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Breathing Room – Carb Jets and the DR650

Sometimes, you just gotta give a bike some breathing room, as our Tech Advisor confirms while working on the carb jets of his DR.

Anyone who knows me understands how hard it is for me to leave a motorcycle that is working perfectly well alone. So I decided to spring for the convenience of a Sigma carb tuning kit for the Suzuki DR650 I call Buggs. In the kit are a few main jets, a pilot jet, some small washers to shim the slide needle, a drill bit, self-tapping screw (to remove the air adjustment screw block off plug) instructions and a template for cutting the air box. This kit is similar to the Dynojet kits, which are very good, but I thought it might be fun to check out the competitor kit.

I did the suggested grinding of the header pipe—for some unexplained reason Suzuki leaves a very large and obstructive blob of weld just inside the header pipe. Be careful not to take it down too far and weaken the pipe. 

I explained to the Sigma people what I was going to do and they suggested a 155 main carb jet, three shims on the needle and two turns out on the now accessible mixture adjustment screw. The stock main is a 140: 155 is a big jump but I figured it was probably much too lean. 

Well, the bike ran great on the bottom end (low speed jet), and into the midrange (needle position) until the throttle was opened to the point where the main jet does its thing. Then the missing, bucking and surging started. Something was not right. 

Support staff at Sigma suggested I go to the 160, which actually made things worse. Then I went to the 150 (smallest main jet in the kit) and saw a big improvement—only a little rich at this point, a bit of a miss and surge but minimal. This experiment told me a lot. The Sigma folks thought the big jet would work: with more fuel, more air is required. The light bulb went on. “There’s my problem!” Time to try opening up the airbox. 

I had already removed the snorkel and installed a higher flow filter with little improvement—still not getting enough air. Too much air is “lean,” while too much fuel is “rich.” Perfect combustion is “Stoichiometric” which hardly ever happens but we try to get close. 

Most folks agree the correct ratio of air to fuel is somewhere around 14:1. That’s by weight. What most people don’t realize is that by volume it’s about 9,000:1, so 9,000 litres of air to burn one litre of gas. That’s a lot of air and at altitude it’s thinner so you need even more. My backyard shop in Alberta is at 1,159 metres (3,802 feet) above sea level. The jetting might be good if I can supply enough air. 

The half-inch drill bit had soon bored six holes in the top of the airbox. Zip ties made sure the wiring would not obstruct the passage of air. This reminded me of a Nelson Racing video in which taking the window out of the dyno room added a bunch of HP to one of their monster engines. 

A test ride proved the theory: Buggs is strong everywhere now. With a performance pipe the bigger supplied jets would definitely be needed.

It makes sense now. Think about it. You can probably walk around just fine breathing through one nostril, but running hard you might need to open your mouth.

One nice thing about working on the Suzuki DR 650 is that after removing the tank and disconnecting the throttle cables it’s possible to loosen the manifold clamps and rotate the carb. This provides access to the top or bottom without removing it from the bike. Big time saver. Be careful to adjust the cables carefully or you may have a very high idle you can’t adjust with the screw.

Buggs “uncorked” is definitely stronger everywhere in the rpm range now. It’s a very noticeable improvement and well worth the effort. It’s a job that can be easily done on a Sunday morning if you follow the instructions. The airbox must be modified. With no O2 sensor or dyno my assessment is seat of the pants. The pants do feel good! 

by Rich Burgess Canadian Biker Issue #317


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