Tuning stock 340 Six Pack

I want to check the tune on my stock 70 340-6 automatic. Any good references out there? I have a shop manual but it doesn't have any specifics on the 340-6. Right now it runs ok but the idle seems to high to me and there is a little hesitation from light to mid throttle. I'm running 93 octane and everything is stock. I need to know what timing, idle, manifold vacuum at idle, and anything else that would help so I can get it to run as good as possible.

there's been some comprehensive 6 pack tuning manifestos on here. see if you can retrieve em & Google it also. I'd find your stock specs for a start/baseline. Start with the AP for the hesitation & float level but I'd see what the articles have to say first. what is your idle speed? what cam?

It's all stock. AP? Idle shows about 1250 rpm on dash tach.

Accelerator Pump. 1250 (if the tach is accurate) is pretty high. Can you bump up your initial & lower the idle?

I'm sure I could if I knew what it was supposed to be. That's part of the info I need. The only specs I have are for a standard 340. I don't have the specs for the six pack automatic for initial timing. I need all the basic stock tune up specs, idle speed and timing for the 340 six pack automatic to establish a baseline.

Factory specs were dailed back for insurance reasons and very few performance cars run best at factory specs.

I sent you a PM, I have several of the tuning docs referenced that I pulled off of Moparts, send me your e-maill address and I will ship them to you.

I have virtually no exp with em & was more just providing a BTT for ya so that is as far as I can go. I hear they take time to get right but are a terror when you do get there. I just Googled "mopar 340 six pack tuning info" & it pulled up a slew of sites

Stock end carbs are WAY lean. Should idle about 1000 rpm at 10 BDTC. I ended up with all 3 carbs with the mixture screws out 1 5/8 turn. You definitely have to have some patience tuning them. Need to put your finger tip over each end carb outer air bleed one at a time. If it speeds up it's lean, slows down it's rich. Definitely check that that funky fast idle tab you twist with a screwdriver is not hanging up the fast idle cam from dropping all the way down, and that the end carb linkages are properly adjusted. I chased that fast idle cam issue for a while. Hesitation could be a mis-adjusted accelerator pump, or the end carbs could be opening a little too soon. The end carb springs are really light. Need a selection of vac sec disphragm springs to tune it.

This ! And unless you have a radical cam,1 5/8" on the end carbs is too much.

First get your timing correct, initial, mechanical advance, vacuum advance and limit your total timing for your tune.

Make sure you have no vacuum leaks and your linkage works perfectly.

Set your float levels, select the proper vacuum pod springs and adjust your jetting and idle A/F settings using a wideband.

It's a bit time consuming and you need to do things in order, but everything you need to know about any specific step is available right here.

I need the recommended specs (for best performance) to be able to do this.

Start at 16-20 initial with a stock camshaft. This should allow you to turn the idle.speed down. Stock cam should idle easily at 750-800 rpm when the set up is right with minimally drop when placed in gear.

The factory specs were not close to optimum as Emissions were a concern.

Don't mess with the accelerator pump until the timing is sorted out.

With 16-20 watch your total timing, stock distributors can mechanically advance 24-26 degrees and I think the most you want to safely run is 34-36 total with the vacuum advanced unplugged.

I've finally had a chance to work on this a little more. I have 10 initial timing, idle at 950, vacuum reading is steady at 16 at idle, and the exhaust smells much cleaner. I have the outer carbs set to where there is no difference in idle if you cover the outer air bleeds. It cranks up and idles pretty well and takes fuel well if you goose it from idle. I've adjusted everything according to the references I've gotten from through here and the service manual.

