Mikuni VM38 trouble
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
I still regret running out and buying the S&S.
Sell the S&S on Ebay.
If you price it at a discount compared to list price it will sell quickly. A lot of people think the sun rises and sets with S&S.
It don't, but put that to your advantage........... pg
Sell the S&S on Ebay.
If you price it at a discount compared to list price it will sell quickly. A lot of people think the sun rises and sets with S&S.
It don't, but put that to your advantage........... pg
Cruiser
Joined: Jun 2008
Posts: 200
Likes: 39
From: Assonet MA
PG, Thanks for the offer but I should be able to get my hands on one. If not I will hit you up. I will start by removing the spring and see where it gets me. Whats confusing me is when I have a decent idle, 1200-1300 RPM, and I just sit at idle she will suddenly falter and I have to grab the throttle to keep her going. It then idles OK for a couple of minutes and then repeats. Fuel flow issues? Do you think timing could be involved at all? You may remember my difficulties seeing the mark. When I did the static timing thing I was coming up on the large lobe when everyone says I should be on the small. Bike runs good other than the idle thing so I have to believe I am pretty close with the timing.
Thanks again..Bucaman
Thanks again..Bucaman
Cruiser
Joined: Jun 2008
Posts: 200
Likes: 39
From: Assonet MA
Danny my S&S experiences were somewhat different. Bike had a S&S on it when I bought it. My feelings from years ago are that they are too difficult to keep dialed in as well as a pain in the *** to start. When the Mikuni was giving me trouble I put the S&S back on and it ran as I know the S&S to run, but I had no idle issues which leads me back to the Mikuni.
With the ifo from PG I will take some comparison measurements but I can't imagine why the idle screw would ever be changed..... I am left to repeating myself I know....
I do remember when I first installed the VM everything seemed fine. All settings seemed to be as they should be. I had the bad advance adventure and all seemed to a bit crazy with the VM. Coincidence??
I will keep you all posted with my progress.
With the ifo from PG I will take some comparison measurements but I can't imagine why the idle screw would ever be changed..... I am left to repeating myself I know....
I do remember when I first installed the VM everything seemed fine. All settings seemed to be as they should be. I had the bad advance adventure and all seemed to a bit crazy with the VM. Coincidence??
I will keep you all posted with my progress.
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
PG, Thanks for the offer but I should be able to get my hands on one. If not I will hit you up. I will start by removing the spring and see where it gets me. Whats confusing me is when I have a decent idle, 1200-1300 RPM, and I just sit at idle she will suddenly falter and I have to grab the throttle to keep her going. It then idles OK for a couple of minutes and then repeats. Fuel flow issues? Do you think timing could be involved at all? You may remember my difficulties seeing the mark. When I did the static timing thing I was coming up on the large lobe when everyone says I should be on the small. Bike runs good other than the idle thing so I have to believe I am pretty close with the timing.
Thanks again..Bucaman
Thanks again..Bucaman
OK, you should be able to find a ring nut at a bicycle shop. Take the needle in with you to try it out for thread size. If not, I can fix you up.
The Mikuni should idle smoothly all day long without so much as a hiccup.
What you describe happening makes me think there may be some clogging in the idle and low speed air bleed. The fix for that is a good cleaning of the carb with carb chemical (immersion) in a bucket.
Here is some additional info on the area of the carb we are discussing. You may already have this but in case you do not I will attach it here..........
pg
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
Bucaman......
I forgot to add that I do not believe the timing has anything to do with the idle problem.
When it stumbles at idle it is either running low (lean) on the fuel in the fuel/air mix or running rich on the fuel in the fuel/air mix.
Since a quick blip of the throttle keeps the bike from stalling, I believe it is either too lean, or has clogged passages........ or both conditions.
Give the carb a good cleaning in a pot of carb cleaner. Rinse it out with solvent and blow dry the parts and passages. Then reassemble and see if there is any difference. Be sure to remove all plastic, rubber, gaskets, and O rings from the carb before submerging it in cleaner.
I am still at a loss regarding why the idle needle seems to be so short. I cannot get my mind around that one so far.
pg
I forgot to add that I do not believe the timing has anything to do with the idle problem.
