Budget FCR swap for DRZ400, a new approach

[Jeff Allgeyer]

Budget FCR swap for DRZ400, a new approach

For those owning a DRZ power gains from an FCR swap may seem very attractive, until you consider the cost.  The Keihin FCR MX carbs are expensive and used swap kits rarely sell for less than $450 on eBay.  Prices for the Thumper talk kit and others are rising as Keihin has had recent price hikes.  I talked to Mike at the Power Barn in early October, when I began my search, and he was out of 39mm FCRs.  He told me once they were back, prices would be about 35% higher because “Keihin US had a massive price hike earlier this year”.

 

Limited potential of the Mikuni CV36 means that if you want more power from your DRZ, and FCR is a must.  If you have already made the 3x3 air box mod this and exhaust are the next logical step for your DRZ.

 

For those looking for power there is another way to solve the problem without spending so much money.  Two weeks ago I purchased a Keihin FCR MX version 39mm off eBay for $125.  FCR’s without the removable intake are common on 4 stroke racing four-wheelers and dirt bikes.  FCRs without the removable intake bell are identical internally to those with the removable intake bell.  These can be obtained cheaper than those with the removable bell, hence the reason I was able to get mine for $125 instead of $500+.  I did a lot of hunting to get one at this price, but they sell for $200-300 regularly.

 

As an engineer by trade I was able to design an aluminum adapter to make this happen.  For those on a budget this is a great option.  If anyone wants an adapter to test on their bike, message me and we can work something out. 

 

Keihin FCR Basics

So that you understand more about Keihin carbs here are a few details.

 

Ø  There are 3 body styles to the Keihin FCR:

1.      Keihin Slant FCR

2.    Keihin MX FCR with removable air boot.  This is most commonly adapted to the DRZ400.  The reference “air boot” is located on the air box side of the carb.

3.      Keihin MX FCR without removable air boot.  Not commonly used on DRZ400 but the topic of this discussion. Notice there is no bolt on plastic adapter on the air box side of the carb.  

 

 

Ø  These body styles can have 3 option packages.

1.       Choke

2.       Choke and hot start

3.       Choke, hot start, and throttle position sensor (TPS)

 

 

Ø  When trying to find the appropriate FCR you need to inspect for port size.  When you hear about 37, 38, 39 or 41mm carbs the referenced measurement is the throat size as measured from the position of the flat slide.  See the picture below, this is a 39mm carb.  39mm and 41mm carbs are most commonly adapted to the DRZ.  The smaller the bore the more “torquey” the motor will be.  Some have used 37 or 38mm carbs and had success. Any perform better than the stock Mikumi.

 

 

Ø  From my research 39mm carbs were available from the factory on some of these models:

o   2003-2007 YZ450F

o   2000-2009 KTM 525 XC four wheeler

o   KTM RFS 4 stroke motorcycles 450 and 525

o   CRF450X

Always ask your seller to take the appropriate measurement.  Good reputable sellers will be happy to take your money, for the 5 minutes it takes to check their carb.

 

What you need for an affordable FCR swap

 

Now onto my FCR project.  To adapt a FCR with non-removable air boot you need:

 

 

Item #5, the FCR adapter, is the key component.  This lets you take your carburetor without the removable air boot and make it the same overall length of the FCR everyone else uses.  The adapter’s air box size matches that of the stock Mikuni carb and attaches well to the air box.  On the carburetor side the adapter is slightly oversized (~.007”) for a good fit.  I recommend using 3M weather-strip adhesive or RTV silicon to seal the junction.  Keihn castings vary slightly and no two carbs measure the exact same, making this necessary.

As a side note, I made one of these also from ABS plastic which I actually prefer. Being weaker than aluminum, ABS plastic adapters are a press on fit and you are ready to go (no sealer needed). 

 

 

Thanks to the TT community for jetting suggestions. My DRZ with stock exhaust is running the following jetting configuration.

 

 

The fit of my carb with this adapter was nearly identical to the last FCR I installed on a friend’s DRZ.  Below are more pictures.  Please enjoy and share your thoughts.

 

Here is one ABS adapter.

Edited November 12, 2016 by Jeff Allgeyer

[dg400sm]

fantastic write up, well done, how did you make the ABS adapter?

[William1]

There needs to be a rounded edge (ala velocity stack) on the bell to afford smooth transition from the larger diameter of the air boot to the carb throat. This will also produce a better air 'signal' at the slow and main air jets. Take a look at the removable adapter to see what I mean. Your solution is one of the better ones out there, you just need to refine it a little more.

 

Good going.

[Jeff Allgeyer]

First I designed the part in SolidWorks which is a common 3D CAD program available through my work.   I 3D printed one version from a cheap material to check the fitment.  You can see the cheap 3D printed adapter in the last picture. 

 

Once I got the dimensions right I went over to a friends house and machined two.  He has a lathe at home and a CNC milling machine which worked great!  

 

After machining the aluminum version, just for fun, I made one from ABS.  I should have done this originally. The first 3D printed adapter wasn't very strong, but the ABS version was.  ABS can withstand temperatures around 400 degrees and for this part would have been fine.  It also passed my testing for resistance to gasoline.  I left it under a wet gasoline soaked rag and did not see any distortion.

