72 340 - What Did We Miss

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Canuck340

Canuck340

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Thunder Bay, ON
Done a fair bit of work on sbc's but this is our first sbm build. This winter picked up a 72 duster 340 and we pulled the motor to freshen up. Ended up replacing bearings and rings but rest was good (forged pistons, nice performance cam, etc.). Put it back in car and replaced the old MSD 5 box with a mopar orange ignition box. Also replaced the wiring harness back to factory new with dual ballast resistor. Have gone through the threads and most suggest an advance at idle of 6-10? At first we thought the distributor was locked as could not get to run below 30deg advance. Bought a new distributor (proform) but no change. We have used piston stop to verify timing mark and no matter what we do the motor runs at 30deg, anything less and dies. Have disconnected vacuum advance for now to make sure doesn't go too high. Is there anything with the electronic ignition setup we are missing (lag, etc.)? Any other suggestions that couple of chevy guys might miss? Appreciate the help...
 
Is the distributor gear slot in the correct position? It is specified how to line it up in the FSM.
 
It doesnt happen often but its possible the balancer ring can slip on the rubber.
Also make sure someone has not mismatched the balancer and timing cover. I think 70 up they changed which side the timing marks are on the timing cover. They can interchange as long as you use the correct balancer for the timing cover. Easy way to check all this is to bring up to tdc and see if your balancer lines up at 0 or close.
This would also be a good time to make sure your intermediate shaft was indexed correctly.
 
It sounds like you may have the polarity reversed (wires switched) going to the distributor.
 
From memory, it has to point at #1 cylinder at TDC well before that. (It has been ages since I did one)

Check the FSM to be sure.

If it is off even by one tooth, your timing will not read right.

EDIT - I was of by several degrees, see the correct info below.
 
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I'm guessing your manifold vacuum is very low.What is your idle vacuum and at what rpm?
> make sure the secondaries are closed, pinch off the booster hose, and make sure the PCV is working correctly and is plumbed to the base of the carb on the nipple provided for it.
I would suspect a vacuum leak. It may be sucking air from the crankcase. Put a vacuum/fuel-pump tester gauge on the dipstick, and disable the PCV, and seal the crankcase, then start her up. The engine should now immediately begin to build pressure, NOT VACUUM. Do not let the pressure exceed 4 psi, it could start blowing out seals or gaskets.Don't panic, it doesn't build pressure that fast unless you have really poor ring-seal. If it registers a vacuum, then your intake is sucking from the bottom of the intake ports.
>If it's not there, then
Do a Leakdown test. Your valves may not be closing, possibly because of too long pushrods. This would produce a very low manifold vacuum.
In the event of a large LD number,back off the rocker shafts and retest. You can do the LD test now with the pistons at the bottom, to make it easier on yourself. Still a large number; might require pulling the engine down for inspection.
>And yes it could be cam timing,as mentioned, but ....... You wouldda had to have really messed up. There is a quick test for this called checking split overlap, which should occur within a few degrees of TDC.
>If the engine runs and revs up, it almost doesn't matter where the D is installed. You can make it run in any of however many teeth the I-shaft has. The only issue you will run into is with where the Vcan ends up, and it's adjustability between hitting the firewall and hitting something else. And of course you have to re-clock the wires, and sometimes custom wires have to be shuffled around to fit again. I wouldn't spend a lot of time on that, if it runs and revs up.
>Pick-up polarity is another issue, but this is also not your problem. If your polarity is reversed, you can still set the base timing exactly as normal, and it will idle exactly as normal; it just won't take revs very well, and the timing light will show a highly unstable advance system , plus numerous random misfires and skipped strobes, it's crazy to see.
 
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I do not have a '72 FSM, these are from the '70 version & I was off by several degrees with my last comment.

1970 318 340 distribuor timing 1.JPG


1970 318 340 distributor timing 2.JPG
 
Alaskan_TA said:
I do not have a '72 FSM, these are from the '70 version & I was off by several degrees with my last comment.

