Setting initial timing

[FINOCJ]

Figured I post this here for those who might have some old jeep experience:
A couple questions as I am setting initial timing and idle for my V6 - it has a new comp cam with the 252 grind (just a bit more lift but not extreme - good for 1000-4000 rpm), so I am kind trying to find new norms. (note: dwell set and reconfirmed multiple times at 30, and using a timing light on #1 wire with carb vac advance line plugged).

1) How much initial timing is too much timing? Here at 5000' I ran 5-8 BTDC with the old stock style cam. ALthough 5 BTDC is FSM direction, given the extra elevation here, and the use of a vac gauge, I went up to 8 BTDC over a few years. I think my vac guage (reading manifold vac) probably suggested I could go higher, but was a bit hesitant. Now with the fresh engine and new cam - figure I should start from scatch. So can I place initial timing where the vac gauge reads maximum? This would put me at 14-16 BTDC - That seems like a lot of advance even for 5000'+ elevation. My ear also tells me this is the range where the rpm smooths out the most as well as reaches max rpm. I am just hesitant to run that much advance. Maybe the new cam likes that? Don't want to burn a valve - will be diligent listening for pre-ignition and keep an eye on temps when idling and for hard starting.

2) What is the expected amount of manifold vac when idling at these high elevations? I am getting about 13 Hg with all the idle mix and speed re-adjusted for the 14 BTDC advance. I think this is pretty much what it used run...but just wondering if any other high elevation jeepers see similar numbers.

maybe...just maybe...getting closer...

[dieseldoc]

my experiance in old AMC iron were 401v8 4barrel carbs.
they liked 10+ (vac guage used) the book said 7.
so your odd fire with the cam may like that, high a setting.
you should be able to ask the builder.

[FINOCJ]

Just re-set the idle mix screws and idle speed screw. Good news, it will definitely idle smoother at lower rpm with the extra advance - put at 650rpm with 14 BTDC. At 8 BTDC it seemed to need about 750-780 rpm for decent idle. Was also able to turn the idle mix screws in about 1/8 of a turn. Think this is all good news. Maybe after dinner I can take it out for test drive.

ps - its kind of funny seeing all the diagnostic tools wired to the engine with a timing light, vac gauge, and my external multi-meter for the rpm. Kind of interesting to be monitoring all of them at once.

[FINOCJ]

Been driving it around town a bit - seems pretty good with one issue. With no load (in neutral), the engine seems to have a misfire between 1500-2000 rpm or so. I don't notice it driving around (where I am mostly accelerating through the 1500-2000 rpm range), but when sitting at a redlight or in the garage operating the throttle by hand. As I am concerned about getting too much total advance when the 14 initial is combined with the mechanical and the vacuum, I pulled the ported vac hose from the distributor and plugged it - just like you would do to set initial timing and idle etc. The engine seems to run at 1500-2000rpm much smoother (again with no load, just in neutral).

As I understand it, at mostly closed throttle (e.g. holding constant rpm with minimal load), vacuum would highest and the vac can would be adding most of its advance, whereas when accelerating with open throttle the vacuum drops and the vac can would reduce advance. Could I be getting too much vac advance. I am going to remove some of the initial timing and see...or maybe the vac can (new) doesn't work.

[FINOCJ]

Seems as though I found the limit of too much total advance with the initial set at 14 BTDC. At cruising rpm with slightly open throttle, the combination of the initial, centrifugal mechanical weights and the maximum vac advance was causing pre-ignition (that I couldn't hear) but easily see and and feel the engine rattling. Backing off the initial timing has removed the higher rpm issues. Of course, I ended up with that much initial advance originally as the engine liked idling there the best. So, The idle isn't as nice or smooth as it was, but no concern of pre-ignition damage now. I might try to find a vac-can that has a bit less total advance in it so I can move the initial timing back up. Of course, I probably should curve the distributor and adjust the weights and springs - but it does come in smoothly for a total of about 20 degrees advance and it looks to be all in at 2500 rpm or so. Add in the 16 I get from the vac can and the 8 initial, that is about 34 total with everything all in. Guess it doesn't like 40.

[Hypoid]

You are on the right track now, get the base timing and mechanical advance dialed in first. The only thing that should affect those is engine RPM. If there is vacuum at the timed port at curb idle, the carburetor is out of adjustment. Do some research on the vacuum advance canister, it might be adjustable.

After that, move on to the carburetor. Take your time researching operation and adjustment.

There is much more to a cam change than just lobe lift: http://www.lunatipower.com/Tech/Cams/CamSpecTerms.aspx

Like Brandon said, the manufacturer may have some advice for changes to the fuel and spark systems.