Idle Rpm High in Park, Low in Drive

[Newolddart]

Old cars are new to me so hopefully y’all can help me out...

Bought the car some time ago now and am working through lots of random issues.

69 Dart 360 Magnum swapped with a custom cam (no specs but appears pretty mild), edlebrock heads with 350 roller rockers (relashed), hei ignition conversion with stock 360 distributor (vac advance) and msd 6al box/wires, 650 double pumper (no choke), 727 with unknown converter stall (7/16 bolts in flexplate with half moon cutout). Motor is mechanically sound and recently rebuilt with limited miles. Trans has been “flushed” several times in searching for several leaks, fluid has always looked good. 91 in the car (highest we have here).

Crank does not have timing marks so timing is being done by ear/feel (I know I know) even though i do have a timing light. I have a vacuum gauge (15-16” at idle) and a wideband in the car (idle IN GEAR at about 13.5-14.0 afr, slightly richer in park). Intial timing, idle speed and idle afr set for highest vacuum, smoothest idle and best afr.

New issue im working through:

Car all warmed up and idling in park with vac advance capped, i set idle for 900-1000 rpm, put in gear she drops to 750-800. All is well.
Reconnect vac line in park, rpm spike to 1300-1600 rpm. Vac line connected and dropping into gear, rpm drops to almost 600 and car wants to stall and die, when it does die it sputters out the carb. Vac hooked up and idling, im hearing some engine noise I can only attribute to pinging (near #8 or 6) because it wasn’t there before and almost entirely goes away with vac disconnected.

Vac canister diaphragm confirmed good. It’s connected to full manifold vac (per everything im reading, including professional recommendations, full manifold is perferred to port). I have tried retarding/advancing timing with vac hooked but it either increases rpm slightly or makes the car die.

Warmed up, car is hard to start and pops some out of the carb unless I hold the pedal down and crank. Cold, engine will crank right off.

Is this huge disparity in park idle vs drive idle normal for vac advance? Am I missing something easy? Very new to distributors so I don’t know if the vac advance is simply too much for the car at idle or if I’m missing something...

I have read through numerous forum posts, articles etc and am still at a loss (now at a loss with a headache). I obviously don’t want to cap the vac and bandaid fix it but I’m getting close...

I plan to dump the oil and verify the noise i am hearing is not motor related. Will pull plugs and check for pre det/pre ign.

Any help would be great.

 

[OldmanRick]

That's a lot to digest. Can't believe the balancer does not have at least a TDC mark? You need to figure that out first and then get the timing right without the vac. advance. It can be added in due time. Vac is certainly high enough that the power valve in the carb is not the issue.

Another thought is voltage to the distributor. Not sure what that one needs, but if it's lacking, that may be the issue?

 

[Professor Fate]

Hook the vacuum advance up to ported vacuum. Think about it. At idle is when your manifold vacuum is at it's highest, and that's when you're pulling full vac. advance now, probably to the point where your mechanical is kicking in. As you increase the throttle, manifold vacuum falls off and decreases your advance. Just the opposite of what you want. Hook it up to ported vac., and you have no advance at idle- vacuum signal increases with rpm and so will your advance- just what you want.
As far as your timing mark goes, are you running a Magnum or '70 or newer timing cover, or a pre-'70 timing cover? Is your water pump a driver's side (early iron pump) inlet, or a passenger side (later aluminum pump) inlet?
If you've got an iron pump on a post '70 timing cover, or an aluminum pump on a pre '70 timing cover, then the pump inlet is covering your timing marks. They're there, just covered up.

 

[pishta]

advance the timing a little and see if it does anything. It may pick up idle RPMS. just dial it back with the idle screw. My 273-4 idled at 700 and dropped to about 690 in gear. Could not even tell it was in D. What AFR are you looking for at idle? 13's are about right. Turn the idle mixture screws for highest vacuum, then turn the idle down to get back to baseline and keep doing that until there is no more vacuum to be gained. Dont tune for AFR at idle. It is what it is.

