Stock 340 TA build with Mr. Six Pack cam (114 lobe separation). Car starts, idles and runs great, but is slow to rev under load. In neutral it revs fast and once up to speed and driving it pulls hard, but from a stop the engine is slow revving / lacking power. Not really bogging as not running right, but bogging as if I'm pulling a trailer. No smell of excess fuel, but just doesn't have much power until about 2,500 rmp and then it takes off fast.

Had a friend that works at a auto shop sniff test (with a Snapon machine) the car under load and it was running super lean, but at idle was perfect. The center carb had a 61 main jet in it when I got it back from the guy that restore them (and that the jet it was sniff tested with). I've gone up to 64 then 68 and now I'm all the way up to 80. It would seem like that is way to much for a center main jet on a 340, but every time I add more main jet it runs better and gets faster on the top end. But it still revs slow on the bottom end and takes off like it's waiting to wind up. I think the big main jet on the center is trying to mask the real problem that might be the outboard carbs not getting enough fuel to the engine when launching from a stop. Once the RMPs get going its like a raped ape chirping the tires as it grabs gears.

In the outboard carbs vacuums, I've tried the lightest springs (white), both yellows (short and long), purple and the plain spring. The long yellow seemed to work best. But the way it feels to me is the outboard carbs are not putting enough fuel in whenever the gas is mashed regardless of the spring / timing they open. Really feels like a 2 barrel 318 stock engine for a few seconds until it winds up.

On the accelerator pump I started with the white cam, then orange and green. It like white and orange best. The intake vacuum at idle is about 15 to 16 in neutral and in drive (at idle) is 13 to 14. Was using a 6.5 power valve, but went up to a 7.5 and it liked the extra fuel a little sooner. On the engine run stand if I opened all the carbs at the same time (manually) it revved SUPER fast, but that wasn't under load so doesn't help me now.

The car is running the Rev-N-Nator electronic ignition, stock intake and exhaust manifold with stock T/A heads. At 2,500 it's at 35 degrees of timing and at idle it's 15. Just for fun I put a different brand new 340 center carb (same jet set up with the same cam as I was running) and it the engine runs the same as the first original carb. So doesn't seem to me it's the center carb that causing this.

I'm out of ideas guys.

Thanks in advance for any thoughts or comments.

What is your transmission, and gear ratio? I have a friend who built a 340 that isn't much better than his old 318 off the line, but pulls hard once it's going. He is running 3.23

How are the float levels and fuel supply?
Have you changed or fixed the kill bleeds on the outboards yet?
I and a bunch of other six pack racers use a screw and nut on the return slot to help open the outboards quicker

Did you degree the cam in... what is it.. it could
maybe be advanced to help it rev in the low RPM range

What's the initial timing set at?

It's a stock 727 floor shift Auto with stock a 8 3/4 (3.55) sure grip rear. The trans was built by a good friend that has passed away. He was the Dodge factory Auto transmission guy for Beaverton Dodge in the 70's and built transmissions on the side after he retired. Can not say for sure what the stall converter is, but it feels like it's hooking up solid.

As for the fuel pressure it's great, but not sure about the float levels. That something I have not checked yet. Thanks for the reminder and I'll take a look at them to make sure they are set correctly. Not sure what the Kill Bleeds are so I can assure you I have no clue where they are are how they are set. It seems to me that if I can get the outboards to put more fuel in quicker it should solve the problem.

I'll have to find my machine shop spec sheet to look up the degree of the cam. Something I haven't thought of yet. Good suggestion.

The initial timing is 15. At 2,500 it's 35. I do not have the vacuum advance hooked up either. I've always set my timing with them disconnected. Maybe that's part of my problem?

You say it likes more jet from 61 to 80, this screams vacuum leak. Either it has one or the outboards are leaning the mix due to no squirters and street gears.

What timing chain set did you use?
How tall rear tires?

The reason it feels like a 318 2 bbl is because it is one. Same stroke, and starting out it is on only a 2bbl. Larger ports, valves and exhaust manifolds lead to doggy low RPM performance.

Stock to stock a 340+6 does not rune like a 440+6. The smaller engine needs to get the RPM up to get air flowing and all carbs open.

I'm not bashing your ride, I own one too.

Your base timing could be higher but you would
need to recurve the advance so you dont go too
high at total... but about 19* should be pretty
good... cam timing would help it a lot(advancing
it)... more than likely it would say about 108-
110 installed but if it wasnt it will be slow..
even 104 to 106 should really help low end.. but
if you do that it moves the whole curve lower in
the rpm and you loose top end rpm.. but if you
dont ever get up in the high rpm you dont need it

Do you know the real compression ratio? Was it measured or just put together?

Contact Bob K. he knows these carbs as well as anyone. especially if you have his cam.
Doug

I thought vacuum leak first off too. No way a 340 should be happy with an 80 main jet is what I was thinking. Not sure on the timing chain brand, but I always use quality parts. I would need to look up the receipt to know for sure. Tires are stock reproduction Polyglas.

The compression is 9.1 to 1. The engine runs and idles fantastic. It's just that pesky lazy launch. I think it's going to be best to put it on a dyno and watch how the outboard carbs react under load. And that way I can get some good A/F reading under heavy load too. The next dry day we get I'll take it down and see what it does on the dyno.

Thanks for the help guys and I'll be sure to update what I find.

Good idea.

I've driven and tuned bunch of the 340 six pak cars, both automatic and stick tranny, none of them with stock carbs. want bigger jets like your does
The last stock AAR Cuda like yours I tuned like the stock jetting on the center carb. it didn't like 61 or 63
I wonder if your power valve isn't working

NANKET is exactly right. Until you get the RPM up to start moving air the engine will not create enough vacuum to open the outboard carbs. If you had mechanical outboard carbs it would be a completely different animal. I have tuned on a lot of them and unfortunitly that was a down side and automatic trans. cars are worse than 4-speeds.

The initial timing is 15. At 2,500 it's 35. I do not have the vacuum advance hooked up either. I've always set my timing with them disconnected. Maybe that's part of my problem? [/quote]

Definitely try running it with the vacuum advance first,it helps responsiveness.It should be adjustabale to limit it on a performance dist also.If that doesn't work,I'd try a different ignition box as a next step.All this after a float/fuel level check!

It could be as simple as the converter is too tight

Monte's right.

It could be the convertor is a tad tight, but lets think about this for just a second...

With vacuum carbs, your motor needs to draw enough signal to open the vacuum pods and apply the needed fuel. This means, it has to go very lean to even draw the pods open.

If your not launching at or near your shift point, its going to be lean until the engine rpm rises enough to draw in the fuel which its not getting....We call it a bog. And that is what vacuum carbs do.

We have a 572 Hemi we dyno'd with a six pack, same issue...jet it all you want. Doesn't help. We had to raise the loading point of the dyno just to get past it. A standard 950 race carb, was over a tenth quicker just because of the bog.

Three center carbs is your best bet...

That's something I've never considered. When you say to tight you mean a stall converter is to low, right? And it makes sense. I notice when I shift from Park to R or D it really grabs hard. It's what I would describe as a very noticeable "grab" in to gear even with your foot on the brake.

If you chassis dyno it, you should be able to get a pretty good idea of what the stall is...... Along with power, a/f ratio, and be able to see how the secondaries are operating.