OP
UTC

Enthusiast
180ss with moded engine
Joined: UTC
Posts: 68
Location: Montreal
 
Enthusiast
180ss with moded engine
Joined: UTC
Posts: 68
Location: Montreal
UTC quote
Hi gang, i was trying to work out the jetting on my carb. My setup is:
Old polini 208 (single ring kit), ported engine, JL exhaust and the 28mm carb.
On the carb i have:
Jjg needle (middle clip)
40 Pilot jet
132 main
At idle, I find that if i open the trottle even a bit and close it, the engine revs up and down for maybe 2-3 seconds then slowly comes back down.
As well, if i raise idle setting a bit and turn in pilot screw, the engine actually starts to race, and if i back it out past 1 turn, the revs go down. This seems to be the complete opposite of what should be happening, right? What am i doing wrong? Any ideas? Thanks
Fab
@christopher_55934 avatar
UTC

Ossessionato
2007 Stella 225
Joined: UTC
Posts: 3547
Location: Rochester, Minnesota
 
Ossessionato
@christopher_55934 avatar
2007 Stella 225
Joined: UTC
Posts: 3547
Location: Rochester, Minnesota
UTC quote
Re: Help jetting my 180ss (pwk 28 carb)
chef wrote:
Hi gang, i was trying to work out the jetting on my carb. My setup is:
Old polini 208 (single ring kit), ported engine, JL exhaust and the 28mm carb.
On the carb i have:
Jjg needle (middle clip)
40 Pilot jet
132 main
At idle, I find that if i open the trottle even a bit and close it, the engine revs up and down for maybe 2-3 seconds then slowly comes back down.
As well, if i raise idle setting a bit and turn in pilot screw, the engine actually starts to race, and if i back it out past 1 turn, the revs go down. This seems to be the complete opposite of what should be happening, right? What am i doing wrong? Any ideas? Thanks
Fab
Not familiar with a PWK,

High hanging idle is usually lean or an air leak. Revving up and down is also usually a sign of an air leak. I would try a pressure test to rule out an air leak, otherwise you're going to be chasing your tail with jetting.
UTC

Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
Joined: UTC
Posts: 4598
Location: London UK
 
Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
Joined: UTC
Posts: 4598
Location: London UK
UTC quote
Where is the airscrew set at? Could just be that.
OP
UTC

Enthusiast
180ss with moded engine
Joined: UTC
Posts: 68
Location: Montreal
 
Enthusiast
180ss with moded engine
Joined: UTC
Posts: 68
Location: Montreal
UTC quote
Right now, it's about 1 turn out (air/mix screw). As i said, normally, when you turn air screw in all the way, the engine dies out...but on this carb, it's the oppisite... the engine revs up🤷🏻‍♂️.
I had also raised the idle a bit before I adjusted air mix screw.
Fab
UTC

Enthusiast
Vespa T5 251, Lambretta GP200
Joined: UTC
Posts: 74
Location: Leeds, England
 
Enthusiast
Vespa T5 251, Lambretta GP200
Joined: UTC
Posts: 74
Location: Leeds, England
UTC quote
Most of the carbs we commonly use the mixture screw is metering fuel. The PWK type carbs are metering air (mixture screw before slide). Even given the symptoms you've described that pilot could be too rich. I'm no bonafide expert but I do know some Lambretta owners that have had to go as low as a 30 pilot to dial that in correctly. Until you get close to where you need to be you get confusing symptoms so it could not hurt to try maybe a 36 for 2 mins and see if it improves.
OP
UTC

Enthusiast
180ss with moded engine
Joined: UTC
Posts: 68
Location: Montreal
 
Enthusiast
180ss with moded engine
Joined: UTC
Posts: 68
Location: Montreal
UTC quote
Well, i was thinking of the system and how it may be lean. I then thought of fuel system and its' delivery.
I undid the fuel line at the carb. I noticed that the fuel line had a kink in it ( where it goes into carb), and had solidified from being like that for a few years. I took a container and opened the fuel tap, the flow was steady, but not really coming out quickly (maybe 1/3 to a 1/2 of fuel line diameter). If i switched tap the "reserve" the flow increased but slightly).
Could i have a blocked line? Or maybe bad fuel tap? If it's the tap, is there a way to clean it out, unblock it? Should fuel line be 24" long?

On a side note...i do have a mikini fuel pump, but not sure if i should use it?
Thanks
Fab
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