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Help me ID these? and can they be tuned? Mikunis

Discussion in 'The Garage' started by gggGary, Mar 4, 2011.

  1. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    OK so a main jet shouldn't be just laying in the drain fitting? Oops. Anyways the plugs which had idled up the driveway after a moderate blast looked pretty white so I threw a pair of 260's I had in there and ran it to town and back. did a real speed chop this time and the plugs are still pretty bare looking.
    plugs 260 mains 001.jpg

    But it ran cleanly all the way up with WFO and it sure feels powerful even subtracting the loud = fast effect.
    So any opinions? My stab in the dark guess is try going up some more from here?
    won't go too crazy till I put an exhaust I can live with on it. Snuff R nots? LOL
     
  2. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    I'm thinking at least #280-#290. Are you indexing the plugs?
     
  3. grizld1

    grizld1 Grumpy old man Top Contributor

    You guys are way out of range on the mains unless you're racing. On a cammed and ported high compression 700 with 1-5/8" pipes and straight-through glasspack megaphones, I ran the following in VM36's for some years. PJ: 22.5. NJ: 159 P-5. JN: 6F9-3. MJ: 190. Those jet kits you were looking at are 2-stroke oriented; really, a couple of mm. of venturi size doesn't make a huge difference in jetting. gggGary, I have some suggestions for you. First off, confirm your float level. On your 38's it should be 18 mm. +/-1 mm.; try 17 mm. and see what happens. Second, due to the weaker vacuum signal caused by the big venturi you may need to use air correction jets to raise velocity in the main air bypass and help with atomization. You might stick #2.0's in there and see what happens. Now--if the motor doesn't fall on its face when you roll the throttle full open, you're not far off on the MJ's, so if I were you I'd be looking at the mid-throttle mixture. What NJ's are in there? There's an inverse relation between NJ and MJ sizes, and I've seen some pretty strange things stuck in there. And I strongly advise you to ditch those 6DP1's for 6F9's.

    The grossly oversized headpipes won't help your tuning efforts!
     
    Last edited: Jun 21, 2013
  4. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Grizld1, glad you brought this up. Back in the day, there was some confusion over certain Mikuni jet numbering systems, and somehow got my hands on a chart showing orifice size, and vague explainations for the numbering system. Mainjets in my vm36s held a slosh/baffle, maybe different number was used for those not utilizing that. Hoping someone with better memory could chime in on this.
    On my VM36s (1975 model):
    Pilot - #35, orifice: 0.54mm, 0.021"
    Main - #270, orifice: 1.40mm, 0.55" (a bit lean for gas mileage, #280 for performance)
     
  5. grizld1

    grizld1 Grumpy old man Top Contributor

    The splash guard has been in every VM 34 and 36 I've ever used, but my experience with them on the XS650 goes back only around 20 years, though I'd had VM's decades before on various 2-strokes. I'm honestly surprised that your bike runs with MJ's and PJ's that fat; where's your float level set, and what JN's and NJ's do you have in there? The first VM's I dialed in on a Yam twin were used 34's. Had to go crazy rich on mains and pilots before I figured out that P-2 NJ's weren't gonna cut it!
     
  6. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    When I ordered my vm36s back then, I purposely ordered the 2-stroke variant, so I could grind the cutaways to my liking. That's when I noticed the odd jet numbering system. The #270s translate back to #140, on the normal system, just a bit on the lean side. Gonna drag out and dig thru the boxes to find the needle #'s and such, will post later.
     
  7. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Okay, just emerged from the cave, brushed-off the scorpions and black widows, got some numbers:

    Mikuni VM36 ( 36E / 4 ) - outlet diameter 37.5mm
    Manifold VM36/200 (carb/manifold/head port matched)
    Pilot - #35 (0.54mm, .021" orifice)
    Main - #270 (1.40, .055" orifice)
    NJ - 159 Q-5
    Needle - 6FJ6 (clip mid position)
    Slides - Started as #1.5, final cutaway at 6mm, .240"

    Spent a lot of time setting this up, idled fine, explosive throttle response, idle to WOT no bog, got 70 mpg @ 70 mph. These are matched for this engine, has some "oh, I wouldn't do that" tricks inside, may not work on others...
     
