How To Tune PHBG Carburetors

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Here are some resources for those of us new to the world of PHBG Tunings. . .

Dellorto carb diagram phbg.jpg

A la Harold form the performance forum (Re: tuning a dellorto phbg 21mm racing type --Date Posted: 02-05-08 14:17)

"david is right about the air leak but totally wrong about the main jet. as you can see in the diagram below…. the main jet starts functionality about 1/3 of the way through but doesnt become truly effective till 1/2 throttle. if you are having issues at 1/3 throttle the best thing you can do is to adjust the needle setting. “lowering the needle” by “raising” the clip up one position makes the the gas flow leaner in the low to mid throttle range. its tapered so when the needle is 1/4 to 3/4 of the way out, the angle of taper is what determines how much gas can escape through your mainjet. Once you reach WOT the amount of gas is completely controlled by the main jet. actually there is one other element to the equation. the air/mixture screw. look at the chart see how it is moderately effective all the way through? that is because it uses a separate jet which also sucks fuel that doesnt have a needle. the adjustable screw thats flush with the carb body controls it. by turning that counterclockwise you will let a minute amount of more gas flow in to the system, by tightening it you will cut that off. so you can make minute adjustments by using the screw or major ones by changing the jet.

wow, i’ve never typed that all out at once, shits crazy."


Also, from Brett in the same thread

"Think of the clip as being stationary and the needle moving up and down along it like a window sash. The higher it is the more open and therefore rish, the lower it is the less open and therefore lean.

Top clip setting, leanest. Bottom clip setting, richest. Standard, second from top.

I wouldn’t mess with your needle until you’ve gotten the main jet figured out and have set your idle. Elliot seized his polini by leaning out his needle."

Add to this as needed and good luck!

Dellorto carb diagram phbg.jpg