Idle Rpm High in Park, Low in Drive

[MopaR&D]

Start with getting your initial timing set right and go from there. Anything else you do with that not set properly is just wasted time and effort.

I run MVA on both my Duster (modded 450-hp 360) and D200 pickup (stock 360) and it works great but you NEED to have everything else in the timing dialed in (initial, advance curve, total mech advance). It needs to have enough initial timing so when the vacuum drops and reduces some of the vacuum advance the idle speed doesn't get affected. So get it dialed with VA disconnected and capped so the RPM drop is minimal then experiment with ported vs. manifold vacuum to the advance can.

 

[Mattax]

My guess is that it has a stock mid 70s distributor with a lot of mechanical advance and a similar vacuum can .
So when you set the intial timing via a vacuum gauge you are getting too much timing too quickly . The hotter cam likes more intial timing than the stock one for that distributor but then total timing ends up around 40*+.
This is why Mattax says to use manifold vacuum ... but you need to set intial back around 5-8*. The vacuum advance will bump the idle timing up into the range that the new cam likes and wont over advance it mexhanically .
I bet there is a 15L or 17L advance plate in that distributor....
The right way to do this is to recurve your distributor using a FBO plate and lighter springs. Set the initial around 14-16 and total around 32* . That means you will use the #8 slot on the plate ( it has several different length sets of slots on it ) .
Then you can readjust your carb for idle mixture.. which may be wrong now due to the timing situation .
And take the carb off , flip it over and set the throttle/transfer slot to just about square .

How to limit mechanical advance in a mopar distributor, tuning for street, strip or all out racing, cure that rich stinky idle, win races

Just to be clear, all I was suggesting is that this is one way to get going right now.

I do not agree that the right way is to recurve the distributor by using an FBO plate (or welding).
That may be a useful approach, but if it will depend on the shape of the advance curve. IF the vacuum at idle is strong enough, and the sahpe is right, no further mods may be needed. Or it may be that welding up the inside of the slots is the right approach. That's the one I would suggest is usually the right approach for those that can do it. It's not for everyone and every situation is different.

The first step is to find out what the timing is at idle rpm, or even better rpms (plural)
600 rpm, 750 rpm, are idle rpms mentioned. 1300 + was also mentioned and when possible I always encourage measureing the timing vs rpm as high as the person feels comftable and has the ability to do so.
From that informations, an appropriate approach can be determined.
I just feel that is all down the road.
The big challenges right now are establishing top dead center and base timing.

 

[mbaird]

The limiter plate would be the easiest way . I have recurved some using the plates and others by soldering the inside of the slot then shaping with a Dremel but really cant tell a difference

 

[Bewy]

Mattax,
Are you able to read? My comments about Chrysler & MVA was that they didn't get it wrong very often but they did with MVA. That is NOT a criticism of everything Chr, just one thing. Hardly a 'crusade' criticising one thing. All manufacturers make mistakes. For the record, in my opinion, out of the big three, Chry was the best & got it right the first time more than the other two.
And apparently you forgot, or didn't know, that Chrys used MVA "When coolant temps at idle reach 225, the valve opens automatically & applies manifold vacuum directly to the dist....This increases engine idle speed".

And your nonsense about load on the engine & losing rpm. If you drive up a long hill [ load ] your rpm will drop UNLESS you increase throttle opening to produce more power & keep rpm up. Get it now?

I provided one possible reason for the magnitude of the OPs rpm drop going into gear. That scenario is veeeeery common & the usual cause that I have seen in 30+ yrs of tuning. Since the OP provided no timing #s, all I could do was take a guess in this case. Other possibilities would be a very lean idle mixture &/or the centri weights were activated in Park, but then dropped back when placed in gear, reducing timing in gear.

 

[Bewy]

From D. Vizard,
Author & racer. From his Holley book: 'The optimum idle advance is typically 35-40* for a short cammed street engine'.
Idle timing in a carb book?

 

[mbaird]

From D. Vizard,
Author & racer. From his Holley book: 'The optimum idle advance is typically 35-40* for a short cammed street engine'.
Idle timing in a carb book?

Please explain what that means.
Optimal idle advance?

 

[Bewy]

Optimal idle advance.
The highest idle rpm, smoothest idle, highest vacuum with least amount of fuel used.
Two more quotes.
[1] Same H book." The more vac an engine pulls at idle, the more rarefied the charge is as the piston approaches TDC.......To get an optimum idle for the lest amount of fuel consumption, timing often needs to be as much as 45-50* BTDC'.
[2] This one is closer to home. MM magazine Oct 2015. 416 stroker. "Initial timing at 1200 went from 15 to 26 while adding an inch of vacuum, Total timing didn't change but a DRAMATIC gain in low rpm throttle response was realised. If tweaked correctly, you will see your idle vac creep up, as we did here''

 

[Newolddart]

So an update on this...

