High altitude jetting question

Hello All,

I have a 78 KZ650 B that is has Mukuni VM 24 carbs with stock exhaust. It has 102.5's and #15 jets, and the needle in the stock position. I've just cleaned the carbs(via the dunk it method) and made sure all the ports were clean. Float levels were set with the tube out of the bottom of the carbs. Timing is spot on, and the advance is working well, and I just replaced 1 shim under an exhaust bucked on #3, and all other valves are good.

The bike starts really easy, and sounds great. I've syn'ed the carbs with sticks. Low end runs great but between 5000-9000 rpm's I get some hesitation, or little bobbles before I get to high RPM. Doing a plug chop in 3rd gear for 20 seconds, the plugs are just a bit black overall, and grey at the tip, and they are dry.

I'm using regular gas, not premium, and running at 5300 feet ASL(Denver). I've turned the air screws(located under the carbs) 1 turn each way from the 1 1/2 out when I started, with little effect.

I have a factory air box, brand new boots, new air filter, and have checked for air leaks with WD 40. Gas tank is really clean, with new Hose, and I've tried with, and without an inline fuel filter, gas tank open and closed, and petcock is in good working order. Vac line from #2 carb is routed to the petcock, and it's working properly. Fuel line is not kinked.

I'll try with some premium fuel next fill up, and I'll see what you guys think. I'm guessing the higher altitude and stock jets and stock settings are cooking my noodle.

Maybe a main jet , and needle position change????

What do you pro's think??

I haven't ruled out weak spark either, I'll selling the bike, so I don't want to go nuts on the $$ for news coils, but would like smooth power delivery for good selling points for the new owner.

I'm going out the the garage now to triple check the timing, and then to fill another bike with some BLOODY $4.09 /gallon GAS. Where are all the electric bike with solar panels(I'd guess in less than 5 years, they will be everywhere).

Thanks guys.

Almost sounds like a great canditate for WG's coil mod! Will cost less than $20 and maybe a couple hours of your time. Check it out here !

Bob475 wrote:

...I've turned the air screws(located under the carbs) 1 turn each way from the 1 1/2 out when I started, with little effect... Maybe a main jet , and needle position change????...What do you pro's think??...I haven't ruled out weak spark either, I'll selling the bike, so I don't want to go nuts on the $$ for news coils, but would like smooth power delivery for good selling points for the new owner....

I've turned the air screws(located under the carbs) 1 turn each way from the 1 1/2 out when I started, with little effect-- actually these are pilot circuit mixture screws, not air screws. For higher altitudes would be turned in more thereby allowing lesser fuel to better match the thinner air ).

Whether top-end mixture might be improved by reducing mainjet size should imo be based on comparing the plug readings after high speed throttle chop procedure with each set of mainjets. Also, too small mainjets typically feels like the bike is simply not getting enough fuel at wot. And way too large mainjets will sometimes cause mis-firing due to excessively rich mixture at wot.

Whether midrange performance might be improved by clipping the jet needle further away from the pointed end, thereby allowing lesser fuel to better match the thinner air, is often determined by a seat-of-the pants test ride. It will either pull better or worse through mid-range, but not the same.

Most probably a waste of time to fool with any mixture adjustments without first assuring good spark quality.

Good Luck!

Thanks for the posts, I'll do nothing to the carbs until I do the WG's coil mod.

Thanks for the link.

When that's done, I'll do more plug chops, and try adjusting the pilot circuit mixture screws.

Thanks again, this forum ROCKS.

Bob