Graham Jarvis Mikuni carb mods and jetting specs

[KTM753]

Graham Jarvis Mikuni jetting specs and modifications:

I eluded to these mods and jetting specs in another thread so I thought I would post these in a separate thread. I found these pictures and description on KTMTalk.com forums (not the awful Facebook group). They are from a member named "AfricaOffroad". He says these modifications and jetting specs came from another factory Husky rider he knows in South Africa who got them from Graham Jarvis's mechanic. He has posts as far back as 2008 from the Roof of Africa hard enduro race (Lesotho - 5000ft) so I am guessing he really does know someone who got this info from said source. Additionally, I have read somewhere along the way that when Jarvis or Johnny Walker come to ride/race in the USA, they bring 3 things with them from their UK race bikes: Suspension, Head, and Carb. Install those 3 things on a stock bike in the USA and go to town. 

Any text in italic text is a direct quote from "AfricaOffroad" and pulled from lots of different posts from different threads. My Liberal Arts degree taught me that plagiarism is wrong.  My later earned Mechanical Engineering degree taught me a lot more interesting things ?

Here are the jetting specs:

Left column is sea level +
Lesotho jetting would be for 5000 ft +

You want about 1 to 1 and a quarter turns on the air screw with the jetting spec I gave at sea level

Here are the float bowl modifications:

Description of these mods, "The drain bolt holes connect to the outer holes in the bowl. The drain plug has a groove filed in it to act as a feed. This all ensures a good fuel feed to the main jet, similarly for the plastic splash guard.

Brass pipe is bent up slightly and partially crimped closed. Helps eliminate wasteful spillage from the bowl.

Good friend of mine is a factory sponsored Husky rider.
He got these mods straight from Jarvis' mechanic.
He did the work for me."

"I wasn’t happy with my buddy squeezing my overflow tube like that but happy to report it does work. Note it is also bent slightly upwards"

Here are mods to the splash guard:

Here are the notch mods to the slide:

"Note regarding the slide notch, only the small notch gets made bigger."

"All the mods were done with hand tools."

"Try it, every person I know who has used this set up raves about it.
Two very good riders previously running a Lectron and Keihn have reverted back to their Mikunis with these settings"

Quotes from James Dean of JDJetting.com (A great resource on that forum and why its worth going though the hoops to join it)

"Remember that the 2017 KTM 250 has a needle jet size #R-8 and it's smaller than the #S-4 needle jet of the 2018-19, so some of the differences in needle settings are coming from this. Explained: 2.79mm (R-8) vs 2.82mm (S-4), making the 2017 leaner by .03mm overall on the needle." 

"Maybe the float level has been lowered too much. Try 8.5mm instead, and then a #35 pilot jet, taking 1 step at a time."

I am unsure if the 2017 Husky vs. 2018/2019 Husky are similar needle jet changes. 

Another telling post from KTMTalk member "towerjack" about his Mikuni experience and these mods:

"My son bought a new left-over 18 300XC mid September. Brought it to my shop and was leaking from carb before ever being ridden. Tore it apart and found a broken o-ring under the float needle seat. Took the bike for a first ride and it ran terrible, just as described in this thread. Read entire 70-plus pages here and started in on it.

Checked reeds and could see daylight like crazy, replaced with VForce4. Did notch in slide mod and went to work on the taper on air box side. Drilled feeder holes in the bowl drain plug, enlarged the holes in the splash guard and bent the overflow pipe and crimped it.

Installed 43-75 needle, first clip - 25 pj - 440 main - 1-5/8 on AS - 8.8mm float height - closed plug gap gown to .019
Above work took about 2-1/2 hr.

Rode bike over to a harvested bean field and rode easy for 15 min. to warm it up good, then adjusted AS and idle. Turned the smoker loose and no worries mate. Runs as well as my '12 300XC that is mildly ported, 225 comp. and aftermarket head, .040 deck height and 36mm Keihin. Running 110 Turbo Blue with 40:1 Amsoil Dominator. Rode for 2 hrs. in a gravel pit pulling hills over weekend and performance never changed. Maybe we got lucky and got a Mik with no mfg. flaws, but after the mods I've got no complaints."

