A few factors, the main one being the carbs, their size, cfm flow and general overall design.

It has to do with air flow and velocity..."suction" I believe is the word describes it best... Thats what pulls the fuel thru the needle jet and main jet.

 The 44 certainly flows more than the 41... and makes more HP.

I could explain it, but it would sound like jibberish.
Instead, consider this:

To me, all that BIG equals less signal. I thought that would require a larger jet, but I guess magic has no laws.

I'm with you. But how do we know if either one is "right on" with jetting, and what was the target jetting for (drag, trail, dune)?

I'm with you. But how do we know if either one is "right on" with jetting, and what was the target jetting for (drag, trail, dune)?

I'd have to say that it would only take one or two sizes to be "right on" with jetting if it's at all out of tune.
It surely doesn't account for the difference between a 500 and 620 main jet.
Try putting a 620 main jet in a carb that was tuned for a 500 main jet and it'll be ridiculously rich, regardless of what type of riding you're intending to do.

The first question should have been if this is a typo. I too have a Hall port slightly less than Dales with a #19. Jerry sent jets in the mid 600's for a VM44. I dont remember seeing any one run a 500 main with this pipe.

Did you pull those spec's from the "old" pipe shootout runs?  On monday I can review the specs on that old gas drag motor. Been a couple years since it ran that way, but I'm thinking those are good numbers.  I switched to alky on that motor.  My small reed with the 44 is running at 560/580.  I like em a little on the edge far as the main goes...I do make sure the needle position and needle jet are safe for duning thru the bowls.

I've posted in my build thread, but I'm also running a TM38 modified by HPR to a 41mm.  I'm running a main jet of 660 the last time I was in the dunes. It was just a little rich, so I figure a 640 should be good for Glamis and the temps between 70-90.

I'm heading out again for Thanksgiving, so figure it will be colder out there, so my 660 should be fine for the colder temps.

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.

Quote from: Toydoc on November 10, 2012, 05:12:00 am

I'm with you. But how do we know if either one is "right on" with jetting, and what was the target jetting for (drag, trail, dune)?

I'd have to say that it would only take one or two sizes to be "right on" with jetting if it's at all out of tune.
It surely doesn't account for the difference between a 500 and 620 main jet.
Try putting a 620 main jet in a carb that was tuned for a 500 main jet and it'll be ridiculously rich, regardless of what type of riding you're intending to do.

Not always true. Every needle jet has a flow limit. I've had some with no flow change in MJ after 480. So could you add more needle jet to the 38 and bring it down to the 500? They are all combos that work, but is it the only one that works, and is it the best or right on? I really don't know so I'm asking.

Quote from: WestTexasKing on November 10, 2012, 05:23:24 am

Quote from: Toydoc on November 10, 2012, 05:12:00 am

I'm with you. But how do we know if either one is "right on" with jetting, and what was the target jetting for (drag, trail, dune)?

I'd have to say that it would only take one or two sizes to be "right on" with jetting if it's at all out of tune.
It surely doesn't account for the difference between a 500 and 620 main jet.
Try putting a 620 main jet in a carb that was tuned for a 500 main jet and it'll be ridiculously rich, regardless of what type of riding you're intending to do.

Not always true. Every needle jet has a flow limit. I've had some with no flow change in MJ after 480. So could you add more needle jet to the 38 and bring it down to the 500? They are all combos that work, but is it the only one that works, and is it the best or right on? I really don't know so I'm asking.

I agree that there is a needle jet and needle combination that works best for a given engine.  I have encountered the same problem many times when increasing the main jet size or when removing the main jet will not richen the mixture.  When this condition exists, the flow area between the needle jet and the tip of the needle is controlling the fuel flow and not the main jet.  The solution to this problem is usually corrected when I install a needle that has the same shape up to about Ύ throttle and has a steeper taper from Ύ to full throttle. 

If the flow area between the tip of the needle and the needle jet is restricting the fuel flow at full throttle, installing a larger needle jet would help the main jet problem but will probably make the mixture too rich at small throttle openings.   If you have a needle that has a very sharp point and increasing the main jet size has no effect, then it is time to increase the needle jet diameter and the needle's major diameter.

We make our own needles for our alcohol carbs because of the above problems.  Most gasoline carbs have needle jets and needles that have their largest diameter around .100”.  Our alcohol needle jets are usually in the .150” to .160” diameter.  Our alcohol needles have a major diameter in the range of .140” to .150” diameter.  As the needle jet and the major needle diameters increase, it provides more flow area at Ύ to full throttle while allowing the correct flow area at the lower throttle openings when the needle's major diameter is optimally sized.