1968 340 timing

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Great advice @AJ/FormS . I had forgotten to mention about the transfer slot setup. The attached photo shows the correct relationship between the throttle blade and the transfer slot. The optimum setting for the blade is to uncover an approx square opening on the transfer slot. It is a real pain to keep removing the carb to check this setting, so I found the best way (with the carb off) is to use the idle screw. Back it out until the blades are completely closed, then turn it in and count the number of turns until you see the square opening. Then you have a reference point. As an example, lets say that the optimum idle screw setting is 1.5 turns in. After advancing the idle timing, the engine rpm will increase and you will thus find you have to back out the idle screw to get your desired idle rpm. This upsets the transfer slot setting!! ie you need to get the idle screw back to 1.5 turns in. A couple of ways to do this:
- On a Holley 4 bbl, you may have an adjustment for the secondary throttle plate idle position (under the throttle plate!!) You can adjust this to allow the sec plates to go further closed (ie reducing the idle speed) and that allows you to screw in the idle screw(to increase the idle rpm) to get back to 1.5 turns out.
- If there are holes drilled in the throttle plates, you may have to reduce their size to allow the correct idle scre setting.

It is a lot of work to get the correct setup when you significantly change the idle timing!!!!
Jim
PS There are a couple of guys who are really good at recurving and/or restoring distributors.
Joe at Joe’s Mopar Distributor Restoration
and Ray at Halifaxhops https://www.forabodiesonly.com/mopar/members/halifaxhops.33547/

Holley transfer slot008.jpg
 
Lets explain why the '68 340 timing spec is 5* BTDC with some graphics.
Short version: It's a part of a package of changes to reduce the emission of HC and CO at idle. For it to work correctly, the rest of the package has to be used. Then it will be 'ok'. It won't work well if there ares changes to the cam, to the advance curve, to the idle fuel mix, or to the idle rpm used for setting the initial timing.
Chrysler explained this in their Master Tech Series, on this page 1967 Chrysler Imperial CAP System from the Master Technicians Service Conference Session 241, 1967 Chrysler Imperial Engine Combustion fom the Master Technicians Service Conference Session 240

Long version:
For best fuel economy and power at idle rpms the fuel-air mixture should usually be relatively rich, between 12:1 to a little more then 13:1 air to fuel.
The timing that best matches the mixture at idle speed is that provides the time (in fractions of a second) to get the best leverage on the crank.
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To reduce the carbon monoxide at idle and deceleration, timing at idle was delayed.
The results is more of the cylinder walls being exposed to the flame resulting in hotter combustion conditions and more thorough burning of the fuel components.
The slight loss in power was covered by slightly higher idle speeds.

Above idle rpm, timing was advanced very quickly so it would be virually the same as a non-emissions engine.
Unfortunely 340s were never released in a non-CAP version, so we can't compare directly. But we can look at other Chrysler hi-perf engine packages that had non-CAP and CAP versions.

So lets look at timing for 1967 273 Power-Pak using specs from the Plymouth FSM.
This is the range of acceptable timing when initial was set to the target 10* at 600 to 625 rpm.

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And now overlaying the timing for the same engine CAP equiped with a manual transmission.
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And here's an overlay of a '67 440 4bbl non CAP an dCAP with Automatic transmission.
upload_2020-8-17_17-43-47.png



Getting to 1968, here's the 440 CAP timings
upload_2020-8-17_17-46-56.png


Now looking at the 340, we can see how the 5* initial worked with the matching distributor, when set at 625-650 rpm.
upload_2020-8-17_17-48-58.png


We can also see Plymouth was relatively conservative with the timing in general. This probably was done to ensure there would be no pinging under any conditions, location, or fuel. So even with an original distributor advance, you could experiment with additional initial timing. For example if the distributor advance rate was near the maximum, setting initial 9* at 650 RPM would result in a 34* at 4000 rpm, WOT. That's not unreasonable. In fact its typical starting point for performance tuning the timing.
upload_2020-8-17_18-2-51.png



 
SorryAJ, I disagree about the "can't have too much initial timing"
Most certainly can. See people do it all the time.
With an automagic transmission, I agree we can get close to most efficiency by minimizing rpm loss and vacum drop, when going from idle into drive.
And we still need to play the balancing game with the fuel mix as well as the timing.
With a stick its more difficult to tune a hot engine for more power at idle since we can't easily give it a consistant load.
 
Dartswinger70 said:
I am setting my 340 up,I have bnoticed that it doesnt "like" the 5 BTDC initial setting, at frist I thought my damper had slipped,but i verified TDC and the mark lines up. My engine seems to like 10 or 15 btdc. It has been converted to MOPAR electronic igintion with Orange box. It will run at 5 btdc,eith 16 inches of mercury on a vac guage, but it doesnt start smoothly,it long cranks. I am thinking it needs more advance.
Click to expand...

My suggestion is to measure timing from the lowest rpm to the highest timing/rpm you can measure with whatever you have on hand.

