Carb jet size question

[Diggerauditor]

Here's a quick calculation for CFM.

351x 5000 x .80/ 3456= 406CFM
CI x Max RPM x VE / 3456

This number would get you in the ballpark. As an example, I built a 302 for a buddy for his 4x4 and a manual 3-spd trans and only had a 600cfm w/vac sec and it dyno'd at 368 horses. So bigger is not always better. If it's a daily driver you can easily get by with a 600cfm carb. If it's mildly rebuilt you can use a 750 with a little rework.

 

[flaminranger]

a couple things I was wondering
1. when is it running rich? at idle or cruise or wot?
2. how is the engine built what heads, cam, intake, exhaust?
3. what do you want the truck to do? race, street, 4 wheel, rock crawl?

if it is a street setup I totally agree with an 850 demon is too big.
and a 650 dp should work just fine.
it can get decent mileage if you keep your foot out of it. it is in the secondaries is where you will kill the mileage. It don't matter whethor or not it is mechanical or vacuum. if you open it up your going to use more fuel!!!!

if you like to floor it from every stop you make, then yes a vacuum sec. carb is the way to go .
the dp uses more fuel when you start to open those secondaries as that is when the second accelerator pump starts squirting more fuel.
if it is tuned correctly and you are just cruising around running off idle it won't get any worse mileage.
but we all know if you got more power to use it is hard not to use it.
Duane

 

[doorgunner]

I know only what I learned from these guys....but I was wondering......

is your Holley IDLING rich?

or.......

CRUISING rich?

or.....


BOTH?

I had to drill a .062" hole in each butterfly to stop the carb from IDLING rich (Like the guys said---Holleys aren't designed for fuel economy........and....I don't have enough experience to "jet-down" the idle circuit/so--it was simpler for me to drill holes to provide more air at idle--since adjusting the idle-air screws wasn't helping the raw fuel smell at the exhaust pipes)....

Now--according to the heated O2 sensor that I recently installed--I can now adjust the idle-air screws so that the mixture is just about perfect at 700rpm IDLE, 0.4 volts according to the voltmeter in the cab....

Next--I have to rejet the primary jets by using the readings that I get from the O2 sensor/Voltmeter set-up (it's CRUISING lean now at 60mph according to the readings, 0.1 volts.....I need to slightly increase the jet size to get the O2 reading up to approx. 0.5 volts)

(I won't even say what I'm doing to the secondary at the present time LOL....that's another experiment )

The guys are correct about NOT needing a large cfm carb on a 302/350/351 stock/mild engine...I can get all the fun I want out of a 600cfm---and pass up a couple extra gas stations too.....

If none of the above makes any sense....I completely understand.

 

[motorbill87]

that all hepls a lot guys. i'm running a holley 650dp now, just incase you over read that part but I totally agree that the 850 was waaaayyy too much for the motor.

I street drive the truck and do some pretty serious off roading with it too. i'm not 100% sure if it running rich under full throttle, but it does have that raw fuel smell at idle and at cruise. It breaks up a little between 3,000-4,500 rpm if im not under full throttle tho. I plan on throwing a wide band o2 sensor on it so i can monitor it and get to the root cause. The engine was freshly rebuilt about a year ago and i've probably only put 5,000 miles on it since....(probably less, no odometer)

stock stroke and bore, 448/472 lift cam, single plane high rise intake, full msd ign. system, 1/2 carb spacer and, uh, ya, thats it so far lol.

i guess thats the next step I need to take is finding out where its running richest at?

thanks again for the info guys......you've really got my gears turnin in the brain lol

 

[doorgunner]

Let us know each step you go thru and the result....I know it will help me learn some stuff (I already know that I'm doing it the hard way...but...too late to stop now )

 

[motorbill87]

well I finally got a chance to play with the carb. it had 72/78 jets in it and i went down to a 67/73 set up. runs like crap! lol.

I guess the guys who rebuilt the carb put too small of jets in it so i'm going to play with it and maybe try a 75/81-82 setup and see where that take me, dont have a A/F guage yet so i'm going by trial and error