Post Number: 3
|Posted on Sunday, November 09, 2008 - 07:58 pm: ||
Ahoy- any advice on tuning a Schebler Model D carby for a Blaxland Chapman X3.5HP. Correct me if I am wrong,it has been a while since I tuned the carby and it is running rich.
To begin with ,then do I
1: open needle valve, (turning to the left from fully closed), one complete turn
2: then turn air valve screw 4 turns to the left(opening up)?
If carby is still running rich what is the adjustment to lean out mixture?
Post Number: 44
|Posted on Sunday, November 09, 2008 - 08:26 pm: ||
Firstly, forget about the numbers. You tune the carburettor by ear. If the mixture is too rich, turn the needle in - allowing less fuel to flow - until the engine runs at its smoothest and fastest speed.
The air valve setting is not critical. A bit of resistance to the air flow is all you need. The more you increase the spring pressure, the richer the mixture.
Some so-called experts may wish to baffle you with science, but just listening to where your engine sounds happiest is all you need when tuning. You are in control, not a set of numbers.
Post Number: 102
|Posted on Sunday, November 09, 2008 - 11:37 pm: ||
Eric , you are mostly correct,but air valve setting
does help with starting(including the correct weight spring)Me and my mates(so called experts)
spend a lot of our time getting things just right
Assuming the air valve is ok adjust the main jet
lean until the motor starts to backfire through
the carb,then adjust it until it stops backfiring
1/16th turn at a time until it stabilisers,then
turn between 1/32 and 1/16 extra(depending on
condition of carb ,float etc).Once the engine
is running the air valve(with correct weight spring)stops being a part of the mixture ratio,
and you need not bother thinking about it
Post Number: 45
|Posted on Sunday, November 09, 2008 - 11:56 pm: ||
I know you don't need to think about it, but how is it possible that the air valve is no longer in operation when the engine is running? All the air entering the engine has to go through the air valve, so I would love an explanation of how it doesn't affect the fuel mixture.
Post Number: 4
|Posted on Monday, November 10, 2008 - 01:35 am: ||
Many thanks Eric and Todd for your assistance.
I am based in the central west so I do not see many other putters and the discussion board is a great help.
A few years back a friend fiddled with the air screw and since then I suspect the fuel runs rich as shown by black plug colour and sometimes smoke through the exhaust..
There is no worry with starting usually first go.
However one day a while ago I was trolling and the motor quit due to a fouled up plug.
So a rich mix needs leaning and I assume the air valve is okay and it is the needle I will adjust in first up as you suggest.
Things are just still just okay with water in the dams around here but the rocks, trees and old fences are always a hazard.
Post Number: 103
|Posted on Monday, November 10, 2008 - 04:03 pm: ||
Eric, once the engine is started the air valve
is sucked back hard against and fully compressing
the spring(hence the requirement for the correct
weight spring,4 stroke simplexes run a much heavier
spring than single 2 stoke blaxlands for example.)
This is achieved at very low rpms(lower than normal idle),the result of this is a orifice with a much
larger air flow potential than the throat past the
main jet.In fact some engines we have had to
rebuild for people have had no air valve at all,
or in the case of early carbs,the air valve has
disintegrated and gone through to the sump(blaxlands)but the engines still ran,and with
the same mixture setting.They were just a bugger
to start.Hope this helps
Post Number: 46
|Posted on Tuesday, November 11, 2008 - 05:00 am: ||
In Victoria a spring-loaded valve impedes the flow of air. Perhaps engines in NSW can defy the laws of physics!