yes, another carb size thread

[GMC400]

I know I haven't posted much but I've come to the point of asking a question. Last summer I built a 434 small block.

Stock GM Block Bored .030" stroked 4". 9.7:1 compression w/ 0.051" head gaskets (for pump gas). Dart Iron Eagle Platinum 230cc heads, 2.08/1.60 valves. Victor Junior Intake MSD HEI distributor w/MSD 6AL box. Isky Racing Solid Flat Tappet Cam 264/264 Duration @ .050, .588/.588 lift w/ matching Isky lifters. push rods are oversized to head specs. 1 5/8" headers w/3" true duals w/ Flowmaster 40's. Right now it is running an Edelbrock 750cfm Carb tuned somewhat richer w/springs and rods for throttle response.

I am currently using the stock jets that came in the carb. I have the tuning kit for the carb. The only problem is the engine begins cutting out around 5,500-6,000 RPM. Would the jets help this or given the specs would I be better off to step up to an 800-850 cfm? I am getting ready to buy another carburator soon anyway for another engine that will use a 750, so right now the price of a new carb is not a factor. I will also be stepping up to 1 3/4" headers soon w/the same exhaust.

Sorry for the only post (besides the first one) being a question. I've been lurking for a while but haven't had any decent input to add.

Thanks, Joe.

[Itoldyouso]

How much time will it spend on the strip and how much on the street?


Don

[GMC400]

Mostly strip/mud drag. Probably 80% strip/mud drag and 20% street. The main reason for the pump gas is the rules for street classes in the mud drags.

[tango]

Sounds Like your fuel pump may be the problem . That 750 Should be big enough . But with them heads and that big camshaft You most Likely will have to step up the Jets . Read the spark plugs right after you get on your engine . Yes a 750 Holley D/P with the bigger fuel bowls would be a better carb for this build .

[kitz]

IMO if you want to tune your carb best install an O2 sensor. Then install a gage and you can readily see the difference at any throttle level without pulling spark plugs. Then later should you feel the need for EFI you already have the sensor!

Digital Air/fuel Gauge, Part # SUM-G2986 $27.95
O2 Sensor, Part # SUM-G2989 $29.95
Weld-in BUNG for O2 Sensor, Part # SUM-G2990 $4.49

I also agree the Holley D/P would be much better suited to that motor.

Kitz

[Racecar100]

Sounded like you may have too small of fuel line or a pinch line somewhere.

[skids72]

I would take another look at the CFM... For racing applications, you want a higher flowing carb (my rule of thumb is cubic inches x 2 for 7000).

The formula I've seen is CI * RPM / 3456 * VE where VE is volumetric efficiency... we use about 85% for street cars and 110% for race cars.

I would use a 850cfm carb at the very least on your motor and a 950cfm would be better.

With 230cc heads, the carb is your bottle neck....

BTW: welcome aboard

Two cents,

-Chris

[Dave Severson]

I'm with Kitz!!!! At the insistence of the guy who dyno's are drag car, we put Oxygen sensors in the collectors last year and installed a gage. Slickest tune up analyzer invented since the timing light!!!! Takes all the guess work out of jetting and carb tuning in general!!!!! Only problem now is getting the driver to remember to look at it on the big end!!!!!! Oh well, what do you expect from a driver with CRS??? Had to put a tattle tale tach on the car,,, suppose the make an A/F gauge with recall and a memory?????

[Dago Red]

out of curiousity, and sorry to hijack, why do sometimes small block engines, 350's for instance, run more carburetion than big blocks? I bought (haven't setup yet) a 770 street avenger for my 429. I read an article this morning and the guy had a 350 running dual 400's.

just curious.

Red

[Dave Severson]

Carburation is dependant on a lot of things other then just cubic inches. The cam and valve train, cylinder head runner size, and intended usage are just some of the factors. For example, a 460 with a mild cam in a light weight street car with a 750 vacuum secondary carb will work fine in a light weight car on the street.... Then like my old trusty solid roller cammed 408 Windsor cammed engine with almost too big of heads, and a 5 speed takes an 800 (flowed to 830) mechanical secondary carb to pump enough E-85 through it to make it run really strong.....

My observation on Hot Rods has also been that the 2 biggest mistakes are over-cammed and over-carbed engines.... Bigger is not always better....

[GMC400]

I had planned on using the air/fuel ratio gauges on the test stand when the engine comes back out of the truck for a windage tray install. I didn't know if anyone would have had a better way or not. Thanks for the advise.

The fuel line is 3/8" running through a Holley 130GPH pump w/the fuel regulator set at around 7psi.

[techinspector1]

I had planned on using the air/fuel ratio gauges on the test stand when the engine comes back out of the truck for a windage tray install. I didn't know if anyone would have had a better way or not. Thanks for the advise.

The fuel line is 3/8" running through a Holley 130GPH pump w/the fuel regulator set at around 7psi.

In my opinion, you're overpowering the needle and seat at 7 psi. Edelbrocks don't like more than about 5 1/2. Are you running a deadhead regulator or is there a fuel return?

[techinspector1]

IMO if you want to tune your carb best install an O2 sensor. Then install a gage and you can readily see the difference at any throttle level without pulling spark plugs. Then later should you feel the need for EFI you already have the sensor!

Digital Air/fuel Gauge, Part # SUM-G2986 $27.95
O2 Sensor, Part # SUM-G2989 $29.95
Weld-in BUNG for O2 Sensor, Part # SUM-G2990 $4.49

I also agree the Holley D/P would be much better suited to that motor.

Kitz

Kitz, thank you very much for this. Excellent information.

[mooneye777]

I am with skids72 on this one, its undercarbed, big valve heads and tall cam and the victor jr. I also think the edelbrock carbs are more of a street carb then a strip carb. since 80% of this engines life will be under load its time to upgrade the carb to keep up with the rest of your combo. an HP series carb is a better choice for hard running motors.

[techinspector1]

"By the way, using the o2 sensors is fine, but, you want to know what, how rich it is on a pull. You won't get that with an 02 sensor."

Help me out here Denny and explain please. I know nothing about the limitations of O2 sensors.

Thanks