Better to tune slightly lean or slightly rich?

I know you want a leaner idle ("bunsen burner blue" via colortune plug), but when higher in the RPM range, better to keep it slightly lean or to go slightly richer? Looking for a balance between economy and power. Also, is it ok to use a colortune plug at higher RPMs in a no load situation (just revving the bike) or is the best way to do a plug chop when the bike is under actual load or is it all based on "feel" when picking a main jet?

To give background on why I'm asking, I'm going to be experimenting with single carb set up on a 400 once my manifold comes in. The VM34 I got for around $20 "used" (it was used once) is not a genuine Mikuni. It takes all mikuni jets, but the needle jet is not removable (mine is pinned in). So I may have to counteract that with a slightly larger or smaller main jet or needle then what the people with genuine Mikuni VM34s  Ive been in contact with are using. I am willing to give this chinesium carb a swing before I go forking out 100+ for a genuine carb.

I'm currently setting up my VM34 single carb conversion on KZ440.
I have to wait for a couple of bits before I can get any further with my project, but I've got it to idle steadily. The next step is to choose a main jet. It has a 150 in there at the moment. It runs and works well on that main jet (revs up very nicely), but it will cut out if revved up a bit too sharply. Now, bear in mind the engine is not warmed up and I've not taken it for a ride, on these jets. So it's likely my pilot jet is slightly rich, and that my main jet is correct but will have to be upped in size anyway.
The problem is the switching between pilot and main circuits. This is why I think you need to go slightly richer than the engine would like, on main jet, with the conversion. If it gets a bit more fuel when it switches between pilot and main, that will probably stop it cutting out.

I've watched the Youtube videos by that guy who struggled with his conversion, and he's just... well, the less said about him the better. The info he gives out is all wrong, even after he corrects himself. You need a much bigger pilot than a 20, for example.

I am still in the process of setting up mine, but I expect I'll have to go a little richer on the main jet. I need a new aux testing bottle for fuel. When that arrives, I'll know more.

Err on the safe side and go slightly rich if you must.
Colour tune is only good for getting the low end fuelling in the ball park but personally think they are a waste of time and once under load you will most likely find the results from that put the fuelling on the lean side.
Best way is real world under load plug chops or Dyno runs.
 

I agree with zed.  You will decrease the chance of damage by going rich as opposed to lean.  

wrote:

The next step is to choose a main jet. It has a 150 in there at the moment. It runs and works well on that main jet (revs up very nicely), but it will cut out if revved up a bit too sharply. Now, bear in mind the engine is not warmed up and I've not taken it for a ride, on these jets. So it's likely my pilot jet is slightly rich, and that my main jet is correct but will have to be upped in size anyway.
The problem is the switching between pilot and main circuits. This is why I think you need to go slightly richer than the engine would like, on main jet, with the conversion.

the videos I've been watching said 27.5 pilot, 150 main, Q0 needle jet and a 6F9 needle set to high position for the flip. Since my chinesium VM34 doesn't have a removable needle jet, I'm probably going to go with the 27.5 pilot, the 6F9 needle set to the top clip, and probably experiment with somewhere in the 155-165 range for main since the Q2 is smaller than the planned Q0 (159 vs 166), just give it a LITTLE bit more bump in fuel to make up for it.

As for the "choppiness" with sudden and generous amounts of throttle: that is natural, and is more indictive of rider error than anything else. Most slide carbs don't have accelerator pumps, which give a squirt of fuel when switching wildly between the main and secondary jets, or in this case the pilot and the main, in order to smooth the transition. With these carbs, you need to gently roll into WOT as opposed to just whipping it out like people are used. The CV carbs use vacuum to gradually fluctuate the needle into a WOT position, whereas with slides, you throttle is the needle / slide, so if you just "crack" it open like you're used too, there will be a split second of having too much air but not enough fuel, creating those chops.

wrote:

 

Yeah those videos are by someone called KzDom - he probably has an account here (I can imagine him fitting in well) and thousands of "thanks". I don't expect to get even one. But I know I'll get my bike running right within the next couple of weeks, so it's horses for courses... That's the difference.

The guy's videos go on and on. He talks slowly, he spends 75% of his time looking for tools, and his tuning of the carb on that 2-into-1 KZ440 of his appears to have taken him 4 years in all.

