Help an old man tune a new to me 340

[HS Duster]

I just got back into a Mopar after almost 50 years. Trying to match my high school Duster. I purchased a 72 340 904 Duster 4 months ago. Been trying to tune to get it running right. It has a miss/skip that starts about 1600-1800 rpm. It cranks ok, idles ok with approximately 15# vacuum. Starts stumbling then smooths out and runs OK on up. Here are the specs on the motor from the person I bought it from.
69 340 block, Keith Black pistons .020 over, X heads w/ stainless steel valves, Mopar purple cam (slightly larger than stock), Mopar electronic ignition orange control box, HP exhaust manifolds, Mopar aluminum intake, 750 CFM 1407 Edelbrock carburetor with an electric choke added. This is a list of the things I have tried with basically no change.
I just replaced the plugs with champion RN9YC-no change from the Bosch that it had
Set timing from approximately 0 TDC to 4-5* BTDC- no change
Replaced fuel filter, pulled carb air horn, cleaned out bowls and orifices, set floats to spec-no change
Tried many combinations of set up springs and metering rods-no change
I'm frustrated and looking for things to check from this vast knowledge here. Thanks for any help.

 

[RustyRatRod]

The first thing that jumps out is the age old mistake of not enough initial timing. Other than that, it would be helpful to know static compression ratio and "WHICH" Mopar camshaft it is, as there were many different ones. Specs or a part number would be good. Single or dual plane intake?

 

[skep419]

Someone smarter will chime in soon but I'd start by swapping out the carb (I had a 750 ede many years ago that always had an off idle bog)

 

[66Valiant528]

Not an expert but I'd like to see some more initial timing like 15 degrees. And what kind of distributor and ignition system do you have? How do the plugs.look? Cold fouled? Try some RN12YC . Dizzy advance springs? Vacuum advance? Ported or intake? Need more info!

 

[crackedback]

Initial timing in the 14-20 area depending on cam, limit the distributor mechanical so total stays in a safe range. Bigger cam more initial is the way I tend to go. Purple cam is about as descriptive to what you have as saying help me tune a 340 with nothing else for info. Which grind, there are a bunch of them

That carb is a PIA... probably the source of you miss.

 

[HS Duster]

Don't have the specs on the cam. The guy I got it from doesn't have the information. The guy he bought it from built the motor and didn't pass on all the info. Haven't pulled the carb off completely to see if single or dual plane intake. It's more of a skip or miss than a bog I think.

 

[HS Duster]

Vacuum advance is on ported vacuum, tried it on manifold vac with no change. Stock Mopar ignition system. Plugs are new and look ok with very little time on them. I will try more advance and see what happens. I tried a little more yesterday and either didnt help or made worst. What carb would you guys recommend for street cruising if I have to replace it?

 

[HS Duster]

What am I looking for on the distributor springs? It's been a long long time since I have done this.

 

[harrisonm]

Someone smarter will chime in soon but I'd start by swapping out the carb (I had a 750 ede many years ago that always had an off idle bog)

Gotta disagree with that. Over the years I have had trouble with several Holley carbs. I have NEVER had a problem with an Eddy carb. I will NEVER use a Holley carb again. I know there are hundreds of you that will disagree, but all I can say is what has worked for me. A perfect example is my BAD experience with Michelin tires. 99% of the people out there will say that they love Michelin tires, but I had a really bad experience with them. I had a 2007 Jeep Grand Cherokee that needed tires in about 2010, and I got some Michelin car tires on sale. My Jeep went from riding like a luxury car to riding like a lumber wagon. I put up with it for about a year and went back to Goodyears. Wow, what a coincidence. My Jeep went back to riding like a luxury car. So no matter how many people tell me I am wrong for not liking Michelin tires, all I know is that I had a big problem with them once and will never use them again.

 

[mopar head]

There for a while, the orange boxes were/are bad.
Agree with the more timing advance and limit. Where I`d start.

 

[HS Duster]

How do I limit total advance? All I know on the cam from the previous owner is slightly larger than stock.

 

[mopar head]

How do I limit total advance? All I know on the cam from the previous owner is slightly larger than stock.

Either weld up the slots and file or press the easy button.
Some here argue that they`re not accurate but mine seems alright.
A lot of threads on this subject to find with the search function.

FBO Mopar Distributor Limiter Plate

 

[RustyRatRod]

Either weld up the slots and file or press the easy button.
Some here argue that they`re not accurate but mine seems alright.
A lot of threads on this subject to find with the search function.

