Husqvarna 73′ CR400 – Carburetor Clean & Jetting

First off, I have to say is that this bike is a real arm jerker! Not to mention what a PITA it is to start. When kick starting, if often kicks right back, potentially leaving you with a sprained ankle. From on now, I’m going to wear MX boots during startup.

Service Manual

Here is a link to the service manual: Husqvarna MK ML Workshop Manual

Selecting Pilot Jet Size for Bing 54

So in this post, I want to cover pilot jet sizing. I discovered that the pilot air screw on this Husky was set at about 3 turns out. Since the pilot screw meters air (turn CW to richen) on the equipped Bing 54 carb, it was an indication that the idle circuit was rich. Also, throttle response from 0-1/8 opening was very heavy/rich, and seemed to load up quite easily. The pilot air screw effective range is 1-3 turns out, with 1.5 being perfect. If the screw is set below 1 turn, go up/richer in pilot jet size. If the screw is 3 turns out or more, go down/leaner in jet size.

The stock pilot jet size is a 35. I found a 45 installed, way too rich! I ended up with a 30, which is one size leaner that stock. The pilot screw setting ended up just a hair over 1 turn out. Needle and main jet were setup properly. I chose to go one size leaner due to the fact that the oil/fuel ratio that I decided to run in this bike is 32:1. I’m certain that in the 70’s, 2 strokes ran richer in oil, possibly 20:1 or 25:1. Oils have come a long way since then, and we no longer have to use that much oil. Now, the leaner oil/fuel mix will have a thinner consistency, and the bike may run richer in fuel/air, since the fuel will flow easier through the jets.

The video below will show you how to take apart a Bing carb and swap pilot jets. I also had to take it apart and inspect for paint chips that came loose in the fuel tank. Don’t forget to watch all of it, I take this bike out for a before and after comparison.