Changing Carbs, What jets?

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805moparkid

805moparkid

Slant and AFX Guy
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so im looking at a 750 vac secondary car instead of my 600DP...

currently run 66/75 on the street and 71/75 at the track....

do i run the same jetting in the 750?
 
I would slap it on unchanged for a good baseline and work from there.
 
In phoenix..i would probably start out with stock jetting...

a carb is jetted for sea level with a 29.92 absolute barometer 29,92 at 62 degrees...
 
70aarcuda said:
also..does the 750 vs come with a secondary metering block or metering plate?

Quick Tech Fuel make a excellent adjustable vacuum secondary kit...you dont have to play with springs..

http://www.summitracing.com/parts/QFT-63-1/
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wells its a 3310 dual inlet so i dont know... i hope so... if not i'll make it one... and i will be doing that for sure!
 
If it's still stock and unchanged, it has a rear metering plate. Changing the 3310 750 to a rear metering block system is how you get a 780. It's a good mod to do though...and that secondary diaphragm Tony posted is da sheeot. That thing is nice.
 
Original 3310 750's have 72 primaries and the secondary plate is equal to 75 or 76 jets if you convert.

As for the CFM, the original 3310 was rated at 780. Subsequent versions were 750 because of changes to the boosters. I believe the newer ones had a "bridge" across the booster opening while the originals didn't. It apparently hurt airflow but helped the vacuum signal to pull the fuel through making more responsive.
 
Locomotion said:
Original 3310 750's have 72 primaries and the secondary plate is equal to 75 or 76 jets if you convert.

As for the CFM, the original 3310 was rated at 780. Subsequent versions were 750 because of changes to the boosters. I believe the newer ones had a "bridge" across the booster opening while the originals didn't. It apparently hurt airflow but helped the vacuum signal to pull the fuel through making more responsive.
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when i get it i'll let you guys know... what metering block should i get for the rear?
 
There are a few outfits out there that make conversion metering plates with provisions for jets or you could go with a block. If you stick a metering block on it, you'll need a new carb line if you already have one.

I don't really have any input on the blocks, the VS carbs I've run already had a Holley metering block installed.
 
Any idea what you engine pulls for vacuum? The 750 I had on my slant needed a plugged power valve, as it had 3 inches of vacuum at idle in neutral. Once I found that out and plugged the power valve, I had to jet WAY up. It ran really well, and I drove it almost daily for 2 summers.
 
73Scamp318 said:
There are a few outfits out there that make conversion metering plates with provisions for jets or you could go with a block. If you stick a metering block on it, you'll need a new carb line if you already have one.

I don't really have any input on the blocks, the VS carbs I've run already had a Holley metering block installed.
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i just cut my rail in half and used some fuel hose so i didn't have to remove the whole rail at the track for jet changes...

BBBCUDA said:
Any idea what you engine pulls for vacuum? The 750 I had on my slant needed a plugged power valve, as it had 3 inches of vacuum at idle in neutral. Once I found that out and plugged the power valve, I had to jet WAY up. It ran really well, and I drove it almost daily for 2 summers.
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yes... about 11 at 1000rpm idle, about 7 at 700 idle, and about 15 on cruise @ 50mph/3000rpm lol... my converter is just a little light so i throw it in N at lights or two foot it...

EDIT: and running a 6.5 PV
 

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I don't know if any regular type of metering block will work. But the kit Holley sells is part # 34-13. A power valve is not used in the secondaries. Primary power valve should have a LOWER number than the vacuum reading warmed up IN GEAR. Depending who you ask, it can be anywhere from 1.5 #'s less to splitting the difference between in-gear vacuum reading and neutral vacuum reading.

It may idle better in gear if you add 1/4 - 1/2 turn more on the mixture screws from the best neutral setting.

http://www.summitracing.com/parts/HLY-34-13/


(Post edited due to mis-wording.) ;)
 
Locomotion said:
I don't know if any regular type of metering block will work. But the kit Holley sells is part # 34-13. A power valve is not used in the secondaries. Primary power valve should have a LOWER number than the vacuum reading warmed up IN GEAR. Depending who you ask, it can be anywhere from 1.5 #'s less to splitting the difference between in-gear vacuum reading and neutral vacuum reading.

It may idle better in gear if you add 1/4 - 1/2 turn more on the mixture screws from the best neutral setting.

http://www.summitracing.com/parts/HLY-34-13/


(Post edited due to mis-wording.) ;)
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yesi know this... for an auto divide idle vac in half and thats the PV but my converter is to tight... even at 1100 rpm... so i just put it at 800 so it lopes really hard...

and thanks for the part number on the metering block... what is the difference between a 3310/4160/4150 anyways??
 
Basically -

3310 is a 4160 style carb which doesn't have a secondary metering block

4779 double pumper would be a 4150 style carb because it does have a secondary metering block.

Metering plates are used to save money on initial production. They are cheaper than a block. You can buy and/or modify the plates, but a secondary block is so much easier to tune with.
 
Locomotion said:
Basically -

3310 is a 4160 style carb which doesn't have a secondary metering block

4779 double pumper would be a 4150 style carb because it does have a secondary metering block.

Metering plates are used to save money on initial production. They are cheaper than a block. You can buy and/or modify the plates, but a secondary block is so much easier to tune with.
Click to expand...

cool thanks... and yea i know about the plates... we gave up on the dyno trying to tune using them on the landy 528 hemi motor... we try to keep little stuff like that correct but for that car i needed the ease in adjustments...
 

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