 

[CFD244]

Get ready for it

 

[Dana67Dart]

Vac canister diaphragm confirmed good. It’s connected to full manifold vac (per everything im reading, including professional recommendations, full manifold is perferred to port).

IMHO.

that is your problem. At idle manifold vacuum is high 15 to 16 as you pointed out.

That means the vac can is full in adding 10 to 15 degrees.

These cars were designed to have a ported vacuum source for the vacuum advance. Very low to no vacuum at idle to the can. Then the instant the throttle is opened it sees vacuum and adds advance.

You can make a timming mark. You just have to determin when the #1 piston is at TDC of the compression stroke, then make 2 marks one on the balancer and one on the front cover. Get a timing tape and you have all you need to time it properly.

 

[Dana67Dart]

In answer to your original question.

It is normal for a carborator car to drop rpm when in gear.

Go to mymopar.com and down load your cars Factory Service Manual. It's free no excuse.

Also
Look at the (very corny but informative)

Chrysler Master Tech Service Conference Library – MyMopar

And watch the engine tuning movies. You would be surprised at how similar these old carborated cars are to modern EFI electronically controlled cars. They are just mechanical computers

 

[fishmens67]

First thing you need to do is find TDC and mark the damper. initial timing is very important when tuning the idle circuit.
I adjust auto transmission cars in gear.

 

[Professor Fate]

And a comment unrelated to your problem-
Those 350 rockers are hurting you, unless they're an aftermarket ratio. Chev 350s run a 1.5:1 ratio, Magnums (and their cams) are designed to use 1.6:1 rockers. Swap to the proper 1.6 ratio and that "pretty mild" cam will wake up a bit.

 

[Mattax]

with stock 360 distributor (vac advance)

For the moment we'll have to assume the advance curve has not been messed with.
However, unless the tag is still on the distributor, we don't know what that curve is.
If its from the early smog era, then it may have a very fast and long primary advance.
IF that's what it has, and the cam is mild enough that the manifold vacuum is strong at 650-750 rpm, then
set the base timing much lower, maybe around 5*BTDC and use manifold vacuum on the vacuum advance.
That will bring timing at idle speeds into the range 12 - 18* BTDC.

a wideband in the car (idle IN GEAR at about 13.5-14.0 afr, slightly richer in park)

A little lean for non-smog idle but its in the ballpark. WBO2 are not always accurate interpretors of AFR so don't sweat the number. Just use as a reference when you make changes.

Intial timing, idle speed and idle afr set for highest vacuum, smoothest idle and best afr.

Best AFR is the one that produces the most power. That is checked by losing the least amount of rpm when placed in gear. Another way to look at it is the highest manifold vacuum in gear.

Reconnect vac line in park, rpm spike to 1300-1600 rpm. Vac line connected and dropping into gear, rpm drops to almost 600 and car wants to stall and die,

Timing and fuel mixtures work hand in hand.
Adding that much advance lit off the cylinder so early it wasn't even in the ball park.

It’s connected to full manifold vac (per everything im reading, including professional recommendations, full manifold is perferred to port)

Well you can stick with them, or go with what works.

I have tried retarding/advancing timing with vac hooked but it either increases rpm slightly or makes the car die.

because its not close enough to adjust anymore.

 

[Newolddart]

You gents are great!

It’s now my work week but I’ll try plugging away at her and get updates.

The rollers are Jegs 350 1.6 ratio.

The front cover is post 70 with the serpentine belt setup. There is timing pointer off the front cover that reads a tdc and 5 ticks above and below, but the crank damper itself doesn’t appear to have any timing marks. *edit*
A closer look and the damper has some timing marks that are very difficult to make out as they have been painted over. Looks like it ranges 0-30 but very hard to see and didn’t show with my timing light...

Thanks so far!

 

[Mattax]

It is normal for a carborator car to drop rpm when in gear.

A little. Like 50 rpm, not 150 or 250 rpm.
Some older, pre-smog trucks will just shift into gear with a manual transmission and need no throttle.

First thing you need to do is find TDC and mark the damper. initial timing is very important when tuning the idle circuit.

