Can someone explain why a BIG intake, BIG carb, BIG V2 reed engine would run a smaller main jet than an oppositely setup motor?

 1988 LT 500
Owner –Dale Fry
Bore: 86.5mm
Stroke: 86.00 mm
Cylinder – small reed cylinder with HPR porting
Head – HPR modified with o-ring head gasket
Reed – V Force 3
Piston – Wiseco machined to HPR specification, (with one season of riding on it).
Carburetor – Mikuni TM 38 HPR modified 41mm with 620 main jet
Power Valve – Non operational and set to the fully closed position
K&N air filter mounted on the end of a stock LT 500 carburetor to air box hose
Fuel – VP 110 with Maxima 927 oil at 32 to 1



1987 LT 500
Owner –Dale Fry
Bore: 88.5
Stroke: 86.00 mm
Cylinder – Large reed cylinder with HPR porting
Head – HPR modified with o-ring head gasket
Reed – V Force 2
Piston – Wiseco machined to HPR specification (with one season of hill racing on it)
Carburetor – Mikuni VM 44 HPR modified with 500 main jet.
Power Valve – Non operational and set to the fully closed position
K&N air filter mounted directly on the carburetor
Fuel – VP 110 Maxima 927 oil at 32 to 1

I would have to go back and look at the dyno sheets and the engine specification sheets to see if there is a typo.   

The 389 series needle jet used in a TM 38 Mikuni has a 4mm high shroud.  The 224 needle jets used in a VM 44 uses a 8mm high shroud.  A taller shroud should communicate more vacuums to the main jet with equal velocity over it.


Dales drag engine has a straight intake manifold as well as a smaller air filter than his small reed dune engine.  I would have liked to have used a larger air filter but space limitations required a smaller air filter.  I hope that the new suspension on his drag bike will allow a larger air filter and maybe give it some more power. 

On a given engine setup, a smaller air filter will usually require a smaller main jet to optimize the power for that air filter and engine combination than the same engine with a larger air filter. 

A rule of thumb is:  As the air filter surface area increases main jet size also increases.  Or we can say it another way:  As the pressure drop from the outside to the inside of the filter decreases, the main jet size increases.

 
Different style carburetors have different air flow characteristics over the needle jet shroud.  Different intake manifolds, different style reed valves, how many dividers are in the reed cage and where the dividers are in the reed influence where the highest velocity occurs at the needle jet shroud even though these are down stream from the carburetor.

Angled intake manifolds will often require a larger main jet to reduce the chances of detonation occurring on the left or right side of the combustion chamber.  The left or right bias of fuel distribution within the combustion chamber many times will follow the left right angle of the intake manifold.

It takes a certain amount of fuel to generate one horsepower.  A 50 HP engine takes 50 times as much fuel as a 1 hp engine with the same thermodynamic efficiency.  This does not mean that the 50 hp-carbureted engine has a jet with 50 times as much flow area as the 1 hp carbureted engine.  We can get a lot of fuel through a small hole if we put enough vacuum on it.

Looking at engine specifications can be very confusing and can sometimes cause expensive failures when all of the details are not understood.  There are many times I do not have the time to understand why some carburetors require jetting that does not follow logic.  I just have to give the engine the jetting it wants regardless of what I think it should need.