View attachment 1715182244

View attachment 1715182245
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This only works if the distributor is clocked like an OE distributor. I've seen OE distributors not cooked like that and countless aftermarket distributors. The rotor can be pointing at 12 o'clock and the drive is a 9 o'clock.
 
yellow rose said:
This only works if the distributor is clocked like an OE distributor. I've seen OE distributors not cooked like that and countless aftermarket distributors. The rotor can be pointing at 12 o'clock and the drive is a 9 o'clock.
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no matter where the distributor clocks, it can't hurt to have the intermediate shaft clocked correctly per FSM, can it? Once you clock the IS correctly, install the distributor with the vacuum advance(if it has one) in approx correct position (see FSM or Motors Manual), and rotate dist body CCW slightly until points open or magnetic pickup lines up with reluctor ridge. This should be close to your particular #1 terminal on cap. Install #1 spark plug wire there, and continue with firing order(18436572) in CW rotation(sometimes that info is cast into the stock CI intake manifold). If you are unsure of cylinder numbering, consult FSM or MM.
See if that works for you.
 
rustycowll69 said:
no matter where the distributor clocks, it can't hurt to have the intermediate shaft clocked correctly per FSM, can it? Once you clock the IS correctly, install the distributor with the vacuum advance(if it has one) in approx correct position (see FSM or Motors Manual), and rotate dist body CCW slightly until points open or magnetic pickup lines up with reluctor ridge. This should be close to your particular #1 terminal on cap. Install #1 spark plug wire there, and continue with firing order(18436572) in CW rotation(sometimes that info is cast into the stock CI intake manifold). If you are unsure of cylinder numbering, consult FSM or MM.
See if that works for you.
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It does matter. If you clock it per the FSM and the distributor isn't clocked the same way you won't get it timed.
 
reread OP. I noticed he stated they installed a new orange box. If it's one of those fake chinesium orange boxes, it wouldn't surprise me if they have issues from that, too. I saw a YouTube video, where the "transistor" on the outside was fake. Just a shell, no guts, not connected to anything!
Put a points distributor in it, rough time it like I suggested and see if it works.
 
rustycowll69 said:
reread OP. I noticed he stated they installed a new orange box. If it's one of those fake chinesium orange boxes, it wouldn't surprise me if they have issues from that, too. I saw a YouTube video, where the "transistor" on the outside was fake. Just a shell, no guts, not connected to anything!
Put a points distributor in it, rough time it like I suggested and see if it works.
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I noticed that too, lots of people having issues with the Mopar boxes for years now. It might be worth a shot putting that old MSD box back on. If the timing marks have indeed been verified with a piston-stop tool there's no reason mechanically the timing would read something that it isn't, since a timing light reads the ignition pulse and shows that directly in relation to the crankshaft. I'm betting it's some signal or electrical issue...
 
You speak in vague terms about this engine. Do you know the exact MEASURED static compression ratio? Do you know the EXACT camshaft specs? 6-10 degrees initial timing is where a stock engine is timed. The 72 340 is a low compression engine. It can probably stand 18-21 degrees initial timing with a total of around 36 degrees all in by 2500-3000 RPM depending on vehicle weight and rear gear ratio. We need concrete information if you expect concrete answers.
 
crackedback said:
This is a very strong possibility
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I was thinkin that too, that's why I asked for more info. All these others already recommendin stuff with half the story. lol

....and as usual, ran all over Jim. Probably one of the most experienced people on here.
 
RustyRatRod said:
I was thinkin that too, that's why I asked for more info. All these others already recommendin stuff with half the story. lol

....and as usual, ran all over Jim. Probably one of the most experienced people on here.
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Kk...just used factory dist plug so didn’t think could reverse. Will check wiring in FSM.
 
Canuck340 said:
Kk...just used factory dist plug so didn’t think could reverse. Will check wiring in FSM.
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There has been cases where replacement pickups were wrong polarity

Guys THE DISTRIBUTOR GEAR or placement of the distributor DOES NOT HAVE **** to do with timing.

YOU CAN LITERALLY close your eyes and throw in the distributor gear.........and then throw the distributor in right after it.

Then just bring up the marks no1 ready to fire. This means both valves closed

Turn the dist. so the vacuum can has "some room" to swing, then stab the no1 wire in wherever the rotor points. Set the timing.

This is likely a cam timing issue, or as said above, a rotor phasing issue (wrong pickup polarity) Either that or the OP has a LOT wilder cam in the engine than he thinks
 
Some of us are not parts throwers.

There is absolutely nothing wrong with knowing how to use the FSM, especially for those of us who rarely do this.

Most of us have an FSM around, but I would not tolerate thrown parts in my shop. ;-)
 
Alaskan_TA said:
Some of us are not parts throwers.

There is absolutely nothing wrong with knowing how to use the FSM, especially for those of us who rarely do this.

Most of us have an FSM around, but I would not tolerate thrown parts in my shop. ;-)
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Is this also tongue-in-cheek? then you forgot the lol.
But if not then Ima thinking you missed the point.
Which was and is ; Oh chit I missed your little ;-)
erase that...... lol
 
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