340 Running super weird

-
Greenmachine225

Greenmachine225

Well-Known Member
Joined
Dec 18, 2016
Messages
62
Reaction score
30
Location
San Jose
Hey guys, so I’ve been having a hell of a time figuring out what’s wrong with my 340 in my 71 Duster. Everything was fine other than an off idle stutter until a couple buddies convinced me to let them tinker with the timing and distributor and now… the car will almost feel like it’s missing a couple cylinders then after sitting in gear idling it goes back to normal BUT it comes back. 340 goes in and out of this problem while I’m driving. WOT it’s fine at higher rpm. The last thing they changed that I haven’t undone is one of the mechanical advance springs in the distributor, they put a super light one on one of the weights and I’m thinking it’s causing the timing to jump around. It’s a mopar electronic distributor and has the plate for adjustment in it. Everything else was working perfectly fine until they messed with my timing system. I have triple checked the timing annd it’s where it should be. Any advice? Does the spring sound like it could be the problem?

IMG_7664.jpeg
 
My first thought would be a lean condition caused the stuttering. Can you put it back to where it was before your buddies messed with it?
 
Greenmachine225 said:
Hey guys, so I’ve been having a hell of a time figuring out what’s wrong with my 340 in my 71 Duster. Everything was fine other than an off idle stutter until a couple buddies convinced me to let them tinker with the timing and distributor and now… the car will almost feel like it’s missing a couple cylinders then after sitting in gear idling it goes back to normal BUT it comes back. 340 goes in and out of this problem while I’m driving. WOT it’s fine at higher rpm. The last thing they changed that I haven’t undone is one of the mechanical advance springs in the distributor, they put a super light one on one of the weights and I’m thinking it’s causing the timing to jump around. It’s a mopar electronic distributor and has the plate for adjustment in it. Everything else was working perfectly fine until they messed with my timing system. I have triple checked the timing annd it’s where it should be. Any advice? Does the spring sound like it could be the problem?

View attachment 1716299991
Click to expand...
Well that sucks. Obviously you don't know what they did and we sure don't know. Ask them and report back.
 
Did they mess with the vacuum advance ? It could be that your mechanical advance is now coming in too fast . The vacuum advance would be coming in on top of that during cruise and you are getting detonation . Then you idle and it returns to initial setting.
You will need to describe when it misbehaves as far as driving type … acceleration, part throttle cruise , hwy speeds …etc .
 
There's only so much swapping they could have done in the distributor. Put the orig springs back in. Taking a dist apart isn't all that difficult.
 
If you can get the timing light on it and give us some specifics, we may can help. Where the initial (idle) timing is for example. Also when (at what RPM) the mechanical advance starts to advance would be helpful information. Also, details about the engine build. Compression ratio (known MEASURED, not a guess) camshaft specs, converter stall if automatic and rear gear ratio. If you do not know the compression ratio, some compression test results can get us on the right track.
 
Boy! Your question opened up a can of worms. You will receive about 1/2 right and 1/2 wrong answers.
Wait til the really “smart guys” wake up although RRR is about the best.
Honestly, without many many details we are just spitting in the wind and hoping it will stick to the front porch.
Way too many things could had been done. No details on what you have (stock or modified and how modified) is your drive train. Best bet is to ask for a new buddy locally that knows what the heck they are doing on here and get one pro from here to help you figure it out. You may just had had a leaking intake gasket or stuck distributor weights and now all heck has broken loose. Good luck.
Syleng1
 
RustyRatRod said:
I'm a thankin ya, but man is that ever a stretch. lol
Click to expand...
Telling it like it is has made me trust you more than any thing Rusty.
I just got back from Tenn last night. If I had more time I would had like to pop over and meet you in person. Since I was a yankee- they only let me stay one week in the mountains. lol!
 
Syleng1 said:
Telling it like it is has made me trust you more than any thing Rusty.
I just got back from Tenn last night. If I had more time I would had like to pop over and meet you in person. Since I was a yankee- they only let me stay one week in the mountains. lol!
Click to expand...
You are welcome as the flowers in May. Anytime.
 
You need to get your friends to put it back to as close as possible to where it was before they started. From there, start with the basics by checking the plug gaps, checking the timing at idle and at around 2000 RPMs. Then make only "one" change at a time a see what happens. If the first change doesn't make a difference, put it back where it was and try something different. The mistake some people make is doing wholesale changes and screwing everything up. It took me a while when I first started to adopt the mentality of "one change" at a time.
 
Demonracer said:
You need to get your friends to put it back to as close as possible to where it was before they started. From there, start with the basics by checking the plug gaps, checking the timing at idle and at around 2000 RPMs. Then make only "one" change at a time a see what happens. If the first change doesn't make a difference, put it back where it was and try something different. The mistake some people make is doing wholesale changes and screwing everything up. It took me a while when I first started to adopt the mentality of "one change" at a time.
Click to expand...
I wouldn't let those "friends" ever touch my junk again. In fact, they'd have a couple of whooped asses.
 
