Edison approach to alerantive XR400 carbs

I'm on strike 2 here but thankfully this isn't baseball.:busted:

The reasoning and my history. If you haven't figured it out yet I've done a crap load of work on these motors over the last 7 years but they have had 4 wheels around them. In the EX world there are a lot of carburetor options and although many are great there's a cheap alternative that runs 95% as hard as the high dollar options, some even liking it better then aftermarket. When I joined up here in the 2 wheel club I was shocked that the only accepted option was $400 for a 36mm Mikuni Pumper that isn't any bigger then stock! That cheap alternative for the EX is an 04-05 trx450r carb. It's 42mm round slide Kiehin piston-valve with accelerator pump and a mildly modded 400 loves it with a surprisingly strong bottom end yet. Yes I said 42mm!

So as I build my XR4 I already know that I'd be shooting myself in the foot if I settled on a 36mm carb because I like bigger bores and cams and this motor likes bigger carbs. Makes sense, it is 400cc. Don't get me wrong though, the Mikuni is a real nice pumper for a stock bike and would seem fine on a well modded machine too but this 4 stroke is like every other one in the world in regards to when you go big inside, you want to go bigger outside or you're missing out.

Whether the info is appreciated or scoffed at I'll post up my findings one by one as I try to find viable alternatives. When I say viable I mean still being able to use the stock environment and not needing a ton of work. I'm sure collectively this is not a new idea to search for such a beast but I enjoy a good project and I like cheap alternatives when they produce results.

Currently the "test bike" is a 2000 Xr400r, snorkel removed, Uni filter, stock engine, stock headpipe and a 4" supertrapp slip-on with 11 discs.

TRX400EX stock carb (any year)

It's a Kiehin 38mm piston-valve with accelerator pump. This carb can be tuned to run real hard in the 416 and under builds. The down side is the size. It's a bulky one. This would offer a similar feeling the 36mm Mikuni pumper offers. The idea was mount it and use a motion pro twist throttle kit.

400excarb1.jpg

400excarb2.jpg

If you look at the red circled area the choke is hitting the shock as the body of the carb is on the frame. Solution is to cut the choke assembly out of the carb to move it back in order to make room for the intake boot. I don't consider this a viable option but if someone wanted it that bad that's what it would take.

400excarb3.jpg

As you can see there is some distance needed to squeeze that intake boot in there. If you go too far back the throttle cable would get into the frame (blue arrow)

400excarb4.jpg

Honda CRF250X carb.

I don't know the history of the crf's enough to tell you what years ran this exact carb but the unit tested is from a 2006. Big thanks to BajaBoundMoto for supplying the carb for testing.

It's a Kiehin 37mm FCR. Offering about 6% more performance over the 36mm Mikuni this carb would also fall into the ideal setting for a stock to mildly modded machine.

The first picture show the problem. Without modding the throttle cable area this carb also has fitment issues. As the area hits the frame you can see the carb needs to move a bit more before the intake boot can fit back on the engine. Pictures 2 and 3 offer more detail. After mods to make it fit, tuning and adapter to connect to the air box boot I would not call this a viable option either.

CRF250x1.jpg

CRF250x2.jpg

CRF250x3.jpg

Keep it coming ATJ :busted:!

While I am perfectly happy with my current engine performance (Gordon mods + Mikuni) which will "easily lift the front wheel in the 1st 6 gears and reverse with proper technique", I do constantly fiddle with the suspension, wheels, tires, etc. and I like your approach.

I have found the XR to be a simple platform that is generally easy to work on, with lots of cheap mods, and responds well to tweaks.

Cheers,

Ben

I think i speak for everyone when i say keep up the good work, we all appreciate your effort as it will benefit all of us seeking more power on a budget (aren't we all on a budget? lol)

Thanks guys.:busted:

I just fitted a Mikuni BSR42 and it fired up on the first kick cold. :thumbsup:

Give me a week or two to source a couple small parts and I'll give a full review. This might be just what the Dr. ordered for my BB build. From my initial tinkering it feels like the early 450r carb on the 400ex. Can't wait to see how she handles even on the stock engine.

