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Old 08-20-2008, 08:23 PM
RLorange RLorange is offline
Motorized Bicycle Elite Member
Join Date: Jun 2008
Location: Australia
Posts: 127
Question 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!
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