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[IrishCJ6]

I've been researching this for a few weeks now, I have an erratic idle (jumps up and down about 200 RPM every 5 - 10 sec) I've looked for vac leaks and nothing, have adjusted the mixture (vacuum reading etc etc) and still the same. A few on the US forum suggested that I should use manifold vac instead of ported as it improves idle efficiency and throttle response a low RPM which suits off roading. I've also read it suit HEI dizzy's and the whole change to ported vac was only for emissions reduction (GM Engineer info post) Just wondering what everyone else is running, before I try.

Mine is a AMC 360 with the MC2100, I have removed all the emissions gear and have a HEI dizzy. Currently running 12* BTDC at 695 RPM and a fairly consistent 18.5 HG of vacuum at idle.

[Bubba]

Dunno what the true techy answer should be but I always use manifold and prefer it  over ported

[IrishCJ6]

Great, thanks for that Bubba, I'm going to give it a try tomorrow I'll have it so that I can change quickly if I don't like it. Out of curiosity what initial advance are you running at?

[Bubba]

I used to run my amc at 9 degrees  but nt run an amc for a few years sadly

[5536paulj]

Definitely manifold, smooths out the idle no end, adds about 15 deg advance at idle.

Mine runs like a dog on ported, you may need to back off your initial a bit though

PJ

[IrishCJ6]

Well had a play today, found that the vac advance gives me 25*, so to get a max of 32-36 meant pulling the initial back to 0. Seems to run good at that. Have to recheck the total again and could maybe advance a couple degrees more on initial. 25* seems a lot.

0* initial - vac disconnected and plugged.
32* total at 2800
25* at idle (750 RPM) with vac connected.

What you guys think

[5536paulj]

Probably a bit conservative on 0 deg at idle, but given the way we all modify jeeps with big tyres, different gears etc. suggest you drive for a bit see if you like it. it'll let you know when it's too far advanced by pinging! Mine seems to like about 6-8 at idle, but I've tinkered with my distributor and have 35" tyres.

Did you try it on manifold at 12, that's the std set up for a HEI?

Regards

[IrishCJ6]

Paul,

Oops, cock up, you would think an engineer can read things and understand but no. I got it all wrong, I forgot that the final reading was un vac'd and is the more important one. So when I first had the initial timing at 12* (un vac'd) and reconnected the vac line up I got a reading of 37* so I thought this was too high and thus reduced it but went too far. I now reread the doc that I initially read (the GM engineer one) that the reading can be above 36* at idle but so long as the non vac'd reading is less that 36* its ok. So on Monday I will redo this again and this time get it right. I'll try 12* (initial unvac'd) and then see what total (un vac'd) reading I get (somewhere between 32-36*)

[5536paulj]

I've tinkered with mine so much I had nothing to compare it to, so I used a vacuum gauge, adjusted timing for max vac at idle and then retarded it by a  couple of degrees, worked the best with no pinging and reasonable power.

But try using the instructions as a starter as you've said. 

[IrishCJ6]

I'm running on of these dizzys

http://www.ebay.com/itm/AMC-JEEP-CJ5-CJ7-304-360-401-V-8-HEI-DISTRIBUTOR-6512-R-/350873011288?pt=Motors_Car_Truck_Parts_Accessories&hash=item51b1a94458&vxp=mtr

Question is, is 25* vac advance normal, can this be reduced

[5536paulj]

Dunno what normal is, mine's 20ish, remember it's only coming in when your foot is off, some vac canisters are adjustable, using an allen key (3mm I think). Only adjust that if you get detonation on part throttle.

Set initial to spec first, then check total which should be in the mid thirties. Adjusting the vac advance is fine tuning at the end.

Loads of info on this on Jeepforum if you've got the time to read it all...

[IrishCJ6]

Paul,

Had another look at this today, I was right 25* vac advance at idle, so I disconnected the vac line and plugged it. Timed the engine to 14* initial and then ran it up to 2900 RPM to see what total I got which was 30*. So I'm getting 16* mechanical advance.
So after that I set about fiddling with the vac canister, I tried adjusting it (both ways) to see what change it made on the amount of advance. I got 20* one way and 25* the other way so still no where near where it should be. I opened up the dizzy to see what was going on and saw the vac advance lever and its adjustment. I then tried turning the canister adjustment and found then only thing that changed was the amount of vacuum required to operate the lever, I way softened it the other hardened it. I had seen on the interweb that you can buy/make a limiting plate and attach this to the dizzy. So that's what I did, it was very much trial and error on the length of the plate but eventually I got to 13* advance. (I had 8* on an early try and took 0.5mm off and went to 13* so very sensitive).

Here is a quick snap of the plate



So at the mo these are the figures:

14* initial
30* total (2900 RPM)
16* Mechanical
13* Vac advance at idle

The Jeep seems to run great, haven't been off roading but seems smooth and pulls happily, question is I'm still off the recommended  32-36* total, but if I advance the initial any more I'll be advancing the Vac too. Ideas?

[5536paulj]

Suggest you drive it a bit now as this will be tyre / gear specific for your Jeep. Especially try it when it's lugging, up a gradient in a high gear for instance, and listen for rattling at full throttle, you've probably got none at 30 deg total, but don't feel you have to advance it more if it runs well. 13 is quite a lot and it might get hard to turn over / start

Your vac advance is for cruising at part throttle mainly, you can't set that at a standstill really, and you need a vac gauge to do it with any accuracy.

Rule of thumb, is you want the vac advance to come in before the power valve on the carb opens, at about 8 inches vac. The adjustment is for at what vac the advance comes in at, not the amount of advance, it needs to be all in by about 15 inches. If I remember turning the screw out makes it come in faster (or the other way around), if it's too quick you'll get rattling at part throttle cruise. Getting it right will give you the best mpgs, whatever that is with a 360.

It doesn't matter how much vac advance you have at idle as there's no load on the engine, don't worry about that!

Hope this helps, but I suggest you drive it a bit and if it's OK leave it alone, you could become obsessed. 

[IrishCJ6]

Good advice, 14*.initial is where I use to run it, the only change is the extra vac advance at idle.