  8. grizld1

    grizld1 Grumpy old man Top Contributor

    I'm scratching my head over your jetting and wondering how in the world your engine runs on it without fuel fouling the plugs in two blocks. The high speed fuel consumption you're claiming is quite a bit better than bone stock. Except for the P-5 NJ's (good choice for the street), you're running what looks a lot like a full-race setup, with the exception that your mains are fatter than anything I've seen or heard of in a racing XS twin engine, and the 6 mm. cutaway (?!)
     
    Last edited: Jun 22, 2013
  9. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    Haha, wish I had a time machine, we could go back to that era and you could be indoctrinated into our 'head scratching' club. Must've been 2" of dandruff on the floor...
     
  10. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Lots of great info sneaking out here. I hate to stab anyone helping but I do have to say the 70MPG seems over the top.
    I will tear down these carbs and fully document what's inside. I also need to get a reliable tach in place, the electronic tach is flaky and I suspect this set up will make power way beyond a safe/sane top RPM. I did find a long annoying youtube that showed how to use a utility knife blade to set the float height. That actually worked very well, and I believe I have them set properly. I have been head scratching over a way to verify actual fuel level though.... (more dandruff on the floor) I appreciate all the input and this may take a while but will keep hammering away til I get it where I want it. I may throw that set of MM 34s on just to see how his tune compares. I just pulled off the mains to see what they were but there are some differences in those, the mains were higher in the sump than on the other two sets of VMs I have.
     
  11. grizld1

    grizld1 Grumpy old man Top Contributor

    Direct inspection method got you scratching your head? C'mon, Gary, you know how to set up a sight tube; you're just trying to scheme a way to do it on the cheap without having to buy another float bowl plug!
     
  12. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Well, yeah!!!!
    I guess I can drill and tap it for a drilled zirk I suppose I could even tee it to the vent and leave it in place. I wonder if an "undrilled" zirk will hold fuel?
     
  13. grizld1

    grizld1 Grumpy old man Top Contributor

    Why use an undrilled zerk? I have a permanently modified bowl plug with a 6 mm. hose barb on it, but if were going to unthread the barb and use that plug I'd block the hole with a 6 mm. screw and a copper washer; the hardware's just as cheap and easily available.
     
  14. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Because I bought a big metric zerk assortment. LOL
    I have three carb sets with three "different" sump plugs.... :banghead:
    Actually teed to a vent might be good, quick drains into a container with minimum fuel on hands and engine.


    Here's what's in the (now confirmed as 34mm) snowmobile?? carbs that started this thread. (not the carbs running on the bike)
    I have no history of these carbs ever working as set up.

    Mikuni VM34 ( 34L 02 )

    Pilot - #40
    Main - #260
    NJ - 247 Q-2
    Needle - 6DH4 (clip top position)
    Slide - #2.5
    float valve 1.5

    Guess VM carbs is a big subject?

    I think I can get a few things ready on the bike tomorrow, starting with a mechanical tach, the external oil filter/cooler lines are blocking the tach fitting screw, snafu.
     
  15. grizld1

    grizld1 Grumpy old man Top Contributor

    Wouldn't run a line to the vent if I were you. The vent is there to keep the float chamber at atmospheric pressure; interfering with that function will cause trouble.

    Re. that jet combo, right, it's for a 2-stroke. The only brass you can effectively use on the XS650 is the slide and the float valve. Re. your electronic tach, try hooking it direct to the negative side of the coil. If it spikes, stick 2 Mohms of resisters in the line. (I'm assuming, of course, that you can program the tach for 0.5 pulses per revolution if you're running an ignition that uses dual coils or 1 pulse per rev with a single 2-tower coil).

    Re. plugs for the plug, rather than counting on an undrilled zerk to hold gas, you might consider packing a zerk with JB Weld; that stuff's impervious to fuel.
     