I have a bog/hesitation issue when driving at low rpm/speed, easing into it until the rpms come up a bit and than it seems to pick up fine. I have the secondary rods set how the factory manual says to, but in some of the references I've read to lengthen the front rod 1
turn and shorten the rear rod 1 turn to help with hesitation. I'm not sure whether to
try this or something else. Seems just right while its idleing, but driving slow its boggy until
the rpms come up a bit. Any tips would be much appreciated.

http://www.mopar1.us/6barreltips.html

950 rpm is way too high for a stock cammed 70 340

Turn the initial to 16*, limit mechanical and it may even get rid of your bog. See if it starts hot at 16* and I bet you it's a bunch snappier at idle. Don't drive it with 16* initial until you limit the mechanical advance

Learn what it takes to limit mechanical advance. Otherwise you will be hamstrung with a crappy running car at idle and low rpm due to the emission requirements these cars were built with.

There is a guide to tuning six paks-find it-use it and you won't have to ask questions here and get piecemeal answers of sometimes questionable advice. The comprehensive guide is work of many including myself that I compiled/edited. Btw the rod offset idea you referred to is bad advice. Read the guide!

Pardon my shortness -I do technical support for a very complicated product so I am very tired and I have no patience for the keyboard cowboys here who have no idea how to retain and store information from people who were kind enough to share and post it here.
Copy...paste...save

Would you be able to provide a link? Why is the rod offset idea bad advice?

I've got a few different references and there is a lot of contradiction and little if any explanation as to why one approach is better or worse than another which is why I'm not sure what exactly to do.

My main issue now is low rpm bog/hesitation. I haven't read anything yet except for the article from Jan 92 Chrysler Power that says to offset the rods to eliminate off idle hesitation that addresses my bog issue directly which is why that seems attractive.

I've likely got a number of factors contributing to the bog and I'll need to troubleshoot each of them. One is a questionable vacuum advance which I'll replace soon. Another may be that it's running too cool. I have no idea what thermostat it has but the temp gauge barely moves up stays low and I've used an infrared thermometer and the intake stays in the 150-175 range. I also disabled the noisy heat riser and the bog seems much worse with it disabled but I'll retry it with it enabled to be sure.

As I continue to tune and mess with timing. How is the best way to modify the distributor to limit advance? Where is the slot that needs to be modified and how is the best way to do so? For each degree that I need to limit the mechanical advance so total doesn't exceed 36 do I need to shorten the slot by 0.015? To find out if my bog is from the secondaries opening too soon would it help to disconnect the linkage and vacuum lines from them and run just on the center carb to find out for sure if it's from those or maybe a faulty accelerator pump or pump setting? I need to check what springs are in the pods but if they are black and the secondaries need to come in even later to stop the bog is there a way to do that?

I'm also still curious as to why offset secondary rods is a bad idea?

If the rods were to work better being offset,they would have been that way from the beginning.Manufacturers spend millions on design and engineering to make things work,why would you change it?

What is the vehicle? What does it weigh, what rear end gears? What trans? Where do you live, what is the ambient temp while you are driving?

If engine temp is too cool it will bog, unless tuned to run at that cool temp. Is the heat crossover in the intake clear and not blocked by a gasket? If you have not even checked the outboard springs yet you are missing important info. Know the basics first, then get fancy.

Why not offset linkage? THINK about it, WHAT does that linkage do? It closes two end carbs, so when your foot says to close them, it needs to happen right now. The linkage does not open the carbs.

WHY do you want offset linkage, what is the gain?

1970 Challenger 340 6 BBL 3.55 gears whatever a stock one weighs probably between 3000 and 3500 lbs TX torqueflite between 50 and 70 degrees I don't know on the crossover and haven't checked the outboard springs.

Why would I want to offset the linkage? I was curious about it and the pros and cons of it since I read that taking one turn out of the rear carb linkage and adding one turn into the front carb linkage may be a possible solution to bog/off idle hesitation in a Jan 92 Chrysler Power article on tuning 6 packs and that's what I'm trying to tackle.

http://a12mopar.com/yabb/YaBB.pl?num=1398047559/0

From the text...

Make sure the timing is 15 - 18 deg btdc [advance] at idle. THIS IS IMPORTANT

Carry on.

Anyone have any tips on how to modify the slot to limit total timing? Where is the slot that I need to modify?

