General Carb. Tuning

[hoodie2]

Anybody got any tips on identifying the subtleties of poor carburation. I often find that my bike gets a bit out of tune, particularly right off the bottom where the power will not come in smoothly and progeressively from idle. On wider throttle openings I can tell if it's weak and searching for fuel or four stroking and obviously rich, but on the pilot jet I struggle to identify which condition causes this delayed responsiveness. I can spend all day trying to tune this problem out on the mixture screw without ever succeeding. Other people have ridden the bike and do not seem to notice a problem but I know how smooth the thing can run, I just can't keep it in that trim for some reason.

Edited December 23, 2003 by hoodie2

[charliechitlins]

If I can't get enough adjustment out of the mixture screw, I go for the choke (enrichener).

It's a little tricky, but I open it a tad while riding at the throttle setting I'm trying to tune to see if the problem gets better or worse with a richer mixture.

Then I know which way to start moving with my jetting.

The other way is to have a bunch of time and a fistfull of pilot jets...but that can get expensive.

[kevin j]

like charlie I play with the choke. then try the fuel tap shutoff and see how it changes as the bowl runs lower on fuel.

[charliechitlins]

Somebody on the BB once mentioned that he noticed real differences in his power with only small adjustments to his mixture screw, and the way he noticed it was while trying to pull wheelies in 4th gear. He said that was a much better gauge of low-end power than just by seat-of-the-pants, which probably doesn't work on a bike you can't sit on. Har!

I keep meaning to try the 4th gear wheelie method.

[jtt]

I think you may be referring to the Billy T technique

Put the bike in gear, and run it up to max RPMs, now without touching the clutch, allow it to coast back down to idle. Once it gets to idle speed, wack the throttle open. If jetting is close, bike should rev quickly and cleanly, usually wheelying.

For setting mixture screw, try this.

After fully warming motor, and "cleaning out", let it idle at a nice low setting. Now turn the mixture screw (most times a fuel screw on trials bikes) in until it starts to die. Then, slowly turn the screw out, 1/2 turn at a time and listen carefully to the idle speed. as you turn the screw in and out you should notice the idle increasing at some point. Adjust idle back to same level as before using the idle screw and continue turning the mixture screw.

What you will find is there is a "sweet spot" where the idle peaks. I usually set mine about a 1/2 turn richer than this point. Now go back and count how many turns the screw is at. If it's more than 4 turns, you should try a richer pilot, then repeat the procedure.

Combining these two techniques will get you pretty close most times.

Hope that wasn't too confusing...after re-reading it, I'm confused

[billyt]

Thanks JTT that was my post.

I have worked on many different carbs throught the years and they all have one thing in common: physics.............I can use my basic physics knowledge and years of black magic carb tweeking.

These basics apply to any and all carbs.

The big problem out there is that everyone and their brother adjusts their carb differently, they can imagin that a carb is running rich when in truth it is running lean and vs versa.

Misconceptions about what particular jet does, and when run ramapant.

There needs to be a standard way of checking your carb for basic set up, fine tunning, and re-jetting. If we all use the same standard the helping each other out would be a hell of allot easier.

Carb tunning is like elbows we all have one and we all have an opinon on how to set it up.

A carb is also a very personal device what works for one bike may not work exactly right for another indentical bike. The same holds true for the rider he may like his carb set up one way and another ride wants a whole different set up.

The bottom line is that the carb basics must be estabslished no matter the rider or bike or even the type of carb installed.

Lets try and establish this standard and then move on and help each other. Any takers for establishing the carb basic, one voice one standard.

The ball is in your court guys, I can help with setting a standard and posting some digital pictures and charts/graphs to help us along.

BillyT

Edited January 7, 2004 by BillyT

[jtt]

Thanks JTT that was my post.

Good to have you back Billy

[terry]

Here is a great article on what each jet does and when. Really cleared things up for me.

http://www.speedandsport.com/jet.html

[jtt]

Good site Terry!

Another interesting exercise is to actually mark your throttle and grip in 1/4 turn increments. A small peice of masking take and a marker work well on the throttle housing, and White Out or white paint on the throttle grip. What you'll find is usually pretty humbling What you "think" is wide open, is rarely more than 1/2 throttle. Meaning most of our riding is on the idle, slow speed (pilot) and needle circuits.

[mark christopher]

so what do you actually look for and gain by setting the mixture screw?

[jtt]

The mixture screw will effect throttle response, right off idle. As you can imagine this is a pretty important area. Your looking for a nice crisp response when cracking the throttle open.

[jrsunt]

When the bike is running lean, it usually makes the power more lumpy and harder to bring the power in smoothly on the bottom end, bad for slippery conditions because it makes the rear wheel want to break traction. When you have it slightly rich, its good for the slippery stuff because the power is more linear and not as snatchy. Although it may not rev completly out every time it is more favourable at this time of the year, (its like running with the timing slightly retarded). When it is set up perfect, the engine should feel strong throughout the rev range, without any peaks or troughs in the power.

The mixture screw makes massive changes, the weather has alot to do with how your bike runs. I agree with charlie, if you ride on a tick over it 3rd/4th gear then crack the throttle open the engine should not faulter, it should want to throw you over the back and keep on pulling like that until it is flat out.

[mark christopher]

power is more linear and not as snatchy. Although it may not rev completly out every time

you just described mine!!! but i do tend to ride on the clutch more! ie pick the revs up and feed the power in to find the grip. so im poss a bit rich, 2000 montesa screw out to lean off??

Edited January 7, 2004 by mark christopher

[jtt]

It's a fuel screw on the DelOrto, so out is more fuel (=richer). In is less fuel (=leaner)

[mark christopher]

It's a fuel screw on the DelOrto, so out is more fuel (=richer).  In is less fuel (=leaner)

No sorry on a 2000 montesa bike its an air screw and out is leaner!