Jet's have little to do with idle mixture. What is your ignition timing and mixture screws set at?
600 cfm holly on a 305
- Thread starter tellboy
- Start date
Have you taken a minute and pulled your plugs to check the electrodes/ They need to be a light coffee brown color to be firing and burning well. Anything else can be a sign of problems elsewhere.
As for the jets, before I'd go there, what model of Holley is your 600? An 1850 or ?? Is it a square bore or spread bore? Is it mounted to the factory OEM intake or aftermarket? Carb to base spacer?
My own experiences with tuning Holleys is that if you do go in and do a jet swap, you will need get a replacement set of the bowl and metering block gaskets as the stockers might commit suicide when you try to dismount the bowl. Speaking only for myself, I have used the Moroso Buna-N bowl and block gaskets and they can be used repetitively for multiple coming and goings into the fuel system without needing to be swapped out for new. Not sure where I came across the tech tip for them but if you apply a light layer of Chapstick to the gasket to metal surface they come and go even easier when you are doing the bowl drop.
The bowl screws ought to be WMS fractional, thinking 5/16ths hex head, maybe 3/8ths. When you go to do the bowl drop, pull one of the lower screws first and be prepared with a lot of paper towel or a small catch dish to have gas drain out of the screw boss.
The jets can be removed from the metering block using a standard screw driver but there is a use specific installation/removal tool that won't goiter the slot in the jet during the process. Common screwdrivers come with tapered surfaces that don't sit down into the jet slot completely square.
As for where to go in sizing, what I was taught so many decades ago was that it takes a minimum of three sizes difference numerically to make an appreciable difference. So from 66, if you plan to go down, then either 63 or 62 would be your possible destination. The jets come in cards of two.
All this is simply what comes next if you go through all the other suggestions that have been offered and nothing positive results. As many others have commented, there is a lot to check over and verify first before moving on to invasive surgery.
Also do be aware that Holleys are highly sensitive to shifts in barometric pressure and humidity. Dunno why, they just are. And any kind of prolonged inactivity where gas sits in the carb bowls is not conducive to good working order because what passes for gas now has a very short working life and decomposes rapidly. The crud from gas that has evaporated can block internal passages and cause havoc with how the carb starts and idles. if you are forced to park the vehicle, adding something to the gas tank and system like Gold Eagle Sta-bil and running the car for a few minutes to get it through the lines and up to the carb is probably a good thing to consider.
Nick
As for the jets, before I'd go there, what model of Holley is your 600? An 1850 or ?? Is it a square bore or spread bore? Is it mounted to the factory OEM intake or aftermarket? Carb to base spacer?
My own experiences with tuning Holleys is that if you do go in and do a jet swap, you will need get a replacement set of the bowl and metering block gaskets as the stockers might commit suicide when you try to dismount the bowl. Speaking only for myself, I have used the Moroso Buna-N bowl and block gaskets and they can be used repetitively for multiple coming and goings into the fuel system without needing to be swapped out for new. Not sure where I came across the tech tip for them but if you apply a light layer of Chapstick to the gasket to metal surface they come and go even easier when you are doing the bowl drop.
The bowl screws ought to be WMS fractional, thinking 5/16ths hex head, maybe 3/8ths. When you go to do the bowl drop, pull one of the lower screws first and be prepared with a lot of paper towel or a small catch dish to have gas drain out of the screw boss.
The jets can be removed from the metering block using a standard screw driver but there is a use specific installation/removal tool that won't goiter the slot in the jet during the process. Common screwdrivers come with tapered surfaces that don't sit down into the jet slot completely square.
As for where to go in sizing, what I was taught so many decades ago was that it takes a minimum of three sizes difference numerically to make an appreciable difference. So from 66, if you plan to go down, then either 63 or 62 would be your possible destination. The jets come in cards of two.
All this is simply what comes next if you go through all the other suggestions that have been offered and nothing positive results. As many others have commented, there is a lot to check over and verify first before moving on to invasive surgery.
Also do be aware that Holleys are highly sensitive to shifts in barometric pressure and humidity. Dunno why, they just are. And any kind of prolonged inactivity where gas sits in the carb bowls is not conducive to good working order because what passes for gas now has a very short working life and decomposes rapidly. The crud from gas that has evaporated can block internal passages and cause havoc with how the carb starts and idles. if you are forced to park the vehicle, adding something to the gas tank and system like Gold Eagle Sta-bil and running the car for a few minutes to get it through the lines and up to the carb is probably a good thing to consider.
Nick