FBO Mopar Distributor Limiter Plate

Why is that the easy button? The distributor still has to come apart the same amount as if you were going to weld the governor slots.

 

[RustyRatRod]

How do I limit total advance? All I know on the cam from the previous owner is slightly larger than stock.

Maybe you can post a video of the engine idling? Might give us some insight. You'll need to post the video to youtube and then post the link here.

 

[Captainkirk]

What is total advance? Mine ran well at 36 degrees full advance, disabled vacuum advance and light spring kit in the distributor. Also, you are running too much carb IMHO for a 340 street car. Stick with a 600-700 cfm carb. A Holley 650 DP is my personal choice. I would look for vacuum leaks around the carb base. Your problem sounds like a lean stumble to me.

 

[HS Duster]

What I don't understand is that the guy I bought if from in Iowa said it ran fine there. He had timing at about 0 TDC. He drove it very little. I wonder if the car setting has scrapped the carburetor. How far could I advance the initial timing and remain safe on total timing?

 

[crackedback]

15" of vacuum at what RPM?

How much rpm drop when pulled into gear from P/N?

Isn't a muscle engine mopar on the planet with even a stock cam that runs good timed at TDC... NONE! Minimum 10 BTDC for even stock engine was iffy.

The ede/holley thing... That particular ede carb 1407 750cfm has a history of being difficult going to back to when it was released. All the others seem to be ok for the most part.

Hillbilly timing tape the balancer if it doesn't have a timing tape on it or marks. Set it to 15 btdc and reset the carb idle settings/mix screws. DO NOT drive it until you have the distributor sorted out. Idle is like building a house, the foundation upon which all the rest of the carb systems build.

 

[RustyRatRod]

What I don't understand is that the guy I bought if from in Iowa said it ran fine there. He had timing at about 0 TDC. He drove it very little. I wonder if the car setting has scrapped the carburetor. How far could I advance the initial timing and remain safe on total timing?

I'd say "around" 10 degrees "or so". It depends on how much mechanical advance the distributor has. Thing is, the engine may need as much as 20 or more degrees initial. They often do.

 

[HS Duster]

14-15# vacuum at 875-925 idle rpm. It drops approximately 100 rpm in gear. Not sure on the drop. Getting rpm off my timing light. I'll get the wife to put in gear for me tomorrow while I got the light on it. There are marks on the balancer. I roughly checked the mark by pulling #1 plug, putting in a piece of wire, rotating engine by hand to the mark, and watching the wire go up to what i think is the top of the stroke. By getting the distributor sorted out, you mean limiting the total advance? I may have access to another carb, waiting on a call back. All the vacuum lines are not very old and look good. Ill spray around tomorrow and try to verify no vacuum leaks on the base also.

 

[Captainkirk]

Total timing (yes, use the timing tape on the balancer) should be in the 34-36 degree range at full advance. By 2000RPM you should be fully advanced

 

[66fyssh]

I'd recommend an Edelbrock AVS2 650 cfm carb. I replaced my 750 with it. The 750 pulled like heck once going but idled like crap and had an off idle stumble I couldn't tune out.

 

[HS Duster]

That carb is the model 1906 I believe. I may have access to a 1406 Performer 600 CFM to try. I'm waiting on a call back. Any experience with the 1406?

 

[MOPAROFFICIAL]

Vacuum advance is on ported vacuum, tried it on manifold vac with no change. Stock Mopar ignition system. Plugs are new and look ok with very little time on them. I will try more advance and see what happens. I tried a little more yesterday and either didnt help or made worst. What carb would you guys recommend for street cruising if I have to replace it?

Does the accelerator pump work? Have you tried moving the rod to the hole closer to the pump?

 

[RustyRatRod]

What is total advance? Mine ran well at 36 degrees full advance, disabled vacuum advance and light spring kit in the distributor. Also, you are running too much carb IMHO for a 340 street car. Stick with a 600-700 cfm carb. A Holley 650 DP is my personal choice. I would look for vacuum leaks around the carb base. Your problem sounds like a lean stumble to me.

Total is initial (at idle) and "however much" mechanical advance is in the distributor. I wouldn't run without vacuum advance on a street car. It'll kill any mileage and also drop off some part throttle response.

 

[RustyRatRod]

That carb is the model 1906 I believe. I may have access to a 1406 Performer 600 CFM to try. I'm waiting on a call back. Any experience with the 1406?

Yeah. Don't use it. That's the economy version. They are VERY lean. I would get the timing sorted first. We already know that's an issue.