Write down each measurement and plot on the timing graph. Use the 273 non-emissions graph as a reference.

Since it was converted to an ECU, clearly the distributor is not the original.
So the only way to know what the advance looks like is to measure and plot.

You can also look to see if the distributor still has its tag. The tags are held onto the side with a screw like this.
upload_2020-8-17_18-29-10.png


MP ones are often colored green.

One final thought. Initial should always be measured with the vacuum advance line disconnected and plugged. You prob know this but a lot of people don't. Golf tee is a good hose plug.
 
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This thread has so much good information in it that it should be in the How To section or a sticky
 
I lost a lobe on a camshaft on my first small block rebuild. The distributor was not factory-oriented and the vacuum advance was hitting the fire wall so I could not get the timing correct to get the engine started for a quick start up and camshaft break-in. Live and learn.
 
Looking at the picture of the Ops balancer, I can't believe that those numbers and the mark to the left don't indicate that the dot is 180 degrees out. It's just too perfect to be coincidence! I wish mine were marked like this. It seems to me that the distributor is 180 degrees out and they then moved all the plug wires to make it "right". IMHO I'd pull the distributor, turn the rotor 180 degrees, reinstall it and put the wires where they belong, and then let the timing begin!
 
It matters not a bit, how the driveshaft is indexed.
It only matters that the wire that goes to #1 cylinder is in the tower that is nearest to the firing-tip of the rotor when #1 cylinder is on at or just before TDC, on the compression stroke. Nothing else matters.

Lets say you have rotated the crank and found compression in the #1 cylinder. And have rotated the crank until the piston has come to the top. So now it is at or near TDC-compression. You don't even have to know where the Timing index marks exactly are, as long as the piston is at or just before TDC-compression.
Next you can drop the Distributor into the hole in absolutely any position, so long as it engages the driveshaft which can be in absolutely any position.
Next, you push the V-can back until it hits the firewall, then pull it back towards the rad until the nearest vane lines up with the little metal bar called the pole-piece, on the magnetic pick-up; or the points just barely break contact. That's where you snug the D down.
Now you have exactly two data points; namely #1-TDC compression, and the lined up vanes. In this position the engine is almost ready to fire up and run . At this time, I like to back up the crank to about 10 to 15 degrees advanced, but this is not required.
Next; you correctly install the rotor, it only fits one way, and it matters not where it is pointing. then you mark the body of the D with a sharpei, exactly where the firing tip is, in such a way that you can see the mark when the cap goes on.
Next, you install the cap,and go find the index mark you just made. Whatever tower is nearest the sharpie-mark, will be the tower for the wire that goes to #1 cylinder. And the rest follow in the correct firing order, in whatever direction the rotor travels, CW for SBMs.
The idle timing is now as correct as you can make it, before ever starting the engine. For idling, you have a huge window of timing to get into, and the engine will start and run; I'll say from 5* retarded to 20* advanced. or until the starter kicks back. That's a 25* degree window.
So now, hit the key.
As soon as it starts, and NOT DURING break-in; you tug on the V-can advancing the timing, until the rpm peaks. Then retard the timing some arbitrary amount until the engine complains about it. Then put it somewhere inbetween those two points.
Now, you can install your timing lite, and read where it actually is
If you cannot find the index mark on the balancer;
1) Either the index mark has climbed under an obstruction, which could indicate too much advance, or
2) your balancer and timing-tabs are not a matched set, in which case you simply swap your timing sensor clamp to the next or previous cylinder in the firing order, until the balancer mark shows up,or
3) you are accidentally not on the #1 wire, or
4) your timing lite is not compatible with the MSD Multi-strike.

If the engine fires up and runs, then as already said, it matters not how you got there, but it's right. You can take the D out and stick it back in in any new position, with the #1 piston at TDC-compression, any old way you want to, and the only thing that will change is where the rotor is pointing. And then you rewire the cap to put #1 wire in the new tower, and then Presto!, away you go. And you can do that all day, and it won't make your engine run any different.
Now, the FSM tells you to put all together in a certain way, principally because......... the multi-length factory plug wires fit best in that position.

I purposely left out rotor phasing, so as not to muddy the waters
 
Another question, if "total timing" (initial plus mechanical) is 32-36 does vacuum advance add on to that ? to make it somewhere near 50 at 2500-300 RPM?
 
RealWing said:
Yes
Click to expand...
Cool, I was playing around with it last night after work, setting total then hooked up vacuum,came in at right around 50. I had heard engine damage may occur, if over 36 total timing etc,but it was running god at Idle, I will next get it going down the road under load and go from there. My goal is to get it to **** and git as best as possible,and even more I want passengers to hold on to the dash and beg to be let out at the next corner;)
 
The statement that too much timing breaks motor parts, applies to WOT; Wide Open Throttle. The thing you have to listen for is DETONATION. This has to be avoided at ALL cost/ no matter what.
Your Power Timing at WOT, depending on your set up, will usually want to be at 34/36, after 3600rpm. Not every combo will accept 36 and not every combo will accept 36 before 3600. Only you or your tuner can figure it out. Nor do you have to push the limit from day one. It is better to be 3 or 4 degrees SHORT of optimum, than even just one degree too much. Your butt-dyno will not feel the difference of 2* short of optimum power timing.Maybe not even 3 or 4 degrees.
So the operative is to sneak up on the max PowerTiming over time. Lots of time.
 
