340 carb size

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kiwitony

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Hi guys,
I am thinking of changing my carb on my 340 duster .Its got a 750 vac sec holley and iI think its too big.when I had the car on the dyno it was down on power.we found a cylinder low on compression.some one ground the rockers to get valve clearance and one rocker pulled the valve wore the valve guide and buggered the valve seat so of with the heads fixed the seat and guide.I looked at the ports and got the dye grinder out and cleaned mhem up.I tryed to find some ajustable rockers but no luck(mopar parts are hard to find in NZ os I ordered some roller rockers to fix that drama and thought I should look at the cam,it was abit small so new cam kit and push rods was ordered from the states.the cam is a .497 lift with 1.6 rockers.the old cam was .420 lift .I thought this will wake it up but not as good as I thought it should.The car is a 73 340 duster with headers full MSD alloy high rise duel plane inlet manifold,shift kitted 727 and 3.91 suregrip rear.What size carb should I be running?
 
I reckon you could tune the 750vac to work but a 650dp would be easier.
You've got the gearing for a dp.
Any stall in the convertor?
 
I have 2 cars with similar built 340's, b-body with an auto, eddy airgap intake and a 750 vac sec carb and an a-body with a 4-speed, MP M-1 intake and a 650 DP. The 340 with the 650 DP seems to run better. I would like to try a 650 on the other motor and see if there is a diff. The Holley 650 DP seems to be a common carb on 340's and other hot small blocks.
 
No carb will come out of the box and run perfect on your engine...ALL carbs need to be tuned
 
What is the compression ? Maybe the rings are worn out. Did you degree the cam ? The spring for the secondaries might be too stiff.

I run two 500 cfm Carters on a old eddy 273 intake on my 340 with 3.91 gears. Pulls really nice, the 10 inch 3,000 stall converter is a big help as well. The 340 needs rpm's
 
I would think that the carb you have should be good. You may have some other problems as 'dodge freak' said. I've got .504 lift cam and 750 cfm is the 'recomended' carb. Your .497 lift shouldn't make that much of a difference.

Thre's always a simple answer to these problems. The forest tends to get in the way of the trees, and I HATE it when the answer can't be seen!!!

BTW....NZ KICKS ***. I used to stage out of Christchurch when I ran supply runs to McMurdo Station, Antartica
 
The 750 will be big on the primary side. The 68-70 340 had a small square bore carburetor. The 71 and up had the 850 TQ and they worked WELL. Making that comparison, the squareness of the 750s venturis will make for some tuning on e stock bore and stroke 340. I would start with 2 steps leaner on the primary jets. You may be able to step up the secondaries, but I would concentrate on the primary side first.
 
I have the same setup as you do. I am not happy either, but my buddy said it is the high rise, too big for the street, don't come alive untill 4 grand. I picked up an ld 340 intake will have better torque for the street. I hope
 
It all depends on what the engine want's/can handle. Stock engine, read the charts- 500 cfm. Now play with cam, etc- more carb. But !!! it wants it at the secondaries, not the primaries. This is where most mistakes are made, like Stroker hinted at.

I used to use Holleys, re-jet, check vacuum, new power valve, new pump circuit, etc. Always had to drill holes, with a cam; square or spread. insane.

Play with idle bleeds- touch and go, ruin a carb's orifices. Then a bud bought an Edelbrock, what an eye burner that was. 750 on a 440 with a cam. So learned the Carter carb, ordered the kit, got it to almost not burning eyes at idle, plugs good, better response; ran out of rods and jets to lean the primary one more time.

The whole trick is: timing, then carb. Then smaller primaries the better; let the secondaries give the fuel (they only give the engine what it wants;vacuum).

so it boils down to- smaller primaries, let the secondaries open, when the engine needs it. That's why TQ and Holley 3 barrels where made. But the secondaries still need a tweak, depending on the engine.

If you feel/hear the secondaries cut in; you are getting over a bog, where they cut in too early.
 
thanks guys,I have put a lighter spring in the vac diaphram so hope this will help It had a black spring in it.compression is 180 psi at cranking so that seems to be plenty and we think about 2200 stall. Cam is set 0 degs.the graphs say 600-640 cfm.was looking at demon carbs are they any good?I was thinking maybe a 625 road demon,what rare tour thoughts
 
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