Mid-Range Stumble Cure

Hey Gang - I had been having a stumbling problem in the 5-7,000 rpm range on my bike when running around 1/4 to 1/2 throttle. The bike would just bog if I suddenly whacked open the throttle. This problem was most noticeable at highway speeds in 5th gear.

I thought that because I was running velocity stacks air turbulence on the highway might have been an issue but through a systematic check of my leg postitions around the stacks didn't make any difference to the air flow.

The stumble didn't appear if I let 'er rip from the low rpm's and held the throttle wide open which is how I normally run the bike, but the motor was running a little rich according to a few plug chops I did to try to pinpoint the problem which was rather frustrating to chase down.

I have since solved the problem by moving the clip up one slot on the needle. The clips are now set at the second from the top. I also put on a new set of carb clamps.

Previously I had the worm-gear-type pipe clamps on the carbs but ended up getting a set of used carb clamps from our local Kawasaki dealer. For carbs #1 and #4, I had to grind down the outer lip to have the carbs fit correctly as these two outer carbs have what appears to be a casting nipple that I suspect would have stopped the carbs from completely snugging over the boots the final 1/8 of an inch or so.

The carbs are Smoothbore 29's from Wiredgeorge and they are fantastic on this engine combo. The bike fires with the first or second turnover every single time. Other riders are rather surprised when they see the bike fire so effortlessly.

When I received the carbs from George the clip was in the middle and the air screws were out 1/2 a turn. This worked well on his test bike which is a stock 900 motor (if I am not mistaken) but the 900's have little more cc's than my 810 which might have needed to be accounted for, thus the slight richness on the clipping.

If anyone is looking at Mikuni 29 smoothbores for their 810 project, I have found this set up to be perfect with excellent throttle responce and no bogging.

The mains are 122.5's, pilot jet 17.5, 1.5 slide cut-outs, 5DL31-3 needles at the second clip from the top and the air screws work best at one full turn out from light seating.

Cheers-Colin Firth-Ontario Canada

Glad you got this sorted out. Sync the carbs again hehe Also, you wrote, "5DL31-3 needles at the second clip from the top"... 5DL31 is the badge on the jet needle. The -3 is the clip position and this is the standard way to write that the jet needle is clipped to the 3rd slot with slot one on the top. If you reclipped to the 2nd slot you now have 5DL31-2.