 

For anyone on here who wants to do the swap I can give them one of my ABS adapters.  I still have one left over.

[Jeff Allgeyer]

There needs to be a rounded edge (ala velocity stack) on the bell to afford smooth transition from the larger diameter of the air boot to the carb throat. This will also produce a better air 'signal' at the slow and main air jets. Take a look at the removable adapter to see what I mean. Your solution is one of the better ones out there, you just need to refine it a little more.

 

Good going.

 

I thought of this when I was designing the part, but figured since the stock Keihin carbs don't have the fillet/bevel I would not put this feature on mine.  A lot of the aftermarket ones have the bevel falling into the slow/pilot air port and the main air jet port.

 

Tell me what you mean by "signal"?

Edited November 12, 2016 by Jeff Allgeyer

[William1]

I thought of this when I was designing the part, but figured since the stock Keihin carbs don't have the fillet/bevel I would not put this feature on mine.  A lot of the aftermarket ones have the bevel falling into the slow/pilot air port and the main air jet port.

 

Tell me what you mean by "signal"?

The stock ones (like the Yamaha FCR) do not because that shape is integral to the airboot.

 

The 'signal' is the strength and consistency of the air pressure that results from the velocity of the air entering the carb. A flat design like you have is the difference of smooth air flow comparing a Fighter jet to the Space Shuttle. You want a smooth transition. Not an abrupt edge.It is one of he reasons many of the conversions you are doing here have not been 100% satisfactory.

[Jeff Allgeyer]

The stock ones (like the Yamaha FCR) do not because that shape is integral to the airboot.

 

The 'signal' is the strength and consistency of the air pressure that results from the velocity of the air entering the carb. A flat design like you have is the difference of smooth air flow comparing a Fighter jet to the Space Shuttle. You want a smooth transition. Not an abrupt edge.It is one of he reasons many of the conversions you are doing here have not been 100% satisfactory.

Mine runs very very smooth, but I spent a lot of time making sure the through hole to my flat slide was concentric with the adapter.  I have access to a lot of precision measuring tools and spent a good amount of time checking dimensions.  That is also why I 3D printed it first to verify fit.

 

I can understand what you are saying.  I might try making this mod and see if I can tell a difference.

[Anthon Berg]

awesome; thank you for sharing thw knowledge

[ohiodrz400sm]

Excellent thread. You went above and beyond here.

Keep up the great work.

[William1]

This is a thread about a guy who made a part for himself. It is not (nor can it be) a thread talking about the development of a part for retail sale. That steps over the boundary of helpful knowledge to commercial enterprise and to do that, you must contact the ite owner and become a vendor.

[Jeff Allgeyer]

I have been looking online for more details on the velocity stack.  Do you all have any experience using an adapter with the horizontal sheet metal piece?  The description here makes it look like some hoax.

http://www.ebay.com/itm/55mm-PowerNow-Adapter-Keihin-Fcr-Drz-400e-Klx-Yzf-Wrf-250-426-Yz426-426-400f-/272443853201?hash=item3f6eeb4991:g:g9MAAOSwNRdX9A~j&vxp=mtr

[Joshz400_03]

Hey Jeff,  I plan to make this carb work as well.  I have one on my LTZ400 currently from a YFZ450 quad.  I would be interested in one of your rings if possible.  mxrob has showed me this adapter as well. - https://procycle.us/bikepages/dr650.html#jns-fcr-adapt

 

If you look on the LTZ400 forums there are tons of threads for tuning this carb as it is very popular there.  specifically suzukicentral.com  The quad version comes with a bunch of tps bs that can be deleted and I will include the link below.  I will also include a master thread to making the FCR work on the LTZ.  For those of you who don't know the LTZ shares the same engine as the DRZ.  

 

http://www.suzukicentral.com/forums/15-performance-parts/55895-fcr-carb-mods.html

http://www.ttbomb.com/z400/fcr/

http://www.suzukicentral.com/forums/17-maintenance-repairs-tuning/93921-fcr-jetting-guide.html

 

Jetting database -

 

http://www.suzukicentral.com/forums/17-maintenance-repairs-tuning/32548-jetting-database-5.html

Edited November 15, 2016 by Joshz400_03

[Joshz400_03]

Also page 10 here talks about flat vs tapered adapters --

 

https://docs.google.com/presentation/d/19EF5I7EtbxMLLIBLBlWZPWtRs1Kz9yU1erOBsic6gjs/edit#slide=id.i101

[Anthon Berg]

I have been looking online for more details on the velocity stack.  Do you all have any experience using an adapter with the horizontal sheet metal piece?  The description here makes it look like some hoax.

http://www.ebay.com/itm/55mm-PowerNow-Adapter-Keihin-Fcr-Drz-400e-Klx-Yzf-Wrf-250-426-Yz426-426-400f-/272443853201?hash=item3f6eeb4991:g:g9MAAOSwNRdX9A~j&vxp=mtr

 

 

FWIW I've never seen scientific measurement results for any of the PowerNow or Boyessen gear. Scientific measurements as in any form of repeatable quantified measurement of either power increase, engine transient response to throttle input, flow speeds, or anything. No patents mentioned either that I could see just now, which indicates to me that manufacturers would do this if it worked.

