Keihin FCR39 on 2000-2002 KTM 250/400/520 (Slant FCR)

Deceleration Popping on 4-strokes

Setting the fuel screw mixture

Accelerator pump timing screw

Accelerator Pump Leak Jets and PowerPump Adjustment

Fuel screw #JDFM040

Accelerator Pump Adjusting

Yamaha WR250/WR450F Stutter Condition

Dust Under the Carburetor Cap

Leak Jet Change vs. Diaphragm Mod

Question About Fuel Injection Tuners

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Keihin FCR39 on 2000-2002 KTM 250/400/520 (Slant FCR)

Deceleration Popping on 4-strokes The most frequent cause of loud popping on deceleration is an air leak at the header pipe connection to the motor. Even a very slight air leak can cause the deceleration popping. This is usually harmless, but can be annoying. As the pipe develops carbon/soot, the popping usually reduces with ride time.

The jetting can be adjusted richer at idle to help reduce the popping in most cases. A lean idle mixture can cause popping on deceleration, but the deceleration popping is usually more mild. Try turning the fuel screw on the bottom of the carburetor outwards by 1/4 turn steps up to 1 turn. The next option would be to switch from the Red marked needle (if currently being used) to the Blue marked needle in the same clip position (or 1 step leaner) to make the low-end jetting richer and keep the mid-range unchanged. Be aware that richening the low speed jetting excessively will reduce mileage and make the motor more difficult starting when hot. If your motor doesn't need the choke at all when cold starting, then the idle may be on the edge of a rich condition when hot.

Note that all the Japanese motorcycles have a bolted header flange and gasket to prevent exhaust air leaks. Yamaha WR450s and WR250s have a air-cut coast enrichener to reduce the same deceleration popping too.

Setting the fuel screw mixture- Try adjusting with the motor fully hot, and turning the fuel screw inwards until the idle slows or runs rough, then turn outwards 1 1/4 turns from this position. For example, if the idle slows or runs rough at 1/2 turn from closed, then set the fuel screw to 1 3/4 turns out (1/2 + 1 1/4 = 1 3/4).

See-  HERE   to understand how the fuel screw can affect the idle speed and air/fuel mixture at idle. The target range for the air/fuel mixture is near 12.5 to 13:1 at idle.

Accelerator pump timing screw- Use the pump timing screw by the linkage spring to readjust the pump starting point. The screw needs to be turned inwards until the linkage has freeplay and the rod no longer is compressing the diaphragm. Turn the screw inwards and lightly push down on the right side of the plastic to feel the freeplay (where the rod goes down). Once freeplay is found, turn the screw outwards until there is only about ¼ to ½ turns of freeplay. This allows the pump to completely refill and keeps the squirt from starting too early as you open the throttle.

See- HERE (the setting shown may be near 3mm when using an aftermarket replacement spring for the linkage)

Accelerator Pump Leak Jets and PowerPump Adjustment- Watch the following video to see the effects of the PowerPump adjustment screw

View Video

Fuel screw #JDFM040- The JDJetting fuel screw is more accurately machined and maintains the same tolerance as a Keihin fuel screw from the factory. The idle mixture is better controlled and more accurately set.

Power Pump-Adjustment- Install the float bowl and set the pump adjustment screw to about 1 1/2 turns out. Ride the bike in an open area where you can accelerate and feel the power as you roll the throttle open. Compare the pump setting at 1/2 turn out, 1 1/2 turns, and 2 1/2 turns, and then decide what you prefer.

In most cases, the pump needs to be further out for woods and slower riding, and even more at higher elevations. Try 1 1/2 to 2 1/2 turns above 6000ft. When riding in faster conditions with more load on the motor, use 1/2 turn to 1 1/2 turns, as an estimate. These are just suggestions, and you can decide what works best for you. The main thing to watch for is a slight sputter or stumble when opening the throttle quickly or when shifting.

Use the pump timing screw by the linkage spring to readjust the pump starting point. The screw needs to be turned inwards until the linkage has freeplay and the rod no longer is compressing the diaphragm. Turn the screw inward and lightly push down on the right side of the plastic to feel the freeplay (where the rod goes down). Once freeplay is found, turn the screw outwards until there is only about ¼ to ½ turns of freeplay. This allows the pump to completely refill and keeps the squirt from starting too early as you open the throttle. See-  HERE (the setting shown may be near 3mm when using an aftermarket replacement spring for the linkage)

Yamaha WR250/WR450F Stutter Condition-

This is an ignition problem that Yamaha has designed into the bike to make the motor run cooler at light throttle and medium speeds. The stutter is VERY COMMON, and can sometimes be reduced by disconnecting the TPS electrical lead from your carburetor. The condition is most common on flat roads at light throttle positions, when producing under 10hp at the rear wheel. The stutter is harmless and will not affect the power under acceleration, if it is not too annoying, leave the bike as-is and just ride it. The problem is mostly noticed when riders want to do road riding with their bikes.

