340 Tuning - I need advice

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Robb

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This is my first 340, and I am not familiar with any of their running characteristics (if any), and I have some tuning questions…

Motor Info/History
The motor is a stock bore ‘69, and I am told it was a low mileage stock rebuild (X heads, stock intake, and manifolds) with a mild mopar cam. It has about 10-15k on the rebuild. I have receipts verifying the bottom end rebuild, but I have no details on the cam or head work. It’s running mopar elec ignition, with all new stock replacement tune up parts. The old plugs seemed a tad rich, but none were fouled or oiled. Also new is a 650 edelbrock AVS w/elect choke.

Issues and Questions
It has ran with what seems like a slight miss (seems shaky or shuddery for lack of a better description) and poor hot starting since I bought it. The new carb and tune up has helped (mainly with the throttle bushing gas leak), but it still has a rough idle. We have verified that the vac advance is working, firing order is correct and the timing is at 10 degrees. Where do we go from here? We have a few theories, but my first question is should we assume a smooth steady idle without shaking/looping is possible, or is this just how a 340 runs? I hope that’s not too dumb of a question.

Any additional recommendations on timing?
Could we have a vacuum leak or intake leak?
Worn timing chain?
Harmonics issues?
Valve adjustments?
All of the above?
Are we just over thinking it?

Keep in mind, I am a novice mechanic and a lone mopar owner surrounded by GM people, but my main help is my buddy who is a marine mechanic by profession, so he has a lot of experience with all sorts of naturally aspirated V8’s, the least of which are unfortunately mopars!
 
A couple of things come to mind.

Depending on the cam, 10 degrees initial may not be enough. If you increase initial timing in 1 or 2 degree increments while mainiting the same idle rpm and the manifold vacuum increases you don't have enough timing (I need 17 on my 360 for a clean idle). Just remember it you add more than a few degrees initial you will likely have to much total and will need to limit the total by welding/epoxing the slots if it's a stock distributor or adjusting the stops if it's Mopar Performance unit. Also getting the initial timing right will make tuning the carb easier too.

If the coil is mounted on the intake in the stock location the hot start problem could be related to the coil starting to fail when it gets hot. Otherwise an insulating gasket/spacer between the carb and intake can help.

If you are running headers and have to snake the wires between the number three tube and the head for number 5 & 7 you could have baked the wires. I use cut to fit multiangle wires that allow the number 5 & 7 wires to have the boots bent at 90 degrees and routed under the tube.
 
Sounds like Dave's got you covered. Temporarilly plug the vacuum advance hose at the vacuum pod on the distributor, and add more base timing. For almost stock engines I like to be between 12-14° initial. If it doesnt ping, go up to as much as 16. If it does, back it down 2° at a time until it stops. After the initial timing is set, use a vacuum gage to set the idle mixture on the carb while keeping the idle speed as close to 650rpm as you can. Once you've got that, and it's pretty steady, increase the idle speed to 700, reconnect the vacuum cannister at the distributor, and test drive it.
 
Good stuff... thank you both for the info, now I have something to go one!
 
Oh BTW... I have mopar performance elec ignition. How do adjust the stops for total timing? This is new territory for me...
 
Robb said:
Oh BTW... I have mopar performance elec ignition. How do adjust the stops for total timing? This is new territory for me...
Click to expand...

Get the Mallory kit P/N 29014. It has springs to chang the curve and advance limit tabs so you can limit the advance in the distributor. It will work in the MP distributor.
 
Robb,

I'm certainly not an expert tuner, but would like to add my 2cents worth.

Any additional recommendations on timing?
>>>Yes:
1. If you knew your entire engine was stock I'd suggest timing it to factory specs;
2. Since it's now, time it by total, not initial; with the vac advance plugged, try for between 34 and 38.

Could we have a vacuum leak or intake leak?
>>>Certainly could. Search that down first just like any engine.

Worn timing chain?
>>>Worry about this later. Check the easy stuff first.

Harmonics issues?
>>>Ditto above answer.

Valve adjustments?
>>>Only if it's a mech cam - is it?

All of the above?
>>>One thing at a time.

Are we just over thinking it?
>>>No, Other than the details of timing, tune it like any other engine.

As mentioned, you need to keep your eye on idle speed and vacuum while timing it. But that too isn't any different than any engine.

Suggestions:
1. Check for vaccum leaks. Make sure all carb connections are properly done or plugged. Make sure distributor vacuum can is connected to the correct vacuum connection too.
2. Remove one plug wire at a time to see if any don't make any difference, if so you have one or more bad plugs or wires or wire connections.
3. Set the initial idle to about 700 rpm, and adjust the carb, reset idle.
4. Make sure you use the correct timing mark on the flywheel when timing.
5. Disconnect the vacuum can on dist and plug the open hose.
6. Set the total timing at about 2500 - > 3000, start at about 34 deg and work from there.
7. Check your idle speed, and reset if needed to 700 - > 750 or therebouts.
8. Recheck total timing to what you just had or therebouts.
9. Make sure your vacuum is where it should be.

... Etc, ... back and forth re-adjusting idle speed and carb mixture and total timing and checking vacuum.

When done reconnect dist can vacuum hose.

Is it better? If not either more expertise is needed or go on to mechanical issues.
 
Timing a street car using the total timing method gets you a mixed bag at best. OK for track cars, not so much for street stuff IMO.

If you have two distributors, one with 28*, another with 18* of mechanical, the two will run totally different when installed. Using a 34 total number the 28* distributor would have 6* initial which isn't or barely enough even for a stock engine IMO. The one with 18* in it will have 16* initial, which for most cammed SB mopars, is about the right area for initial. Even stock, most of the SB's I've have run at least 12*. Crisper better response than the factory settings.

