CV Carb Tuning

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By Patrick Zambori, H-D tech at Wisconsin H-D

This applies to CV carbs, but some of the theory will apply to any carb, once you understand the adjustment options and circuits.

Anyway, here goes.

The first thing I do is get it about halfway warmed up, 2 or 3 miles. Then I slow down to about 25 mph in 2nd gear. This is roughly the middle of the transition range. I do it a bit cold so that if it's good here, it will be a bit rich when fully warmed up, which is a good thing. 

Steady cruising, steady throttle, 25 mph, enrichiner off, it should run smooth, no coughing, surging or hesitation. If it does, a change in pilot jet is in order. I'll start small and work my way up until it smoothes out.

Each time the pilot jet gets changed, the mixture screw should be  readjusted using a lean drop method. I use a digital tachometer. Turn the screw in until it stumbles and back out until you find the fastest idle speed. Adjust the idle speed screw so the engine is at 1000 rpm. Turn the mixture screw in until it drops 50 rpm, then ease it back out just enough to touch 1000 rpm again. With a 48 or larger pilot, the idle may not drop at all and in that case I leave the screw open just a half turn.

Next is the midrange, back to the halfway warmed up stage, this time starting slow and slowly raising the speed of the bike in steps, say 10 mph increments and stopping for a steady cruise check at each stage, and feel how the engine runs. Again it should be smooth, no cough or surging, hesitation, etc.

About 60 mph cruising is the middle of the midrange. running a steady ten miles here and reading your plugs is the best way to check it. Shim the head of the needle up as necessary to richen the midrange. The needle determines the 'personality' of the carb much like the cam does for the engine. I like to use the stock needle that came with the bike, as it's profile is based on the weight and gearing of the bike.

Next is the main jet. I get the bike up to the 60 mph range, then nail it full throttle and feel what the engine does. It should pick up on power. If it slows down, your main jet is off. Start with a smaller main jet. If it starts coughing at full throttle, then go the other way.

Just my personal and professional opinion, but I have had great success with Dynojet products, and have many many happy customers after my having installed the kit. Of course you can rejet the stock carb with stock jets, but that will only correct the mixture, it doesn't 'improve' the carburetor in any way.


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