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Old 04-04-2012, 01:29 PM   #1
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Mikuni pumper carb for the XR400

Well I ordered one just now. I think I pissed off the guy that answered the phone. As he was asking me what my bike was equipped with (stock internals, airbox, exhaust, etc.) I mentioned that my bike was totally stock with airbox snorkel and all but, was thinking of later putting on an FMF Q4 slip on. He instantly went into richard-cranium mode with his butted in and mumbled response of, "Junk". Instead of acting like I didn't hear him, I confronted him about his comment and the product's quality and what not. His response was, "Well I know what works and makes power and that aint it.". I came back with, "So the materials and quality are fine but it's no good anyway?". He said that since it's his competitor, it isn't any good. I ask him what he offers in the same decibel range, to which he has nothing and explains to me that to make power you will have to have sound. Nevermind that if I ran his pipe I'd be banned or cited at any public land riding area and wouldn't be allowed to run it at a lot of mx tracks. Or, I'd no longer be able to commute to work on my XR since I leave at 4:45 in the mornings and would really tick-off the neighborhood. I think i'll be fine with a tiny bit less peak hp and a load more tolerance for my bike from everyone, including myself.

So in light of the current mood, I move the conversation on to some questions I had which involve jetting and altitude. I live on the coast so I'm rarely above 50' asl. But I occassionally attend a TAR or other ride in the hill country and will eventually head farther west into the rockies, etc. I inquire about how many jets I'd need to cover a full spectrum of altitudes and hint to maybe buying a whole set of jets for the carb and having it thrown in with my order...saves on shipping right? He barked back, "It comes with two extra jets... one leaner, one richer for fine tuning.". I ask if he considered 11,000 feet up would be covered by fine tuning on a carb jetted for sea level. He said I'd be good. I dunno. I've read these carbs aren't as sensitive to changes because the pump makes up for a lot of it. Since I don't know for sure, I didn't argue it.

With the purchase of this carb, I'll have more money tied up in parts I've bought from them than the bike cost me initially. Customer appreciation isn't what it used to be.
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Old 04-04-2012, 01:52 PM   #2
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Re: Mikuni pumper carb for the XR400

Heck, why wait. I went ahead and ordered the pipe too.

Not mine... but I like pictures.
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Old 04-10-2012, 05:34 PM   #3
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Re: Mikuni pumper carb for the XR400

Time to find out if what is said about this carb is true.

Exhaust comes in tomorrow.



Tinkered with it a little more before running out of time. Decided to go ahead and pull the entire rear subframe off to install the carb. It's pretty tight in there.







I'm sure some of you are intimate with this sucker. So far, I've been really lucky. The guy who owned it before me didn't let if freeze up. And, I pull it and grease it occasionally. The horror stories being my motivation.

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Old 04-11-2012, 06:15 AM   #4
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Re: Mikuni pumper carb for the XR400

I do believe I would have found another place to purchase the carb.
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Old 04-11-2012, 08:42 AM   #5
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Re: Mikuni pumper carb for the XR400

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I do believe I would have found another place to purchase the carb.
Yea, that guy wouldn't have gotten any money from me. Plenty of tuners out there that know their stuff who'd love to sell you some product.
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Old 04-11-2012, 10:17 AM   #6
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Re: Mikuni pumper carb for the XR400

Finding the carb from somewhere else isn't necessarily a problem. But, finding the carb with the machined down ventury bell housings is. If you look at the pic of the carb you'll see the shiny parts on the front and rear of the carb.... that's not stock. Also, most places only sell the carb itself for at or near the same price. This is a kit that came with the machined pieces I mentioned, the correct jetting for specified altitude, a couple extra main and pilot jets to fine tune, and both push and pull throttle cables with the correct throttle assembly on top of the carb. Believe me, I looked into it. I have a ride next weekend to attend and didn't have time to play scavenger hunt to get it all together. They had it in stock and got it to me in three days. Sometimes you just concede for the sake of convenience.

I should have it on today, gonna clean up everything that can't be reached in there otherwise, check/adjust the valves, and change oil. Old girl should be good for a while. These are some tough dirtbikes.
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Old 04-11-2012, 10:57 AM   #7
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Re: Mikuni pumper carb for the XR400

Been on the road so I haven't been posting much. I've had the Mikuni installed for 7yrs now, it still works! Outstanding performance!
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Old 04-11-2012, 02:58 PM   #8
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Re: Mikuni pumper carb for the XR400

I pulled the exhaust header off to clean up the welds on the inlets according to Gordon's mods. >http://www.thumpertalk.com/topic/373...ods-for-xr400/. They must be the sloppiest factory welds I've ever seen on a Japanese motorcycle. One of the four header studs came out of the head instead of the nut loosening. I found one at a local hardware store along with four new nuts. Minor setback but I almost expected at least one to do that. At least the ports will be much closer matched when done. Anyone that regularly builds engines recommend using a thread locking compound on the head end of the studs?
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Old 04-11-2012, 03:40 PM   #9
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Re: Mikuni pumper carb for the XR400



I'm installing one on my '96 XR but ran into a problem with the o-ring between the rubber boot going to the intake and one of the 3 bolts that was rounded when I started to pull it off. Here I sit twiddling my thumbs while Central Texas Power Sports orders the parts. Maybe by Wednesday I'll have mine running.