Sorry, but I haven't really got the time for his videos. He goes on and on and then corrects himself later. The first video he explains to the viewer which jets to use, and that Q0 needle jet. The next one he says, sorry, I was wrong. The one after that, he changes it yet again. Another one, he cranks and cranks on a bike with an obviously dead battery.

I don't think his pilot jet recommendation is even close to correct (27.5). I know there are differences in temperature, altitude and air pressure, but 27.5 isn't right.

With regard to what you're saying about choppiness, get your bike running first and then teach me. Thanks!

wrote:

Yeah those videos are by someone called KzDom. his tuning of the carb on that 2-into-1 KZ440 of his appears to have taken him 4 years in all.

I don't think his pilot jet recommendation is even close to correct (27.5). I know there are differences in temperature, altitude and air pressure, but 27.5 isn't right.

With regard to what you're saying about choppiness, get your bike running first and then teach me. Thanks!

Oh my bike runs on the factory carbs, I just want to simplify working on it. The choppiness is the nature of cracking the throttle on slide carburetors. You read posts and watch videos explaining left and right. There is no accelerator pump so you have to roll the throttle on or just deal with the very brief chop. You can tune them meticulously in a way that it's mitigated, but it will always be there. It's the nature of the slide.

As far as elevation, pressures, temperatures, etc. I'm at 2400-2600 feet, so I feel like it'd be a good starting point as someone living at "high" altitude, especially since he doesn't mention any of those details in his videos.

As far as 4 years to finish his KZ, maybe it's a hobby that he does over time, maybe he has a lot of projects, maybe his job takes him away from home for months on end. Hard to say. Videos are lengthy, I will give you that. As far as him going back and forth on the jets, the 27.5 pilot / 150 main / Q0 needle jet / 6F9 needle set up was his final video on the KZ440 single carb finessing, so I guess he's satisfied with it?

By having an air-cooled engine and asking that question. I would say rich.
Lean is not good for longevity if you ride hard. Especially, if you are modifying the stock setup and not verifying with dyno, plug chops, wide band o2, or 5 gas analyzer.
If you were liquid-cooled, that would be a different scenario. You could get away with a leaner mixture.
Play it safe and run it on the rich side till you get it on a dyno.

wrote:

As far as elevation, pressures, temperatures, etc. I'm at 2400-2600 feet, so I feel like it'd be a good starting point as someone living at "high" altitude, especially since he doesn't mention any of those details in his videos.

As far as 4 years to finish his KZ, maybe it's a hobby that he does over time, maybe he has a lot of projects, maybe his job takes him away from home for months on end. Hard to say. Videos are lengthy, I will give you that. As far as him going back and forth on the jets, the 27.5 pilot / 150 main / Q0 needle jet / 6F9 needle set up was his final video on the KZ440 single carb finessing, so I guess he's satisfied with it?



 

I'm sorry to say this as I know most people ascribe value to putting one's face out there in Youtubeland (a sort of skin in the game), but that guy doesn't have a clue what he's doing.
I'm an amateur and even I can detect this.

I get the feeling he may be a nice guy, a bit of a loner, an outsider, ex-military and maybe deals with PTSD with excessive cannabis use. But that's no excuse for his laborious, contradictory and flat wrong advice.

In his latest one, he's gone down to a 20 pilot:


If you watch that whole video and listen carefully, you will hear when he begins to talk about his journey tuning up that bike, you get to a nugget of truth about him having misread the Mikuni manual, and then he conveniently cuts himself off.
Same thing happens when he's doing his WOT runs, and he's about to say how many turns out of the air screw he has. You can almost hear the internal dialogue he has with himself ("I've said too much, better shut up").
His bike's top speed reads 85 but judging by road marking, the landscape and the speedo all over the place, it's doing a lot less than that.
So the fruit of his labour after 4 years is a bike that performs worse than a 125cc scooter.
Also, in his many videos about that project over many years, he never addresses doing any work to the 11th hour manifold. That manifold needs to be burred out. It has huge ridges and stops inside because it's made from lengths of pipe. Has he dressed them, or jb Welded them? No. So his bike will be impossible to tune anyway.

Sorry to have to say this as I know it means I seem mean. But I get things done when it comes to my project bikes.