Nice thing about the BSR is there are plenty of 36mm's out there too.

Gas for ATJ.....

Very Cool! I have 440 that could use a non stock carb... interested to see what you come up with.

I had my first success today. The up side is it's a monster! This thing just pulls like a freight train. The front came up so fast I was caught off guard (rode it out :busted: ). As an added plus it's big enough to feed even a 440 (my biggest peeve with the 36mm pumper). The down side is that it takes some modification and you're going to need a couple parts.

Before reading any further assume your own liability if you choose to do this mod to any machine.

What we're talking about here is the Mikuni BST40SS. It's a 40mm modern CV style carb that will pop the front wheel even if you're luggin' in the wrong gear. I was very impressed! The carb can be found on any........... just ignore the brand.......... Suzuki DR650 made after 95. It can also be found on just about any KTM LC4 from 1999 to 2005.

The dirty deeds:

First thing we need to do is move this big 90 degree elbow (red arrow) that feeds a breather for the slide. It currently hits the right frame rail. The good thing is the place it needed to go was already casted in the housing (blue arrow) and just needs to be drilled out. The KTMs use the hole we need but I have a DR650 carb here.

portplan.jpg

First thing you do is remove the top, spring, slide and needle/ Then stick something big and solid in the 90 and twist it out. The 90 has little barbs on is and you're basically ripping the aluminum up getting it out. Comes out easy once you get it moving and it won't hurt anything for our next step.

remove90.jpg

Tape off anything you don't want shavings in unless you're doing a full teardown and cleaning later. To plug this hole you need a 1/4" pipe thread tap and a 1/4" allen head plug. You will NOT need to re-drill the hole you just opened up for the tapping. Run the tap in far enough that the plug will mount flush when installed. You can see how far I had to run it in.

quartertap.jpg

Now that we have a nice tapped hole you can use either pipe thread sealer or a generous amount of loctite. I Used a big glob of red loctite because I never plan on taking it out again. Tighten up the plug and move on to drilling out the new hole.

tapped.jpg

plug_red.jpg

plugged.jpg

You will need to be accurate with your drilling here. The 90 is a press in fitting. The correct drill bit needed is a 15/32". Easily found in most hardware stores or possibly where you bought the 1/4" plug for the previous step. Line up your drill bit the best you can and keep a clean hole. I did it with a hand drill.

drilledport.jpg

Now you can re-use the 90 you pulled out of the other hole. line it up so it points near the center line of the carb. This will help with routing your breather assembly later. Start it in by hand and tap it the rest of the way with a hammer. You don't need to sink in all the way, just make sure it's secure. IDK why but I put some red loctite on this too. It can't hurt.

installport.jpg

I was able to get the breather tube and filter from the previous owner. It's kind of a must have unless you want to make an aftermarket set-up. The OEM unit can be purchased separately, works very nice, sits right on top of the air box and only needs a simple mod.

First pic shows the original configuration. The red pic indicates the breather filter and it simply needs to be moved to the other end of the tube where the green arrow is. In pic 2 the bell created from the breather should be trimmed off with a knife and then when the carb is mounted on the bike you connect that end to your 90. Some sort of clamp should be used.

orig_breath.jpg

mod_breath.jpg

We're half way there. This next step is almost an option. The bowl vent on the right side of the carb runs right into the right frame rail too. The hose is a little kinked and although would work it doesn't look good. The good thing is Mikuni casted in what we need once again.