AJ/FormS said:
Be careful what you wish for, I ride alone, not by choice.
No one will ride with me any more, and nobody wants to drive my beast. Nor would I ride along with anyone ,lol,
Click to expand...
Yes, I have found ,working with young folk, people are leery if you don't have airbags. I have a 86 Mustang GT, I offered a ride as we were going out to lunch the one person was like "nah,Ill drive myself etc." its like you are nutz for driving an old car without airbags I told them"I grew up driving these cars" Funny thing also,they dont know between HP and torque,I had a guy with a hyundai tell me he had more HP than my 86 Mustang. I said "yeah but Ill beat you off the line with torque" They were like Huh? I said "Torque is what throws you down the road quick, your 0-60 time..." not a clue :)
 
Without rereading the entire thread ...did ported vs manifold vacuum come up at all ? It seems to me that with certain engine combos manifold vacuum can get your timing up at idle without changing the upper curve.
 
mbaird said:
Without rereading the entire thread ...did ported vs manifold vacuum come up at all ? It seems to me that with certain engine combos manifold vacuum can get your timing up at idle without changing the upper curve.
Click to expand...
now that you mention it...I was loooking into it s I was always under the impression that vac advance should be on ported, as in, you want the advance when getting into the throttle.
 
Dartswinger70 said:
now that you mention it...I was loooking into it s I was always under the impression that vac advance should be on ported, as in, you want the advance when getting into the throttle.
Click to expand...
Actually, it's the opposite;
The vacuum advance has to progressively drop out as throttle is applied. Else you could rattle the pistons to pieces.
 
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Your distributor does at at least 4 different things.
1) Power-Timing
2) Idle-timing
3) PartThrottle, and
4) Cruise timing.

Of these,and IMO, the idle-timing has to be set first, to establish a starting point for the Power-Timing. Then, we already know that the end-point is gonna be at around 34/36. So you have to modify your D,in some way,to connect those two points. Then,you have to figure out between idle and WOT, what rate of advance the engine will tolerate without detonating .
Now you have established just two timings. There are, so far, just two times when the Timing is right, namely idle, and WOT. AT ALL other throttle settings, the timing is guaranteed to be wrong. Not an error on my part; AT ALL other throttle settings, the timing is guaranteed to be wrong.
This is where your V-can comes in. You the tuner ,have to figure out how much more, and sooner, that your engine wants PT(PartThrottle) timing . You can never hit all the PT points that your engine will call for, using the Factory-TYPE Distributor, So you just hit as many as you can, And IMO one of the most important of those, for me at least, is the Cruise Timing.

And I am always willing to modify or give up Power-Timing, or Idle-timing, or rate of advance;
to get the PT and Cruise timing that my engine wants;
Because ...... my engine spends most of it's life NOT AT WOT. lol.
 
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I have been thinking about this,and doing mild research( I say mild because I dont have alot of time for research LOL!)
in my mind,running the vacuum advance on manifold vacuum makes sense. In this scenario, I could set my initial timing by the book at 5 btdc at idle because having the vac advance on manifold would put me at about 15 btdc,then add 20 for the mechanical for 35 total at 2500 3000 rpm. Then the rest is basically what is mentioned above by AJ/ form S. for power you want that advance always there untill total timing at 2500-3000 rpm, at which point the manifold vac drops off, the mechanical advance is already there. It makes sense now.. I will say though this is debatable and there are those even to this day that will say "Ported vac" is the way to go, ( i read in my research that ported was done as an emission control or something back in the 70s,maybe this is where that comes from, but we are talking performance here) but that dont make sense, the way I am seeing it is, on ported vac, by the time the vac advance kicks in, the mechanical advance is already there or behind by maybe a second. I also want to do the "distributor curving" with the X Y axis graph and be certain where my distributor advance curve is.
 
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IMO the only time a streeter would run manifold vacuum advance is with a cam that is waaaay more strip than street.
When yo do that with a typical street carb, it is impossible to correctly set the transfer slot exposure, without also simultaneously having a way too fast idle speed.
Your engine does not care one bit about Power-Timing, until it hits stall. Punching it up below stall is nothing but trouble...
With a manual trans and a moderates cam, it will just exacerbate bucking.
But hey, you've thought about it so your way must be right. And it'll only cost you time,to start over.
I think it was @toolmanmike who said more is not always better, and
when it comes to timing,
that is 100% true.
 
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