MP DISTRIBUTOR TIMING CURVE 340

[CFD244]

Hi Folks

Still playing around with my "mule" 318 getting Fuel and ignition straightened out before I break in the 340.

Today, I ran my Mopar Performance distributor (bronze internals). Here's what I found.........

Advancing began at approx 1500rpm. By 1600, I was at 5*, by 1800 at 10*, and at 2000 I was at 14*. I believe the advance is locked out at this point (14* advance) with the plate inside the distributor since going to 2500rpm yielded no further timing increase. I will be moving that lock point eventually to accommodate the idle timing (which I expect to be around 14* btdc to maintain a civil idle speed) that I will have with the 340, shooting for 34* all in by 2500.

At 15 inches of vacuum, the can added 17*. This was pulling vacuum with an external source from idle.

So.........Does this sound like the type of curve I am looking for? Mild 340, 9.3:1(XE 262), 2500 stall, 3.23 gears, auto transmission. This is going in a '71 Demon cruiser. I am hoping the engine will be punchy, and have the balls when I pull out to pass.......No track time.

Thanks FABO

 

[4spdragtop]

No help Troy, but I'm watching

 

[Bewy]

14* of idle timing will NOT be optimum with the rest of the combo. Hard to give an exact number, but 25-30* at idle would likely be optimum.
See the link......

 

[CFD244]

I agree, but at 15* I was idling at 1000rpm. I could get it down, but the throttle plates would be almost closed.

 

[RustyRatRod]

There's really no other way but to drive it and put it under a load. I agree with our friend down under that 14 initial is likely not enough, though. No two combos are alike, so you're going to have to experiment around with it.

 

[Kendog 170]

My 340 4 speed has a stumble when letting out the clutch. Been thru the carb. (Tranfer slots,accel. pump,EctTiming at 14 BTDC. If I bump it up to 20' or more , no more stumble. I'm waiting till spring to do a recurve.

 

[CFD244]

But what about idle speed with an automatic car? How do you keep that down near 850 or 900?


Also, are the advance numbers that I posted acceptable? (When advances are happening and how much is happening relating to engine RPM) ie, The shape and speed of the current curve.

 

[Johnny Dart]

12/14 initial with 34/36 all in, is where your mild 318 will run best all around. Remember to set your timing, with vacuum advance disconnected. Once in this ballpark, you will be able to set your idle.

And before I get the red X's, and all the hate mail, read the original post. Cruiser and no track time.

 

[Mattax]

Today, I ran my Mopar Performance distributor (bronze internals). Here's what I found.........

If it looks like this, its a Mallory built version.

Changes don't do what you think they they will do. In particular changing the advance also changes the initial spring tension and degrees out before the secondary spring joins in.

The YT, YH advance is a PIA design to make it do what you want. I was trying for something similar and was moderately succesful using heavier springs and bending loops and/or spring pivots.

It came up a few times in this thread with some pictures of springs and weights (skim through you'll see a little on page 1, then bottom page 2 into 3).
Too much advance at light throttle?

and a discussion about modifying the weights in this thread.
Modifying YH Advance Weights
Which I haven't got around to trying myself yet.

Advancing began at approx 1500rpm. By 1600, I was at 5*, by 1800 at 10*, and at 2000 I was at 14*. I believe the advance is locked out at this point (14* advance) with the plate inside the distributor since going to 2500rpm yielded no further timing increase.

Advance should begin at 1000 rpm or less. Stock, non-smogged 318 should start advancing by 800 rpm.

I will be moving that lock point eventually to accommodate the idle timing (which I expect to be around 14* btdc to maintain a civil idle speed) that I will have with the 340, shooting for 34* all in by 2500.

That's too much timing at 2500 rpm to use with vacuum advance.
2 bbl 318 is a pretty efficient engine. Doesn't need that much advance until high rpm.

I'll promote reading up on what timing does, a post from this thread: total timing reading accurate with no load?

Since your timing measurements started with TDC, lets plot them onto a chart with a factory 318 distributor that was meant to be set at TDC. Some others graphed here.

You want the advance to begin at a lower rpm. Then response off the line should have more pep. For your 318, if you want to learn how the adjustments inter-relate, would be worth changing the advance from 14 to 18 or 20. If I remember this right, increasing advance range this will increase tension on the primary spring and also also allow the long loop spring to engage earlier. If you observe inreasing the advance is putting more tension on the primary spring, then you will have to bend the tab in a little. The mechanism already has too much force on it at rest, and this is holding it back from advancing until 1500 rpm. If it has two short loop springs, exchange one for a long loop spring. Make sure the long loop spring is a little loose at rest.

At 15 inches of vacuum, the can added 17*. This was pulling vacuum with an external source from idle.

