I wanted to show you guys some pictures of the fcr I made for mixxer's sponsored pro rider. It's a 43mm with a rat stack installed on the bell. I also wanted to show you what a straight bored fcr looks like and one that was taper bored. It's common to see the 43mm and bigger fcr's just straight bored. And it works that way but I know mixxer has put on straight bored and taper bored fcr's (same size) and the tapered fcr's out flow the straight bored on a flow bench. The taper bored models are also nice if your adding a rat stack because it's machined in and matches the bell on the fcr. The exit side is straight bored with 43mm and bigger. I wish I could taper bore it but if I did the wall thickness would be to thin.


This picture shows what the bell would look like straight bored to 43mm. You can see where it has been cut and where it has'nt. There is a good % of the bell not touched and the same size plus the rat stack has'nt been machined. You can also see I have the venturi install to prove it's out to size (43mm)




The three pictures above show the bell has been taper bored and you can see 100% of the bell is machined. This is the same 43mm that was straight bored. I did this so you could see for yourself and you can also see the rat stack is fully machined and matches the bell. Also the finish is the machined finish from the cnc mill,not polished yet.



Here's a venturi insert. It's a aftermarket venturi,not the stock one. You have to have one if you want to go 43mm or bigger. This one pictured is 43.5mm. Just took it off a cnc wire edm machine. Thats the way I machine the 43mm bigger. No tool pressure like a end mill and it cuts with brass wire and DI water.


Here's the finish the edm leaves. Looks like a fine bead blast finish IMO.


Here's a 44mm insert by a 43mm insert,you can see the one has a thinner arch. The 44mm is not for everyone. IMO it's for over 500cc or high rpm use.


Here is a 43mm insert on the mid section. You can see the shield sticks up where the top of the nozzel is. With the 43mm and bigger fcr's the bottom of the venturi drops down and the exit side of the shield should be machined down.


Machined shield

I'll post some more pictures so you can see the finished 43mm FCR (below)






This picture shows I trimmed out the stock boot to match the new 43mm bore.

If any of you guys want to buy the aftermarket venturi inserts and install them into fcr's yourself,you can buy the inserts from me. Please keep that in mind. I don't get pissed if anyone has a standard mill or cnc mill for that matter and wants to do there on work.

If you want your fcr bored to 43mm you can contact mixxer or KBR. Both can offer you this service.

KEWL!!!! been real busy lately... haven't got them out to you yet.. hopefully today or tomorrow... great pics and all

absolutely beautiful work..... thanks for sharing the pics and info....

a little info on the tapered venturi on the OEM carb bodies (all carbs... not just the FCR's)

the taper from the bell to the center of the venturi is designed into the carbs after many years of research by the carb manufacturers..... when the venturi is the smallest portion of the carb throat you will have the best throttle response...and best metering and atomization of the fuel... it is no mistake...or coincidence....that all oem manufacturers of carbs have a taper from the back of the carb to the center\venturi

a straight bored carb is much simpler to do...and doesn't require a 3D CNC mill... and while it may flow a couple CFM more due to less diameter change before and after the venturi....the couple cfm difference isn't, IMO\E, worth giving up the throttle response of the tapered carb body...

the way that Sredrum\Venom has settled on for carb boring is called "Hybrid Bore"...

the back of the carb to the venturi ring is 3D CNC bored with the OEM taper angle still intact.... and from the venturi to the carb outlet, the carb is straight bored for less volume\ less reduction of vacuum signal strength...and also less turbulance for more cfm....

lots of work....but worth it if you want the best possible peformance carb.... RESPONSE + CFM

and don't forget...if you are going to run an aftermarket air filter arrangement.... you will be ruining the flow if you don't have the same blended inlet to the carb bell as the oem intake offers...thats where the Venom Rat Stack comes in... without it...you will be down by 1~2HP from the square back of the carb causing turbulence at the entrance to the carb...

