Carburetor tuning!
 

So I've finally put my Ninja 250 back into service after nearly 3 years of downtime. Don't ask. :p

Anyway, before I put it up, I had added K&N pods, gone with 108 mains and 1 shim under the needle. Stock with the airbox is 105, no shim.

I've been accelerating with wide throttle openings between 3-4K, and today I took out the shims. Power below the point the main jet opens seems a little reduced, the engine falls off much faster when dropping the throttle, letting the clutch out while revving needs to be done a little more carefully to avoid stalling the engine and the engine still cruises fine, no hiccuping.

I don't have any method of measuring my air/fuel ratio, so I'd like to know from those who tune carbs if the above sounds like its running lean enough to cause damage.

Tell tale sign is to go a little ways from your house, get into 4th gear, and hold you throttle at a position where the jet in question has the most influence. Hold it there for 5-10 seconds and then immediately kill the engine. Pull the plugs and see if they show lean or rich signs.

What you are looking for is the BASE RING.... that is the ring on the bottom of the plug that the ground strap (the arm) is welded/soldered to. What you want to look for is a full turn of darkness all the way around the outside of that ring. If its not all the way around, the bike is lean.

I'm no expert but I've been recently buggering around with modified intake and carbs on 2 stroke scooters. The bigger main jet/pod mantra is oftern bandied around as a good thing, but for cruising I think not.

I have found results confusing, to say the least. Try sliding a sock over the pod, try different thicknesses of fabric, 2 socks, thinner socks. This will restrict air flow AND improve filtration. Try no pod or filter, this will increase airflow.

Going to 108 is not much of a jump. Often I go from 65 to 80 on a 50cc. (I have one bike with a 120!)

My feelings are that increasing fuel and air flow create starting and running issues including bogging at low revs. Done right, top end is improved, but I fear that all else suffers. If you cruise at small throttle 90% of the time, you may piss around for ages with no gains whatsoever. Many modified scooters get sold by owners who have bought all these go fast bits, but never got the machine to run right.

I race around on my 50cc, and am often at a speed, much higher than previously attainable.

My guess is that with a thin sock or 2 things will improve for general cruising and top output may see gains too. Duct tape to cover some of the pod, stays on for months too, and you can fine tune your air intake by adding/subtracting strips of duct tape.

Then of course there is the intergrating of other important factors like intake manifold exhaust valves timing plug heat range....however that all said, a pod and main jet up size is a good cheap power upgrade only if you ride to the tuned settings. Namely, increased top end.

If you cruise at small throttle 90% of the time..... you keep pulling the carby off, chop needle heights and jets and socks for ages and ages with little gains..... Sock it to them and see what happens, thats what I've just done and come away thinking too much air isnt always good unless you go a whopper main jet and ride flat out always.

The base ring on both of my plugs are black, not heavily deposited black but a thin, even film of black. The black goes to the base of the ground strap, and less than 1/8" up the ground strap it suddenly changes color to a very light tan/off-white color, and the rest of the ground strap out to the tip is the same color. Both plugs are identical in appearance.

Yay, nay?

At the time I installed them I was more concerned with cramming as much air as possible in while firmly in main jet territory, so if I feel I can do significantly better by going back to the airbox, I will do exactly that.

Going from 105 to 108 is all that was needed to get the pods running smoothly. And I was basically going off the Ninja 250 forum's advice, which the consensus was 108.

Was the bike fully fully hot? Do you have large variations in temperature/humidity where you live? Were you riding at a speed where the throttle opening are such that the main jet, or the needle was predominant? Have you read the K&N thread on at the moment?

I've got a full radiator block, and just drove 5 miles, and the return coolant line was hot to the touch, although my temperature gauge was only registering 75% of normal summer operating temperature, it was about 50-55F at the time, and I was riding right around 4K in 4th, and the main comes on fairly noticeably just past 6K. Throttle opening was very light for the last 10 seconds before I killed the engine.

And I have not read the K&N thread yet.

I really dont want okay comment....But your method is not correct. A only slightly opened throttle does not allow for upper speed jet testing, you need to go faster!