In pic 1 find a drill bit close to the vent hole diameter but not bigger, a little smaller isn't a bad thing. Take the bowl off the carb and if you have a long enough bit you can do what I did and drill from one side to the other for perfect alignment(red arrow). The middle has already been drilled out my Mikuni. You're just drilling out the blank on the left side of the carb (blue arrow). In pic 2 you can see up in where you are drilling through. Be sure to blow out all shavings (green arrow and pic 2). It's a real easy clean up and accessible.

ventdrill.jpg

ventplan.jpg

Cap the right side vent (blue arrow) with a vacuum cap. Do the same for the emission port next to the mixture screw (red arrow). It won't be needed.

capvent_emiss.jpg

Okay, so far it's just been some fun with regular tools. I have talked to Motion pro and for $70 they'll make you a set of cables to connect this carb to your stock twist throttle. I went another route and used a twist kit for a 400EX. I had the cheap-o Motion-Pro kit and it was scary close. Just had to mod the cable mount plate on the carb (red arrow). I cut the top off and welded a fat nut on the plate to accept my cable end. This was about $35. If I didn't have this kit in the shop I would have ordered a nicer kit (around $50). I know the carb is set-up for push pull but the spring is plenty strong to operate as a single cable system. The blue arrow is the new vent you just drilled.

vent_mount.jpg

Here it is mounted. The choke (blue arrow) is a cable operated system that sucks and needs Suzuki handle bar equipment to work. I currently have the cable removed and the choke piston blocked in the off position. The solution is to convert it to a knob style choke. I have one on order, it cost me about $20. I have 2 more coming that were closer to $10 that I think might do the same thing but those are pure guesses. After paying $20 for the one I know works I figured there had to be something better out there and I was paying someone's mark up for having already figured it out. I'll keep you posted.

mounted.jpg

The issues have been covered pretty well above. There is one more and its that the carb is shorter. I was able to stretch the air box boot enough to fit for testing. I've already sent the blueprints to my machinist for a carb boot adapter to be turned out of aluminum. Hopefully by next weekend I'll have that. I'm having 5 of them made (hint, hint). I also have a design on a carb mount plate that would just bolt on and accept any 400ex single cable system. PM if you need help with your conversion.

This is without a doubt light years ahead of stock. It's too small for a modded DR and just right for us. I was able to buy a used unit from a member in the DR650 forum here and have another member offering to sell his to me too. My total so far with the carb, twist kit and little parts is around $185. If you had to buy new the carb is around $410 from cheapcycle.

Great work and doesn't seem too hard apart from the cables, which really isnt hard...more of a hassle to get cables made up etc.

Just a question, does this carb have an accelerator pump? and if not, isn't that the whole problem with the xr's bogging due to no pump? Im no expert and my question probably doesnt make sense lol

good work mate :busted:

EDIT: was just checking the manual, it says the carb bore is 38mm, so does this mean the mikuni pumper is actually smaller than stock bore size?

2nd EDIT: I was reading just a bit on cv carbs and found this interesting which also answers my question above

But how does this improve things over the simpler slide carb?

When the throttle is cranked on the slide carb, the slide is pulled up immediately by the throttle cable, expanding the variable venturi suddenly, and causing the lean stumble described above.

When the CV butterfly is opened, the slide does not immediately jump up to a much more open position. It raises gradually as the increasing engine revs provide the needed depression (at the venturi), which is then transmitted to the chamber above the diaphragm. As the slide rises, the increasing depression also encourages more fuel to enter the carb bore and combine with the greater air supply now available. And the higher the slide goes, the more fuel the tapered needle permits to flow. In other words the genius of the CV carb is that the fuel from fuel hole can now "keep up" with the increasing air available--maintaining the mixture at proper ratios during the accelereation process.

Full website here

1) I just measured the stocker with a caliper and came up with 37.4mm. The efficiency of the pumper carb is just better. That with the accelerator pump makes it a good improvement over stock but IMO it's what Honda should have mounted in the first place but for aftermarket it's not exactly going in the right direction. If XR's Only set up a 38mm version of this carb you'd really have something to write home about. I'd say this BST40 is very equal to that thinking.

2) Exactly Tek! CV = Constant Velocity. Your quote up there explains it to a tee. When paired up properly like this the response is incredible.

