carburators humidity?

[dwmarlin4114]

I'm having trouble with my bike flooding and am wondering if the carb is just not gonna work right because the weather has been 90-100F with around 90% humidity lately. fuel vaporizes better in warm weather right? but i think the humidity is throwing it off. How bad does this effect carb adjustments?

I've dialed in the carb to the point where it idles fine with no choke but it bogged down after about 1/8 throttle open also unburnt fuel spits out the exhaust. So in an attempt to lean my mixture I moved the clip in the needle jet needle one position up right? It ran one time where idle was fine, it kinda bogged at low rpm and then ran great at high rpm but after killing the motor and trying to start it a different time I didn't get those results.

my other question is air/fuel mix if I'm running rich I need more air but wont allowing more air at higher velocity through the carb just draw that much more fuel with it? Also I'v adjusted the float tab in an effort to keep it from flooding but that didn't really work

If anyone can answer any of these questions I would really appreciate it the bike is 79 ke125 with mikuni vm24ss

 

[DrJ]

Yes, I've experienced poor performance on humid days due to lack of oxygen in the fuel mix. I wouldn't bother readjusting carbs for conditions.

 

[Sonreir]

Humidity and temps will affect your carbs, but not as drastically as it sounding in your case.

Also... I may be repeating what you already know, but this information may be useful to anyone reading along. All of the following assumes you have an engine with good compression and your timing is properly set. A problem in either of those areas will affect your carb tuning success.

Step one - Warmup
Get the bike running and warmed up. Do what you need to to get the engine hot. Tuning a cold engine is an exercise in futility.

Step two - Pilot Jets and Air Screws
At this point, your idle is controlled by a combination of your pilot jet and your air screw. The pilot jet controls how much fuel is entering the system and the air screw controls the air. First up, set the air screws to 3/4 turns out. Next, from a steady idle of around 1200 RPM, blip the throttle. If the engine isn't responsive or even dies, you're probably too lean. Go up a step or two on the pilot jet. If the engine is responsive but you're getting a bit of smoke out the exhaust or the exhaust note sounds "tinny", you're probably too rich. Drop a size.

Once you think you have the pilot jet correct, take the bike for a spin. Your goal on this trip is to hold a stead speed of between 15 and 25 mph in third gear without moving the throttle. If you can't do it, your pilot jet is probably still too small.

After the pilot jet has been selected, you may need to tweak the air screw. Up the idle to about 1500 RPM for this test. Next, turn the air screw in 1/4 turn and see if anything happens. Try out 1/4 now and see if anything happens. One direction should cause the engine revs to increase and the other direction to decrease. Drop the idle back down to 1200 RPM and repeat the test with the air screw. If you are able to increase the revs using the air screw, dial the idle back down to 1200 RPM. If there is a range at which the air screws don't seem to affect the RPMs, opt for having them turned further in than out.

Step three - Main Jet
First, find a nice long stretch of road and get the bike up to 30 mph in top gear. Now, crack the throttle all the way open and see how long it takes to get to 60 mph. As soon as you hit 60 mph, kill the engine, pull in the clutch, and come to a stop. Immediately pull the spark plugs and take a look. You want a very light tan color around the base of the ceramic insulator. Dark brown or black means you're running rich and pure white (or even worse, metallic-looking flecks) means you're too lean. Adjust the main jet and retest. A decrease in the 30-60 time means you're headed in the right directions. Stick with the main jet that gets you the shortest time from 30-60 mph.

Step four - Needle jet and jet needle
The combination of the needle jet and jet needle control operations between 1/4 and 3/4 throttle opening. For this step, you basically want to repeat step three, three different times. Once at 1/4 throttle, once at 1/2 throttle, and once at 3/4 throttle. If you're lean on all three settings, try raising the needle a step or two. If you have no more room to raise the needle, you'll want to go larger on the needle jet. Opposite if you're rich; drop the needle and then, if necessary, downsize the needle jet. If you're getting a combination of rich and lean, try swapping out the jet needle. A needle with more taper will richen the mixture at 3/4 and a needle with less taper will lean it out.

 

[dwmarlin4114]

hey thanks guys, more info though I'm not just adjusting the carb for humid weather I am only trying to get the bike running after a complete rebuild and it just so happens to be mid summer with high temps and humidity. also Sonreir, you mentioned

"If the engine isn't responsive or even dies, you're probably too rich. Go up a step or two on the pilot jet. If the engine is responsive but you're getting a bit of smoke out the exhaust or the exhaust note sounds "tinny", you're probably too rich. Drop a size."

If I am too rich do I go up or down a sive in the pilot jet?

thanks

 

[Sonreir]

hey thanks guys, more info though I'm not just adjusting the carb for humid weather I am only trying to get the bike running after a complete rebuild and it just so happens to be mid summer with high temps and humidity. also Sonreir, you mentioned

"If the engine isn't responsive or even dies, you're probably too rich. Go up a step or two on the pilot jet. If the engine is responsive but you're getting a bit of smoke out the exhaust or the exhaust note sounds "tinny", you're probably too rich. Drop a size."

If I am too rich do I go up or down a sive in the pilot jet?

thanks

If you're too rich, you go down in size. Bigger numbered jets mean bigger flow.

Also... it might be worth mentioning that going WAY too big on jets will actually mimic a lean condition, though this is usually the case with main jets and not pilots.

 

[eram]

"If the engine isn't responsive or even dies, you're probably too rich. Go up a step or two on the pilot jet. If the engine is responsive but you're getting a bit of smoke out the exhaust or the exhaust note sounds "tinny", you're probably too rich. Drop a size."

If I am too rich do I go up or down a sive in the pilot jet?

This was just a typo in the original post...first line should read too lean and go up a size. Otherwise solid info.

 

[Sonreir]

Yes... damn typo... going back to correct it now...

 

[dwmarlin4114]

haha yea i was concerned the one piece of info i needed from that didn't sound right but I got it now though thanks.

I guess my plan now is to get the float level adjusted per the maintenance manual, try to find a smaller pilot jet, and hope for dryer weather.