Mikuni VM36
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Stage IV
Joined: Aug 2010
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Mikuni VM36
First I would like to say that i have read a lot of your posts on here and they are all very informative. You guys seem to know what you are talking about and that is kind of what i need right now.
Just bought a Mikuni VM36 that came off of a '72 sporty 1000 that supposedly started well and ran well. I didn't realize that it was a VM36 when i bought it as I would have just found a VM38 instead. Regardless I have mounted it on the side of a '71 Sportster 883. I have kicked and kicked and kicked and all I can do is get it to sputter. I have played with settings and have gotten it to "want" to start, but that's about it. The jetting is all factory from what i can see. I have played with the air screw and the throttle screw. Seems best if the throttle screw is way in and air screw open 2 turns or so.
I have adjusted timing with a multimeter, set it to .018 and have adjusted the push rods as well. I have sealed the intake with the o-rings, taped with the stretchable self adhering tape and the clamped with stainless aircraft clamps. All of this said the Tillitson that I took off was a 1 to 2 kick start. Usually 1, so I don't think it is the timing or push rods. Unfortunately that is where the Tillitson stopped was at the start. Could never get it to run right.
All in all I would love to get this bike started again. I know it will, they always do. It is just getting it there that is the problem at this point. i am looking for any input you may have as I am at my wits end here. Thank you in advance!
Just bought a Mikuni VM36 that came off of a '72 sporty 1000 that supposedly started well and ran well. I didn't realize that it was a VM36 when i bought it as I would have just found a VM38 instead. Regardless I have mounted it on the side of a '71 Sportster 883. I have kicked and kicked and kicked and all I can do is get it to sputter. I have played with settings and have gotten it to "want" to start, but that's about it. The jetting is all factory from what i can see. I have played with the air screw and the throttle screw. Seems best if the throttle screw is way in and air screw open 2 turns or so.
I have adjusted timing with a multimeter, set it to .018 and have adjusted the push rods as well. I have sealed the intake with the o-rings, taped with the stretchable self adhering tape and the clamped with stainless aircraft clamps. All of this said the Tillitson that I took off was a 1 to 2 kick start. Usually 1, so I don't think it is the timing or push rods. Unfortunately that is where the Tillitson stopped was at the start. Could never get it to run right.
All in all I would love to get this bike started again. I know it will, they always do. It is just getting it there that is the problem at this point. i am looking for any input you may have as I am at my wits end here. Thank you in advance!
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
If the machine was a one to two kick bike with the Tillotson then with a Mikuni it should start as soon as you just look hard at the bike.
Seriously, let me ask you if you have a lever on your carb?
A Mikuni is a bit different than other carbs. It requires you to use the enricher (some incorrectly call this the choke) to start, even when the engine if fully warmed up. The enricher has a lever that must be applied to start the engine.
And speaking of referring to something incorrectly I just did myself. The Mikuni people actually call this item 'The Lever Type Starter System.'
Of course by now you may have the settings way off what they should be if you have been adjusting everything as you say.
The VM36 carb should have:
#310 main jet
#35 Pilot Jet
Q5 Needle Jet
6FJ6 Jet Needle
2.0 Air Jet
Make sure the Jet Needle has the clip in the middle notch. That is third from the top or third from the bottom.
Turn in the air screw until it bottoms, then back it out 1-1/2 turns.
Apply the lever start (I call this engaging the enricher, others call this applying the choke) and start the engine.
The Mikuni also is different because it is meant to be started using no throttle.
If you give it throttle most likely it is not going to start. That is why it uses a 'Lever Type Starter System.'
Try this procedure and see if it does not start the engine.
pg
Seriously, let me ask you if you have a lever on your carb?
A Mikuni is a bit different than other carbs. It requires you to use the enricher (some incorrectly call this the choke) to start, even when the engine if fully warmed up. The enricher has a lever that must be applied to start the engine.
And speaking of referring to something incorrectly I just did myself. The Mikuni people actually call this item 'The Lever Type Starter System.'
Of course by now you may have the settings way off what they should be if you have been adjusting everything as you say.
The VM36 carb should have:
#310 main jet
#35 Pilot Jet
Q5 Needle Jet
6FJ6 Jet Needle
2.0 Air Jet
Make sure the Jet Needle has the clip in the middle notch. That is third from the top or third from the bottom.
Turn in the air screw until it bottoms, then back it out 1-1/2 turns.
Apply the lever start (I call this engaging the enricher, others call this applying the choke) and start the engine.
The Mikuni also is different because it is meant to be started using no throttle.
If you give it throttle most likely it is not going to start. That is why it uses a 'Lever Type Starter System.'
Try this procedure and see if it does not start the engine.
pg
Thread Starter
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Stage IV
Joined: Aug 2010
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Pinion,
Thank you for your prompt response. The jets are all as described above and the carb does have an enricher system to provide the necessary fuel for starting. The process I have used to start it is to push enricher lever down, turn fuel on, turn key on, leave throttle closed and kick the bike until I either slip off the kicking lever and hyperextend my leg, or come down and cath the back of my left knee on the shock mount. Either of those occurances will bring an end to the kicking!
Where should the throttle/idle adjuster screw be set to start? Or is this something that I will need to play with for my particular bike until it starts? Thanks again for the feedback!
Thank you for your prompt response. The jets are all as described above and the carb does have an enricher system to provide the necessary fuel for starting. The process I have used to start it is to push enricher lever down, turn fuel on, turn key on, leave throttle closed and kick the bike until I either slip off the kicking lever and hyperextend my leg, or come down and cath the back of my left knee on the shock mount. Either of those occurances will bring an end to the kicking!
