dan williams

I don't believe the Mikuni uses a stand pipe like the Keihin. If I remember correctly a bore in the float bowl lines up with a hole in the bottom of the carb body and there is an overflow hole into the top of the float chamber.

Fair enough. Wanted to keep it simple in practice.

No I think you checked your floats and they were good so not the floats. Your float level is pretty good now too. Only happening at 3/4 throttle is the odd thing that still points to vibration in my mind. Possibly a casting defect. This is a tricky one. I will give you credit for being persistent and patient with this. It's the hallmark of a good mechanic.

Ok that's a lot closer to correct though the travel is a bit less than optimum. Perhaps it's time to look elsewhere than the floats. Is your float bowl gasket in good shape?

Nope, angle of the carb slightly affects the pilot mixture on a Beta which is why they tend to stall on downhills. If you look at the main/needle jet tower it is in the center of the float bowl. The pilot is also close to the middle but still on the centerline. Pour yourself a glass of water and stick a straw in the middle. No matter how you tip the glass the water level in the center where the straw is stays the same. So with a concentric carb no matter how you tip the bike the main/needle jet combination always sees the average level of fuel in the bowl. Keeping the pilot circuit on the centerline means the pilot circuit is unaffected by leaning the bike side to side. Your adjustment is way off. When you hold the carb as shown the float arms should be level. Flat. Parallel with the float bowl seam. You should then be able to lift the float arms up to where they are stopped by the stop tang pointing to the right angle where the pilot jet platform meets the main/needle jet tower.

Whoa! That adjustment is really wrong. Pointing to that angle bit on the jet tower should be the limit of travel with the valve open. Resting upside down like that the float arms should be level when the valve closes. Parallel to the float bowl. Are your floats upside down?

Hi James, I know you already mentioned some of this but I'm going to run stream of consciousness for a bit so bear with me. Yes the floats only need to move a tiny amount to open up the float valve to full. It's a conical seat so more than a mm or so doesn't flow fuel any faster. When you pull the carb out and take the float bowl off the seat should close when the floats are level or parallel to the float bowl sealing gasket. Don't be fooled by the difference between a closed valve and compressing the spring in the valve plunger past the closing point. The two adjustment tabs for the floats are the tab the spring clip for the valve plunger attaches to (sets float height) and the little tab that hits the side of the tower the float pivot goes through (limits float travel). There are three tubes coming out of the Mikuni. One on the bottom of the float bowl is the actual overflow. The two that come out of the sides are the atmospheric vents for the float chamber. The usual problem with the Mikuni is the atmospheric vents get fuel splashed up into them and because Beta made the tubes too long they act like siphons and pee fuel until the tank is empty. Usually this only happens when the bike is parked. Someone must have had that happen with your bike and figured they'd cure it by looping the tubes up over the carb. The easiest way to cure it is to put the tubes back where they belong down the side of the carb and nip a small hole in the tube above the float bowl. Inspect your floats for leaks. That can mess up a perfectly float adjustment. Also make sure the floats aren't rubbing on the bowl gasket. If they are trim the gasket with an X-Acto or similar knife. Billy's adjustments and fixes are really the gold standard and work perfectly unless something weird is going on. One other thing I just thought of, make sure there's nothing making the carb vibrate like a fuel line that is hitting the exhaust or the carb hitting a part of the engine. A vibrating carb is an unhappy carb. At this point I think you might have tweaked too much at once and should stop and go back to basics. Straighten the float arms. Set the float level. Set the float travel. Check the floats for leaks and rubbing. Let the atmospheric vents go back to the bottom of the carb and nip a hole just under the bend where they come out of the carb. Make sure the carb isn't hitting anything on frame or motor. Report back here what happens. You and I must be extremely diligent. Ninja Billy is watching!

3D puts on good fun events. Gotta try to make it out there this year.

I'll bring some of my favorite Nashoba Vally, Strawberry Rhubarb.

Can I offer you some cheese with that whine? Don't worry Andy. Grant and I know each other and he'll get me back. Just can't resist the setup.

Message from Trialsmaster Wayne Galvin. Yeah the guy riding over the Union 76 ball in Bernie's book. The King Philip Trail Riders will be hosting the first New England Trials Association trials May 7th at their Wrentham location. Gates open at 7am for riders and spectators . AMA renewal or new membership will open around 8 am and rider sign and neta membership at 9am. The event starts at 11 am, A riders meeting will be at the start area at 10:50. This will be another great event this year with 10 sections within a 10 minute walk from the pits for spectators. This year a little longer loop will be used for the novice +2 tenths mile and experts will have a little more bike time with 2 exp only loop segments. The novice sections will have 1 or 2 gates per class with 6 easy and 2 harder sections. Champ riders will get tested with 13 gates plus 22 expert A gates Advance B gates riders will have a 12 gate course and Intermediate C gates ride a 15 gate course. youth riders start at 10am. food and drinks at the start area.

Sorry doc. He told me he had the parts.

Depends on your yard. I only have a trials bike. I'm pretty sure the problem is my reaction time rather than the bike but...

Did you ever get on your bike for just a short putt around the yard and think to yourself, "Damn this thing is quick!"

