Average elevation is 1000 feet or less above sea level in these parts.

My Dart’s engine is configured as follows:

Holley 390 4v.
Rebuilt Dec 2008 using trick kit 37-720.
Accelerator pump #25 nozzle, orange cam #2 hole.
Jet = 512 or 51 medium
Power valve = 3.5 in gear idle pulling 3-8 inches Hg fluttering needle.
Black secondary spring.
Primary butterflies drilled 3/32â€￾ for idle quality


Clifford 4v intake & shorty headers.
Head ported, gasket matched, over sized ENDB SS valves 1.70â€￾ & 1.44â€￾, shaved 0.075â€￾ giving calculated 9.5:1 compression ratio.
Unknown cam with pronounced lopping in gear idle & 600 rpm, slight lop in “Pâ€￾ at 1000 rpm.
Valve lash: intake 0.012â€￾; Exh.0.024â€￾

Timing:
12 BTDC initial
Governor 11R
R11 vacuum can turned in 4 turns
Advance set to come on at 1600 rpm, full in by 2800 rpm

2600 rpm stall converter, so six cylinder stall is around 2300 rpm
3.55:1 Suregrip

Cruise rpm ranges 2500 to 3000 rpm.

Problem:
Flat, or a slight bog off of idle light throttl imput, very bothersome.
Flat, or stumble at light throttle in cruse rpm range.
Little or no flat over 3000 rpm.

Observations:

Fuel consumption has dropped from a high of 20 mpg to 17 mpg in the last 200 miles.
According to: 4secondsflat dot com/plug_chart plugs are in good to real good range where the front three look to be burning a bit hotter than the rear three plugs. I run a hotter plug in #1 & #6 Gap had expanded to 0.045â€￾ from initial setting of 0.040â€￾ some 4000 miles ago.

The plugs indicate jetting is ok
Idle vacuum suggests a 2.5 power valve may be needed
Perhaps accelerator pump need a bigger initial shot & longer duration

I have been moving up the spring strength since rebuilding the carburetor.
First the lightest spring was installed.
Second brown spring.
Most recently black spring is installed.
My thinking was that the secondary’s were cracking open around 3000 rpm, and these heaver springs would slow that opening, or move it up a hundred rpm improving mileage.

Any help on this quandary would be welcomed

Another ongoing annoyance is the intermittent back spinning of the engine when killing the ignition. Sometimes she shuts right off no back spin, other times a feeble little back spin of a couple of chugs, other times a great many revolutions of back spinning. I have taking to killing ignition while still in gear @ about 600 rpm to lessen spin-back.

When the engine is in a robust back spin mood, she has a bit more snap off the line, and willingly produces black script on dry pavement. When it is in the good behavior mode, willingly shutting down without kicking back, she lacks enough grunt to barely spin the tires. This to me is a sign that perhaps my timing is not falling back to the 12 BTDC range every time. The governor moves freely, no sticking that I know of.

However, during starting it never kicks back.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Flat, or a slight bog off of idle light throttl imput, very bothersome.
Flat, or stumble at light throttle in cruse rpm range.

This sounds like one of two things or maybe both. It could be the vac advance coming(1600 sounds real soon) in too soon or the acc pump is running out of travel due to poor adjustment. You also sound a lil lean for your setup. Check for vac leaks too. This could be part of the issue. I would go lighter on the secondary spring. If you go by Holley rec it should be yellow #2 +/-.

Advance set to come on at 1600 rpm, full in by 2800 rpm
Cruise rpm ranges 2500 to 3000 rpm.

These should match roughly as far as I know, but I'm sure Dan will tell me I'm wrong.

I would start with vac advance and then sec spring if you are sure of no vac leaks.

,Cliff

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'72 4 dr Dart
'63 4 dr Dart

Have you tried going as loose as possible on the valve lash, in hopes of getting the idle vacuum & over-all vacuum signel a little stronger?
DD

Cliff.

Thanks for a quick response.

