tmdog

I have all the necessary tools to adj. the carb (vacuum & tach). Do you have to adj. all four screws for the idle circuit? How? Carb is a Holley 750.

tmdog

TTT- - I think all the carb. guru's are on the water today. I was hoping for some help. Tomorrow's water test is in jeopardy.

articfriends

iTrader: (1)

Fully warm up motor,turn all screws in slowly until it starts to run rough (with vacuum gauge teed in sensing actual manifold vacum). Slowly turn srews out in even increments until you achieve highest vacuum reading then i usually turn them out another 1/4 turn as long as vacuum doesen't start dropping,use tach to set idle back. If you have it right when you hit the key warm it will fire right up without playing with throttle. Its best to set your base timing first before doing this. I'm no carb expert but i have used holley 9381's (my all time favorite carb) on several motors in the past. We'll see when the "experts" chime in if they agree with how ive been doing it,Smitty

tmdog

Why 4 screws for the idle circuit? I was always used to 2 srews. Thanks Artic for the comeback. You apparently have some knowledge in this area. I always thought the idle circuit was adj. on the primary side. Of course my candle does not burn too bright. Let's hear it.

articfriends

iTrader: (1)

A run of the mill holley uses 2 screws on the front but they control the fuel mixture going to front and back of carburetor (the back gets fed thru slots casted in the base plate going from the front to rear). This fuel mixture is let in near the throttle blades resting position thru fuel transfer slots. Without going into carb theory(remember i don't claim to be a expert) the fuel /air mix amount is controlled by 2 orifices,one is a fuel supply hole in metering block,the other is a idle air bleed inside top of carb venturi.The mixture screws just supply the amount of this mix. Anyways,on engines with big ports,radical cams,caressive open plenum intakes etc it is often hard to get motor to idle smoothly feeding idle circuit with front screws and fuel passages in front metering block. A premium holley has 4 corner idle screws,the base plate is cast without the fuel mix passages going from front to rear or in some cases this channel has a little plug to separate the front from the rear. The rear metering block is similar to the front with fuel supply holes/idle air restrictions etc.By having essentially separate idle circuits on front and rear you are able to fine tune the idle better,your also able to feed much more fuel to idle circuit,sometimes on a std holley the idle screws are out so far they can't flow any more mix but motor still won't idle.4 corner mix carbs will usually idle stronger,smoother and better all the way around than a std carb,especially on a radical application like i described earlier. I have had radical drag motors that just wouldn't idle right with a normal carb,you'd get it down to 1500rpm's and it would just quit,dropped a 4 corner carb on and would idle strong down to 1250rpm's. I have played with fuel mixture holes on 2 screw carbs and gotten them to idle stronger but all that is unnecessary on carbs like what your asking about,hope i helped,Smitty

tmdog

Smitty- - - Thanks for the helpful information. Your modesty surely exceeds your obvious intelligence. Jim