 

Marketing is hard though and it's important to simplify, so maybe the tecnical aspect gets buried - but I personally doubt the stuff truly works.

[ohiodrz400sm]

Hey Jeff, I plan to make this carb work as well. I have one on my LTZ400 currently from a YFZ450 quad. I would be interested in one of your rings if possible. mxrob has showed me this adapter as well. - https://procycle.us/bikepages/dr650.html#jns-fcr-adapt

If you look on the LTZ400 forums there are tons of threads for tuning this carb as it is very popular there. specifically suzukicentral.com The quad version comes with a bunch of tps bs that can be deleted and I will include the link below. I will also include a master thread to making the FCR work on the LTZ. For those of you who don't know the LTZ shares the same engine as the DRZ.

http://www.suzukicentral.com/forums/15-performance-parts/55895-fcr-carb-mods.html

http://www.ttbomb.com/z400/fcr/

http://www.suzukicentral.com/forums/17-maintenance-repairs-tuning/93921-fcr-jetting-guide.html

Jetting database -

http://www.suzukicentral.com/forums/17-maintenance-repairs-tuning/32548-jetting-database-5.html

+1

There have been many paradigms about the DRZ that have been perpetuated over the years on this site.

The don't think the jetting data base will be much help for the bike though. The modded ATVs have a much larger and straighter boot between the carburetor and air filter. In many cases, they don't even run an air box. As a result, you will see substantially larger main jets being used because of the additional air flow.

Other than that, the reason I'm experimenting with dual taper needles and running an exhaust cam larger than an RHC 187 on my big bore, is from information I learned over there.

I'm not bashing TT, I just think it's time to bring some facts back to this site and stop repeating the bs that began years ago by a clever business man. That's my opinion.

Edited November 15, 2016 by ohiodrz400sm

[dg400sm]

i agree, i have been looking at other sites too and there seems to be some more out of the box thinking but glad threads like this one offer us more ideas

[Joshz400_03]

+1

There have been many paradigms about the DRZ that have been perpetuated over the years on this site.

The don't think the jetting data base will be much help for the bike though. The modded ATVs have a much larger and straighter boot between the carburetor and air filter. In many cases, they don't even run an air box. As a result, you will see substantially larger main jets being used because of the additional air flow.

Other than that, the reason I'm experimenting with dual taper needles and running an exhaust cam larger than an RHC 187 on my big bore, is from information I learned over there.

I'm not bashing TT, I just think it's time to bring some facts back to this site and stop repeating the bs that began years ago by a clever business man. That's my opinion.

That's a good point.  I know myself and may others run the Honda 400EX airboot as it is much straighter than the OEM Suzuki boot.  I would assume the jetting would be fairly close though or at least a good starting point when tuning this FCR. 

[ohiodrz400sm]

That's a good point. I know myself and may others run the Honda 400EX airboot as it is much straighter than the OEM Suzuki boot. I would assume the jetting would be fairly close though or at least a good starting point when tuning this FCR.

Most recommend 160 for a 400 and 155ish for a big bore.

I'm running a new FCR_MX 39MM, 434 BB, oshc intake, web 540 exhaust cam and MRD exhaust. I just recently dropped from a 155 to 145 main and got a noticeable increase in performance. The bike feels very 450 like. If the weather holds out , I will dyno the bike to validate the butt dyno. I don't know if it's my unique combination of mods or what. In the past, other have said that the recommended jetting recommendations are too rich, but were quickly denounced by the other members.

Edited November 15, 2016 by ohiodrz400sm

[Joshz400_03]

Most recommend 160 for a 400 and 155ish for a big bore.

I'm running a new FCR_MX 39MM, 434 BB, oshc intake, web 540 exhaust cam and MRD exhaust. I just recently dropped from a 155 to 145 main and got a noticeable increase in performance. The bike feels very 450 like. If the weather holds out , I will dyno the bike to validate the butt dyno. I don't know if it's my unique combination of mods or what. In the past, other have said that the recommended jetting recommendations are too rich, but were quickly denounced by the other members.

Dang that is quite a bit leaner then.  I believe I have a 165 in there currently with the 434 and OSHC.  I have the stock airbox but lid removed.  My plan for the drz is to swap the OSHC over as well as the 434 and carb.  Have you looked at the plug at all after dropping to the 145?  Leaner will get you more power but is it still safe?

[ohiodrz400sm]

Dang that is quite a bit leaner then. I believe I have a 165 in there currently with the 434 and OSHC. I have the stock airbox but lid removed. My plan for the drz is to swap the OSHC over as well as the 434 and carb. Have you looked at the plug at all after dropping to the 145? Leaner will get you more power but is it still safe?

No, I haven't looked at it. I might have 20 miles of riding on it. Alot of it was testing it by riding with and without air box door, and by riding with the choke.

The bike runs and sounds great. I'm using 25 years of dirt bike riding experience as a guide.