Try disconnecting the TPS lead and see if it helps your condition. The other alternative is to try running either clip position #2 or #4 on the Red marked needle. This is a work-around that forces you to use more throttle than normal and lightens the stutter, but the bike will make less power when accelerating.

See- HERE and HERE and with replacement CDI and no jetting changes- LOOK HERE

Dust Under the Carburetor Cap-

The carb vent hoses do not lead to the cavity above the slide and below the cap. The vent tubes ONLY go to the float bowl chamber and not above the slide. The only passage leading to above the slide is in front of the carb slide, directly below where you find dust, along the face of the slide. Placing filters on the vent hoses will only keep dust from entering the float bowl cavity, which draws air inwards when the choke is pulled open (a little known fact).

The dust is entering thru the air filter. I don't doubt your meticulous air filter maintenance routine. Try running the stock Honda air filter using the same procedure instead of the aftermarket air filter. The stock filter is a finer foam and will help filter more dust out. You might also use filter skins to reduce the chance of dust passing thru, but plan on lowering the main jet 1-2 steps at the same time.

I've seen the same issues on other bikes, and it shows the limitations of foam filters in extremely dusty conditions. The most fine particles still can pass thru the filter, as the motor needs to flow air and the filtering isn't perfect.

See the internal passages that bring the venting from the top hoses to the float bowl on either side and notch in the carb body next to the side plate (middle top of the intake venturi) that are referenced HERE.

Leak Jet Change vs. Diaphragm Mod

The change to a #60 leak jet with stock diaphragm is an option that will perform nearly the same as installing the diaphragm spacer along with a stock #40 leak jet.  These are each options to consider, but both changes are not need at the same time.  The reduction in pumped fuel would be too much, unless you are riding at high elevations most of the time (primarily above 6,000 ft elevation).

---Installing the diaphragm spacer from the kit with a #40 leak jet will be reducing the accelerator pump's volume using a shorter stroke.  The results will focus the entire squirt of fuel in the first half of the throttle range, which is the approach Honda has chosen for their CRF models.

---Installing the bigger #60 leak jet reduces the accelerator pump's volume by bleeding off fuel, but using the entire diaphragm stroke, which spreads the reduction over the entire throttle range.  This is an approach Yamaha, Kawasaki, and Suzuki use for their off-road and MX models.

The overall result is that the spacer with #40 leak jet pumps more fuel in the first half of the throttle range, whereas the #60 leak jet has more fuel when going to full throttle, and they cross-over somewhere between (near 3/4 throttle the same squirt volume would occur).

The choice is personal preference, and overall they perform nearly the same from my experience (stock diaphragm and #60 leak jet vs. Honda diaphragm mod and #40 leak jet).

See a graph of accelerator pump volume vs. leak jet size, using several different diaphragms HERE.

QUESTION ABOUT FUEL INJECTION TUNERS

Answer:

You will not know how lean the bike is in terms of a measurement, like 14:1 or 15:1 air/fuel ratio. We develop the suggested tuner settings by testing the power on a dynamometer, using air/fuel measurements over each throttle position, and all the rpm ranges.

You will know that we only allow a fuel adjustment range that is not going to be excessive or harm your bike, and that this is using your stock EFI map as a starting point to make changes.

The Power Surge tuner has 6 color/zone adjustment features, each with it's own setting. The light #3 in each color zone is going to use the stock setting, and pass the same signal directly thru to fuel your bike. When the setting is lower, like 2.5, 2, or 1.5, then the jetting is leaner. When the setting is increased, like 3.5, 4, 4.5, 5, etc, then the jetting is richer.

Each model of bike has a recommended list of settings to use for correcting the stock map, and then you can add or subtract fuel from there as you feel works best for your motorcycle and conditions. If you feel the mid-range power might benefit from an adjustment, then move the mid-range (yellow) light up or down and go ride to see how it works. The changes are VERY EASY, and VERY QUICK to make. You can easily try a richer setting, and then leaner, for a comparison.

See- HERE

and

HERE

The great thing about this tuner is that you can choose a power range, go ride, and decide to keep the change, or go right back. You are not forced into some unknown map, or list of maps. You choose a specific range and adjust to your needs, based on your setup, fuels, modifications, conditions, and preferences.

The Power Surge tuner is almost like a carburetor with an adjustment screw for every range, but always knowing that light #3 is stock, and a decal is there on the bike to reference our suggested dyno-tuned settings. The EFI tuner is also similar to having custom suspension work, and suspension clickers. The adjustments are available on the tuner when needed, but after the settings are dialed in, you probably won't be changing the settings very often.

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