C_Rules

iTrader: (7)

Whatsup everyone. My dad has a 12.5:1 355 SBC with an 850 carb. It has a direct port nitrous setup which has it's own fuel pump.

We took it to the dyno yesterday to tune it up. It turns out the car is running extremely lean. At idle it was in the 20s and approaching WOT it was in the high 14s low 15s in terms of afr. So we didn't get any runs in since we didn't want to hurt the engine.

Currently it has size 78 jets. What size should we upgrade to so it doesn't run lean at idle or WOT?

ZONES89RS

iTrader: (25)

Well, a 750 crb would help a bunch, with 74 jest, go from there. That is too much carb.

C_Rules

iTrader: (7)

if the car is not getting enough fuel already won't downgrading to a smaller carb with smaller jets make the situation worse?

ZONES89RS

iTrader: (25)

Slightly smaller venturies are going to pull more fuel down low and help with low end worque also. That is too much carb for a 355. I run a 750 on a nasty *** 383. I have plenty of fule up to 7000 RPM with 73 jets primary.

blk/slvr02ss

iTrader: (21)

Just what the other 2 guys said too much carb for a 355ci,You have to remember the 850 stands for 850cfm which is too much air which leans the motor out.A 750cfm carb is about just right then you can play w/jets & barrel squirters & tuning.

C_Rules

iTrader: (7)

thanks guys i know about about carbs. since he has the 850 already is there anything he can do for his situation? its basically a new carb i know he'd hate to go have to buy another one.

1 FMF

iTrader: (1)

at idle, the main jets have no influence on air/fuel ratio so if you can't tune it down around 14:1 then that's further indicating the carb is too large for that engine.

as long as there is not a vacuum leak the only thing controlling a/f ratio at idle is the idle mixture screws and float setting.
if you're float is set so there is too little fuel in the bowl that will lean out the a/f ratio across the entire throttle setting, so make sure the setting is correct or at least raise the fuel level in the bowl slightly. If fuel drips out the boosters at idle, or the engine doesn't start and there's a super strong smell of gas then you set the level too high.

LS1 Coupe

As what was said above is somewhat correct, the main jets don't come into play untill 3000+ rpm, Holleys, Demons and the like have idle jets that most people dont even know about and often get overlooked and why most people junk out carbs saying they are junk, the idle jets is what controls fuel mixture below the 3000rpm mark as well as even adding some fuel above, Along with these carbs they have low and high speed air jets depending on carb model these can also be replaced, also the power valve will also have an effect on fuel ratio. I have seen guys run these carb on even smaller cube engines but they require fine tuning to get them to perform. I run a 750 mighty demon on a 427sbc without issues. getting the right carb for your displacement is a lot easier to tune than picking something that is to big and working from there. Holley's sight doesnt give much in the way of tuning, BG has a bit more info, http://www.barrygrant.com/bgfuel/def...ge=techsupport , Another excellant tool in tuning carbs is getting the LM1, It will help immensley with tuning. http://www.innovatemotorsports.com/products/lm1.php

invegarating

IS this a vac. secondary or double pump?If its a double pumper back out the idle mixture screws until a correct a/f ratio is obtained at idle,remember the "mixture" that the carb produces cannot be changed only the amount of "mixture" introduced into the airstream(the mixture can be changed by opening up the idle feed restriction but if you go to far it is not easily repaired,also you could close up the idle air bleed but again if you go to far it will need to be drilled back out),I would just back the mixture screws out as needed.
As far as WOT,I think that Holley recommends to not go more than either 6 or 8 jet sizes above "stock",so if is lean I would go up to an 82 and see how it likes it.
I personally don't think that an 850 is to big for a 355(I know of a Camaro racecar that has two 850's on a 331" sbc that runs low 9's)and one of the street cars that I work on has a Quickfuel 850 on it and it runs great,I use either a 950 hp for alcohol or a quickfuel 4150 1050 for gas on my 406.