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@oultondave avatar
UTC

Molto Verboso
Blue 70th anniversary Vespa GTS 300
Joined: UTC
Posts: 1305
Location: Leeds, Yorkshire, England
 
Molto Verboso
@oultondave avatar
Blue 70th anniversary Vespa GTS 300
Joined: UTC
Posts: 1305
Location: Leeds, Yorkshire, England
UTC quote
Rides great and accelerates fine but when slowing down from 70 ish mph it seems as if it's misfiring at about 1/4 throttle around 60 mph. Tried with ASR on and off makes no difference, it feels as if the limiter is kicking in, any ideas peeps please.
@jet_peddler avatar
UTC

Veni, Vidi, Posti
'07 LX150 (Sold), '17 GTS300, '16 BV350, '15 EN650, '09 FXDF (sold). '15 FLSTN
Joined: UTC
Posts: 5664
Location: Home of the Alamo
 
Veni, Vidi, Posti
@jet_peddler avatar
'07 LX150 (Sold), '17 GTS300, '16 BV350, '15 EN650, '09 FXDF (sold). '15 FLSTN
Joined: UTC
Posts: 5664
Location: Home of the Alamo
UTC quote
My BV350 does something similar but at much lower speeds. Caused by the evap system. May be causing your issue as well. I'll let more technical minds chime in and elaborate.
@caschnd1 avatar
UTC

Grumpy Biker
1980 Vespa P200e (sold), 2002 Vespa ET4 (sold), 1949 Harley-Davidson FL
Joined: UTC
Posts: 5423
Location: Sparks, Nevada, USA
 
Grumpy Biker
@caschnd1 avatar
1980 Vespa P200e (sold), 2002 Vespa ET4 (sold), 1949 Harley-Davidson FL
Joined: UTC
Posts: 5423
Location: Sparks, Nevada, USA
UTC quote
When you close the throttle to slow down (especially from high speed), the intake pressure jumps to a high vacuum. If you got a small crack in the intake manifold or another small intake leak (could be a loose vacuum hose), this is the time when it will show up. You get a small gulp if unregulated air from the leak and that can cause a lean misfire. Give everything between the throttle body and the engine a good look.

-Craig
OP
@oultondave avatar
UTC

Molto Verboso
Blue 70th anniversary Vespa GTS 300
Joined: UTC
Posts: 1305
Location: Leeds, Yorkshire, England
 
Molto Verboso
@oultondave avatar
Blue 70th anniversary Vespa GTS 300
Joined: UTC
Posts: 1305
Location: Leeds, Yorkshire, England
UTC quote
Thanks gents
OP
@oultondave avatar
UTC

Molto Verboso
Blue 70th anniversary Vespa GTS 300
Joined: UTC
Posts: 1305
Location: Leeds, Yorkshire, England
 
Molto Verboso
@oultondave avatar
Blue 70th anniversary Vespa GTS 300
Joined: UTC
Posts: 1305
Location: Leeds, Yorkshire, England
UTC quote
So The Beasty is currently languishing at Ron Daley's in Barnsley. The misfire has been diagnosed and we need a new ECU and throttle body. Thank f**k it's still under warranty ( by three days)
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