Listening to all of your suggestions...

I checked my crank and front cover for marks and found some on both. I remarked the crank because it was unreadable. Turned the car over with vac disconnected to check intial and it showed about 26*... really didn’t believe it and figured the crank had been removed. Found tdc for cylinder one, marked it, remarked crank, intial shows around 10* no vac. Makes more sense but again, setting timing this way is new to me.

I tune the carb for highest vac at about 15” and she idles choppy but nice at about 1000-1100 (afr in park is 12.8-13). In gear she drops to 850ish still idles beautifully (afr 13-14).

I reconnect vac, in park she jumps to 25* intial and about 1500rpm. In gear she still drops to about 850 but runs. This is with full manifold vac. My carb does not have a port vac provision.

Timing all in by about 3k at 34* ish.

She seems to run much nicer so I dont know if that was in my carb tuning again or what. At no point does she feel or sound like she wants to stall.

Obviously still having the issue of high rpm in park and dropping a lot in gear. I guess I’m wondering if this is acceptable, im still missing something or if I should just tune her without vac and leave it?

 

[crackedback]

Your going to have to mess with mechanical in the distributor. IMHO, you have an issue that you have no idea what cam is in the engine. 10* initial timing is good or marginal for a stock 318, not for something that has a larger cam in it.

I would try setting the initial timing to 18-20* see how it responds at idle and in gear (RPM drop). Turn some initial timing in and get the idle down if possible. Don't drive it until you get the mechanical/total number under control. Engine timing is a four step deal; initial, total, curve and vacuum (if using). Total timing an engine is a horrible method for a street driven car.

If you are trying to use ported vacuum, put your vacuum gauge on the port at idle. It shouldn't have any significant vacuum reading, should be 0. If it has a reading the same reading as your idle, that's a problem. The front venturis are likely open too far allowing vaccum to hit the port.

I wouldn't mess with the vacuum advance as it's a fly in the ointment.

 

[Mattax]

Glad you made progress.

Obviously still having the issue of high rpm in park and dropping a lot in gear. I guess I’m wondering if this is acceptable, im still missing something or if I should just tune her without vac and leave it?

This is completely up to you. Everyone has a different goals and different views on what is good for their needs.

I checked my crank and front cover for marks and found some on both. I remarked the crank because it was unreadable. Turned the car over with vac disconnected to check intial and it showed about 26*... really didn’t believe it and figured the crank had been removed. Found tdc for cylinder one, marked it, remarked crank, intial shows around 10* no vac. Makes more sense but again, setting timing this way is new to me.

I'm not really understanding what you did here. You want to take another shot at describing?

One thing I didn't want to get into before, before you had the basics worked out, is the advance curve itself.
But now it may be a good time.
IF your initial measured 26* when then engine was idling at 1200 rpm and
then you measured 10*, but the engine was idling at 800 rpm
this would show that the timing was advancing rapidly between 800 and 1200 rpm.
That would be very normal on Chrysler distributor.
I can't over emphasize how important it is to know the rpm that timing is measured.

Here are two example plots illustrating advance on small block engines. Notice qyuickly advance changes the timing above 800 rpm on the 1971 distributor.

I tune the carb for highest vac at about 15” and she idles choppy but nice at about 1000-1100 (afr in park is 12.8-13). In gear she drops to 850ish still idles beautifully (afr 13-14).

Tune for the most power in gear. That is the least drop in rpm when placed in gear and or the highest vacuum in gear as long as the rpm kept the same. Can't use vacuum as a comparison between two different rpms. The engine naturally makes more horsepower as rpm increases.

I reconnect vac, in park she jumps to 25* intial and about 1500rpm. In gear she still drops to about 850 but runs. This is with full manifold vac. My carb does not have a port vac provision.

Do you know what carb? Does it still have a choke tower?

edit
[and I agree with Crackedback. For now the vac advance is distraction. If it was needed to crutch the situation that would be different. That may still prove to be the case if your tuning points that way and you dont want to mess with the distributor advance mechanism. No need to jump the gun. ]

 

[RustyRatRod]

Forgive my laziness for not reading back. Have you removed the carburetor and checked the transfer slot exposure? If the secondaries are cracked too far open, that will cause all of your problem AND it's a very common issue on Holley style carburetors. My apologies if I touched on something you've already addressed.