The lighting and fan both run off the lighting coils. On the older bikes they both ran off the dc out of the voltage regulator. I believe on the newer bikes the lights run directly off the ac out of the lighting coil and only the fan runs off the voltage regulator. The regulator is still fed from the lighting coils so fan and lights are still sharing the available current from the lighting circuit.

I know a bit and still come on here asking questions.

Check your lever adjustment. Hydraulic brakes and clutches are "self adjusting" The reservoir on the top of the master cylinder has a hole that goes down into the end of the master cylinder. This hole keeps the system full of fluid and allows excess fluid caused by thermal expansion to bleed back up into the reservoir so there is zero static pressure when the lever is released. It's quite common for new riders to adjust their levers in so far that the hole never gets open. This causes pressure to build up in the hydraulic system as the fluid heats. For brakes this can cause them to drag generating more heat which heats the fluid more and eventually causes the brakes to stick. For the clutch the fluid heats up and the static pressure acts like pulling in the lever just a bit and more as the fluid heats up more. Look at your levers and make sure the piston in the master cylinders is returning all the way to the stop. It shouldn't be limited by the adjustment screw. If it is back out the screw that sets the resting position of the lever. Oh and welcome to the sport. This is just the first of many frustrations but you'll still love it.

Yup two burps of throttle is the easiest to control. The first lifts the front and the second helps compress the front and rear suspension. The jump up and forward happens just before the second burst of throttle. The jump is actually two motions. First forward then up. If you watch Bou and company in slow motion the bigger the hit the further back they start. The up part of the jump really starts with the front wheel contact. The mistake most people make while learning is staying on the throttle so the rear never leaves the ground and just burys itself at the base of the obstacle.

OK I wrote a long explanation but it was too much so I'll distill it down to this. After the quick burst of throttle at the same time you jump to load the rear suspension snap the throttle off. If you stay on the gas the rear suspension won't release its energy all at once. Try it with bunny hops. Lift the front end and jump while snapping the throttle off. The rear should lift off when the timing is right. Or try this experiment, hold the rear brake on and slowly let out the clutch and note how the rear end squats. That's the stored energy you release by chopping the throttle.

Lookout Billy's grabbed his Claymore!

But what about, "Bold new graphics!"

Only advice I have is find the thieving ba****ds and beat then with the scratched stanchions. Won't fix the forks but you'll feel better.

I don't know. My Beta's rarely need parts. To be honest I'll usually just talk to Stewie at Jack's cycles and he often has what I need. I know he stocks some GasGas parts. If he can't get something I want I just order from across the pond from the motherland of trials. Postage isn't that bad and if the Brits don't have it nobody does. I suggest you check out some of the fine advertisers on this very website. Let's see if Andy pings me for that.

That idle mixture screw is a pain in the A** isn't it. I have a whole bunch of long flat blade screw drivers ground to size and scattered all over my car and garage just so I have one handy at all times. I tried one of the Jitsie mixture screws but it was worse than the stock one. The older bikes had a screw soldered to a cable but I guess Beta gave up on that idea. The carbon reeds won't seem snappier but they will let you pull a gear higher without stalling. It actually will come off idle a bit cleaner because the carbon reeds open and close faster because they weigh less so they will enhance the fuel air charge in the crankcase at low rpm. At high rpm the reeds could be made of lead and it wouldn't make any difference because they just stay open anyway. I always love to read someone bragging about how carbon reeds make their bike faster. Goes along with the, "My bike is faster because it has the RED powerband"

One thing to consider is the current required to start the fan going is considerably more than the current required to keep it going. When the switch first closes you can get an inrush current to the fan above ten amps as the motor builds up a magnetic field in the windings. After the initial demand the operating current usually drops to less than an amp. I know this because I worked on motor control chips for a bit but mostly because I was troubleshooting a fan and put it on a current limited power supply and it wouldn't start up until I cranked the amperage. The problem is usually the thermoswitch. If it is operating normally it will have resistance in the milliohms. As the switch ages the contacts can build up oxides and carbon and pits on the surfaces due to the initial arc when the switch closes. This can cause the switch to gradually start degrading so even when closed it can read several ohms. Doesn't seem like a problem except it will degrade to a point where the required inrush current can't be provided reliably. So maybe you're cranking along and the lighting coils are putting out full voltage, enough to overcome the contact resistance, and the fan starts. Or you're sitting in line at a section with the engine idling and the fan just can't get enough current to start. Comes down to when is an on/off switch not an on/off switch. So yeah you may read the switch as turning on and off but check the on resistance and you may find it's sufficiently higher than a new switch. Just back of the envelope type of calculation, say your switch has got 5 Ohms of contact resistance and your lighting coil puts out 12 Volts. E = I x R (Volts = Current x Resistance) Assuming the inrush current looks like a dead short across the fan motor, which is pretty close to reality when starting, then I = E/R (Current = Volts / Resistance) Current = 12/5 or 2.4 Amps. If the motor needs an instantaneous 10 Amps to start then the lighting coils can't push enough current through the switch to get the fan spinning. Or there might be almost enough current and you end up with an intermittent failure like you have. Keep in mind that resistance in a connector or ground will cause the same symptoms. So the practical takeaway is those few ohms that you normally just ignore on a meter are actually pretty significant when large currents are involved. Even for a very short period of time.