If your bike power and overall rideability improve with a kids sock, you are too lean. If cold running requires more choke than before, you are too lean. High speed lean is the damage problem, so your testing has to be done at high speed.

Sometimes rather than too lean or too rich, it comes down to too much air for the matching restrictive exhaust....

Socks are a good diagnostic tool. Really! Putting them on and taking them off is easier than pulling plugs, and not only gives you feedback re the state of the mixture vrs ambient and engine temperatures, ( as a spark plug 'chop' does)but also allows you to fine tune your CORRECT intake restriction... for that season/time of day....

I often leave them on as they fine tune an air/fuel mixture just perfect. In cold weather I use a thicker sock! Initially have socks with you when you ride, and change them around until you find the bike is at its best. Take a screwdriver and adjust slow speed mixture and idle. Eventually this will be the correct mixture that your bike runs best at. Usually takes around 15min on a hot motor, pulling over and increasing or removing restrictive fabric from the air intake.

If the bike never improved with the thinnest most stretched out sock, it is too rich. But with your present changes I doubt it. Pods with their higher flow will lean an engine, requiring much larger jetting or more fuel. Hardly an economy move.

LOL, that is funny. I ditched my airbox because I couldn't troubleshoot the carbs with it on, and have these glorified clamp on window screens in place now which obviously do not allow enough restriction with standard jetting.

Anyway I've been bodgering cut up paper house furnace filter around them (and spraying them it with paint for fine tuning), an actual sock would have been much easier.

I may try the sock idea. As for going high speed, I don't plan on doing a whole lot of that, I plan on riding around between 3K and 4K, since I work at night, there's minimal traffic on the road, I drive slower.

Why install a high flow filter when you only draw 3-4k revs? Its like putting in a high speed part for no reason? Well I'm glad others have appreciated inventive and experienced advice.

At 3-4,000 rpm stick with your original setup for economy

Like I've said, (three times now already :p), I had different priorities then than now.

Depending on what kind of numbers I get on this tank I may or may not shove the airbox back in.

Well, the airbox is going back in. Had some carb icing a few days ago which left me stuck at work due to fouled plugs. Since I never had that problem with the airbox...

But damn this thing is a complete ***** to get in.

why not use a wide-band oxygen meter to dial-in your carbs?

You can also use a cheap or used narrow band sensor and a volt meter to get pretty close. With the narrow band sensor your mixture needs to be close enough to get it to occaisionally vary between rich and lean of stoich with atmospheric conditions, then you know you are close.

Mount it as close to the head as reasonable so it can stay hot and read accurately.

Nearly all carb'd bikes could benefit from more accurate mixture tuning.

One trick I've been previously criticized on other forums for is disabling the accelerator pump. A lot of small engined economy bikes use off the shelf carbs meant for bigger bikes but then down jetted for smaller displacement. One place that may get overlooked is accelerator pumps. Sometimes they slip through the oem tuning cracks. One prime example is the yamaha vino 125. It's a well known fact that disabling the accelerator pump on them will get you from a relatively gas guzzling 65 mpg all the way up to 85-90 mpg, which is right where it should be as my genuine buddy 125 could achieve 90 mpg.

On my Baja SC50 pep boys scooter, stock it got 75 mpg, terrible especially for a 50cc scooter, after disabling the accelerator pump I get 110 mpg with no hypermiling technique, just day in day out 110 mpg. With hypermiling I can get 120-125 mpg. Now the amazing part is the scooter actually runs better without it. Surprise surprise just like the vino 125

Now I used to have a honda rebel 250. It got about 70 mpg. But by disabling the accelerator pump and being careful of revs and shift technique I did manage to get 102 mpg on it. It wasn't entirely without trouble, if you cracked open the throttle from idle in neutral there was a delay in the revving up of the engine but other that that it ran pretty smooth, I think in these small engine cv carbs the accelerator pump is often pumping too much gas, in the honda rebel it practically was a squirt gun down the carb throat every time to twist the throttle.

Unless I'm mistaken, I don't think there is an accelerator pump on these carbs. They're Keihin CVK30's.