3) This has got to be the best read you can find on the BST40. It's from a KTM LC4 owner and he does an incredible explanation of how to break it down, clean it and mod it. My carb already has the "slide mod". For our bikes I don't think the spring mod is needed. He talks about needle wear and I found none on mine. I have a Dynojet needle that came with my carb but I found the stock needle to be better. The DJ is incredibly smaller and is really designed for the flow demands of a larger engine. In other words, you don't want a jet kit. If you want the adjustable needle that KTM offers (DR650 needle is not adjustable but you could shim it) it's only $8. Didn't find a need for one yet but I have one on order anyway just so I can report on it.

Found a couple ds650 carbs on ebay but they have some weird protrusion on the side. seems to be a different method of control. guess i'll wait on lc4 carb.seems to be better bolt on fitment.

Found a couple ds650 carbs on ebay but they have some weird protrusion on the side. seems to be a different method of control. guess i'll wait on lc4 carb.seems to be better bolt on fitment.

Brain fart!!! I kept saying DS when it's DR. The carb I got is from a DR650 96 and up. DS650 is a whole different beast.

I know of a guy selling one. I started a thread in the DR650 forum here at TT asking where to find them. 2 members PM'd me and offered me good deals. I'll pass on the info if anyone wants to PM me for it.

The DR650 needs a carb upgrade from this carb because it's undersized/designed for a 650. Understandably so if it works so darn good on a 400. They end up upgrade to more efficient FCRs and what not leaving some carbs laying around in garages. When making a deal try to get them to toss in all the breather hoses. This is the thread that got my leads, might work for someone else too.

Thanks guys.:busted:

I just fitted a Mikuni BSR42 and it fired up on the first kick cold. :thumbsup:

Give me a week or two to source a couple small parts and I'll give a full review. This might be just what the Dr. ordered for my BB build. From my initial tinkering it feels like the early 450r carb on the 400ex.

Haven't forgot about this carb either.

It's a Mikuni BSR42 CV style. I think it would come into play in the 440+ builds. No adapters needed to fit, you do need to bump out or move your rear brake resi. The carb is a single line setup from Polaris Predator 500s. Motion Pro makes high end twist kits that would pair up perfectly for this swap. Found lots of these carbs for sale and would still be good on a smaller bore high RPM build but most people build for low end here so that's why I say 440+. Just like the BST40 you'll want to ditch the cable choke and install a knob style.

Damn, now you me wanting to ditch my qwiksilver carb and try this out. Gonna have a heavily modified 440 when I am finished and if this carb set up could squeze more power out of it I'd be a happy camper. Right now I have to worry about some internal parts replacement so I will have to keep this on the back burner, but will definately want to try it out.

isn't that the whole problem with the xr's bogging due to no pump?

The stock carb can be dialed in to have no bog or hesitation, just takes some tinkering. But a dialed in stock carb is nothing like a pumper.

May have an edelbrock for sale shortly.

Damn, now you me wanting to ditch my qwiksilver carb and try this out. Gonna have a heavily modified 440 when I am finished and if this carb set up could squeze more power out of it I'd be a happy camper. Right now I have to worry about some internal parts replacement so I will have to keep this on the back burner, but will definately want to try it out.

The stock carb can be dialed in to have no bog or hesitation, just takes some tinkering. But a dialed in stock carb is nothing like a pumper.

May have an edelbrock for sale shortly.

TDC, what size is the quicksilver?

Not sure, Edelbrock says its a 32x38mm oval and will work up to a 440. My problem is I can't leave stuff alone and seeing your write up, has got me wanting to try it out...........simply because it takes some work to modify the carb to fit.

Not sure, Edelbrock says its a 32x38mm oval and will work up to a 440. My problem is I can't leave stuff alone and seeing your write up, has got me wanting to try it out...........simply because it takes some work to modify the carb to fit.

Makes me want to get my 88.5mm JE installed so I can do a big bore comparison between the BST40 and the BSR42.

4 machines in the stable, this has been my cheapest project lately. I think I better hold out a little before I get the engine torn apart in this one so the wife doesn't kill me......... :busted:

I am curious how you got down the road to ending up with this carb? I never would have thought of doing what you did to make it fit.

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