I have no advance until 15-1600 rpm at which point mechanical begins to come on. I spent several weeks last year with Aggressive Ted holding my hand walking me through a recurve. At the time, because of my cam, getting this engine to idle at a constant rpm with out rolling up & down 50 to 75 rpm for about 125 rpm swing was imposable.. This made adjusting idle mixture futile.

The vacuum advance is coming in at 1800 rpm with 3* of advance above the mechanical of 25 for a total of 28*. At 2200 rpm, mechanical is 26* and vacuum is 19* for a total of 45* which is ok as at the rpm I’m under way in the 45 mph range.

All this trial & error tuning is chronicled here, and here, , and here in a stickey on the engine page.

Now the idle is very steady 600 or so in gear 975 or so in park. Theoretically I can now tune for the proper power valve. According to Holley’s tech. page, should be one half of vacuum reading at idle. Hence, the presumption that I need to drop down to a 2.5 from the current 3.5 value due to the cam’s over lap causing a jittery vacuum gage needle. I have the feeling that when stopped at a light this 3.5 valve is opening and dumping gas for no reason while at idle.

As I type this answer to you, I now am beginning to think that perhaps the power valve is dumping gas while I’m attempting to kill the ignition and causing the spin back. Possible?

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Ok, well just as a quik test, unplug vac advance and plug port on carb to see if you still have the same flat or bog. If so I would guess acc pump...

,Cliff

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'72 4 dr Dart
'63 4 dr Dart

Hi Doc, long time since the last time we talked, thanks for your time.

As you had advised last year, I did slack off the lash to, if I remember correctly, 0.018â€￾ & 0.028â€￾ which lead to a bit of clatter, so I incrementally tighten down the adjustment a wee bit to reduce some of the racket but not all. And, all though I didn’t record the final adjustment at the time I seem to remember that it became 0.012â€￾ & 0.024â€￾ or perhaps 0.014â€￾ & 0.024â€￾. I did enlist the guidance of my vacuum gage to perform the adjustment.

I have to admit that my ear doesn’t pick up the nuance of 0.002 “ steps very well with this lopping cam, and perhaps I need to revisit my valve train with a feeler gage. Could it be some valves want to be a bit tighter or looser than their adjacent neighbor due to varying valve seat depth, or am I taking a walk on the far side with this theory?

This engine has in the past tended to tighten up gaps on select valves. Weather this is due to my sloppy workmanship, or just the nature of the beast, I can’t tell for sure.

Tomorrow is predicted to be sunny & warm as opposed to today's rain. I should be able to get under the valve cover as the dirt guys re-grade my yard & driveway, and the mud dries up here at the New Digs…

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Does the head have hardened exhaust valve seats?

The lash can tighten-up on the exhaust valves, from valve seat recession so make some notes on which valves are tighter and re-set.
If the same (exhaust) valves are tight next time, there is a good chance they are receeding into the head.

I hate to say that it is hard to get carburators to work well, thru all their circuits and transisition points, when using a radical cam.
DD

Doc.

When I had the ENGB valves installed last spring I also had new over sized hardened seats fitted. Come to think of it, the tightening of adjustments were pre head rebuild.

I’ll know more tomorrow after a lash recheck.,

Cliff,

I’ll perform your vacuum advance isolation test as well tomorrow. The accelerator pump was my first guess, but it gives an instant full squirt when operated by hand while looking down into the bowels if the carbonator when at rest.

Also I shall check the gap between the accelerator pump arm, and throttle linkage both at rest, and WOT.

In the past before I was able to get the engine to idle down due to a vacuum leak, I had the throttle idle screw turned in causing too much of the transmission circuit to become exposed which produced a real nasty flat spot. I have since dialed it back so only perhaps 0.020â€￾ or less is now exposed. At the time of this adjustment, the old flat spot diminished by 90%. I’ll also check this setting once again.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

You are def on the right track, just something simply over looked I bet. Good luck!

,Cliff

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'72 4 dr Dart
'63 4 dr Dart

Is that a typo, or are you running a list 0-8001?

