I've tried EVERYTHI

Quote:
Originally Posted by 68 P.O.S. View Post
Your HEI specs sound fine, just have to verify it’s working properly. After your rewiring job, it should be getting the full 12 volts, so just verify that to be sure. Is the vacuum advance adjustable? Normal HEI spark plug gap is .045, but if DUI suggests .050 for their unit, keep it there.

Since you’re a little confused on timing, here’s the skinny. Initial timing is just the engine by itself without any mechanical or vacuum advance added in. Rotating the distributor adjusts it and gets the engine in the ballpark to ensure its working well. Basically a coarse adjustment. Total timing is the engine’s initial timing, plus however many degrees of mechanical advance the distributor has. This fine tunes the initial and mechanical advances together. To adjust it, the engine is rev’d to high rpm (3-4K + rpm, depending on the spring strength) until it stops advancing, which is when full mechanical advance is reached. Personally I just take the springs out (don’t lose them!) and rev it between 1500-2000 rpm to set it because, #1, I’m not too fond of the engine screaming at 4K rpm while my head is under the hood, and #2, full mechanical advance is achieved at a much lower rpm without the springs holding it back. Using your engine and 36 degrees total timing an example….24 degrees mechanical advance, plus 12 degrees initial timing, equals 36 degrees total timing. 24+12=36. That’s your total timing. Then there’s the vacuum advance. The vac advance is used to add an extra amount of timing to increase the engine’s efficiency during low load situations; idle and cruise. Rich mixtures burn quickly, lean mixtures do not. Idle and cruise are lean operating conditions and need the extra timing from the vac advance to burn the lean mixture efficiently. So to achieve this, your vac advance must be plugged in to manifold vacuum. Timed, or ported vacuum, is used for emissions equipment your engine may be required to run, such as EGR valves, smog stuff, etc. Our trucks aren’t encumbered with that, so we can use the vac advance to its full advantage. Adding too much timing will cause detonation, so 10-15 degrees vac advance is plenty. Once hooked up to manifold vac, the extra timing will increase the idle, so you can lower it back down to 750 rpm and you should be able to adjust your idle mixture screws as outlined in the manual. Here’s a timing article written by a GM engineer…more than you want to know I bet, ha, but invaluable info: http://www.camaros.org/pdf/timing101.pdf

Valve adjustment is a great idea as well. Use Comp Cams’ EOIC method for adjusting them dead on.
Thanks 68 POS, I appreciate the informative response. I'm good with the timing now, thanks for the refresher, I just wasn't sure regarding the original calibration they put on the distributor.
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1969 C10 Stepside (PERCY - tribute to CDE)
1996 Chevrolet Tahoe 2-door / 2WD LS5.3/4A (ROBYN)
1998 GMC SLE Regular Cab Short Bed Fleetside 4.3/4A (LUCY) (driver)
2000 JEEP Cherokee Sport XJ 4x4 4.0/4A (RUBY, son #2)
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