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Discussion Starter · #1 ·
Hello,
Great forum lots of help and saved me lots of money.
I have a new to me 1985 C20 surburban with 454. The engine has a new after-market carb ( haven't tuned it yet) and intake( no egr ). All smog systems were removed and the holes were plugged in the exhaust manifold ( still looking for metal plugs, last owner used JB weld??)
Questions: What should the Initial timing be for this engine? Can not find the sticker that states this information on the vehicle.
Did a search and came up with lots of different numbers 12-18.
What is the rpm for idle in park?
Thank you,
Max
 

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Set your timing to TDC, then advance until you just begin to hear detonation under a hard acceleration, then back it off a degree or three. Very difficult to determine a precise number for dist. timing on a used or rebuild engine. If your engine makes too much noise (rumbling exhaust system, or mechanical lifters) for you to hear the spark ping under acceleration, be very careful how much you advance before each test drive...you can easily damage an engine with too much advance. Just move it a deree or so at a time, and when you start to notice any power fall-off at all under a hard acceleration, back it off a degree and it should be pretty close to where it needs to be.

The rule of thumb for pretty much all engines, especially for performance and mileage concerns, is to get as much advance as possible without causing any detonation because when detonation does occur, your engine has stopped being efficient, and started trying to destroy itself (burn holes in pistons, scuff bearings and wrist pins, etc.).

See, detonation is when the fuel/air mixture stops burning evenly, and starts detonating explosively, and rather than smoothly forcing the pistons down on each powerstroke, the flamefront is instead meeting the piston on the way up and actually working against the piston on the last portion of the compression stroke. This is the "spark ping" you can hear in an engine that's getting too much spark advance and detonating, and is extreemly harmful to an engine.

My appologies if this answer is too lengthy, or too in-depth. I tend to ramble on sometimes, and since I'm new to this forum, I'll use that as an excuse.
 

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Discussion Starter · #4 ·
Thank you for your post, It was not too long. This is an almost stock 454 engine minus the Carb., Intake and all smog systems have been removed.
The engine runs strong, but back-fires after heavy acceleration.
Also the first step listed in the Carb tuning instructions is to check the timing. This my first real motor and would like to learn as much as possible towards making it run right.
Thank you for your time and information,
Max
1985 c20
 

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Is it backfiring during the heavy acceleration (as in through the carb), or after the heavy accel (popping through exhaust)?

The former, is generally attributed to lean-burn conditions....either a faulty accelerator pump on the carb, carb needing adjusting/rebuilding, vacuum leaks, bad fuel, etc.

The latter, is normally a rich-burn condition, the exact opposite, and can be from floats being too high, a faulty power valve (for Holley carbs), or any other number of reasons.

Incorrect spark timing can compound either of these problems, but generally doesn't cause them alone. What can cause backfiring that's related to timing is bad plugs, cap, rotor, wires, incorrect dwell setting (for those engines with a points type ignition) or moisture in the dist. cap.

Before I did anything else, I'd make sure the ignition comoponents are in good working order, then I'd proceed to other areas (adjust carb and timing).

There is a way to check whether your carb is burning lean or rich, but it involves adding an O2 sensor to your exhaust stream and reading it with a voltmeter while you're driving the vehicle, but it's kind of a pain, and probably won't tell you anything you can't figure out with far less trouble.

Anyway, if the only symptom you have is the backfiring, your problem may not be timing at all, but rather some problem with the ignition system, or the carb itself.

Also, forgot to address your other question on your initial post: I'd probably set my idle to around 500 to 600 with the tranny in drive, provided you have a nice smooth idle. If not, just move it up until you feel like it's comfortable and won't die at a stoplight. Chilton's probably calls for something very close to these idle RPM's, but book numbers are worthless for anything that's not 100% factory, and in fairly new-like running condition, so you're better off just using your own best judgement, taking into consideration the advice of people who've been around the block, so to speak.....oh, pardon the pun, it was completely unintentional.
 

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Discussion Starter · #6 ·
Your right on a lot of points.
The carb is new and has not been adjusted, so I am sure it is some of the problem, but I am trying to work in order. The manual from Edelbrock says to make sure the ignition system is working right; which leads me to
"what should the initial timing be for this engine".
The popping is in the exhaust, smells rich too.
Thank you,
Max
 

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Ok, to get past the "there is no correct answer for a used or rebuild engine" part, then, I'd just set the initial timing to 4 degrees BTDC. At that setting you know it will be running at least halfway efficiently without being too far advanced to cause any detonation. At that point, you can go about checking your ignition components (gotta have proper fire before you can have proper fuel burn), and then your carb.

Good luck:)
 
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