Hey Mixxer, Sredrum i have a question for ya can you go too big on carb boring with a 450cc machine? I have a port job a HH pistion and was looking to get a FCR carb to add some power. Is there an ideal size or can you just take it to 44mm and jet accordingly? I have a inside line on a 40mm fcr brand new with the TPS from a 07 trx I would love to send it too you guy's to have it cleaned up. Any help from you guy's is greatly apperciated.

actually ...on then 04-05's you wont see any power advantage from switching out the stock 42mm pumper carb...until you take the FCR to 42mm or greater...the FCR has better throttle response for sure...but not more power...


and to answer the other part of the question... YES!!

it is most definately possible to over carb size your machine......

when you test carb sizes ...you will see power increase to a certain carb size per what engine build you have...then once you have hit the peak carb size for your engine, the next jump in carb size will not show any HP gain...and the next size up from that will show a loss in power...plus have terrible throttle response...

from my own testing..... for 450cc's ..... 42mm is plenty big unless you want to go for top end only applications...... 43mm and up are best used with Bigger bore engines ..... big carbs on small engines make for worse throttle response and narrower\ top end biased powerbands...

compression can also be used as a replacement for displacement in the above scenarios...more compression can handle more carb because it gives a stronger vacuum signal...

still.... in my experience....42mm is great for 450cc machines....unless top end bias is what you are building for

another thing to get the power potential out of bigger carbs....

make sure your intake port matches up with your new bigger bore carb\manifold... putting a bigger carb on a smaller\ mis-matched intake port isn't the way to do it

Thanks for the response and ya matching the boot/head to the size of the carb totally is understandable to keep from highjacking the thread. The info is totally apperciated what mods are avaliable for the stock carb anything ? Can it be polished up? or is it at peak preformance already ? Also should I assume that the 05 head is already at 42mm in stock form ?

the stock carb on the 04-05's will make great power ....it is already a 42mm pumper...so it's big enough to feed the TRx adequately until the engine displacement is over 500cc's....or you want to sacrifice bottom end for top end on a 450cc machine....

just get it jetted right and you will have excellent performance.... and spend the FCR cash on some other performance items...

correct...

one thing to remember with bored out FCR's..... the majority of the boring off-sets the the bore to the top of the carb body...and the manifold and port also have to be off-set towards the top of their bore when matching a punched out FCR to the intake tract

I could'nt agree more or say it better than you just did. For mx,dune and some drag use with a 450cc engine it will be hard to beat a taper bored 42mm FCR. And for you guys looking to get your head ported,fcr bored and or a Rat Stack installed at the same time keep Venom and KBR in mind. The head can be ported to your carb for a perfect matched intake tract.

Awsome thread! Thanks for some really god explinations with pictures Chris/John.

great work sredrum.you have a steady hand.
you the man.
the power needs air!
talk too mixx i had a tidd bit to add.
it may be nothing. but who knows.
but i loove the carb work!!!!!!!!!!!!!!!!!
TOP KNOTCH!!!!!

VENOM RULES!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

MINE looks better......


just kidding......but she works AWESOME......

nice work Chris....

and just as I suspected....by using a smaller jet in the main air bleed...im able to keep my jetting very close to where she was before the taper boring.....#168-#170 or so.....and I think she is still a tad fat.....




Click to view attachment

Yo

correct....

the air bleed controls how emulsified the incoming charge of air\fuel mixture is from the main jet tube.....

you want as big as will possible while still allowing not to large of a main jet....

if your jet number gets above 220...then you need a smaller air bleed to bring things back in line

so, with a new 501 set up, do i get my fcr 41mm bored to a 42 then, or go 43? If I go 43, will i have to modify anything else after the carb? I have the stock carb on right now.

Do i ned to re-port my head? What about boring out the intake after the carb where the carb connects to the engine?

everything needs to be totally re-done with a bored out carb....porting...manifold ...jetting....

so is drilling and tapping this main air bleed a necissity or does it just keep your jets close to where they were before?

and you are talking about the 2 holes on the bottom of the carb right and its the one basicly in the center?

Yes, it's the one in the center that needs to be tapped. I just got mine back, and it is a thing of beauty. It helps to keep the main jet size lower, so it's not rediculously huge.
It's already sent out for a touch up by Mixxer.

Glad your happy with it Brad. Can't wait to see the ride report on your monster 501 engine.

bump.... seems a lot of folks ask about the bore jobs and this thread always explained some of the differences, and has great pics

also sometimes i see a few folks seem to be confused about main air bleeds, another good explanation... i added the main air bleed myself and 180 is the biggest jet i have ever wanted, and thats when it was cold and i wanted to be richer, and it was rich

Thanks for bringing this thread back up... I've never seen it... great info here!

ttt....

always nice to see artwork like sredrum's....

i absolutely GUARANTEE if you had your carb work done by someone else... you didn't even get close to this level of perfection....

...and many are just plain hacked up... just the sad facts...

Here's a pic of the yfz carb i just got done by sredrum, 42.5, stock venturi, good price for the work for sure!