Overall thoughts:

I have not personally performed these mods to my Mikuni carb. But I have read a lot of posts at KTMTalk.com from people I respect there and they really like these mods to the Mikuni against a Keihin or Metering Rod carb. 

Additionally another guy suggests using the following needles from an RM125: 16 or 17/62 Suzuki needles. 

If you looking to improve your Mikuni for very little money versus buying a metering rod carb, give these a shot and report back here. 

I hope this helps some folks. Took a fair amount of time to dig all this info up.  

[RichieGoatGruff]

Just pulled this info and pics from the same source, put it in a word doc...  Great minds think alike!

[strokemetwice]

Wow. Thanks. Now I have to put Tybee mik back on and try this. More work. 

[KTM753]

Suzuki 17-62 needle part # is 13383-36FR0

Edited November 29, 2018 by KTM753

[Muffintop]

Hmmm try this or just install the Keihin.  That's an awful lot of work to a carb just to get it to run right. I bought the bike with the shit carb and reeds though so I guess I'm the sucker! 

Thanks for the info, I will have to give this a try over the winter and report back.

[strokemetwice]

13 hours ago, Muffintop said:

Hmmm try this or just install the Keihin.  That's an awful lot of work to a carb just to get it to run right. I bought the bike with the shit carb and reeds though so I guess I'm the sucker! 

Thanks for the info, I will have to give this a try over the winter and report back.

It took me about 45 min to do the mods, pull the keihin out and put mikuni in. That includes pulling silencer, plastics, and suframe up to get the carb out. Why they could not have put a more flexible air box material in like the older bikes i have no idea. 

[Death B4 DNF]

I have a 19 Tx300 and decided to try and tune the stock carb before dropping the 500 bucks on a lectron. I filed the slide installed a JD kit (30 pilot 400 main 2 1/2 clip position on the needle) and a Torque wing. I did replace the stock reed cage with a V-Force because you could see daylight threw every pedals and installed a custom silencer from house of horsepower. The bike ran really good but not crisp enough for me. I could get maybe 50mi out of a tank.  Then I installed a RK head set for 95 octane. I can't believe the difference.  The bike feels ported. Rode 50.14 miles the other day and I still hade just under a gallon of fuel left. 

 

[Muffintop]

It’s hard to tell from the picture but it looks like they drilled holes in the float bowl plug or something? Is that what I’m seeing? Also do you crimp that little tube 100% closed or do you leave a small opening?

[Death B4 DNF]

Not sure on the drain bolt mod. Not completely closed no. And I didn’t crimp the tube on mine I just set the  float height and put a loop in my overflow tube where it plugs into the carb. No more dribble after that. I must say I had a lectron on my 16te300 and am not knocking it at all.  But I like the response I get out of a carb. I have no plans on getting a Lectron at this point. 

[ricky racer]

15 hours ago, Muffintop said:

Hmmm try this or just install the Keihin.  That's an awful lot of work to a carb just to get it to run right. I bought the bike with the shit carb and reeds though so I guess I'm the sucker! 

Thanks for the info, I will have to give this a try over the winter and report back.

Or fix the squish. Stock carb is fine.

 

2 hours ago, Death B4 DNF said:

Then I installed a RK head set for 95 octane. I can't believe the difference.  The bike feels ported. Rode 50.14 miles the other day and I still hade just under a gallon of fuel left. 

Exactly. Even the stock head cut to provide around 1.3mm squish would be better then swapping a carb haha I used a RK and Love it. I also had my bike running pretty good just had a weak idle and soft right of idle. Not anymore! Set my float loop the over flow hose and jetted the dam thing. I did try many needles and like the Blue JD the best. I found no need for any slide cutting.

[Death B4 DNF]

I’m at 5500’ so I had to lean out the slide and go with a red jd needle. Best two stroke I’ve ever owned. 

[motocrotts]

Exactly. Even the stock head cut to provide around 1.3mm squish would be better then swapping a carb haha I used a RK and Love it. I also had my bike running pretty good just had a weak idle and soft right of idle. Not anymore! Set my float loop the over flow hose and jetted the dam thing. I did try many needles and like the Blue JD the best. I found no need for any slide cutting.

