Timing And Idle Tuning

A couple questions as I am setting initial timing and idle for my V6 - it has a new comp cam with the 252 grind, 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) (edited)...What is the expected amount of manifold vac when idling at these high elevations? I just re-adjusted the idle mix and idle speed screws and am getting out about 13 Hg (at 650 rpm and 14 BTDC). I think this is pretty much what it used to pull for vacuum...but just wondering if any other high elevation jeepers see similar numbers.

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

 

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.

 

Heck...I am happy if it just runs some days!

 

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.

Anyone know how much mechanical advance I should get from a standard Delco distributor - need to check the manual. Assuming I get an additional 12 degrees advance from the vac can at max. Anyone have a good maximum limit for total advance (adding initial, vac and mechanical together)?

 

Yup gonna play with it a bit more tomorrow - the mechanical/centrifugal weights are all working fine, but I don't know how much total maximum advance they provide or when it is completely in - I believe it starts coming in at low rpms - say 850-900 maybe. Thinking I might have to back the initial timing off just a bit so I don't get too advanced when holding 1500-2000 rpm. Not sure when my mechanical is completely in, but guessing it is around 2000rpm. I can probably determine this by backing off the initial timing to near 0, then going up through the rpm range recording the advance (all with vac line plugged).

 

I don't have an adjustable (retarding) timing light - just a standard one so its hard to get info on total timing as the mark goes beyond the scale. But in short - backing the initial timing off (back to 8BTDC) has removed the rattling misfire at higher rpm. Runs quite well now at the higher rpms. With the vac line plugged and initial timing backed way off, I could watch the centrifugal come in smoothly with increasing rpm (seems like its about 20 degrees of advance and all in around 2500 - rough eyeball estimate since its off my timing scale). The downside is a rougher idle with initial timing only at 8 BTDC - I opened the idle mix screws a little which removed a bit of the roughness, but it doesn't get rid of all of it. Basically, it seems to like a bit more advance at idle. I guess I could look for a vac can that does not provide quite as much advance (or I could put something on the vac can piston to limit its travel a bit). So, at cruising rpms with 20 degrees centrifugal advance (maybe its a bit more?), and 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.

But I think at this point its more about tweaking the tuning, and its completely driveable with no concern of pre-ignition misfire. I'd also like to get my pertronix ring re-intalled but probably drive it around for a couple days first!

 

James - this is one reason I like the HEI on the motors. I can tune both vac and mechanical advance. I have a doc somewhere that I used for setting both so I have the right curve for the motor.

 

Still fiddling with tuning and timing - re-read my post from #12 - sounds a little confused. As Walt said above, the vac should be relatively low on ported vac when throttle plates are closed. I think I was trying to say that at steady speed cruising rpm with minimal load on engine, the throttle plates would be only slightly open and wondering if that would cause the ported vacuum advance to go too high?

But after further inspection and trial etc, I am pretty sure it was getting too much advance timing when everything was all in. Of course, what was bothering me was how the initial, plus all the centrifugal plus the max vacuum advance just wasn't adding up to all that much (at least not enough to be causing an issue). I eventually backed the initial timing all the way down to only 4BTDC, and while that certainly helped minimze the mis-fire, it wasn't completely gone. Most notable in the 1500-2000rpm range. Again, what was really frustrating was that it ran without a hiccup with the ported vac line plugged and no vac advance.

So after wondering all about possible carb and fuel delivery concerns - none of which seemed justified from the fact it runs great with no vac can attached, and the plugs all looked pretty good in color etc, and no loss of spark along the wires etc (did some night tests with the lights out). So I came back to the vac can and decided I needed to determine exactly how much advance it was providing. As I don't have an adjustable timing light, I had to back the initial to zero, and determine total mechanical which seems to go all in at about 2000 rpm and its only about 16 (not the twenty I thought earlier). Then I added in the vac can - this is the one that came with the rebuilt delco dist (MS 163 16). I can't really find any specs on it - how much advance it provides and at what Hg. I don't know if its the design of the vac can, or if its somehow malfunctioning, but it seemed to provide too much advance. ALthough the crank timing marked moved way above my timing scale, I had a pretty good identifier for 40 degrees (or double my timing scale). When the throttle was initially opened etc, the timing went well above the 40 mark and the misfire would occur (The vac can was providing over 20 degrees of crankshaft timing).

Thought about putting some sort of limiting device on the plunger, but I then just replaced the ms 163 16 with a standard B1 vac can (same as the vc680 or SMP vc24a etc) that I had on previously. Just working the plunger by hand and it seemed to be 'stiffer'. Put it on and ran the engine - seems way better. I even moved the initial timing back up to about 10BTDC - add that to the 16 from the centrifugal for total mechanical of about 26. Add the vac can advance of 16 (at the crank) for 42 when everything is all in. Seems to be okay. Haven't had a chance to really drive it - might need to back the initial off to 8 - we'll see. Its getting better...

ps - its really hard to get specific info on vac can specs. At one time they were stamped with the ubiquitous B1 or B22 etc and that is what all the spec info sheets I have are based on. But its hard to get cross-referenced stuff for the new part numbers. still don't know much about the ms 163 16. I assume the '16' in the part number is the amount of advance, so maybe it was just malfunctioning - or was coming in too soon (at too low of a vacuum?).

 

When I say all in I mean all 3 advances added together using max values...not sure if that is the correct terminology. It runs great with just initial and centrifugal at total of 26. Its the additional 16 (or more) from the vacuum advance that causes the problem...i can back the initial down to 5 which is fsm spec...but it still totals 37. That would be fsm spec using oem distrutor and how it ran before rebuild. The rebuilt engine with mild cam likes a bit more initial to idle but doesnt like the vac can attached. maybe it needs a different vac can that has less advance and comes in at lower vacuum?

 

Thanks Tarry...I get what you are saying and agree. Its the partial throttle (say constant 1500 rpm with no load - in neutral) with vac advance that is causing the unusually high advance timing. Heavy acceleration with WOT does reduce the vac advance and reduces the misfire. So I changed the vac can in case it was improperly functioning. I haven't had a chance to chart out a new advance curve since the vac can change but my ear says its better but not sure the problem is completely solved. I will chart it out and If it still seems goofy I will put my vac guage on the ported vac and see if its somehow a vaccum problem.