^^^This^^^^
Mark the TDC and then use a dialback or timing tape.
Or I suppose it can be scribed on with a bit moer dexterity and patience than I have.

 

[Dana67Dart]

A closer look and the damper has some timing marks that are very difficult to make out as they have been painted over

A paint string stick with some sand paper wrapped around the end, a running engine and VERY CAREFUL APPLICATION will sand off some of the paint. Then a bit of white paint (Whiteout will work) on the timming mark.

You still need to be sure that the timing mark and the balancer are for the same engine. Early passenger side timing mark balancers mark is not on the same spot as the drivers side trimming mark balancers

 

[fishmens67]

a hack saw blade can be used to reclaim the timing mark, cleanout the paint.

 

[Professor Fate]

And being a Magnum balancer, make sure you've got the right timing mark- confirm by finding TDC on #1. Both Magnums I have in my shop right now have 3 different timing marks on each balancer, only 1 is for ignition. The other 2, I believe, are for crank sensor and EFI calibration.

 

[motorpirate]

And being a Magnum balancer, make sure you've got the right timing mark- confirm by finding TDC on #1. Both Magnums I have in my shop right now have 3 different timing marks on each balancer, only 1 is for ignition. The other 2, I believe, are for crank sensor and EFI calibration.

On our '01 Magnum it had a mark that lined up with TDC on the drivers side timing pointer, and another mark that lined up with the older passenger side pointer at the same time!!

 

[67Dart273]

Learn to use a piston stop. I still have the one I built in the mid 70's

Remove no1 plug, bump engine to get the piston "down some". Get a socket on the dampener bolt, and disconnect battery ground for safety. You can remove all plugs and make it easier

Rotate engine CW until engine stops against the device. Make an accurate temporary mark below the TDC mark onto the balancer. Clean, tape, marker, etc

Do same thing, rotate engine CCW and make a second mark same as before.

You will now have two marks some distance apart. True TDC is halfway in between. With care this can be as accurate as it needs to be
==============================================
If you have breaker points igntion, SET DWELL FIRST before adjusting timing. Wide points gap/ low dwell numbers ADVANCE the timing, while the opposite for worn, "narrow" gapped points.


Make sure to get vacuum "off of" manifold vacuum. Determine where timing should be do NOT necessarily use factory "smogger" specs. Retarded timing aggravates this problem. For a "stocker" I'd run at least 8BTC idle/ initial timing, with no vacuum If you have a hotter cam, look into recurving the distributor advance, and go more, at least 12, in some cases 15-20 WITH A recurved distributor.

A lean carb mixture can aggravate this problem. Look for vacuum leaks, cracked or missing hoses, etc. See if carb mixture screws have affect

Warm engine up to normal temperature, and set idle speed. Adjust mixture screws in/ out to fond a "rough" peak starting point. You want them (to start) a bit CCW or "rich." Best to use vacuum gauge or tach

Adjust each screw, and go back/ forth a couple times. Screw them in "slow" and find closer "peak." After doing that, screw both in until idle "just quite" does NOT drop from the peak RPM/ vacuum. In other words you want the "lean side" of "peak."

Now if it does not 'act" right, and after jigging idle speed in drive, some cars "like" to be final adjusted in drive. I would say this is "fairly rare." BE CAREFUL. Chain rear hitch to a tree, use blocks on all 4 wheels, and or a helper. With trans in drive, "jig" idle speed and mixture for best "in drive" RPM and idle quality

 

[67Dart273]

IF YOU BUY a manufactured piston stop

Take steps to make sure that the center plunger DOES NOT WIGGLE in the threads. If so take steps to fix it. Figure out a way to put a lock nut on the thing, or in extreme cases, get a longer bolt that will fit the threads, and put a lock nut out on the outer end of the body.

If you are only going to use it on one engine type, such as SB Mopar, just screw the plunger all the way in until it bottoms and locks "solid" and see how the length is. If it turns out to be too long, !!!!! Just saw it off, grind it "beveled" and smooth and remove burrs

I'm not saying the length I show on mine is "perfect" but it works fine for me.

