Re: [AMC-List] Continuing Saga of Doc the Jeep, Part II.5

[Tom Jennings <tomj@xxxxxxx>]

On Mon, 27 Nov 2006, Nick Lenarz wrote:

> Some of you may recall the problem I've been having with my Jeep, under
> the heading "Weird non-running issue, '82 Jeep CJ". Things are
> better... sorta.

First, you have too many variables.  Changing too many things at
a time makes it hard to sort out cause and effect. Disconnect
vacuum advance until you untangle things. The engine will
run fine, but will lower highway cruise fuel economy, and run
slightly warmer at idle. But you'll eliminate one variable.

Does distributor mechanical advance work? From idle to 3000
rpm, advance should increase 10 - 20 degrees BTDC (depending on
distributor). It should be a smooth change with RPM, and hold
steady. If not it's busted, and a major problem. Measure with
vacuum disconnected!

If above is working, damn right it'll ping under load, with 26
degrees static timing!

Your engine should run smoothly with 4 - 10 BDTC. Might not
make optimum power etc but it should idle smooth and accellerate
smoothly. Factory settings gaurentee that much.

There is Some Number of maximum degrees of advance for a given
engine and state of tune. For AMC and Chevy V8's, its about 36
degrees total advance -- static setting plus centrifugal advance
-- with vacuum disconnected.  For the AMC six it's somewhat less
(30 - 34?) but works exactly the same way. (Some factory
distributors do a lot less than this.)

> If my hypothesis is correct, the spark timing HAS to be advanced
> (rather, is required to be) BECAUSE the valve timing is advanced, in
> order to find that 'sweet spot' where combustion and vacuum is best.

Nope. Ignition timing will vary somewhat with camming, but it
doesn't change the game. More than 32 - 36 degrees of advance,
ping. That's physics.

The cam you have will determine idle/cruise vacuum (I take it
you have a non-standard cam in there), if it's wildly wrong,
you can't fix it with spark timing.

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