which carb for my 340?

[shannon]

I've got a 67 dart with a 340 and was wondering what carb I should be running? The car runs 12.9's at 108, but my 60 foots are in the 2.0 and 1.9 range. It has a 4 speed, 3.91 gears, and micky thomson indy ss tires. If i launch above 3000 rpms it fries the tiers off, so I launch about 2800. It spins a little, but when it stops, i lay into it. Here is the problem, it bogs. The carb is a 750 holly vaccume seconday, Would a double pumper help or would the car even bog more? (Performer RPM air gap, , 10.3 compression ratio, 243 at .50 ultradyne solid cam)

[rumblefish360]

Off the top I'd say delay the secondarys and work that bog problem out.
The answer to your question is that the tire size may also be a problem. Height and width play a big roll in it. And you didn't included it. That makes it harder to answer.

[Tarrbabe]

Rumblefish is right on. Many carbs will work well but I think the 750 vacume sec. servers the best aspects of the 340. The 340 likes to rev. and it needs the bigger cfm of the 750 and the vacume secondarys prevent the bog that comes with larger carbs. Being mid-range in cubic inches the tuning requires a little more care to get the carb to match the engine demands at lower rpm's. Adjust the secondary to come in slower until you lose the bog and then work on your traction problems. You can't control lost ET due to tire spin until you control the lost power due to the bog.
As usual Rumblefish is right on.

[HOLESHOT]

Most likely the accelerator pump shot is too small ie .028 stock.When the tires are hooked up the vaccum will drop off as a result the air flow is less than optimal as a result you need more pump shot to cover up the hole created by lots of air with too little fuel being pulled out of the discharge nozzel .The vaccum secondaries will be closed when this occurs since the vaccum signal is to low to create opening.These carbs work great because of this.With the stock vaccum spring youll not get full opening until around 4500 rpm.So I suggest removing the diaphram and install the short yellow and ensure the resrtictor is drilled out.This will not cause a bog but open the secondaries in the 3800 rpm range and quicker as well.Make sure the power valve is around a 7.5-8.5 depending on idle vaccum. Set the pump arm for the shot to .015 when at full throttle (engine off)This is measured at the arm to lever nut on DS front,push arm down hold lever up and measure clearance between them while at full throttle.Start here and retest.advise

[Tarrbabe]

Shannon, you didn't say what # carb you have? 750,vac. sec. - 3310? Some carbs come with the 50cc accel pump like the early 3310's did when they first came out. I don't know what they have now but knowing the right #'s will help get you more correct answers. Some emission carbs have low-po set-ups.

[rumblefish360]

Thanks Tarrbabe. HOLESHOT has a point to. Pump shots, secondary openings can be a tricky thing to figure out in a chat room. Carbs allways give me fits. Guess I was just lucky to do the right thing first and not far off base.

[shannon]

Thanks for the info guyes. They dont teach you this in collage. Anyway the carb does have the 30 cc pump on it. As for the spring i guess I can play with that. The thing is that my 60 foot times did not change when I disconected the secondarys. ?? So right now I dont have a spring in the diaphram. Guess I need to put one hu? (I disconected the diaphram arm form the butterfly shaft) Also, about what size jets should I be running for a motor like this?

[Dartcustom]

I'm curious about this too.

[mtrv8n]

My 340 did squat with the vacuum holley, and when I went to a double pumper, like a bat outa hell. One big dif is the secondary pump, which dumps as much fuel as you need, provided the squirter is big enough..play with different style squirters too..

Now I'm running a crate 360/380 with an edelbrock eps800, no bogs at all, pulls stumps outa the ground..