 

[Bewy]

Dart,
You are on the right track. Post #3 is bad advice.
The HUGE jump in idle rpm when you used MVA was because the engine is making more hp [ the rpm increase ] & the engine liked it [ smoother idle ]. With PVA, Chrysler missed the boat....Simple as that, didn't happen often, but happened this time with MVA...
Keep MVA. You will need some timing marks to dial in MVA accurately.
With MVA, the engine is already making the extra hp to carry the load as the throttle is opened off idle. That is why many off idle flat spots are cured by swapping from PVA to MVA. With PVA, the extra power is not there to take the load, it takes some time & in that time you get a flat spot.
To check how much idle timing the engine wants, disconnect & plug VA. Engine idling, in gear, rotate dist slowly to advance timing until highest rpm is reached; now check what the timing is & that is the timing reqd. Say it is 32*. Idle timing can be any combination of MVA + initial.
Depending on the above test, you may need to limit the amount of vac adv. [ fabricate a stop ]
The large rpm drop going into gear is most likely from using a fixed factory unit or incorrectly adjusted adj unit that does not work with reduced vacuum. You need an adj VA unit & screw the Allen Key fully CW [ softest setting ]as a starting point. You will then have stable idle in or out of gear.

 

[Mattax]

He put a load on the engine and it lost rpm.
That was because it was more powerful??????

When the engine is running in neutral, this is considered a light load. Engines can usually be run lean at light load.
When an engine is placed in gear at idle, say 600 rpm, this a relatively heavy load. Heavy relative to the power beig produced at that rpm.

The mechanism used to achieve timing under various conditions is NOT a true determinant of whether the timing is correct for those conditions. Many distributors are sophisticated enough in design that vacuum advance was only needed to adjust timing for leaner burning.

Your crusade against Chrysler (and other) engineers really gets tiring. @Newolddart is in this mess at least partly because he read that sort of self proclaimed "expertise". I really hope this thread does not become another one of your crusades. All of the previous posts have been pretty focused on things to be addressed.

 

[CFD244]

.......And away we go!

 

[Oldmanmopar]

Invest in a 3500 stall converter and your drop in idle will disappear. Then Make sure the arm on the distributor vacuum advance canister has 8.5 not 11.0 Plug the hose into the carb above the venturis on ported vacuum after timing is set. Set the timing to 35 Total advance at 2500 RPM or higher with the hose unplugged, This would be where I would start and in that order.

 

[Professor Fate]

Reconnect vac line in park, rpm spike to 1300-1600 rpm. Vac line connected and dropping into gear, rpm drops to almost 600 and car wants to stall and die, when it does die it sputters out the carb.

That statement kind of explains things in a nutshell.
When reconnecting your vac advance to manifold vac, you are essentially advancing your base timing 20-22* (depending on your particular vac can) AT IDLE. Waay too much, which is why you're popping/backfiring out the carb- it's firing too early now.
Cap your vacuum ports. Set base timing with your vacuum gauge, as you have done. Adjust your idle mixture for best rpm/smoothest operation, then set your idle speed. Double check your idle timing.
Now reconnect your vacuum advance to ported vacuum. You should notice NO change to your timing. If you have an adjustable vacuum can, set it so the advance begins just off idle.
You will now have a smooth idle with no popping/pinging, smooth off idle operation with no stumble, and no appreciable RPM drop when put in gear (how much is dependent upon other variables, i.e. convertor, trans. fluid temp., etc.).
Now is when you can start slowly increasing your RPMs until you find your total advance, and at what RPM it occurs- tailoring this to your combo may involve changing springs in your mechanical advance, and/or the use of limiter plates, but there are hundreds of other threads on that subject- so at risk of turning this into a novel, I won't get into that aspect here.

 

[T56MaxTorq]

Perhaps plug the vacuum brake booster if you have one. To Eliminate the booster being a problem. big air leak when you push the brakes and put it into drive will kill any engine.

 

[mbaird]

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