Good info. Should also test for what vacuum is needed to begin obtaining vacuum advance.

Still playing around with my "mule" 318 getting Fuel and ignition straightened out before I break in the 340.

That's fine

So.........Does this sound like the type of curve I am looking for? Mild 340, 9.3:1(XE 262), 2500 stall, 3.23 gears, auto transmission.

For the 340 - No.
For that an advance curve more like the Chrysler built MP distributors came with, or that Rick works out here.

Distributor curve help needed

I just checked the curve of my stock distributor (ported vacuum blocked) and need some help to optimize. Car is a 70 Duster with a stock 360, stock converter, 3.23 gears, 28" tires. Distributor currently has FBO black & gold springs with no limiter plate. 500-24* 1000-24* 1500-25* 2000-27*...

www.forabodiesonly.com

edited since MoparR&D shook my brain

 

[RustyRatRod]

But what about idle speed with an automatic car? How do you keep that down near 850 or 900?


Also, are the advance numbers that I posted acceptable? (When advances are happening and how much is happening relating to engine RPM) ie, The shape and speed of the current curve.

Turn the idle screw down. My Ford truck is up around 25 initial because of the low compression 400 and I can idle it down until it stalls. If you cannot do that, something is wrong.

 

[RustyRatRod]

12/14 initial with 34/36 all in, is where your mild 318 will run best all around. Remember to set your timing, with vacuum advance disconnected. Once in this ballpark, you will be able to set your idle.

And before I get the red X's, and all the hate mail, read the original post. Cruiser and no track time.

You won't get a red X from me, because nobody knows where it will run best until he actually drives it.

 

[-Boosted-]

This isn't for anyone but my own (current) combo that is 11 deg at startup and 36 deg all in on a locked out MSD setup. lol

X

 

[AJ/FormS]

OP said he can't get the idle any slower without shutting off the transfers, Hello!

Op, pay attention to Mattax; Shutting off the transfers will only make big trouble.
If you have proved that the only air that the engine is getting, is coming in past the primary throttle blades and that the PCV system is properly calibrated, THEN.................Just retard the stinking idle-timing already; There is no other way to slow the engine down!
Then fix the timing with a two-step curve. Ram it in hard and fast to 2500/stall, then slow it down to finish up without detonation.

The 340 is likely to want it's own timing curve, I wouldn't waste any time trying to find a one curve fits all deal.

 

[Mattax]

Thanks AJ/
I wasn't suggesting retarding
My suggestion was not to retard or advance. At least not in the sense that most guys mean - which is to change the initial timing.

 

[MopaR&D]

If it looks like this, its a Mallory built version.
View attachment 1716171784
Changes don't do what you think they they will do. In particular changing the advance also changes the initial spring tension and degrees out before the secondary spring joins in.



Advance should begin at 1000 rpm or less. Stock, non-smogged 318 should start advancing by 800 rpm.


That's too much timing at 2500 rpm to use with vacuum advance.
2 bbl 318 is a pretty efficient engine. Doesn't need that much advance until high rpm.

I'll promote reading up on what timing does, a post from this thread: total timing reading accurate with no load?

Since your timing measurements started with TDC, lets plot them onto a chart with a factory 318 distributor that was meant to be set at TDC. Some others graphed here.
View attachment 1716171783
For your 318, if you want to learn how the adjustments inter-relate, would be worth changing the advance from 14 to 18 or 20. This will reduce tension on the primary spring, and also delay the effect of the long loop spring. Retest and the advance should begin at a lower rpm. The response off the line should have more pep too.

Good info. Should also test for what vacuum is needed to begin obtaining vacuum advance.

That's fine

For the 340 - No.
For that an advance curve more like the Chrysler built MP distributors came with, or that Rick works out here.

Distributor curve help needed

I just checked the curve of my stock distributor (ported vacuum blocked) and need some help to optimize. Car is a 70 Duster with a stock 360, stock converter, 3.23 gears, 28" tires. Distributor currently has FBO black & gold springs with no limiter plate. 500-24* 1000-24* 1500-25* 2000-27*...

www.forabodiesonly.com

The Summit billet distributor I have has the same style of internals. I'm trying to wrap my head around how changing the amount of mechanical advance changes the initial spring tension...?

 

[RustyRatRod]

OP said he can't get the idle any slower without shutting off the transfers, Hello!

Then like I said, something's wrong. Hello!

 

[Mattax]

I'm trying to wrap my head around how changing the amount of mechanical advance changes the initial spring tension...?

That's why its best to do it and observe the mechanical relationships.
Without digging into my notes, etc., with the YH advance when the top plate moves relative to the one below, it also moved the weight pivot relative to the spring perch.
What I really think about the design is exactly what Tuner wrote. But if that's what someone has, thats what they have to work with.

