![]() unless you have a Swiss or US specification bike, in which case you may have a tube running to some valves (all of which can be junked if you wish which will liven the bike up a bit). Is the upper vent on the diaphragm cap closed? There should be a cap over the little stub. Including the tube which connects to it? Blow through the tube to check. ![]() Is the lower diaphragm vent open? The large bore tube at the rear of the carb, just under the diaphragm cap (you can see it in your picture above). Anything which is causing the second scenario is likely to cause a big hiccup when you open the throttle, so let's stick with the first scenario for a first investigation. Something is allowing the carb slide to drop, or something is restricting (or adding) fuel flow only with constant throttle. If it's the same, we need to look elsewhere. Going back to your original problem, I doubt this is your cause, but why not try? Find something to cap the port in question, take the bike for a run and see what happens. and it's pretty limited experience at that as these carbs are usually junked pretty quickly in favour of a Keihin FCR. However, I'm just guessing based on my KTM experience. I believe it is just a vent port and normally has a tube which points directly down at the ground, hanging out the bottom of the bike. The bike and the engine all seem to be in working order and I'd quite like to make it run properly. It's a strange bike but I rather like it. Is my fault likely to be something else completely? Should this open hole be attached somewhere and not left open to air? (I've been told it should go to the crankcase breather but who knows? I was told this by a man whose ability I doubt). Have I missed something blindingly obvious? The shop manual and parts lists are no help. But I don't know what and they are so rare I can't find another to look at. I'm pretty sure it should have, and it shouldn't just be open to atmosphere. There is a pipe from the carb to the airbox, providing vacuum, all correctly in place but there is also an outlet on the LHS of the carb roughly level with the carb/airbox pipe that has nothing connected to it. As a bonus, using it like this has doubled the fuel consumption! Something's not right.Ĭlearly something is wrong with the carb. ![]() Relax to a cruise and it loses power again. Take it out and under steady power, at say, 40-50-60 mph, it starts to lose power and won't respond until I give it some throttle, when all becomes well. The bike is 100% standard, including all induction plumbing and exhaust.įitted the carb, it starts and idles fine. I checked it over, made sure all the jets, float height, etc., were as per the manual. The old carb was completely shot so I bought a new one, a Mikuni BST40 and genuine Aprilia. For my sins, I have a 1995 Moto 6.5 fitted with the Rotax 650 engine. ![]()
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