Mikuni Flatslide Pilot Air Jets

Prior to yesterday I had the carbs set up with .7 pilot air jets, 117.5 pilot, and 135 main jets. The spark plugs were showing a little rich at the base ring, and would get an occasional backfire when letting off the throttle between up and downshifts. So I decided to try upping the air jets one step to .8 considering it would lean the mix slightly, but that resulted in harder starting, and more frequent backfiring. Attempting to compensate, I tried adjusting the pilot screws and put the street baffle back in the tailpipe.

Well, it was far-better before I started messing with it, so I'm going back to square one. At this point I need help visualizing how the pilot system of the RS carbs works.

What does the pilot air jet have to do with the pilot fuel jet, or what happens to the air/fuel inside the carburetor when the air jet size is changed, but the fuel jet is left alone?
It is my understanding that inserting a bigger air jet actually has the effect of making the fuel jet smaller, but I'm a little fuzzy how that works :S






What effect does changing air jet sizes at the bell mouth have on the pilot fuel jetting circuit?

"Backfiring could be a lean mix or carbs out of sync.
Steve

Thanks Steve.
I'm looking for some insight into how the fuel systems work specifically on the RS Flat slide carbs, considering the variety of jetting configurations available for tuning them. Right now I'm trying to focus on the pilot system though.

The pilot system should only work at Idle to 1/8th throttle, do you think your at that throttle position when you hear the backfire, which is likely where you may be at normal riding conditions.
My experience is at the point where you are actually driving the needle is coming into play also . And also the there is a bit of an overlap between the pilot and needle even at idle and 1/8th throttle. Flat-slides have a very strong signal and they can pull fuel through the needle jet and kind of fool you once in a while.

I thought the pilot air jet help emulsify the fuel by mixing in a touch of air before the fuel hit the main air stream in the carb throat.
Go to 20 pilots and see what happens maybe….
Do you have your throttle marked so you know what circuit you are on when riding?
Have you adjusted the pilot fuel screws for highest idle?
Working with the air jet is usually the last place to start, but maybe you have already been down those roads

Just some thoughts

Hey JOE MKII, that's some awesome nice bit of info there, and I'll digest it for awhile.
The backfires happen above 1/8 throttle, mostly when I let-off the throttle quickly, which probably injects a gulp of lean air into the cylinder when I do that.

Just took one of the best rides out of town I've had all year, and the bike ran fantastic. The air was crisp, around sixty degrees and sunny all day, with a mild westerly breeze. This year's Fall-color has been beautiful around here, and hopefully have a few more rides like it before the snowman comes to town.

I wonder how many 0.1 steps in the air jet is equal to one step up or down in the pilot fuel jet? The carbs came with 0.8 originally, and I've experimented with 1.0 and 0.7 air jets to this point. When I had the 20 pilot jets and 0.8 air jet combination, the spark plugs showed richer than they do now with 17.5 and 0.7 respectively. It sure is convenient to change those little air jets out at the bell mouth, vs having to pull the bowls off to access the pilot jets. I only wish I understood the relationship between those jets better.
After putting the .7 pilot air jets back in the carbs and the competition baffle back in the tailpipe, the engine ran better than it did before I messed with it. :S Maybe screwing around with it straightened something out.
It did remind me about fixing things that work, but I don't know why I have to keep telling myself not to do that. :whistle:

I suspect you are having the same issue I am with weak intake signals. Even though I have a cam setup and 10.25/1 pistons, my shorter stroke crank has lowered the compression to or close to stock. 8.5/1. I really won't know until I get a proper crank and get the compression back.
Steve

swest wrote: I suspect you are having the same issue I am with weak intake signals. Even though I have a cam setup and 10.25/1 pistons, my shorter stroke crank has lowered the compression to or close to stock. 8.5/1. I really won't know until I get a proper crank and get the compression back.
Steve

Lower compression = regular gas, right? Does carb size have an effect on the signal you are referring to, Steve? If so, perhaps a set of RS34 carbs would be better than the RS36 on the bike all year?
This evening I pulled the plugs out for pics after the brisk eighty-mile ride today. They have been in the motor for a couple months.
Does it look like #3 is a little lean, and the others possibly a tad rich?

Pretty damn close. Leave it for a while. It will run better in the cold weather.
Steve

I would be really happy with plugs that looked that good.

Those look really good ...if anything try raising the neddle up one clip.

spdygon wrote: Those look really good ...if anything try raising the neddle up one clip.

Well, now you tell me... :whistle:

Now that I have replaced the RS36's with 34's, I'll have to wait until Spring. But from the 100 mile ride I took before the snow blew in, the engine seemed to prefer the 34's. The power didn't feel as strong up top, but I sensed much crisper throttle response down low!
And the spark plug insulators were showing a touch lighter color.