I graphed an example in that thread quoted above. This one Distributor springs

So IF this is a Mallory YH advance @CFD244 may actually find is that increasing the degrees of advance allowed may actually delay the start to an even higher rpm. I've edited my earlier to reflect that is probably the relationship. This is where observing on the bench is really worthwhile. If you want to get fancy you can put a 360 degree 'wheel' underneath and place a pointer on the distributor top shaft. Buts its not neccessary just to see what is going on. What @CFD244 might also find is the 14* setting was stopping the advance before the secondary spring did anything. No way to know what is in there and how far out or in the perches are bent until one looks. Sorry about not being more specific, but its been over 5 years since I've messed with Mallory stuff and I just don't recall the details off the top of my head anymore.

 

[CFD244]

Turn the idle screw down. My Ford truck is up around 25 initial because of the low compression 400 and I can idle it down until it stalls. If you cannot do that, something is wrong.

My idle rpm drops to about 550 with the throttle blades closed and 15* of Idle advance.

 

[CFD244]

PCV system is properly calibrated,

Well, I have a 3/8 hose running from the nipple on the front center of the carb to the PCV valve. I figured that was all that was needed. I can say, that the breather fitting is open to the atmosphere since my tinkering is taking place without the air cleaner installed.

Am I missing something?

 

[4spdragtop]

Shouldn't pcv connect to front of carb? Rear port for power brakes?

Well, I have a 3/8 hose running from the nipple on the rear center of the carb to the PCV valve. I figured that was all that was needed. I can say, that the breather fitting is open to the atmosphere since my tinkering is taking place without the air cleaner installed.

Am I missing something?

 

[AJ/FormS]

I have never had any success with plumbing the PCV to the Secondary side. and
that PCV hose has to be rated for vacuum, else it will get sucked flat when the vacuum rises. Fuel-line will NOT cut it. Transmission line maybe. But your jobber should have a thick-walled hose just for that.
Plumbing to the back, under the fully closed secondaries, that incoming PCV air is dry, as in, there is no fuel in it. Thus whatever cylinders get that air, will run a hair lean. You can sorta fix the front with the mixture screws, at idle, but as soon as you tip in the throttles, the Messed up AFR makes trouble.
I tried this once, and that's how I found out.
Many years later, having forgotten all about it, I tried it again, Bam!, then I remembered.

2500 stall, 3.23 gears, auto transmission. This is going in a '71 Demon cruiser. I am hoping the engine will be punchy, and have the balls when I pull out to pass.

With 3.23s and 27" tires 60 mph = about 3850 at WOT. At a cylinder pressure of 185psi, your demon would positively scream past any slowpoke in your sights.
However, if your pressure is closer to 150, well, let's just say, the balls are not gonna have reached puberty yet, lol.

 

[CFD244]

Shouldn't pcv connect to front of carb? Rear port for power brakes?

LOL...........That's what I meant. PB's are off the manifold. Edit coming, thanks Steve

 

[CFD244]

I have never had any success with plumbing the PCV to the Secondary side. and
that PCV hose has to be rated for vacuum, else it will get sucked flat when the vacuum rises.

With 3.23s and 27" tires 60 mph = about 3850 at WOT. At a cylinder pressure of 185psi, your demon would positively scream past any slowpoke in your sights.
However, if your pressure is closer to 150, well, let's just say, the balls are not gonna have reached puberty yet, lol.

See edit...........PCV off the primary side.

I actually dropped my comp ratio because of the small cam I am running. Calculators told me 7.6 DCR if I recall correctly. With 10.2, I was getting near 8.

I'll be running 245 60 14's........25.5 inches tall if I recall?

Yes it is vacuum rated.

 

[Bewy]

What carb?
The engine is no longer near stock, so stock timing curves go out the window. That is why the engine is going to need 20+* for best idle.
If you want to dial in the timing curve with this modified engine, it has to be done on a dyno; anything else just becomes a good guess, which might be good enough.
I have run plenty of carbs with the t/slots closed off....& never had a problem. Many TQs & AVS carbs had bypass air holes in the base which can be blocked/reduced in size to correct the T slot position. The problem with T slots is when there is too much exposed at idle.
PCV can be run from front or rear of carb, whichever is more convenient. It is all manifold vacuum. On Pontiac V8s, the PCV does not connect to the carb. It goes into a junction in the intake, about 2" in front of the carb. The junction accesses both planes [ dual plane intake ].

 

[Bewy]

Kendog,
Post #6. Yep, fixed so many tip in stumbles by just giving the engine the timing it wants that I have lost count.....

For those wondering why this fixes the stumble, it is actually pretty simple: the engine makes more HP with the extra idle timing. The engine now is making the HP it needs to carry the increased load.