As you said, the options are pretty slim to get one of these carbs. I guess when you have a monopoly, you can have whatever attitude you want (much like Central Texas Power Sports.....but I don't want to hijack your thread with my )
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Old 04-11-2012, 03:41 PM   #10
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Re: Mikuni pumper carb for the XR400

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Anyone that regularly builds engines recommend using a thread locking compound on the head end of the studs?
You can use blue Loctite if you want but it shouldn't be necessary . Don't use red Loctite unless you never intend to remove the fastener. I personally wouldn't use any thread locking product on head bolts, just torque them to mfg specs and you should be fine. Pay particular attention to the criss/cross pattern in the service manual. Head bolts are the only thing I use a torque wrench on, I pretty much German torque everything else.
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Old 04-11-2012, 03:53 PM   #11
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Re: Mikuni pumper carb for the XR400

You're reading too fast. Or I'm not real clear on what I meant. I'm speaking of the exhaust header studs that stick out of the head to bolt up the exhaust to. On the 400 there are four of them.... two for each exhaust port..... two exhaust ports. One of them came out when I tried to loosen the nut that holds the exhaust header pipe on. It was rusted to the stud so I got a new stud and four new nuts.
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Old 04-11-2012, 04:40 PM   #12
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Re: Mikuni pumper carb for the XR400

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You're reading too fast. Or I'm not real clear on what I meant. I'm speaking of the exhaust header studs that stick out of the head to bolt up the exhaust to. On the 400 there are four of them.... two for each exhaust port..... two exhaust ports. One of them came out when I tried to loosen the nut that holds the exhaust header pipe on. It was rusted to the stud so I got a new stud and four new nuts.
You wrote "header", I read 'head'. Yes you can use a thread locking compound on studs but you might investigate why the stud came out in the 1st place, rather unusual. It makes sense if the nut was hopelessly rusted to the stud though. Go ahead and laugh but one of the best hi temp anti-seize compounds out there is Phillips Milk of Magnesia "(regular flavor, not mint flavor)". Seriously, it's FAA (aviation) approved!
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Old 04-11-2012, 05:20 PM   #13
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Re: Mikuni pumper carb for the XR400

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You wrote "header", I read 'head'. Yes you can use a thread locking compound on studs but you might investigate why the stud came out in the 1st place, rather unusual. It makes sense if the nut was hopelessly rusted to the stud though. Go ahead and laugh but one of the best hi temp anti-seize compounds out there is Phillips Milk of Magnesia "(regular flavor, not mint flavor)". Seriously, it's FAA (aviation) approved!
It's official.... I have learned two things today that I've never heard of in my life. One being your example of Milk of Magnesia.... and two, a man was talking in the hardware store earlier and mentioned that a water heater zinc anode supplied with certain well water sources is what makes the "stinky rotten egg" smell when you turn on the hot tap. He showed me identical anodes made with raw aluminum instead of zinc and said those were designed for just that purpose. It's been a productive day!
On another note: the stock exhaust header is stainless, right? I'm thinking about polishing mine. It's looking kinda ratty with all the baked on iron oxide (red) clay. Most notable between the tubes where they cross over.
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Old 04-12-2012, 06:57 PM   #14
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Re: Mikuni pumper carb for the XR400

Got everything cleaned up and put back together. Probably the cleanest it's been since it rolled off the assembly line. Sometimes, there's just no way to get to everything without taking it all apart. I spent more time cleaning than I did actually working on it. Oh well, cleaning is the way I find things.... potential problems, broken stuff, etc. I gave up on polishing the factory header. The jaws from the mandrel tube bender left lots of imperfections on the tubes. The more I polished, the more they became pronounced. I didn't want to or feel like removing enough metal to make them go away.
The porting went smoothly. Took off quite a bit but didn't cut it too close. Better safe than sorry. A pic of what I'm talking about. It's kinda blurry... cell phone. What I'm grinding is the inside weld located just inside the shiny circle. It's supposed to be round... and about an 1/8th" bigger all the way around. It is now.



The exhaust went on without a hitch. I've almost become tolerant of ill fitting aftermarket exhausts but, this one was a true "slip on".



I'm real impressed with the quietness of it. It mainly sounds "different", not louder. Which is a huge plus for me. I don't know if it adds any power or not since I put on the pumper carb, removed the airbox snorkle, and ported the header all at the same time. But I will say this. I think my XR is running as good as it's gonna get without adding displacement, cam lift/duration, or noise. It's a big improvement from bone stock... and I like it.
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Old 04-12-2012, 07:03 PM   #15
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Re: Mikuni pumper carb for the XR400



My '96 XR4 had a White Brothers E-series muffler on it when I bought it. It was so loud it literally hurt my ears to have the bike running in the garage. It's possible I can repack it and such but I found a stock muffler on Ebay for less than $100. And I'm pretty sure it's the type that I can remove the spark arrestor IF I wanted to. Right now it's at a sound level I can tolerate.

First, I want to get it running smoothly with the pumper carb.........then I'll dig into the little things.
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Old 04-12-2012, 07:09 PM   #16
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Re: Mikuni pumper carb for the XR400

You're gonna like the it with the pumper carb. I can already tell it's WAAAY easier to start. I did like they told me and just bolted it up. If it's off tune... it's not much.
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Old 04-12-2012, 09:59 PM   #17
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Re: Mikuni pumper carb for the XR400

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You're gonna like the it with the pumper carb. I can already tell it's WAAAY easier to start. I did like they told me and just bolted it up. If it's off tune... it's not much.
And as Rman pointed out in my other thread, the pump can be adjusted to come on at a specific point in the throttle.
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Old 04-12-2012, 10:15 PM   #18
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Re: Mikuni pumper carb for the XR400

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And as Rman pointed out in my other thread, the pump can be adjusted to come on at a specific point in the throttle.
The pump linkage set screw is bottomed out on the spring. I have read in a lot of places that that is the way it runs best. Don't know if someone down the line posted that and it's been reposted numerous times though.
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