 

[Bewy]

Dart,
Congrats! You are going in the right direction.
Couple of comments/thoughts:
- stop messing around with total timing & dist curves, get your idle timing sorted FIRST.
- I have never seen a 650 rpm drop going from N/P into gear in your situation. I guess it is possible that it is just 'your' combo, but to me it seems something else is amiss. A large stock c'ter loads the engine more than a stall c'ter, so if this is what you have you get a bigger rpm drop going into gear. Still seems excessive though.
- you got 25* in Park? Did you check in gear, at idle? If not, this is likely your problem. Less than 25* means either the centri weights in the dist were activating &/or the VA unit was unable to hold with the reduced vac in gear. It is imperative that you use an adj VA unit & set the Allen Key fully CW as a starting point.
- the choppy idle, high @ 1100 rpm low vacuum I assume is from a sizable cam in the engine. 25* in gear is not a lot, & your combo is probably going to need a LOT more for best idle. Like as much as 40*. GM used MVA. Pontiac V8s with 10.75:1 CR, small cams that idled smoothly at 600 rpm left showroom floors with 26* idle timing.
- while I do not think it is the cause of your problem, you should check/verify/the transfer slot in te carbs primaries. 0.040" or less is what you want.
- mixture screws. Adjust for best idle quality [ sound/feel ], not highest vac, although vac will be high.

 

[AJ/FormS]

Go to posts 36 and 37, and

Tune for the most power in gear. That is the least drop in rpm when placed in gear and or the highest vacuum in gear as long as the rpm kept the same. Can't use vacuum as a comparison between two different rpms. The engine naturally makes more horsepower as rpm increases.

now, I gotta tell you, that the biggest cam I have tuned is the Mopar 292/292/108 , in an Eddie-headed, [email protected] Scr, in my personal engine. If your cam is bigger, the following might not apply.

The First thing to do is to put the vacuum gauge back into the toolbox. And Immediately followed by the Timing light.
The Second thing is to lay the groundwork:
>Make sure the valves are closing, sealing, and that the cam is timed reasonably close to correct. That is to say that split-overlap occurs within ABOUT 4 to 6 degrees of TDC-Overlap cycle.
>make sure that no air is getting into the intake from anywhere else than thru the primaries, past the throttle blades.
>Make sure you have a working PCV system, plumbed to between and below the primary blades. You MUST have a working PCV; this will be your Idle-Air Bypass. If you have a brake booster, PROVE that your engine is NOT sucking air thru the charge line. I like that line NOT to be plumbed to any one specific intake runner. If your carb has a port on the Secondary side or a port in the Plenum; use it. If there is not one there, put one there.
>Disconnect the V-can and plug the port on the carb.
>make sure the fuel is fresh, and that the WET fuel level is stable and Not low. Fresh fuel is somewhat critical. If the Light-weight VOCs have evaporated you will have trouble.
The Third thing is to prep the carb;
>Take the carb off and Set the primary exposure per posts 36/37; to a little taller than wide; visibly taller..
>Close the secondaries up tight but not sticking.
>Set your mixtures screws full rich; on a Holley this is about 1turn out. On anything else, set them to 2.5 turns out.
> If you have a big air-mover coolingfan, install some kind of non-combustible baffle on the carb's vent-tube, so the fan cannot affect your float level; the level must remain stable.
Fourth thing;
>Start the engine, and warm it up, For an alloy-headed streeter, try to get the engine heat up to in the range of 185 to 205, AND stable. Do not attempt to tune your idle until the temp is stable., whatever it is.
>After the engine is warmed up, and stabilized, and NOT before;
If the rpm is too slow DO NOT change the speed screw. Instead, advance the timing.
If the rpm is too fast; DO NOT change the speed screw. Instead retard the timing.
In either case, do not install a timing light, just rotate the distributor to get whatever Idle speed your engine likes, but as slow as it will comfortably tick over, or no less than 700. Let it warm up.
Take a walk behind your car. If the exhaust burns your eyes, you got some work to do, and;
Now you can begin the idle tuning.

As for that 292 Chrysler cam in my engine (it's long gone now); I got it to idle at 550 in First gear with a manual trans, pulling itself on a flat level, hard parking lot; but climbing over a dime was sometimes a lil iffy,lol. I measured that cam at [email protected]. The starter gear was 2.66 x 3.55=9.44
IMO; This is about the biggest cam you should ever consider installing in a streeter, and most definitely not with 3.55s, which I found out the hard way.
To get to that point; the Neutral idle was 750@ 14*advance, and with additional Idle Air bypass amounting to one hole drilled in each primary throttle valve at 7/64ths inch.
Your bypass will have to be determined by trial and error, because
we have NO IDEA as to the size of the cam.