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Discussion Starter #1
Ive got bucket loads of long hex FCR jets as a by product of my rejetting exercise.

I still need more (bigger) to play with.

Does anyone know if it's OK to drill them out with number drills?

I have noticed that the "jet size" seems to be determined by the id of the middle part of the jet. i.e. both ends are the same bore, then the middle of the jet is smaller again.

If i drill the jet out parrallel inside will it still function properly?

Also i was hoping to get hold of a chart that directly relates jet size to ID in Thou or mm. I tried measuring a 195 main jet & found that it's bigger than a 1/16 (0.0625") drill bit, but smaller than a 5/64th (0.078") but i have nothing in between.

Thinking of buying a set of number drills & having a play.
 

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You can get a (good) set of number drills and try it.
I did it on the pilot jets on my last bike, and it worked fine.
You may find that if you drill out two jets, one may actually run richer or leaner than the other, due to small variations in how it actually drills out. I'd put the richer one on the rear cylinder.

Some carb jets are rated by flow, so the number has no direct relation to the hole diameter. Others, the number is actually the size. I suspect your 195 main jet has a 1.95mm hole, that would correspond with it being larger than the 1/16" and smaller than the 5/64".

Metric drills come in steps of 0.05mm down in that size range, that can get you some of the in-between sizes that the number drills may not address.

P.S. Chris, you've got mail......
 

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Discussion Starter #3
Nick,
That makes perfectly good sense, the number & the sizing. I will get out my Pocket full of jets & do some referencing & sizing.

Anybody got a theory as to the reduced bore in the centre of the jet?

Is the reduced bore required for proper operation?

Or is it just a result of manufacturing proceedures, i.e. one jet is factory drilled to cover several sizes?
 

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Discussion Starter #5
OT,
my thoughts exactly :-/

Can Anybody elaborate on this?

Chris, if your there, maybe some words of wisdom? [smiley=help.gif]
 

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I've never had a Keihin FCR main jet in my hand to look at, so I'm just speculating about the shape....

I think they try to get a short length of well controlled diameter with consistent inlet and outfeed characteristics, hopefully to be cheaply and repeatably manufactured.

For your needs, as long as you can drill 2 jets that are the same size, that's all you need. If you want to go richer, you'll just drill another set larger yet.

The best part is that you're likely to not hurt anything going richer, eventually the engine will feel 'dull' or 'lazy' as you get to too rich.
 

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Discussion Starter #7
Yea Nick,
I'm with you, can't really hurt anything, hopefully only go faster ;D

Have ordered the drills today, 1.95mm, 5/64, 2.00mm, 2.05mm, #46, #45, 2.10mm, 2.15mm, #44, 2.2mm, 2.25mm, #43, 2.30mm, 2.35mm, #42, 3/32, 2.40mm, #41, 2.45mm & #40

so assuming they correspond, as we suspect, I'll have plenty to play with!

Will let you know the outcome.
 
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