Input Appreciated, Vacuum Issues

[Mike 70RT/SE]

Hi, all. Sorry for a longer post, but here goes. I just got my Charger on the road, and it is awesome. I love my beast.
440, 906's with big valves, performer RPM, 9.4:1 actual, Mopar 284/.484 Cam. Mopar Performance vacuum advance distributor. 727 with 3.55's.

My problem is that I don't draw much vacuum while the car is at idle and in gear. (Also have poor power brake response at idle in gear). Therefore, I don't get any vacuum advance at a stoplight in drive. However, if I put it into park or neutral, I get some vacuum and the RPM shoots up considerably from advance and it will diesel if it's not shut off in drive. It seems I can only get a good idle compromise by disconnecting the vacuum advance unit entirely and just setting idle speed with the throttle position adjustment on the carb.

My questions are as follows:

1. Do I need vacuum advance? What is the point of it? I hear it increases fuel mileage, but why? It did not seem to affect driveability when disconnected.

2. If I need it, is there a way to adjust the sensitivity of my vacuum advance? I know you can adjust the absolute amount of advance it puts in with an allen wrench, but how about the sensitivity. (I.E. puts in advance in vacuum conditions similar to those when I am in drive at a stoplight).

3. How can I increase vacuum? Can I? I've done a cursory check of vacuum lines and I don't think I have any leaks. Does the cam need to be advanced? It seems like blocking the pcv valve is a really bad idea. Would a vacuum reservoir or pump work? My brakes work fine except under loaded low rpm conditions, like parking. Is this unsafe?

Sorry for the tome, but I'm really not sure what to do. My impulse is just to live on mechanical and make do with odd low speed brakes. The idle is somewhat rough, but that can be expected with that cam, right?

Thanks,

Mike

[mopar_muz]

Mike, sounds like a really nice rig you've got there. Bet she goes like the clappers. Question; what type of carby have you got on it, and have you got the vacuum line connected at the right place for the distributor? I had an experience once with a 351 Cleveland (AAAUGH, FORD, BOATANCHORE), anyhow, it had a 700 Holley, and I got all these confusing reports about which vacuum nozzle I should hook the distributor up to. Remember, base of carby is direct manifold vacuum, top of carby, ported vacuum. I believe ported is the one you want - if others read this and I'm wrong, shoot me. If you would like, try and experiment with the different outlets, if there are different outlets on whatever carb you are using. Now the vacuum problem, or lack of. Sound like a fairly big cam, and with the lumpy idle you were describing and the large duration, you will have a lack of vacuum problem. The cam may need to be advanced, but I think that's just the nature of the beast. Due to considerable overlap in the valve timing, and the fact that it's probably not ment to pull until 2500 - 3000 revs, it will be lacking in vacuum. And yes, you can pull the vacuum line off the dist. if you don't mind a little bit more fuel going down the carb. Just remember, with the vacuum line on the dist., the only time when you need it is going down the highway, cruising. With 3.55's, you'd be doing around 3500, 3600 at 60 mph, so your vacuum there would be fine. Just test it. Grab a timing light, look at the timing mark, and rev it out to four grand - should stop advancing before then. If it does advance standing still, it will do that on the highway. If it doesn't, than you may entertaing the idea of either getting the unit fixed or ripping the line off. If it doesn't advance, put your finger over the vacuum line feeding the dist. and check for suck. I think that's about all I need to say, or know to say. Keep me posted

[Blaine Peterson]

You didn't say what carb you're runnin', but it sounds like this cam is causing atleast a couple problems for you. Did you degree the cam when you installed it?

Low engine vacuum at low RPM may be a characteristic of this cam. In that case you may have to purchase an after market device that amplifies engine vacuum and stores it for use with your power brakes. I've seen them advertised in the car mags.

As for the vacuum to the distributor, you don't want any there at idle. I could be wrong on this but it almost sounds like because of the cam you're using your carb isn't operating on the idle circuit at idle. This may be why there is vacuum at the distributor when there shouldn't be. Sometimes with a bigger cam the idle screw on the carb has to be turned in quite aways so the engine will idle. This takes the carb off the idle circuit. Not good. I saw a fix for this awhile back in MoparAction Magazine. They drilled a small hole in each primary throttle plate (I think thats what they're called), then the air mixture screws on the carb could be adjusted and the engine idled fine and the vacuum advance was no longer coming in at idle.

