468 only runs w/ 45 degrees advance at idle - Hot Rod Forum : Hotrodders Bulletin Board
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Old 03-14-2011, 03:48 PM
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468 only runs w/ 45 degrees advance at idle

My Chevy 468 will not run unless the timing is advanced around 45 degrees. The cam has around .500 lift and the duration at .050 lift is 260 degrees. The distributor is a mechanical advance Mallory Unilite. I got both of these items used, so I don't know part numbers or anything like that. I think the cam timing is good, piston 1 is at TDC when the timing pointer reads 0. Does this sound normal, or do I have something wrong?
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Old 03-16-2011, 03:51 AM
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45* Initial advance at idle?
May have installed the distributor off a tooth, Rotor out of phase with Cap.
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Old 03-16-2011, 07:32 AM
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Quote:
Originally Posted by Dave468
My Chevy 468 will not run unless the timing is advanced around 45 degrees. The cam has around .500 lift and the duration at .050 lift is 260 degrees. The distributor is a mechanical advance Mallory Unilite. I got both of these items used, so I don't know part numbers or anything like that. I think the cam timing is good, piston 1 is at TDC when the timing pointer reads 0. Does this sound normal, or do I have something wrong?
0.500" lift and duration of 260° @ 0.050"? Why such a low lift w/that much duration?

The engine should not need anywhere near 45° to run. If you're using a dial back timing light, be sure it's at zero so it's reading true timing. If that's not the problem, I suspect the damper is slipped, or that TDC isn't where you think it is, or the timing light is way off.

DETERMINE TDC, then MAKE A TIMING TAPE. Once you find the true TDC, mark the damper w/a line that matches the tab. Then align the timing tape to this line. From there you can see the actual timing w/o needing a dial back light.
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Old 03-16-2011, 11:34 AM
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Hi,

I agree with SSedan64, you likely are off a "tooth"

This will make it appear like the timing is at the "12 o'clock" position (behind the water pump). And make it appear "advanced".

(Everyone's made this mistake once in a while-- including me.
)

The fix is easy. As the last post indicated, get exactly to TDC.

Then, take off the cap, leave rotor installed, remove the distributor hold down clamp.

Then, gently lift the distributor body up, about 3/4" of an inch and "rotate" clockwise while gently pressing down on the rotor go get it to "notch" into the next tooth on the cam and "align" the oil pump drive. Continue to do this for each "tooth" until you get it to align to the number 1 firing position.

If this is an HEI, the "rough" reference point is to use the cap "power block" where the 12v lead plugs into the distributor body and have it roughly "parallel" to the firewall. (Or, roughly "pointed" at the drivers side fender). This will give the vac. "pot" on the other side enough play for timing adjustment and allow you to "reference" where the #1 firing position is located. Mark it with a permanent ink pen or "wrecking yard" paint pen.

If you "get it off", simply rotate the distributor rotor around again.

Incidentally, make sure you are on #1 and on TDC or you'll get a nasty carb backfire through the engine.

Good luck,

Matt
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Old 03-16-2011, 11:59 AM
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If a distributor is "off a tooth", the timing light will show the timing being off by that amount. Once the timing is set w/a timing light (if it CAN be set, due to the chance that the distributor will hit the intake or firewall before the correct timing can be set), the tooth being "off" is irrelevant.

Now, if the timing chain is off by a tooth the timing could be out of whack- but the engine would be running poorly.

Last edited by cobalt327; 03-16-2011 at 12:07 PM.
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Old 03-16-2011, 01:02 PM
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How easy is it to jump a timing chain gear tooth in a mid 9.5:1 compression big block ?

You cannot crank my 440 manually without the plugs removed.
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Old 03-16-2011, 01:06 PM
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What BBC cam has .500" valve lift and 260°@ .050" duration?
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Old 03-16-2011, 01:13 PM
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Some cam that uses 1.7 rockers.
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Old 03-16-2011, 01:20 PM
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Check your cranking compression. Lots of overlap and duration with no compression may be your problem.

The old L-88 cam is 260's at .050 and runs like crap. There is a guy that restores old Vettes that puts that cam in any BBC Vette he restores.
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Old 03-16-2011, 02:07 PM
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if the cam is in fact the L-88 or ZL-1 cam it will need locked out mechanical advance to idle correctly with a auto trans. Will need not less than a 10" "3500stall" converter minimum.

If so, set the locked out timing at 38deg BTDC at idle. When the distributor is locked out the timing does not advance with rpm.

L-88cam 257-270 @.050 .537 ..559" lift 112LSA
ZL-1 264-271 @.050" .560" .580" 112 LSA

Both have a very rough racey idle and low manifold vacuum at idle.
Neither will work with a stock distributor advance curve.

A compression ratio of less than 10:1 will result in very lazy power.
These big cams need 12.5:1 +
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Old 03-16-2011, 02:18 PM
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Guys,

I don't disagree with all the posts, but without having the right reference points getting the timing "dialed in" will be guess work at best.

If you think about the "off a tooth" on the distributor issue, it addresses the crazy timing readings and doesn't require a timing light with advance/retard features....

(Sorry, I apologize if this starts a flame war...)

But getting the timing marks, and the reference points, and the number one firing position correct is important.... (If only for the next guy that has to investigate the issue, or tune it up.)

Since it runs it is likely he just needs to get the distributor on "true" number 1 firing order. The other items related to the cam profile, etc. can be investigated after that....

Poster?

-Matt
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Old 03-16-2011, 06:54 PM
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The whole off a tooth theory is out to lunch.
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