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BSA A65 Big port valves #715081
11/14/17 6:27 pm
11/14/17 6:27 pm
Joined: Dec 2013
Posts: 9
France
P
pete Offline OP
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pete  Offline OP
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P
Joined: Dec 2013
Posts: 9
France
Hello to everyone,
Finishing actually the restoration of my BSA lightning, I'm waiting for the delivery of my head that had the following
modifications :

- Inlet port to 32 mm with 43.5 inlet valves

I have twin AMAL carbs 389 1' 1/8 (Main jet 220 / Throttle : 3 1/2 / Pilot jet : 106)

My questions :

Do I need to rebore my carbs to 32 mm to maximise the performance of my engine ?

Thanks

Pete

Re: BSA A65 Big port valves [Re: pete] #715091
11/14/17 7:37 pm
11/14/17 7:37 pm
Joined: Mar 2005
Posts: 10,057
Boston, Massachusetts
J
John Healy Online content
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John Healy  Online Content
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Joined: Mar 2005
Posts: 10,057
Boston, Massachusetts
Quote
maximise the performance of my engine


What are you going to do with this engine? Are you looking for top speed, maximum acceleration, or a good all around motorcycle?


Re: BSA A65 Big port valves [Re: John Healy] #715165
11/15/17 5:36 am
11/15/17 5:36 am
Joined: Dec 2013
Posts: 9
France
P
pete Offline OP
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pete  Offline OP
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Posts: 9
France
=> => => "a good all around mtorcycle".

Re: BSA A65 Big port valves [Re: NickL] #715234
11/15/17 5:53 pm
11/15/17 5:53 pm
Joined: Dec 2013
Posts: 9
France
P
pete Offline OP
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pete  Offline OP
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Joined: Dec 2013
Posts: 9
France
Thanks Nick for your reply.
What settings do you recommend => Throttle cutaway, main jet, pilot jet ??

Re: BSA A65 Big port valves [Re: pete] #717545
12/03/17 8:58 am
12/03/17 8:58 am
Joined: May 2013
Posts: 4,730
Rotherham - S. Yorkshire
Allan Gill Online happy
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Allan Gill  Online Happy
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Rotherham - S. Yorkshire
Not sure what Nick has posted as it wasn’t on the thread,

For a good all round bike the standard head is fine, however the earlier 68-466 head is quite tractable (it has a 1,1/8” port tapering to 1,1/16, about 1/4” in is the taper) these work well with the change from 38mm valve to 40mm and I have 32mm carbs mounted to mine, although they do pull very well with 28mm and for standard road use will be more tractable still.

In comparison the bigger port head will seem flatter at certain points in the Rev range and will work well... but only within a smaller RPM window, the smaller port head will pull much stronger over a bigger window. Cam will make a difference also.


beerchug
Re: BSA A65 Big port valves [Re: pete] #717562
12/03/17 2:35 pm
12/03/17 2:35 pm
Joined: Mar 2011
Posts: 833
derby england
wak Offline
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wak  Offline
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derby england
the carbs should always be bigger than the port or at least the same size,


BSA lightning
BSA B50MX
TRIUMPH TR6C
BSA BUSHMAN
BSA Gold Star Daytona
BSA Gold Star Scrambler
BSA Rocket Gold Star
BSA C15S
BSA Cyclone
Triumph T120
Triumph T100 Daytona
Triumph 5TA Trials
Triumph T100 Scrambles
Cheney 560 TT
Re: BSA A65 Big port valves [Re: pete] #717620
12/04/17 12:46 am
12/04/17 12:46 am
Joined: May 2013
Posts: 4,730
Rotherham - S. Yorkshire
Allan Gill Online happy
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Allan Gill  Online Happy
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Rotherham - S. Yorkshire
Thunrbolts have 1,1/8” ports (28.5mm) yet they still match them with 28mm carbs, so half an inch is neither here nor there,


beerchug
Re: BSA A65 Big port valves [Re: pete] #717781
12/05/17 2:36 pm
12/05/17 2:36 pm
Joined: Mar 2011
Posts: 833
derby england
wak Offline
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wak  Offline
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Posts: 833
derby england
That wasn't a serious answer was it Allan , thunderbolts are single carb with a Y adapter which goes a lot smaller , having a smaller carb than the port drops the gas velocity


BSA lightning
BSA B50MX
TRIUMPH TR6C
BSA BUSHMAN
BSA Gold Star Daytona
BSA Gold Star Scrambler
BSA Rocket Gold Star
BSA C15S
BSA Cyclone
Triumph T120
Triumph T100 Daytona
Triumph 5TA Trials
Triumph T100 Scrambles
Cheney 560 TT
Re: BSA A65 Big port valves [Re: pete] #717816
12/05/17 6:42 pm
12/05/17 6:42 pm
Joined: Apr 2005
Posts: 1,621
Mississauga, Ontario.
A
Adam M. Online content
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Adam M.  Online Content
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A
Joined: Apr 2005
Posts: 1,621
Mississauga, Ontario.
Khm, smaller ports give higher gas velocity, but less volume.


Moderated by  Allan Gill 


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