A/C idle question

[734406pk]

Where is your initial timing set at for idle??? Are you running stock distributor setup??

You might be experiencing similar problem I had with my 74 340 w/mild cam... if you try to set timing at close to factory 'normal', cam won't like it, and falls off under load... advance it enough (like 16-18 deg advance) for best vacuum at 800 rpm in Neutral (tranny pumping). You should only get about 200 RPM drop when A/C kicks on, according to my manual... (I don't have A/C on my 73).

At 1400 RPM, you are already on mechanical advance curve on stock dizzy... so any drop in RPM pulls timing back, which compounds the RPM drop with that cam!

Check timing shift at idle for load/no load variation... you might have to add stiffer advance springs to move advance curve higher in RPM range so you aren't fighting yourself with that cam...

Just IMO...

  I am running this same cam in a 440. Initial timing is 22 deg BTDC, 35 deg total at 2900 rpm. Idle speed 1000 rpm My idle droops 175-200 rpm when the a/c compressor clutches in.  Idle vacuum is 14.5 - 15 InHg. Im not sure what the specs would be for a 340 but i would expect a lower idle vacuum to start with.

[RCCDrew]

Quick update. I checked out the brake booster, no rpm change between it being connected or disconnected and blocked off. I reset my idle for 850 rpm but haven't been able to readjust the mixture just yet. I will reply this weekend when I get that done.

[734406pk]

Good luck! Check your throttle plates (primary and secondary). Make sure they are not hanging up on the gasket etc.

[RCCDrew]

Got the idle set to a good 850 rpm. Adjusted the idle screws for max vacuum and rpm. The car does 850 rpm at idle, about 350 with the AC and fans on, and dies in gear. Suggestions?

[734406pk]

Check/adjust your base timing and idle mixture for the highest stable manifold vacuum at your idle speed. My idle is set at 1000 rpm and the engine likes 22 degrees BTDC base timing, but this is a big block w/o electric fans. You have too much rpm drop (droop) when the A/C and fans come on for some reason.

[Bullitt-]

No A/C but mine died when put in gear.... solution was two things...Idle Transfer Slots & timing... solutions below

Follow brother CP's timing method
http://www.cuda-challenger.com/cc/index.php?topic=4430.msg33565#msg33565

Adjust the idle screw so your transfer slots are square (I pilled my carb off to set..)
http://www.superchevy.com/how-to/45638/

[RCCDrew]

Okay, so I finally got to work on the timing some on the car.  Here were my initial readings:
Idle with No vac advance 32* with vac adv. 41*
2500rpm No vac advance 45* with vac adv. 68*

I don't know how the timing got that far advanced.  The only thing that I can think is that I installed an FBO ignition module and coil and didn't check the timing afterwards.  I was expecting about 10* less on everything. 

After adjusting, I ended up at:
Idle with No vac advance 20* with vac adv. 21*
2500 rpm No vac advance 38* with vac adv. 58*

I ended up setting up the idle in gear with the AC on.  I set that up at 600RPM with a total idle of 1030 rpm with no ac in neutral.  Better than where I was at.  I also took a lot of turns out of the vacuum advance.  I think that too much advance at idle was making the timing jump around when turning the AC on and putting it in gear.  Thoughts on that 58* total advance???  Everything I have read (Mopar Action, internet) says that you can run a lot of timing under high vacuum.  The CP thread said 38* total advance??  Is that enough?

[734406pk]

Okay, so I finally got to work on the timing some on the car.  Here were my initial readings:
Idle with No vac advance 32* with vac adv. 41*
2500rpm No vac advance 45* with vac adv. 68*

I think you have your vacuum advance connected to manifold vacuum instead of ported vacuum. There should not be any advance on the vaccuum side at idle. 68 degrees total  is way too much timing!

[Chryco Psycho]

I disagree , manifold vacuum will drop with RPM

[RCCDrew]

 Good article on manifold vacuum. This should put ported versus manifold vacuum to rest.

http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm

So, what should total, all in, no load advance be?

[734406pk]

Good article on manifold vacuum. This should put ported versus manifold vacuum to rest.

http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm

So, what should total, all in, no load advance be?

Yes that article has been around for a while. Back when manifold vacuum was used, the distributors typically had much less centrifugal advance that maxed out at a much lower rpm, say 5-1/2 degrees at 900 rpm and vacuum advance was 8-10 degrees additional. Some timing specs were with vacuum hose connected and some disconnected @ 5-15 deg BTDC. So generally there was not as much advance in play as with a more modern ported vacuum style. You can tune your distributor to work with manifold vacuum, but the timing procedure will a little different. You will have to experiment a bit. Check online for tune up/timing specs on early to mid 60's engines that used manifold vacuum to operate the advance. Or try disconnecting and plug the vacuum line and set your base where it runs best (20 deg BTDC? a vacuum gauge works great for this). Adjust your RPM/idle mixture to say 850 rpm and turn on the a/c and see how that works. You should see less rpm drop (droop) when the compressor activates since the base ignition timing holds steady. So base timing @ 20 deg BTDC, 38 deg total at 2500 (which you have now) and disconnect/plug the vacuum advance temporarily. Adjust your carb and try it.

[734406pk]

I disagree , manifold vacuum will drop with RPM

Usually it would, but this 340 with a .484 purple camshaft is probably idles with 10-12 InHg or so, my guess. The vacuum should go up with rpm somewhat. My 440 has the same cam, makes more vacuum as the rpm increases and moves away from that long duration/overlap.   

[Chryco Psycho]

I can understand added advance with some vacuum at cruise & not adding any advance  @ WOT
 But conneting to manifold vacuum seems pointless , so you set your timing @ 5* at idle connet vacuum & now you have 16" advancing the timing as far as it will go so lets say 20* at idle now you add throttle & as the rpm climbs the mech advance is working but the vacuum is dropping so what the timing doesn't move ? might as weld up the dist .

[YellowThumper]

I can understand added advance with some vacuum at cruise & not adding any advance  @ WOT
 But conneting to manifold vacuum seems pointless , so you set your timing @ 5* at idle connet vacuum & now you have 16" advancing the timing as far as it will go so lets say 20* at idle now you add throttle & as the rpm climbs the mech advance is working but the vacuum is dropping so what the timing doesn't move ? might as weld up the dist .

X2 on this.
So with manifold vacuum your idle timing will be inconsistent and immediately off idle you will end up retarding the timing.

Backwards imop

[734406pk]

X3 for me. Hopefully disconnect and plug the vacuum advance will verify this. It appears that when the a/c clutches in, the load reduces idle speed, which in turn reduces vacuum and retards the timing which lowers idle speed and on and on until the engine stalls. Ported vacuum would be better if it is even needed. I've read this article before and it still makes no sense to me either.