That's not a 390 if such is the case. It's a 450 cfm, and your primary is too lean.

CJ

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Part of Tyrde-Browne Racing

Ceej: Tyop…

0-8007 is what it is.

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67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Oh good! I was wondering how you'd run an economaster spreadbore this long without really bad vacuum leaks!

That cam has to be really radical to cause this much trouble. Have you considered a nice 2300 Holley?
With the black spring, it's doubtful your going to get the secondary open until 6000 rpm anyway, if at all. I don't think you have enough primary jet. If your running 15" of mercury at cruise, you really need a deeper power valve. The idle vacuum is way too low. It's not going to tune with those numbers.

Maybe a nice 280/270 RDP cam from Erson? Get yourself some idle vacuum. Have you run the numbers on the cam that's in there? I think your going to chase your tail until you do. A dial indicator and a nice long lifter replacement tool. Measure that cam with a degree wheel.

Pitch that 11R. Weld it up or find a 9R. Make up your cruise advance with the vacuum can when your foot isn't in it.

CJ

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Part of Tyrde-Browne Racing

Just swapped out the old power valve that I thought was a 3.5, but turned out to be an 8.5, and installed an old 2.5 valve. Than took it for a ride, flat / bog spot is gonzo. The black secondary spring was still installed at this point. I figured one adjustment at a time.

No wonder this thing has been getting crappy fuel mileage. Every light, and light throttle opening she was dumping gas. Could the main jets be too small now that the power valve is staying closed for longer periods?

After test run, I adjusted isle mix using the vacuum gage. Now the engine is pulling a twittering 9 to 10 inches of Hg at 950 rpm, and an aggressive fluttering needle bouncing between zero and 5 inches of Hg. at 600 rpm in gear. Running the idle in park to 1050 rpm the vacuum swings up to 12.5 inches of Hg. with a very steady needle.

Also install the lightest secondary spring, but did not get a chance to whined her out as Mrs. A was on board for a trip to the grocery… Testing, and matrimonial bliss do not go hand & hand around here. Poor thing is a wee bit embarrassed with all the noise, thrashing, and the yellowness of it all...

Since this carburetor adjustment, kick back has diminished. I’ll know better after driving her for a few days how much better it is.

Tonight or tomorrow I will examine the valve lash while the vacuum gage is attached. Perhaps I can dial her in a bit more. It would be nice to steady that needle during in gear idle. I suspect the power valve is still dumping fuel at every needle swing below 2.5.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC

Yes your mains could be be too lean now. I have 53 mains and need to go down on squirter.(based on readings from my o2 sencor, cruise is stoic w vac adv, acc is rich) Secondaries were too rich w metering plate so I bought the jettable sec plate(not sure what sec yet, car is apart lol).

I would follow Dougs advice and check the valves again, if vac. flutter is that bad. I would just put the #2 yellow in also, it won't change your primary jetting.
Do a double check for vac leaks too after checking valves. Do you have HEI or points still? If points, check gap again, have had weird things happen when loosing gap. I always TRY to cover my bases and double check things. Not always successful though, we all have brain farts. I've always been told to write my changes down(still don't). And ya, one change at a time.

,Cliff

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'72 4 dr Dart
'63 4 dr Dart

hellion_locdogg:
I have the orange box and a MSD Blaster 2 coil making plenty of fire in the hole.

As for the secondary spring biz, I’ll run the lightest for a while, than work my way up, probably end up with one of the yellow springs.

Aside from getting this engine to have smother throttle response, I would like it to pull harder in the higher rpm band. One of the ways to get more high end grunt is to get those secondary butterflies to open up.

I’m going to revisit the valve lash settings tomorrow, and than go for a spin long enough to get a plug reading. Than address jet size if necessary.

_________________
67' Dart GT Convertible; the old Chrysler Corp.
82' LeBaron Convertible; the new Chrysler Corp
07' 300 C AWD; Now by Fiat, the old new Chrysler LLC