I have initial at 10 right now because there is 24-26 mechanical and if I go over 10 I would have too much total timing.

To rule out the accelerator pump should I disconnect the linkages and vacuum to the outboards and see if it bogs on just the center?

(1) the FBO plate ($25 to your door) is the fastest/easiest. You can weld the slots up then file em down to the right amt which is time consuming but free (exc for the welding) and the FBO plate has slots for 10-12-14-16-18 (not mine but I eyeballed one) & for dead on dist tuning if the vac gauge method for initial then wants an odd # to get to say 35 total (tried & true SB amt)then the plate will be slightly off (OCD anyone). Jimmy G the JB weldmaster uses that instead of welding. What I would suggest NOW is bump the initial up to 18/slow the idle/reset initial to 18 (if it changed) & see if you can get a good/low(er) idle. it'll take 5 minutes and you can set the initial back in no time if you ain't happy with the results. EDIT the slots are the 2 oblong holes in the flat cam plate in the dist (pop the cap & you can see em & each one has a pin in it & you are limiting how far that pin(s) can move in the slot by shortening the slot on each side)

Here's some pics of the springs in the pods. Not sure if I need different springs for the bog issue. They look pretty light but don't have a color like black, white or yellow. Anyone know?

I just saw a chart on here for them (color VS strength) I'd sure think that one would be the "plain" one, iirc the lightest which would as you know come in the quickest/be the most likely to bog

Anyone have any black or brown springs from a kit that they don't need? Just thought I would ask before I buy a spring kit. Also what part number kit would I need should I not score some on here?

I can get Edelbrock springs for the secondaries locally. Part #12412

http://www.oreillyauto.com/site/c/detail...3&ppt=C0359

Do they have to be Holley springs?

Holley PN 20-13 you'll need to order 2 kits, total approx $30 use genuine Holley parts, you don't need any further issues

Generally I start with the white springs, and move up the spring range to tailor the opening rate/rpm of the secondaries, rather than working downward from Brown or Black springs...I prefer the secondaries coming in as earliest as possible, and basically as demanded by my right foot, rather than "waiting" for the system to overcome high spring tension in the secondaries

Mike

I've read that the Edelbrock and Holley kits are the same. Can anyone confirm?

If you want only springs you can get a pair of any color from Ouick Fuel for $10.00 + shipping !!

Did some more research on this. Holley makes the Edelbrock kit. They are all the same. BIG-10258, 20-13, B5006K, Edelbrock 12412.....and probably some others.

Buy two sets of springs. You will need one for each carb. Additionally, you should buy two new vacuum diaphragms as they are often damaged due to frequent installation and removal of springs. I was told a long time ago to adjust the springs so that you don't have a noticeable "zoo-whaaaaa" when kicking in the secondaries in high gear at 45-55mph. If there is a noticeable bog, and then a pickup, the springs are too soft. All my experience is with a modified RB and six pack set up. I run brown or purple springs in my outboard carbs. If you are getting too big of a hit of fuel off the line, and that is leading to your bog, you can adjust the amount of accelerator pump hit by adjusting the nut on the lever that actuates the pump. Loosen the nut to eliminate some of the hit at launch.

Most of what I read says 15-18 initial, but then I've read some other info that says not to go that high and keep it around 12 with my cam so I'm not sure what is best.

The engine is a 340 with 10.5:1 compression and the cam has 114 centerline. I'll be running 93 octane pump gas, Autolite 65 spark plugs and have brown springs in the vacuum diaphragms. Should I still go for the initial of 16? I have a limiter plate that will allow me to limit it to initial plus up to 18 more degrees.

I'm (was) running 18 on a bone stock 83 318. you might do the "vac gauge method" & see what that gets you for what it wants for initial. No way would I think 12 would be better than 16 even WO a cam & with it even more reason to bump it up. At least see what the VGM comes up with & try both ways if desired & tune for both (idle speed back to your lowest reasonable hot idle speed along with mixture screws) & see which acts the best then change the slot choice in the plate then springs then adj the can.