Where should the throttle/idle adjuster screw be set to start? Or is this something that I will need to play with for my particular bike until it starts? Thanks again for the feedback!
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
OK, sounds like you have the starting procedure down pat.
Turn in the air screw until it lightly bottoms. Then turn it out 1-1/2 turns. This will be close enough to get the engine started. You never want to turn this screw further out than 3 turns.
If the engine will not start at this point, then I would remove the carb and take it apart.
Give it a complete chemical cleaning (let it soak overnight) with carb cleaner. Most likely the small passages in the starting system have clogged up. It does not take much to have this happen.
Then reassemble the carb and see if it will start. It should.
pg
Turn in the air screw until it lightly bottoms. Then turn it out 1-1/2 turns. This will be close enough to get the engine started. You never want to turn this screw further out than 3 turns.
If the engine will not start at this point, then I would remove the carb and take it apart.
Give it a complete chemical cleaning (let it soak overnight) with carb cleaner. Most likely the small passages in the starting system have clogged up. It does not take much to have this happen.
Then reassemble the carb and see if it will start. It should.
pg
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Stage IV
Joined: Aug 2010
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Well I got the bike to start. Soaked the carb, compressed aired all the passages etc. I also realized why it wouldn't start. It had nothing to do with the carv, but moreso the intake valve that was stuck open thanks to a nut the fell into the intake unknowingly. That was a first for me and hopefully a last. Now that it starts its hard to set the idle. It either idles at like 1500 or falls down to around 750 and hangs there. I have tried playing with the throttle needle to raise the slide but like I said. I either get it to jump up or stay down. Kinda confused. The bike is responsive when you snap the throttle back so the other systems on the carb o running good. The needle is preetty far in too. Almost bottomed out on the spring. Any help would be appreciated. Also what kind of plugs are you guys running? Just curious. Thanks again!
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
Well I got the bike to start. Soaked the carb, compressed aired all the passages etc. I also realized why it wouldn't start. It had nothing to do with the carv, but moreso the intake valve that was stuck open thanks to a nut the fell into the intake unknowingly. That was a first for me and hopefully a last. Now that it starts its hard to set the idle. It either idles at like 1500 or falls down to around 750 and hangs there. I have tried playing with the throttle needle to raise the slide but like I said. I either get it to jump up or stay down. Kinda confused. The bike is responsive when you snap the throttle back so the other systems on the carb o running good. The needle is preetty far in too. Almost bottomed out on the spring. Any help would be appreciated. Also what kind of plugs are you guys running? Just curious. Thanks again!
As far as the idle goes, you have to set the fuel/air mixture screw first before you can obtain a decent idle.
I do not understand your explanation of what you are doing to cause the rpm to run high (1500) or what it is that you do to lower the rpm to 750.
If you are playing around with different notches on the needle, then that is doing you no good.
The clip should be in the middle notch, period.
The tapered needle end comes up out of the main jet as you raise the slide. This transitions from the low speed (idle) to high speed (wide open) stages of throttle opening.
You should be able to reduce the idle speed of 1500 rpm to something lower by simply unscrewing the idle speed screw. This screw does nothing but raises and lowers the slide from it's full closed position.
Once you get the bike to idle at about 900 to 1000 rpm you can then adjust the fuel/air mixture screw. It is set right when it causes the idle speed to be the highest.
Once you do this the first time, you will have to readjust the idle speed down to 900 and then do the fuel/air adjustment one more time.
pg
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Stage IV
Joined: Aug 2010
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I checked that the valve moved freely and was seating correwctly and fully closing in its seat. I also checked that the valve and seat weren't damaged before I put the head and rocker covers back on.
As for the needle, I guess I am using the wrong term. The needle has the clip in the enter, haven't touched it. I was refering to the idle screw. Turning that even slowly I can't get it to set at 1000 or 900 where I want it. It sems to either go high or drop low. Doesn't make sense to me. Doesn't seem to be a middle point for me. Air screw is set to 1.5 turns out.
As for the needle, I guess I am using the wrong term. The needle has the clip in the enter, haven't touched it. I was refering to the idle screw. Turning that even slowly I can't get it to set at 1000 or 900 where I want it. It sems to either go high or drop low. Doesn't make sense to me. Doesn't seem to be a middle point for me. Air screw is set to 1.5 turns out.
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Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
OK, here is a pic of my VM38. Yours may be a bit different.......
The pencil points to the fuel/air mixture screw.
The larger screw to the right is the idle adjustment screw.
The idle screw screws in and as it does so the needle rids on a sloped surface of the slide and this forces the slide upward, raising the idle speed.
Pic of the idle screw out of the carb and of the slide.....
So, 1-1/2 turns on the mixture screw sounds like it is close enough to me.
The fact that the idle varies tells me you have an air leak at the manifold O rings.
Especially since you have had the carb off the bike. Additionally, you need to have a carb brace on the carb to keep it from moving.
This (not having the brace) will cause an air leak in the O rings.
So check the O rings.
I believe that is where your trouble is.
pg
The pencil points to the fuel/air mixture screw.
The larger screw to the right is the idle adjustment screw.
The idle screw screws in and as it does so the needle rids on a sloped surface of the slide and this forces the slide upward, raising the idle speed.
Pic of the idle screw out of the carb and of the slide.....
So, 1-1/2 turns on the mixture screw sounds like it is close enough to me.
The fact that the idle varies tells me you have an air leak at the manifold O rings.
Especially since you have had the carb off the bike. Additionally, you need to have a carb brace on the carb to keep it from moving.
This (not having the brace) will cause an air leak in the O rings.
So check the O rings.
I believe that is where your trouble is.
pg
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