Dyno Graph - Small Frame

General scooter chat, any scooter related non technical info.
tony172
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Posts: 138
Joined: Sun Jan 13, 2013 10:50 pm
Main scooter: vespa t5
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Mon Sep 23, 2019 10:02 pm

Hi Guys,

Dynoed a small frame I built today, runs a champ but has a jetting problem I can't quite grasp.
AFR leans out at around 65mph in 4th not sure of RPM without looking it up.
What I'm not sure about is why it seems to only happens in 4th and not at the same RPM in each gear.
Dyno operator thought it could be that the power jet is too small (which I'm not discounting at all).
Just wondered if anyone had any ideas?

Setup (if memory serves)
Pinasco zuera 135
180ex/125trans
140btdc/60atdc inlet
1.2mm squish
Stage 6 26mm pwk powerjet carb
JL sei giorni pipe
Sip vape race ign set 22°retarding to about 14°

Let me know if you have any ideas

tony172
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Posts: 138
Joined: Sun Jan 13, 2013 10:50 pm
Main scooter: vespa t5
Contact:

Tue Sep 24, 2019 8:36 am

Here's the graph, any ideas:

Image

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drunkmunkey6969
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Main scooter: '69 Lambretta GP
Location: North Yorkshire
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Tue Sep 24, 2019 9:49 am

The load in 4th is far greater than any other gear, thats when fuel flow issues are exposed. This graph looks like the carb is running out of fuel in 4th, so perhaps fuel flow from the tank/tap, or the float needle valve would be good starting points for further investigation? Did the dyno operator do a single 3rd gear run also? The same problem may possibly have occurred if held WoT in 3rd.

First, check fuel flow from the petrol pipe at the carb... it should flow 1 pint per minute (or close). If it doesn't flow at that rate..... the petrol tap is an issue.

If fuel flow is OK, look at the carb settings. To help people make suggestions, you'll need to list the settings.

Carb spec:

Carb make/model:
Main jet:
Pilot jet:
Atomiser:
Float needle valve:
Slide:
Needle
Clip setting:
Airscrew (or mix screw) turns out:
Air filter Y/N:

tony172
registered user
Posts: 138
Joined: Sun Jan 13, 2013 10:50 pm
Main scooter: vespa t5
Contact:

Tue Sep 24, 2019 10:25 am

Thank you Mr munky! I've contacted the owner to ask for full carb spec and will be back ASAP.
I also thought fuel flow but as the scoot is fitted with a fast flow tap and with the carb being situated down by the kick start I didn't think it likely--though now you've said it I wonder if the fuel inlet at the carb is a possible bottleneck...

Anyhow I'll be back as soon as I get the specs

tony172
registered user
Posts: 138
Joined: Sun Jan 13, 2013 10:50 pm
Main scooter: vespa t5
Contact:

Tue Sep 24, 2019 9:08 pm

Hi so I've had a reply from the owner


Carb make/model: Stage6 PWK 26mm
Main jet:118
Pilot jet:38
Atomiser: unknown
Float needle valve: unknown
Slide:unknown
Needle: E
Clip setting: top
Airscrew (or mix screw) turns out: 1.5
Air filter Y/N: N

Have asked him to hang the carb over a pint glass with the float bowl off and time how long it takes to fill, and also to drill out the tank cap breather

Any ideas would be appreciated, and also any recommendations for a clutch would be nice as it makes more power than expected!

Thanks again
Tony

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