GM V6 HEI timing

Scott B

Jr. Member
Joined
Jul 23, 2008
Messages
25
Points
1
Make
Chevy
Model
C10
Year
1985
Miles
350000
Engine
4.3L V6
85 C10, 4.3L V6, 3sp manual, 350K miles, one owner. The truck runs pretty good but the gas milage is down and it feels sluggish under the throttle. The base timing matches the emissions label (0 deg at 650RPM with the vacuum advance plugged). With the vacuum advance connected I see 20-22 degrees at 2200 RPM. Hand vacuum pump shows 2 degrees with 5" applied and it seems to max out at 8 degrees with 10"-15" applied. Manifold vacuum looks OK from idle to 2500 RPM and is steady at 15-18" under steady throttle. I have the GM repair manual but have not found any specs on what the mechanical advance or vacuum advance should be. Do these numbers look reasonable?
For what its worth, I think the advance weights had the number 47 stamped on them. The vacuum advance was replaced many years ago due to pinging (did not keep the original). The Electronic Spark Control was also replaced many years ago and seems to work--the advance mark retards with light tapping on the block with a ballpeen hammer and the engine speed at 2000RPM. The distributor is original to the vehicle. I have found some info on the V8 distributors but not the V6. Are they generally the same?
 

billr

wrench
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Mar 12, 2007
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Am I correct in assuming the degrees you posted are crank degrees, not degrees at the distributor (dizzy)?

Yes, the dizzys were all virtually the same for all Chevy engines back then, but the vacuum/centrifugal curves were different for each specific engine type/tune/year and market being sold into. There were probably several different spark "maps" for just the '85 4.3L V6.

Those figures you posted seem rather tame, but may be "reasonable for that era and vehicle type. I would like to know what the centrifugal advance alone is at 3000 rpm (am expecting about 30 degrees).

Have you taken compression readings? That is the first step in a case like this. Have you tried blocking off the EGR ports?
 

Scott B

Jr. Member
Joined
Jul 23, 2008
Messages
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Points
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These are crank degrees taken with an Innova 5538 timing light. Mechanical advance only (vacuum advance line plugged) is 0 Deg at 1000 RPM, 2 deg at 1500 RPM, 7-8 at 2000 RPM and it seems to max out at 12-13 deg from 2200-3000 RPM. Manifold vacuum during this test was steady 14" at 1000 RPM and steady 16" from 1800-3000 RPM. The mechanical advance was not very smooth and came in slowly with advancing RPM.

The EGR valve has 60K miles or so on it and does move when snapping the throttle. I have not tried blocking off the ports--does this mean removing the valve and attaching a homemade cover of some sort in its place? The spark plugs (R45TS) have 28K miles on them. The distributor cap, rotor & plug wires were replaced at the last sparkplug change. I generally replace the plugs at 30K and will plan on getting the compression readings at that time. Thanks very much for your response.
 

billr

wrench
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Messages
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Often, EGR ports are blocked by simply putting a thin piece of steel (soup can, or such) under/over the regular EGR valve gasket then re-attaching the valve to clamp it down.

Was that a typo, did you mean the light is a 5568? Regardless, if it is a "dial-back" type I would not trust it. There are too many ways for those to give bogus results. I would verify with a simple (non dial-back) light. Point is, if total mechanical advance is only 16 crank degrees, that is about half of what is need for the engine to run well.
 
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