If you total time a car, you CAN NOT just pick an RPM and call it good. You MUST do it at a point where the distributor no longer has any advance left in it. Some may be in excess of 4000 rpm. If you set it to 34* at 2500 and there is still 6* left from 2500-4000, you really have 40* total timing, which isn't good in most cases with todays pump gas
.
It's a foundation type thing. You have to crawl before you walk, walk before you run. Think the same when timing a car. Initial is the crawl part. Time the engine for initial FIRST, then worry about limiting the mechanical advance, if required, to hit a desired total timing number. It's a multi-piece puzzle and ignoring initial will get you less than desired results, especially, when a larger cam is involved. I've fixed a bunch of street cars that were timed using the total timing method over the years and the owners couldn't believe it was the same car when done.

Your mileage may vary.
 
Thanks again everyone.... I will take some of this timing/tuning advice and give it try. I am sure I will have more questions, so please look for this thread to pop back up!

THANKS!
 
mopar head said:
The MP dizzy allready has the tabs built in and allready has a performance curve built in.
Click to expand...


Every motor is different Still need to tune it to the motor.


Another thing to look in to is a carb spacer to help with heat soke. My 340 started acting a lot better after I put a 1/2 Phenolic spacer on it.
 
Update: I think I have it timed and tuned better, but there may be a slight studder, or maybe I am just too picky...

The timing is at 12-14 degrees and I used a vacuum guage to help tune the carb. I am getting about 16 on the vac guage but it is still fluttering up and down a unit or two. I can't get it to peg steady. Any recomendations?

Also I think it idels a little high. I don't have a tack or RPM guage, and my idel set is all the way out! Is there any adjustment to the cable or is something way out of wack?
 
My 340 did the same thing, very shaky idle, and hesitated off the line, and diffucult to restart when hot.

you may need to set the initial as much as 20 degrees before TDC to get a smoothe idle. Keep in mind that you will have too much total timing when its "all in" so you will need to restrict the mechanical advance in the distributor.

340's need a 1/2" spacer between the carb and intake to prevent boiling the fuel after shut off.

Without some kind of insulation from the heat of the engine, if you look down the carb about 5 minutes after shutting off a hot motor, you will see fuel dripping from the boosters because it is boiling in the fuel bowls.
 
Thanks.... that's two votes for a carb spacer, so I will get right in that!

Anyone else have any additional timing advance advice? I had it up around 20* but it didn't sound right to me, but I didn't readjust the carb either. Also I can't put more timing advance in it because I can't adjust my carb idel down any further. I there a linkage or cable adjustment?
 
disconnect the cable and see if it will go lower. Might have to bend the cable bracket, make a change there.

As you add advance the idle will probably go up. Need to readjust the carb. Carb settings at 10* and 20* will be very different.
 
crackedback said:
disconnect the cable and see if it will go lower.
Click to expand...

The throttle cable.. Also have a friend sit in it and floor it to make sure when you do tighten it down that you have full throttle too.
Every time you adjust the timing, the carb will need to be re-adjusted. Always.
 
moper said:
The throttle cable.. Also have a friend sit in it and floor it to make sure when you do tighten it down that you have full throttle too.
Every time you adjust the timing, the carb will need to be re-adjusted. Always.
Click to expand...

Additionally, you can not properly tune a carb until you have it at proper idle speed.

I would seriously consider getting a timing light with built in rpm readout. It will make you job way easier.

Whatever your timing is at idle, check your total timing. You might - as suggested - need to back that off, but you might not as well. Conversely, you can time it to total and then check the idle timing. See where you're at. Even if you need to make adjustments, at least you know what you got then.

And one more thing, tell us more about the engine.
 
bigdemo1 said:
Robb since no one else has mentioned it here is a good link for you to understand your vacuum guage better http://www.secondchancegarage.com/public/186.cfm
pay particular attention to reading 3
Click to expand...


This is great! I am saving this or I will try printing each senario and adding to my vacuum kit.... I think #3 is right where I am at, and I am happy that there does not appear to be any major issues... Everyone who doesn't know this stuff inside and out should save that site!

Thanks!!!!!!!!
 
Robb,if you do a little more digging you'll find some nice articles on setting your timing with vacuum guage,also buy one for your dash they tell you so much.
 
I think I was sniffing too many exhaust fumes after my last update. This was my first night back to run the car after my last post, and this thing still doesn't run right! I swear it has a miss. :angry7:It ideals rough/shaky and sounds like it has a miss even as the RPMs go up. I am at a loss. Below is a reminder of what I have done so far...

*new 650 edelbrock carb
*new plugs, wires, cap and rotor, new orange box and resistor
*initial timing is set at 12-13 before TDC
*does not appear to have any vacuum leaks according to vac gauge (thanks bigdemo1 for guage reading tips)
*pulled one wire from the cap at a time and none of them seemed to make a difference (I know that sounds weird).

Now what? I need help, Please!
 
The fuel is new and I have ran many tanks with the rough idle. Thanks for the site on the distributor tuning too, but I am hoping to avoid attempting anything too far out of my comfort zone, which I think that might qualify...

Any other suggestions? Is there any sort of distributor pickup or contact adjustment I can look into? I think it is a mopar performance ignition kit, but for all I really know it was pulled from an old 70's car and put into this 340... How can I eliminate distributor issues or test the distributors working condition?
 
Did you verify that the TDC mark on the damper is in fact correct? Use a piston stop if necessary to verify.

Have you tried the setting the initial in the 20 to 25 degree BTDC range to see how it idles?

I have a similar combo and it idles very poorly when the base timing is anything below 18 BTDC.
 
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