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Old 08-24-2008, 09:51 AM
RLorange RLorange is offline
Motorized Bicycle Elite Member
Join Date: Jun 2008
Location: Australia
Posts: 127
Default Re: Need advice tuning PHBG carby

Mr. Right I totally understand your logic and that is exactly what I thought BUT... It didn't work out that way. What I have come to realise is that the stock HT does not work the same way as my PHBG and the jet size is not comparable. Let me explain:

Fisrstly, The stock carby has no proper circuit which mixes air and fuel nor has is a Venturi to lower the pressure further thus drawing fuel faster. With the stock carby the fuel is drawn out of the top of the the needle valve straight from the well below. With the PHBG and most others there is an orifice separate to the main air channel which draws air into a chamber surrounding the needle valve. The needle valve itself has a number of holes drawn into it letting the air in so it is premixed with the fuel before exiting into the main channel. All of this increases the vacuum a lot over the HT.

Secondly, the needle of the HT has a very narrow taper, so narrow in fact that the jet in the HT never determines the full flow at open throttle unless it is at the lowest possible notch ( beyond this actually). At this setting it is far too rich. The PHBG has a steep taper so once you get beyond half throttle it opens up the needle valve fully. At this point the jet solely determines the fuel flow and an 0.85 is way too rich, I have tried it.

Thirdly, the PHBG has a separate idle jet which is 0.50mm and this supplies two separate circuits the idle and progression circuit thsese dominate at low throttle BUT they still supply some pre-carbed fuel at full throttle which of course means a smaller main jet is required.

Lastly, euro mopeds have the most similar engines to these motors similar stroke and similar compression and almost identical port sizes. They are commonly fitted with PHBG's and the most common jet size I have seen is 0.55.

For anybody else reading this it is important to get a Dellorto specified for a Two stroke engine because they require a different taper needle and and needle valve emulsion tube than 2-strokes.
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