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Mikuni VM 36 (Read 88 times)
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Mikuni VM 36
07/20/21 at 11:33:43
 
Trying to setup a new Mikuni VM 36

Main, 190 (4/042)
Pilot,  27.5 (VM22/219)
Needle Jet,  Q-5 (159)
Jet Needle,  6FJ6
T.Valve, 2.5
Air Jet, 2
Needle V., 3.3 (VM34/39)

Stock engine and muffler (1995)

QUESTION: Should it stumble when making it lean (counter clockwise)

AIR/FUEL Mixture
Starting 1-1/2 turns out. Idle screw as low as possible, without dying.

Can only get engine to stumble turning in (clockwise) about 3/4 turn. Can never get it to stumble (counter clockwise). Idle does goes up for about 1-1/2 turns out then stays the same.

It tried different pilot jets one up and down resetting each time to no avail.


Mid-Range
It was a HUGE bog after 1/2 throttle, no load just revving the engine on all the pilot jets I tried. I did move the clip on the jet needle (up & down) lean backfires. One clip richer, still bogs. Plug is not black or sooty.

Trying hard...


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LANCER
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Re: Mikuni VM 36
Reply #1 - 07/21/21 at 05:19:29
 
Drop your pilot jet down to a #20
Put the needle clip on the middle (3rd groove)
#190 main jet is ok for now, but I install a #200 to begin with unless you are at a higher altitude than sea level

Try that setup
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Re: Mikuni VM 36
Reply #2 - 07/21/21 at 11:28:19
 
I'll give it a try, Thanks!
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Re: Mikuni VM 36
Reply #3 - 07/21/21 at 19:49:30
 
I tried the setup, #20 on pilot and #200 on main. Still bogs, backfires after 1/2 throttle, no load just revving.

Tried the the clip down another notch #4, reset - same bog.
Repeat on all the clip settings (down & up), reset air mix each time (waiting 10 sec for each half turn), same results. I'm about wore out. many hours but I'm getting faster.

I'm missing something??? Wonder if the new carb is faulty. It's not a knock off.
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Re: Mikuni VM 36
Reply #4 - 07/22/21 at 10:12:28
 
Before adjusting the settings on the carb are you riding your bike a few miles before doing so to give your engine time to fully warm up ?
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Re: Mikuni VM 36
Reply #5 - 07/22/21 at 10:54:55
 
Bad petcock ?
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Re: Mikuni VM 36
Reply #6 - 07/22/21 at 13:59:16
 
New fuel thingy (On, Res., Off). The original was vacuum activated.

Sad to say Yes, I have ridden it after many changes, but not all. I wait until the engine is at 180 degrees or higher (thermal gun) before adjusting the air/fuel mixture. I do this after ever change. Then ride most of the times but I can tell by just revving under no load, it's not right. Go for a ride and still the same (I've log the response more than 25 times, then quit.)

It works marvelous riding around town, no problem, but I never go pass 1/2 throttle (I have a template I taped to the throttle so I know where it's at. IT corresponds exactly with the slide inside).

When I hit the county road and roll over 1/2 throttle (and it is almost always half throttle) I get a HUGH bog (throws you hard forward) I back off and instantly I have power, a responsive throttle unless I roll it or crack it again pass 1/2, then dead bog it will kill the motor if I keep it there. (I'm not a young buck so I'm not jumping up and down on the throttle). If I feather it around 1/2 slightly back and forth I can gain speed, but no full on.

I've ridden it during the bog and open the choke (just for grins)  nothing, backfires. belches. Same response even with choke off.

Being it's mid-range, apparently I need a thinner jet needle. Not sure if it needs to be match to a particular needle jet or if I can try it with the same needle jet. I worked on car carburetors back in the day but first go around with Mikuni.

I have boo-coo hours trying to get this thing figured out. Could be beyond me OR it's a bad new carb, rare I agree, but it's still a possibility or something beyond my mechanical abilities, I choose the later.

Wife says, "Take it out behind the barn and shoot it!"  I think I would sell first, at least for parts.

Thanks-
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Re: Mikuni VM 36
Reply #7 - 07/22/21 at 15:12:40
 
Rider, whatever is going on has nothing to do with your idle circuit.  Since the beast falls flat on it's face (i.e. as if you turned a switch), the problem could be a grossly rich condition or a grossly lean condition.  Whatever happens shuts your engine "off", so you don't seem to be getting any of the telltale signs that indicate which way to go.  Too lean and it would surge, buck & pop-back through the carb.  Too rich and it would blubber and run slow.  Something is happening at 1/2 throttle that literally shuts it down.  Do I have that correct?

Your 6FJ6 is the standard needle delivered with the VM36, as are the Q5 needle jet and the 2.5 slide.  Ask Lancer if those are the components he delivers in his 36mm VM.  If Lancer agrees that those jets should work, then your needle jet, needle and slide should work.  The 190 main jet shouldn't come into play until your throttle is at least 3/4 open.  It also seems like a reasonable size to start with.

Note that your 6FJ6 needle is a compound needle.  The second taper starts at 35.2mm, mid-point between the D3 & D4 measurement points, right about 1/2 throttle.  That may be a valuable clue.

The main air bleed helps control when your main circuit starts to pick up.  If it is too small, or obstructed, it can cause the condition you describe.  Inspect the main air bleed.  Remove it.  Tell us what size it is.  Verify that the passage behind the main air bleed is not plugged.  Remove the needle jet and blow through the main air bleed passage.  Make sure the passage is not obstructed.

The main air bleed is located in the front of your carburetor.

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Re: Mikuni VM 36
Reply #8 - 07/22/21 at 15:16:40
 
Sometimes it's the simple stuff.  Please don't take offense.  Make sure you don't have a piece of rubbish in the float bowl drain plug.  A foreign object in the drain plug will get sucked up into the main jet every time fuel starts to flow through the jet.  It's a common problem on dirt bikes.  Junk gets into the bowl through the bowl vents.  It behaves exactly as you describe, give it throttle and it's as if you turned the switch off.
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Re: Mikuni VM 36
Reply #9 - 07/22/21 at 15:24:43
 
While you have the needle jet out, verify that the air bleed hole in the jet is not obstructed.  This hole ties into the main air bleed circuit.  If this hole is plugged, it's just like plugging up the main air bleed jet.  It will cause the main fuel circuit to pick up earlier than it should, and also result in an overly rich mixture.
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Re: Mikuni VM 36
Reply #10 - 07/23/21 at 11:05:23
 
Everything looks good BUT this is the first time I took the Air Jet out. Blew it out with compressed air and went smaller on the Main Jet (your description of rich symptoms seemed right on) and VIOLA, for the first time I got past 1/2 throttle but it seems lean backfiring coming to a stop sign. Need to order a few larger jets and I think I can get it going.

This has been a world of help from many...Mucho Gracias!!

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https://drive.google.com/drive/folders/1AkSKwbI6cHzObv9fpKHCnPe5F1UGvRa5?usp=sha
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Re: Mikuni VM 36
Reply #11 - 07/23/21 at 16:28:38
 
Don't assume backfiring means a lean mixture.  Keep turning for performance.  I'm running my bike a lot leaner than the guy I bought it from and my bike doesn't backfire anymore.

Also, you never stated you elevation.  That matters.
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Re: Mikuni VM 36
Reply #12 - 07/25/21 at 10:54:42
 
Thank you OhioMoto! I believe you are correct. After a hard acceleration I get backfire only then. I can easily tune that out. Still waiting on a few new jets, but its running good. May not even need them, what a relief. I'm just below 1k in altitude. I'll keep thumping.
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