Greenmachine225 said:
Hey guys, so I’ve been having a hell of a time figuring out what’s wrong with my 340 in my 71 Duster. Everything was fine other than an off idle stutter until a couple buddies convinced me to let them tinker with the timing and distributor and now… the car will almost feel like it’s missing a couple cylinders then after sitting in gear idling it goes back to normal BUT it comes back. 340 goes in and out of this problem while I’m driving. WOT it’s fine at higher rpm. The last thing they changed that I haven’t undone is one of the mechanical advance springs in the distributor, they put a super light one on one of the weights and I’m thinking it’s causing the timing to jump around. It’s a mopar electronic distributor and has the plate for adjustment in it. Everything else was working perfectly fine until they messed with my timing system. I have triple checked the timing annd it’s where it should be. Any advice? Does the spring sound like it could be the problem?

View attachment 1716299991
Click to expand...
Put the distributor back to the way it was. The older (Chrysler built) DC/MP distributors have a great timing curve for a stock or close to stock 340. Then you'll have to reset the timing. Either set it to 12 around 750 rpm or follow the instructions from MP setting it at 2800. That will be your new baseline. Tweak it from there.
 
RRR, I couldn't agree more. The only reason I said use the friends to put it back was that only they know what they did. There was only one person other than myself that I trusted to work on my junk. He was my former race car partner and had more Mopar knowledge in his head than I will ever have. Unfortunately he passed away a few years ago, and I miss discussing things with him.
 
Demonracer said:
RRR, I couldn't agree more. The only reason I said use the friends to put it back was that only they know what they did. There was only one person other than myself that I trusted to work on my junk. He was my former race car partner and had more Mopar knowledge in his head than I will ever have. Unfortunately he passed away a few years ago, and I miss discussing things with him.
Click to expand...
Also, I am assuming from his post, he may have limited knowledge of what they did, or how to undo it. But he's here now and if he returns, we can walk him through whatever he needs to do.
 
Probably adjusted the timing without disconnecting the vacuum advance.
 
Mattax said:
Put the distributor back to the way it was. The older (Chrysler built) DC/MP distributors have a great timing curve for a stock or close to stock 340. Then you'll have to reset the timing. Either set it to 12 around 750 rpm or follow the instructions from MP setting it at 2800. That will be your new baseline. Tweak it from there.
Click to expand...
Is there any chance that you could post that MP curve?
 
Agreed, he may have limited knowledge. I don't think his friends did him any favors and it seems they have limited knowledge also. I don't mean to bash his buddies, but I wonder how their cars perform.
 
The first thing that has to be done is get the timing back to where it was. That begins with restoring the advance curve. Hopefully they still have the original springs.

Greenmachine225 said:
the car will almost feel like it’s missing a couple cylinders then after sitting in gear idling it goes back to normal BUT it comes back. 340 goes in and out of this problem while I’m driving.
Click to expand...
This could be fueling but I think it sounds like too much timing.
Greenmachine225 said:
I’m thinking it’s causing the timing to jump around. It’s a mopar electronic distributor and has the plate for adjustment in it.
Click to expand...
It very well could be .
OK. I missed that its a Mallory built distributor before getting my coffee.
Those are more difficult to set up and what I wrote before about Chrysler Built MP distributors does not apply to these.
This is because changing the advance 'stops' also changes the tension on the light spring, and the degrees in before the long looped spring engages. I can walk you through this with photos.
Greenmachine225 said:
Does the spring sound like it could be the problem?
Click to expand...
Yes and no. See above.
 
Were your “friends” Chevy guys by chance? Were they worried that your Mopar was faster than theirs?
 
Best I can do right now is point you to previous discussions.
They will make more sense when you are looking at hte mechanism.

Mattax said:
Recent discussion of the Mallory YH advance.
Slot adjustment post. LA Mopar electronic distributor retaining clip, now YH adjustments.

When you increase the advance allowance slots, it also increases the tension on the primary spring. Increasing the slots 2* will also bring the secondary spring into play 2* sooner.
Click to expand...

The instructions were writtern by people who didn't understand what I wrote above.
When you increase the advance allowance slots, it also increases the tension on the primary spring. Increasing the slots 2* will also bring the secondary spring into play 2* sooner.
Mattax said:
Reading the graph, we think that reducing the advance to 18* will result in something like sketched here:
upload_2018-11-23_21-5-16.png


But what actually happens is it starts advancing earlier and ends up more like this:
(using orange and purple springs for example.)
upload_2018-11-23_21-10-34.png

Preload can be regained by bending the spring perches out.. But we want to limit the number of times we bend them back and forth!

*PS. Ignore that "factory standard v8" nonsense. No such thing I've seen.
Click to expand...


Cooling Issue Solved & Some Tuning Questions

With only one spring the timing is going to be all in RIGHT NOW off idle is my guess.
board.moparts.org board.moparts.org

Changing the limit changes up the spring tension and therefore changes when the weights move out.
If you reduce the limits a shorter long loop spring is needed. OR the tabs (spring perches) need to be bent.

CFD244 said:
Is there any chance that you could post that MP curve?
Click to expand...
www.forabodiesonly.com

Carb choice for a mutt motor.

Hi crew. I'm a little green when it comes the venerable 318 LA motor. I tortured a couple of 340 motors in my misguided youth, however a huge volume of water has passed under that bridge. I recently acquired a 67 Barracuda. Mix-n-match 318 of unknown bloodline. Factory 273 exhaust manifolds...
www.forabodiesonly.com www.forabodiesonly.com
 
Last edited:
You must log in or register to reply here.
-

Back
Top