[Mike 70RT/SE]

Mopar_Muz,

Thank you for the reply.

The carb is a Carter AFB 750. Brand new. (I know, should have gone with Holley, but I distribute Federal-Mogul stuff and got it CHEAP!)
I followed briefly the whole debate on ported vs. manifold vacuum for the distributor. What a pissing contest that was! Anyways, I will play around with various inputs tonight.

However, like you, I'm starting to think that it is, in fact, the nature of the beast. Low vacuum is a fact of life with that much overlap...

Regardless, I'm going to check again for vacuum leaks with propane method tonight, hopefully I can get something figured out soon. I guess it won't be too bad to part with the vacuum advance as I get decent mileage (10 mpg, with this car and cam, that's a feat).

Any thoughts on the brakes? I've read posts about the 280/.484 and everyone said that there was plenty vacuum for the brakes. Perhaps this was just subjective opinion on what qualifies "working" power brakes.

Finally, on timing, I've done the test you recommended and what the MP Engines book recommends. The distributor works just fine, I just need to find some way for the advance to respond to less vacuum.

Hope that made sense and thanks again,

Mike

[Mike 70RT/SE]

Blaine,

Thanks for input. Looks like we posted at the same time. Again, carb is AFB 750.

What you're saying is interesting. So, I should not be pulling vacuum at idle?

I will have to do some research on getting off the idle circuit. The screw is not turned in a huge amount. It idles around 850 RPM in park, but really slows down in drive. I just thought that this was a symptom of not enough vacuum going to the distributor in drive. What leads me to believe this is:
1. I set everything up with no vacuum advance and it idles properly (if a bit rough), both in drive and in park.

2. I hook up the vacuum advance with above setup, and I have to back off the carb idle speed adjustment as RPMs swing up to around 1200-1400. I adjust carb to get proper idle with vacuum connected in park and so it runs at a slower speed. I put into drive and the engine runs VERY slow and rough.

Is this a symptom of being off the idle circuit. I have to admit I'm quite a novice to carb and distributor tuning. (Though I can proudly say I put it all together).

Thanks again,

Mike

(If only I could tune like I can do bodywork and paint...)

[Mike 70RT/SE]

Anyone else have thought?

Here's the summary.

I have a 750 Carter AFB and the mopar performance ignition w/orange box.

I can set up the engine without vacuum advance so that it runs nicely both in park and drive. I do this with mixture and throttle position screws.

When I plug in the vacuum advance, idle RPM in park soars. So, I turn down throttle screw. But after this, when I put it in gear with brakes on, it runs very poorly.

What gives?

Thanks in advance,

Mike

[DAVE JONES]

Just a personal thought here.
1....check to make sure the cam is degreed in properly.
2....I've heard from quite a few folks that with this cam, you probably will have a fairly large drop in RPM when going form idle in park to idle in gear. Alot of folks run a moderate stall to help this condition out. I have a 340 6 pack and i had to run my idle at 1200 which bypassed my idle circut, just to hope for 600 in gear. I went with an 11" 2800 stall and I was able to drop my idle to 850-900 to get 700-750. Problem solved..now my idle mix screws reacted the way they should with the lower idle in park.
3.....I personally didnt want another variable to mess with so I romoved my vaccum advance. recurved my dist so that at idle it is 16 and at 1900 I'm in fully at 36. I lost very little gas milage and gained a whole lot of happiness. Lets see what the pro's think about this stall idea and theory......good luck

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[Mike 70RT/SE]

Final update--I fixed the problem. It appears that the advice that I was off of the idle circuit was dead-on. I went back and read the sections in the MOPAR engines book about carburator adjustments and it covered my problem in detail. I was wrong in using the throttle adjustment to get more idle speed. The problem--engine was too lean. Turning in the throttle screw simply took me off of the idle circuit. So, I drilled out the jets and put in smaller metering rods. All of the sudden, my cam is tractable at idle (well, at least relatively) and the RPMs only drop about 100 when I put it in gear! It was an amazing change. Oh, and I can now run vacuum advance b/c I'm not getting crazy vacuum through the ported carb vacuum at idle anymore.

Thanks everyone for their input!

Regards,

Mike