Single cylinder sizing?

johntm · Post by **johntm** » Thu Nov 22, 2007 7:39 am

Can I assume that if I am changing from a carb to a throttle body on a single-cylinder engine that I should use the same size throttle body as the current carb? (40mm in this case on a 450 cc engine)

FIntruder · Post by **FIntruder** » Fri Nov 23, 2007 7:45 am

You may wanna see what is available first as this will limit your options, that is unless you have already found a 40mm throttle body. When I was searching Ebay I don't recall seeing much smaller than 45mm.

Is the carb you are replacing a CV carb or direct pull? A CV carb has more parts in the throttle bore than does a direct pull or throttlebody and this effectively reduces the size of the bore from a flow standpoint.

johntm · Post by **johntm** » Sat Nov 24, 2007 5:50 am

A little more info -

The engine is in a Kawasaki KLX450R dirtbike. The current carb is a flat-slide model Keihin FCR-MX40. I ride at lots of varying elevations (from 600 ft to 14,000 ft) and I am hoping that FI will make that easier by eliminating the very time consuming jet and needle changes required to ride in those conditions.

Is airflow velocity critical on a FI system, or will the loss of velocity of adding another 5mm of bore not matter?

Post by **efahl** » Sun Nov 25, 2007 6:29 am

The EFI part of the system doesn't care a whit about velocity, that's purely a "mechanical" aspect of the motor. Jumping the size of the throttle body 5-10 mm should have little or no impact on power, it will change throttle response though, in that a smaller motion of your hand will result in a bigger opening at the throttle.

You could start by taking the cheap route, simply pull the fuel line off the carb and it magically becomes a throttle body...

Eric

johntm · Post by **johntm** » Sun Nov 25, 2007 6:10 pm

Kinda hard to jam an injector into the main jet hole though.....

FIntruder · Post by **FIntruder** » Fri Nov 30, 2007 5:40 pm

What do you have in mind for a ThrottleBody?

johntm · Post by **johntm** » Mon Dec 03, 2007 6:01 am

I haven't even started looking. Figured I'd find out what size I needed first.

I'm new to this whole EFI thing - I need to find out what size body, what size injector, how to map it all, etc.

FIntruder · Post by **FIntruder** » Sun Dec 09, 2007 3:29 pm

What is the outside diameter of your carburetor where it mounts to the head/intake manifold?

You might search Ebay for BMW R1150 throttlebodies. One has the TPS on it and both have an injector. You will only need the one with the TPS but might have to get the set.

A CanonDale 4-wheeler had FI and the whole setup might be transferrable to your bike. The fuel pump would be the right size as well.

wildwestsydney · Post by **wildwestsydney** » Mon Dec 10, 2007 7:08 am

You may want to search ebay for a throttle body set from a suzuki sv 650. I was going to use one of the TB's from that but I could not make one fit using the stock air box. I then machined my own TB from Billet using the guts from the SV TB. I used this to succesfully convert my 1998 DR 350 dual sport ti FI using a MSI ver 2.2. Maybee the SV 650 set could work for you.