42 or 45 Mikuni?

[Tattoo]

My currant build is a 95" 10.6:1 compression ported heads, TW55 cams, 42mm Mikuni. I'm going 103" this winter with the same heads and maybe same cams. should I go with the 45mm with the 103"? looking for any feedback from anybody running one.

Thanks in advance.

[ICANTD55]

I would go with the 45 for the 103. I used a 45 for a 114ci and it also ran strong on my 124ci.JMHO
48mm on the 124 now.
Rick

[Hillside Motorcycle]

45 should work great.

[RevFastEddy]

What size is your intake port??? Going to a carb larger than the intake port does not do very much more. If you have a 1.60" port a 42 mikuni.. If you have a 1.80" port a 45 mikuni...

[Tattoo]

What size is your intake port??? Going to a carb larger than the intake port does not do very much more. If you have a 1.60" port a 42 mikuni.. If you have a 1.80" port a 45 mikuni...

I have 1.90"

[Admiral Akbar]

What size is your intake port??? Going to a carb larger than the intake port does not do very much more. If you have a 1.60" port a 42 mikuni.. If you have a 1.80" port a 45 mikuni...

I have 1.90"

Then go with a 48..    What manifold are you going to run?

Max

[Tattoo]

The mikuni manifold ..

[Fxstchewy]

I vote the 45.

[fuzznut5197]

What size is your intake port??? Going to a carb larger than the intake port does not do very much more. If you have a 1.60" port a 42 mikuni.. If you have a 1.80" port a 45 mikuni...

Knowing nothing about EFI, why does EFI not follow this general rule? Like, if you have 1.8" heads, alot of people will have 57mm throttle bodies. Is it the advantage of having more plenum volume, or something else?

[RevFastEddy]

What size is your intake port??? Going to a carb larger than the intake port does not do very much more. If you have a 1.60" port a 42 mikuni.. If you have a 1.80" port a 45 mikuni...

Knowing nothing about EFI, why does EFI not follow this general rule? Like, if you have 1.8" heads, alot of people will have 57mm throttle bodies. Is it the advantage of having more plenum volume, or something else?

.......................................................................
Bigger sells a lot more than smaller. Show me a 57 or 61mm tuned to the max and there are 10 on the street that wont make any more hp than a cleaned up unit. The torque would likely suffer as the air mass entering the cylinder would be entering at a slower speed in the intake track.  Kind of like someone runing 2.2" valves when a 1.9 would work best. Big sells... JMHO

[kevmac]

45 on antthing over 100 cubes

[GoFast.....]

45 on antthing over 100 cubes

I'm running the 45 on my 98 WT 115hp and I really like it and it runs perfect with a SE mainifold which has the 1.60 intake manifold.

[Mike52]

Knowing nothing about EFI, why does EFI not follow this general rule? Like, if you have 1.8" heads, alot of people will have 57mm throttle bodies. Is it the advantage of having more plenum volume, or something else?

EFI and carbs are different animals on an engine, the fuel flow characteristics are different. The Carb requires air passing thru it to draw fuel out of the carb then into the intake ports, the EFI shoots fuel directly from the injectors into the intake tract near the entrance to the head at precise intervals, fuel doesn't have to flow completely thru the intake manifold like in a carb set-up. The butterfly on the EFI unit only controls the amount of air entering the engine, the carb butterfly controls both air and fuel.

Mike

[Admiral Akbar]

The mikuni manifold ..

Are you sure it's 1.9 ID at the head? Stock flanges won't take a much more than 1.9 OD.

Max

[fuzznut5197]

EFI and carbs are different animals on an engine, the fuel flow characteristics are different. The Carb requires air passing thru it to draw fuel out of the carb then into the intake ports, the EFI shoots fuel directly from the injectors into the intake tract near the entrance to the head at precise intervals, fuel doesn't have to flow completely thru the intake manifold like in a carb set-up. The butterfly on the EFI unit only controls the amount of air entering the engine, the carb butterfly controls both air and fuel.

Mike

Well, I shouldn't have said I know nothing about EFI, I mean, I know how they work, just never had an EFI bike and never worked on one. If a set of heads+manifold flows 300cfm, why put on a 400cfm TB if a 320cfm TB will work and not reduce that 300cfm flow? Is it just sales like RevFast said?

[Tattoo]

The mikuni manifold ..

Are you sure it's 1.9 ID at the head? Stock flanges won't take a much more than 1.9 OD.

Max

I'm sorry that was the valve size.

[Admiral Akbar]

"If a set of heads+manifold flows 300cfm, why put on a 400cfm TB if a 320cfm TB will work and not reduce that 300cfm flow?"

Very good point. Max

[fxdx2k]

"If a set of heads+manifold flows 300cfm, why put on a 400cfm TB if a 320cfm TB will work and not reduce that 300cfm flow?"

If you are going strictly by dry flow and static flow data, then that does seem to make sense. However, in a dynamic situation (engine running) that may not be the case. Not everything is always quite so black and white. Look at some race vehicles and calculate the cfm capability of their injection/carb setups and then calculate the theoretical cfm needs of the engine based on displacement and operating rpm. You may be in for a bit of a shock. I'm not saying that a large carb or throttle body is a guarantee of increased power, it's often the opposite is true. But, if by "test and tune" or some other method of controlled data collection the engine in question does produce higher power with a larger carb or throttle body, well, why not? The downside may be driveability or difficulty in tuning. Those are the choices one often has to make when pushing the limits with a street driven vehicle, at least in my humble experience