  16. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    I drilled and tapped a plug from the ugly snowmobile set. All three sump plugs more or less interchange. After a couple of checks the float valve showed it's flaws and I replaced it. (stole it from the MM set, will go get a new one)

    I pulled the speedo and tach and put in a stock set for now, much better! It was running good enough to go put 75 miles on it this evening, nice riding time as the sun went down. She is making enough power now to overpower the clutch at at about 5K RPM. I have the 260's in it and I am not sure if they are too rich or not yet. I gotta make up my mind and order replacement pipes, these are just too obnoxious. Guess I gotta drop the headers and see what's up at the port. Then see if I just change mufflers or get new headers. And look into the clutch. Not sure what oil is in it so will do a change and look at the sump filter.

    Here's the brass list
    First the 38s on the bike
    body 38 N9
    N 6DP-1 3rd slot
    NJ P8 166
    MJ260
    PJ 25
    Slide 1.5
    Float valve 3.3

    The MM VM34s pristine, never had gas in them yet.
    Body 34B 168
    N 6F9 -3
    NJ P6 159
    MJ 180
    PJ 25
    Slide 2.5
    Float valve 3.3

    May swap those in next and compare how it runs with them.

    Here's a shot of the "utility blade" float setting method.

    madness 005.jpg

    The blade is about 18.7mm
    about halfway down the notch is just strong of 17mm
    Say can you guys tell me where the fuel level is supposed to be? I am down about 6-7mm from the bottom of the body.

    Shots of the MM main jet, float bowl and floats.

    madness 009.jpg madness 008.jpg

    Floats are not captive on the rods and are oval in shape now. the float arm no longer has the "jog" in it
     
  17. grizld1

    grizld1 Grumpy old man Top Contributor

    Float level on the VM36 and VM38 is the same--18 mm., +/- 1 mm. Float level on the VM34 is 23 mm. +/- 1 mm. The range is a tuning range, not a tolerance. In VM's the level is measured from the gasket surface on the carb body to the point on the float fork that makes contact with the horizontal pin on the float when the carb is assembled.

    Re. your brass, the P-8 NJ may be what you'll want to use to balance the greater volume of air through the bigger venturi. The 1.5 slides are probably too rich. The 6DP1 JN will run, but you're likely to get some flaky behavior in roll-off testing.
     
  18. TwoManyXS1Bs

    TwoManyXS1Bs BBQ Hunter Top Contributor

    The VM38 slides may have been cut, if this was an old-school tune. Measure the cutaway height, see if they're more than 1.5mm. I should probably re-mark mine...
     
  19. gggGary

    gggGary Stop that! Top Contributor XS650.com Supporter

    Well I violated a tuning rule tonight changed two things changed mufflers but the new ones are way restrictive quieter than stock! And went back to the 200 mains. they are just too lean I had to nurse it past 5K. Power was way off no more overpowering the clutch at 5K. Probably will drill some holes in the mufflers, quiet, still quiet then at about 5K it sounded like a two stroke coming on the pipe brrrriiippppp. Thought it might get a hernia!
    I measured the slide cutaway the slides look factory no evidence of filing. but look at the caliper. Something I don't get here?

    madness 005.jpg

    Intake and exhaust ports after 75 miles with the 260 mains.
    Not sure if-what porting was done. I have not narrowed down just how hot a "webcam #2 grind" is, they now use 59 and 59a grinds. Has inner and outer valve springs and alloy caps. Guess I'll have to degree it to know for sure.
    I was wrong about the headers they are 1.6" ID
    I'll have to get the exhaust somewhere's close, then I guess get mains from 210 to 250 and start swapping?
    Thanks again for all the help I really appreciate it.
     

    Attached Files:

  20. grizld1

    grizld1 Grumpy old man Top Contributor

    Never take the PO's word for anything--there's no such animal as a Webcam #2 grind and there never was. Maybe a Shell #2? If it's a Shell, inspection is with no valve lash at .050" lift. When you get your valve event numbers, the guys at Hoos Racing can tell you if they correspond to a Shell #2.

    If the caliper doesn't lie that's a #2.5 cutaway no matter how the slide is stamped, and that's what ya want.
     

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