Plugged the vac advance and no change, plugged the brake booster and no change, plugged the pvc and no change. Thinking about trying to block the mech advance in the dizzy, then the timing will be what initial is at that 1600 rpm? But then i wouldn't have any advance on up the rpm range?
Help an old man tune a new to me 340
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There should be a # stamped on the vac adv arm. Dbl that # and that is crank degrees, knowing that may help.
You can also verify that # by putting a Mighty-Vac on the vac adv.
You can also verify that # by putting a Mighty-Vac on the vac adv.
I'd leave the mechanical advance where you put it and pull in about 18-20 initial.
cookietruck
Well-Known Member
Can you temporarily make the idle too rich to see if it smooths out the 1600-1800 rpm range?
Also, maybe try a different ignition box?
I replaced the plugs from 9 to 12 heat range, advanced the initial to 18-20*, may be a very thin hair better. I think the new carb is just smoother and the ignition problem still exists. Dizzy is still restricted to all in to about 30* as best as I can tell with my marks on the balancer. The restriction slot in the dizzy is about half covered. I checked the gap on the magnet in the dizzy and it's about .0008. Could the orange ignition box, coil, or the ballast resistor create a miss like this? Or would they completely fail? I hear some valve chatter or pinging at some throttle positions on my test drives. Running 93 octane ethanol free fuel. It cranks when you bump the starter, runs ok up to 1600, stumbles/misses then smooth out with more rpm. Could valve issues create this? Thanks, guys, for all the input. The search continues.
I tweaked the timing again and got it some better. I backed the initial down and it got worst, so I went back to about 18-19* and it's running better now. The total now is about 28-29* with the dizzy slots about halfway open. Someone had asked about the drop when put in gear. Its idling at about 920 and drops to 680 when put in gear. Haven't tuned on the new carb yet. Will try later this week. Got a tune kit for this carb on the way. Thanks again for all the ideas, suggestions, and feedback. Keep any ideas or suggestions coming to continue to get it right.
Captainkirk
Old School Mopar Warrior
Here are a couple questions for you...
1) What is your compression ratio?
2) Solid (copper core) or resistor plug wires?
3) Solid or resistor plugs?
4) Are you running MSD?
I am a novice and don't know the compression ratio. How can I get it? the plugs in now are new RN12YC champions. Mopar distributor with orange box. The distributor is the later one where you can limit the total weight advance. I have them in the middle of the slot. I think I need to open it up a just little to get a little more total. It gets to the total real fast to in the rpm range.
Captainkirk
Old School Mopar Warrior
If yours is a stock '69 the CR is 10:1
Here's the thing; the RN12YC plug is a resistor plug (denoted by the "R"). If your plug wires are also resistor (carbon core) wires they can cause misfire at mid to upper range RPMs, especially in higher gears with higher compression pistons. Don't ask how I know. Just sayin'.
Spiral core [ also called heli core ] plug leads would be a good idea, not very expensive, if the leads are old or unknown. Eliminates another variable.
No change when the PCV was blocked? There should be. It could jammed. It should rattle when you shake it.
And this is the 2nd or 3rd time I have mentioned this & is so important for your problem: you need to remove the carb & check how much transfer slot is showing above the pri throttle blades. This dimension is fundamental to idle/off idle operation.
No change when the PCV was blocked? There should be. It could jammed. It should rattle when you shake it.
And this is the 2nd or 3rd time I have mentioned this & is so important for your problem: you need to remove the carb & check how much transfer slot is showing above the pri throttle blades. This dimension is fundamental to idle/off idle operation.
If you run a compression test on each cylinder and post the numbers, we can get a lot closer to what may be in the engine and help you tune it more accurately.
The guy I bought it from in Iowa got it from the original owner in Texas that built the engine. So, the guy I got it from didn't have all the info. I asked him what the compression ratio was when I bought it, and he wasn't sure, but thought 10:1. So that lines up with a stock 69. I should have said I wasn't sure and have not confirmed the compression ratio. The engine came to me with Bosch wr8dc+ plugs, I replaced with RN9YC and no change in problem. I changed to the RN12YC along with timing change and its better but still some rough. It's in the lower rpm range, 1600-1800. The plug wires are orange Mopar electronic suppression G.X. 1-00 AATH. You can see them in this picture.
In that case you’ll be lucky if it’s 9.5:1. It may be closer to 9:1. If they didn’t measure the volumes and do the math they had no idea what the CR is.
The PVC valve rattles when you shake it. It's not plugged. I blocked the end of the hose coming off the carb port to the PCV to check for vac leak and no change. I installed a new carb and it's not much different with the lower rpm problem than the old one. It is smoother, but still some rough at 1600.
furrystump
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So, I run the Summit Billet distributor that uses plastic “keys” that you insert into the advance plate to adjust your mechanical advance. The difference in the keys thickness changes the advance. I run 23 initial and 34 total, which was easy to get. For me, I just taped a degree wheel to the distributor gear and held the distributor in a vice. Made a pointer and moved the advance weights out. Looked at how many degrees it turned and x 2 the number. Thats how much mechanical it has. Got me close enough to only do it once. My initial isn’t hyper critical to me. So I got it close and set my total after. Was shooting for 10 degs. Got 11 degs Understand my engine is NOT apples to apples to yours by any means. But I like the distributor.
Summit Racing SUM-851006 Summit Racing™ Billet Electronic Mopar Distributors | Summit Racing
Summit Racing SUM-851006 Summit Racing™ Billet Electronic Mopar Distributors | Summit Racing
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PRH
Well-Known Member
At this point, my WAG is that the ruff running at 1600-ish is related to a transition circuit “situation”.
An easy test to try would be to run less initial timing, and no vacuum advance, which would require opening the throttle blades more to maintain the current idle speed, which would expose more of the slot.
I’d just drop the timing down to like 5-8btdc, unhook the vac advance, reset the idle speed, and see if it changes anything (better or worse) in that 1600 area.
An easy test to try would be to run less initial timing, and no vacuum advance, which would require opening the throttle blades more to maintain the current idle speed, which would expose more of the slot.
I’d just drop the timing down to like 5-8btdc, unhook the vac advance, reset the idle speed, and see if it changes anything (better or worse) in that 1600 area.
That is a Mallory advance. They can be tricky to get what you want for a curve.
Personally I’d rather have one of the several Mallory advance mechanisms than a GM style advance mechanism.
According to Rick Ehrenberg, tech editor of Mopar Action Magazine, those new style mopar distributors have advance weights that are too light and cause your timing to bounce around. I have never used one so I'm just relaying what he says that may me causing your problems.
That tells me he has no idea how to set a timing curve.
Virtually every distributor has weights that are far too heavy for a curve for even a stock engine, if you are tuning for performance.
If you are tuning for emissions then that’s a different story.
BTW, the weights he says are too light are still to heavy.
I’ve thought about having aluminum weights made up and installing a steel sleeve in them for durability.
It would certainly save me time and money.
Thanks for the suggestion, I'll definitely try it. Probably be Wednesday morning when I get back home. Still haven't tried any turning on the new carb either. I'll try some on it after I try that.
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