Need advice tuning PHBG carby

Hello all! I have just been testing a dellorto replica PHBG carby. This carby is 19mm and conveniently fits over the stock manifold but this restrict is down to around 16-17mm (I have the thin walled crome type). Will this greatly affect performance and tuning while waiting to get a full sized one made up. Incidently my 70cc engine has a inlet port way bigger than the stock 14mm its more like an oval 19mm.

Now it came with a huge 0.90mm jet, I used my stock jet (0.70)mm which was still far too rich but it did run. I bogged up the 0.90mm carby with a fuel proof epoxy putty and put a hole in it using guitar string which I am guessing is 0.50mm. This time it ran much better on the leanest clip setting in fact at low revs it seemed like the right mixture, no smoke or anything! BUT... as soon as the throttle opened up it bogged down losing power.

Now I know it is pretty close but I need advice as to where to go from here.

Would it be that the jet is now too small and the bogging is due to fuel starvation once the jet rules at open throttle?

OR, is there fuel collecting at the step down in diameter to the manifold?

OR, is the taper on the pin too steep (it looks steep)?

OR, is the jet still too large. This carby has a proper circuit which draws air into the emulsion tube surrounding the pin unlike the stock carby which only draws from the top only. I went to such a small jet because mopeds, which have similar engine specs run Dellorto's using 0.50- 0.55mm jets.

My feeling is that it is still too rich because I am running at the leanest clip setting. Maybe I need a smaller guitar string!

Good advice thank you! I just studied the Dellorto tuning manual and I understand what you are saying about the needle.

There is no room for a higher notch on the needle but I could drill into the slide body a bit to lower the needle more. As for the cut away I am assuming you are talking about the angled cut at the back of the slide well this is 3-4mm high. There is no cut away at the leading edge at all.

Hmmmmmmm.

I might try to lower the needle by 1mm and increase the jet size a bit.

Sorry the cut-away is on the air filter side and yes that link is for my carby.

Yeah I studied the tuners handbook from Dellorto and was wondering about the atomiser. I think it has got the 4-stroke type which is no good, I guess I'll have to block some up.

By the way Quick Steel epoxy putty seems to hold up under constant fuel and sticks to metal well I have had success bogging up jets with this stuff.

I ran it with the jet from the stock carby first and it was rich as **** which is why I have gone so small. I'm guessing 0.50mm. I feel the jetting is nearly there I'm going with the advice on the pin. I noticed that the clip is too wide to fit in the indentation on the slide so it is out 1mm further than it should be. I am going to drill into the slide a bit and then top the clip with a washer so I can get another couple of settings leaner.

If this fails then it looks like I might be bogging my atomizer holes.

PHBG 15mm AS Carb (Clamp On Style) 2 Cycle [D0-2518] - $95.17 : Dellorto Carburetors, And parts for you.

This one looks like the perfect carb but its $100! pitty nobody in china is knocking these off.

Good link Datz! Awesome news that you got the 15mm PHBG AS. I reckon it will be the winner. Did you get to choose what jet it comes with? Interested to see what comes as stock. Remember to try your jet off the Ht carby as they fit the PHBG when tuning.

I am guessing that a 0.55 will be right for my 19mm PHBG. Doesn't necessarily mean that your smaller one will though.

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.

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!