Need advice tuning PHBG carby

I dont think a .55 is going to be large enough with the 19mm carb. The stock HT carb is about 13mm and the stock jet for that is larger. Your flowing more air so you need more gas. If you get the bike up to speed and start it with a
.80 or .85 jet ill bet the bike would run WOT, then you need to work on the slide and needle....When you do get it running you can check the jetting by going at WOT for a bit then pulling in the clutch and shutting the engine off still at WOT and coasting to a stop, letting the engine cool and looking at the condition of the plug (called plug chop), it will tell you exactly how rich or lean you are.

Didnt get to choose what jets it came with, so I'll report back once I get it. I have a good assortment to start out with for the main jet (steps of 5), but will need to place another order for a few more when it comes time to fine tune it.

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.

Great info, RL!!

Im really looking forward to trying out the PHBG on my bike.

Thanks Datz!!

You know the main fun with these engines as hobby is figuring out how these engines and carby's work.

I recommend you do the head gasket removal mod, works amazingly by boosting compression.

Simply sand down the head and cylinder head surfaces perfectly flat using wet-n-dry glass paper and water on piece of glass on a table. This only took me a couple of hours and with care I ended up with two PERFECTLY flat and smooth surfaces which sealed without the head gasket. Amazing results by boosting the compression ratio slightly. Best performance improvement I have done. Still waiting to get my expansion chamber going!