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Old 02-02-2011, 04:12 PM   #1 (permalink)
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Tillotson HR Carb Issues

I have an old SkiDoo with a Rotax 399 with an HR40A carb. I have been through this thing a dozen times, cleaning and replacing and fiddling around. I am not stupid (or at least I don't think I am) but it seems that every time I do something it gets a little worse.
Recently I obtained another old HR40 and installed a rebuild kit (full set of parts except for the main jet check valve for some reason). It had three different springs for the inlet lever - I selected the one that I thought most closely matched the original (in terms of its spring constant) and with this setup I now I seem to be getting NO FUEL FLOW AT ALL. It will start and run only if I spray fuel directly into the carb throat which is a bit awkward. It seems like either the fuel pump isn't strong enough (it does pump enough to fill the fuel filter) or the spring is keeping the inlet valve closed which I guess means the fuel pressure is too low.
Does anyone know any way of testing any of this stuff? I could try to measure the vacuum from the pulse port I guess but what sort of number would one expect? I also thought about trying to measure fuel pressure at the return line fitting with both idle and high speed needles fully closed but again I don't know what to expect.
Having said all of this the compression isn't great and the engine might have bad seals but it ran OK- not great - before I started trying to "fix" it.
And for what it's worth I am 99.9% sure that the fuel and the filters, tank and lines are fine, and I've done the ignition setup and replaced the plugs, and I don't seem to be out of gas so.............
Feel free to make any suggestions or comments you like. It will be spring soon and I'll have lawnmowers to screw up.
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Old 02-02-2011, 10:26 PM   #2 (permalink)
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Start by pressurizing the tank and see if the fuel will rise to the carb and then try to start it.
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Old 02-03-2011, 11:39 AM   #3 (permalink)
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Tillotson HR Carb

Interesting suggestion - I'll give that a try this weekend.
many thanks
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