chevyho

Member
Nov 13, 2009
6
0
I just purchased a larger jet for my CRF 450R, I went from a 175 to a 178 for the colder weather. I just went to my dealer and asked for a keihin 178 jet... now it has the same stampings in the same location as the stock jet, but a 178. I now read that CRF jets are numbered differant? Is this true? The jet went in fine and runs great, just confused how the numbering can be differant. Any help is very appreciated.
 

chevyho

Member
Nov 13, 2009
6
0
It came from the owners manual.

"Because your CRF's jet size numbers do not correspond with other carburetor manufacture's jet size numbers, use only genuine Honda CRF450R jets."

Am I just interpreting this wrong? The reason I ask is because there seemed to be confusion at the dealer.
 

Rich Rohrich

Moderator / BioHazard
LIFETIME SPONSOR
Jul 27, 1999
22,838
16,902
Chicago
Mikuni hex head jets are numbered by bulk flow, while Mikuni round head jets and Keihn jets are numbered by inner diameter in mm.

That's is what the Honda manual is referring to.


... and for those who are still curious

The flow difference between a Mikuni 175 hex jet and a 180 is about 6cc ~3%) . Keihn jets are numbered by jet id, and the difference between a 178 and a 180 is about .0008" (I'll let those so inclined do the math to determine the flow difference) .

Now to further confuse the issue keep in mind that air/fuel ratio is based on weight of fuel and weight of air, but jets essentially meter by volume. So we really need to know the weight of the fuel flowing through a jet to understand all this. In simplest terms fuel weight is a function of the area of the jet multiplied by the value of the square root of the fuel head pressure multiplied by the density of the fuel.

It looks like this:
weight of fuel = jet area * ( SQR Root (head pressure * fuel density)

None of the above takes into consideration the changes in fuel vaporization characteristics as you change altitude, which can have a profound impact on the final air/fuel ratio available in the combustion chamber ( the only place a/f ratio is really significant anyway) when the sparkplug fires.

As fun as it might be to look at jetting this way, the sad fact is the cross sectional area of two jets marked with the same number can vary FAR MORE than the difference in flow due to small changes in density . Good jets can vary as much as 5% cheap jets can be closer to 10% variance.
 

_JOE_

~SPONSOR~
May 10, 2007
4,697
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Yeah, just simply trying it out and feeling for the change is the easiest way to jet for sure. We could break out the calculator and play around all day trying to figure it out or start swapping jets out and looking at chopped plugs if we want to be exact.
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
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Merrillville,Indiana
You walked into a Honda dealer, demanded a keihin main jet, and they were confused? I got a Honda dealer like that by me also, they are into street cruising. Go tell them you need a few leak jets also. Leaning the ap circuit sure wakes up the off idle response. If it was not for Honda, I may never have seen a Keihin carb! Save some head ache, order the parts your self from Service Honda. There are a lot of alien jet companies out there, hacks with drill bits. Like Rich said, go with oem, or give JD a call. Vintage Bob
 

chevyho

Member
Nov 13, 2009
6
0
Very good info, thanks a lot. So a keihin 178 jet measures 1.78 mm I assume. Well it is definetaly a keihin jet since it has the same stampings on the hex portion of the jet as the stock one... I feel better now, plus plug is looking beauty. Thanks again guys, just joined this site and absolutely love it.
 

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