You guys have a pic of the loop you put in the carb lines?

[Muffintop]

Well this is what I ended up with, the holes drilled in the drain plug make a little more sense once you look at it. The bowl has holes drilled in it but the plug does not for some reason, I just marked mine with a sharpie and drilled the plug so they line up with the holes on the float bowl. 1/16" bit works great. Also you can get some needle files from harbor freight for $2.99 for the slide mod. Hopefully this shit works or else I just hacked up a $250 carb ?

Edited December 1, 2018 by Muffintop

[Death B4 DNF]

Well this is what I ended up with, the holes drilled in the drain plug make a little more sense once you look at it. The bowl has holes drilled in it but the plug does not for some reason, I just marked mine with a sharpie and drilled the plug so they line up with the holes on the float bowl. 1/16" bit works great. Also you can get some needle files from harbor freight for $2.99 for the slide mod. Hopefully this shit works or else I just hacked up a $250 carb [emoji28]

Looks good. Let know if you can tell a difference.

[strokemetwice]

I tried this mod.  Granted i only ride around the block, but back to back, the keihin 36mm has more snap, is less picky about cold weather, revs cleaner, and just seems to have a lot better power. Im sticking with the keihin. 

[Death B4 DNF]

I tried this mod.  Granted i only ride around the block, but back to back, the keihin 36mm has more snap, is less picky about cold weather, revs cleaner, and just seems to have a lot better power. Im sticking with the keihin. 

I’ll agree the Keihin 36mm has a little more snap. (I had one on a 16 te300) But it also signs of sooner then the 38mm Makuni. If you want more snap off the bottom from the 38 install a torque wing that’s what I did. A mod you can feel right away. I’m at 5500’ and have no complaints after I got it dialed in. In the summer we’ll ride southern Colorado up to 12,000’ and yes you can feel a slight power loss but not enough to drop $500 on a lectron.

[motocrotts]

Did all of the mods today. Super easy to do, took less than an hour. Seems much cleaner, i really had to turn the idle down from stock. At least 3 full turns out. I'll get it out this weekend for a full test.

[motocrotts]

Rode the bike today after making the carb mods and all i can say is wow! Tons of torque off the bottom. Much more than the GG and even stronger than the beta. Revs out further than the beta too. Very controllable, certainly don't need to do any head mods or seek any more power. Bike never stalled all day and just had a touch of spooge on the end of the muffler, and thats running 50:1 with 20% 110 race fuel. Most fun I've had in a long time. Might have a lectron up for sale soon.

[rocky4by]

1 hour ago, motocrotts said:

Rode the bike today after making the carb mods and all i can say is wow! Tons of torque off the bottom. Much more than the GG and even stronger than the beta. Revs out further than the beta too. Very controllable, certainly don't need to do any head mods or seek any more power. Bike never stalled all day and just had a touch of spooge on the end of the muffler, and thats running 50:1 with 20% 110 race fuel. Most fun I've had in a long time. Might have a lectron up for sale soon.

Hey motocrotts,  are you still on your GAS GAS ? I have a guy wanting my KTM 15-500, not sure I want to let it go, it would put me back looking for another 19-500 or a another 2T.  I an really liking my TX 300 I was riding almost everyday till the weather turned bad.  

[motocrotts]

10 hours ago, rocky4by said:

Hey motocrotts,  are you still on your GAS GAS ? I have a guy wanting my KTM 15-500, not sure I want to let it go, it would put me back looking for another 19-500 or a another 2T.  I an really liking my TX 300 I was riding almost everyday till the weather turned bad.  

No, i sold the GG. It was a good bike and i really liked the handling and suspension.  Until i rode the tx.  The motor on the GG was just not in the same league as the tx or the beta for that matter. I have several reasons why but i really don't want to make them public. 

If you like the 15 ktm don't ride a new one. Stock, they're weak but after being vortexed they're a completely different bike. Best 4 stk ever imho.