When it stumbles at idle it is either running low (lean) on the fuel in the fuel/air mix or running rich on the fuel in the fuel/air mix.
Since a quick blip of the throttle keeps the bike from stalling, I believe it is either too lean, or has clogged passages........ or both conditions.
Give the carb a good cleaning in a pot of carb cleaner. Rinse it out with solvent and blow dry the parts and passages. Then reassemble and see if there is any difference. Be sure to remove all plastic, rubber, gaskets, and O rings from the carb before submerging it in cleaner.
I am still at a loss regarding why the idle needle seems to be so short. I cannot get my mind around that one so far.
pg
Cruiser
Joined: Jun 2008
Posts: 200
Likes: 39
From: Assonet MA
I do have it but thanks.
I've tried running lean but that does not seem to help, however if I give it just a bit of the enrichner lever it smooths out. Kind of like you would with a choke. Could I be that lean?
I'm leaning toward a blockage of some type. I guess I'll be giving it a good cleaning.
I've tried running lean but that does not seem to help, however if I give it just a bit of the enrichner lever it smooths out. Kind of like you would with a choke. Could I be that lean?
I'm leaning toward a blockage of some type. I guess I'll be giving it a good cleaning.
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
I do have it but thanks.
I've tried running lean but that does not seem to help, however if I give it just a bit of the enrichner lever it smooths out. Kind of like you would with a choke. Could I be that lean?
I'm leaning toward a blockage of some type. I guess I'll be giving it a good cleaning.
I've tried running lean but that does not seem to help, however if I give it just a bit of the enrichner lever it smooths out. Kind of like you would with a choke. Could I be that lean?
I'm leaning toward a blockage of some type. I guess I'll be giving it a good cleaning.
Give it a good cleaning and then we shall see if it makes any difference.
pg
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Road Warrior
Joined: Sep 2006
Posts: 1,605
Likes: 82
From: nj
I have a similar problem that my 84 ironhead has . When the bike warms up the idle goes up, when I put the bike in gear, let the clutch out and pull back in the clutch the idle goes back to normal. It seems when I put a load on her she is okay. If I let her sit in idle she is okay when I give her some gas is when I have this problem I have a s&s carb.The bike runs great and starts great but this problem is just getting to me. The cables are good and the throttle linkage comes back Thanks guys for some help
Stellar HDF Member
Joined: Feb 2006
Posts: 3,035
Likes: 17
From: Houston Texas
I have a similar problem that my 84 ironhead has . When the bike warms up the idle goes up, when I put the bike in gear, let the clutch out and pull back in the clutch the idle goes back to normal. It seems when I put a load on her she is okay. If I let her sit in idle she is okay when I give her some gas is when I have this problem I have a s&s carb.The bike runs great and starts great but this problem is just getting to me. The cables are good and the throttle linkage comes back Thanks guys for some help
Number 2...... You may have better results with starting a new thread regarding your problem with an S&S as the subject matter.
Reason being we are talking about a Mikuni here.
A Mikuni is a round slide carb and (I may be mistaken) I think an S&S is a butterfly type carb. At any rate, they are two different animals.
That said, I will try to help a little here.........
[When the bike warms up the idle goes up, when I put the bike in gear, let the clutch out and pull back in the clutch the idle goes back to normal. It seems when I put a load on her she is okay.]
That sounds normal. A cold engine will warm up and idle faster. Usually we start a cold engine with the choke on. As the engine warms up we take it off choke and the idle slows down.
So, just from your statement there, I would say your warmed up idle speed is too high. Putting it in gear (even holding in the clutch) puts a load on the engine and slows down the idle. So all that sounds normal.
Of course you would not be writing this if things were normal.
[If I let her sit in idle she is okay when I give her some gas is when I have this problem I have a s&s carb.The bike runs great and starts great but this problem is just getting to me. The cables are good and the throttle linkage comes back]
OK, so here is the can of worms. This almost sounds like the throttle cable is binding a bit.
As an example only.....If you were to take all of the slack out of the throttle cable, then start up the bike.
Let's say it idles perfectly.
Now turn your handlebars to one side or the other. Because there is no slack in the cable, the tightness causes the throttle to open just a slight amount. This results in a much faster idle speed.
Here is what I would look for..........
Again, I am assumming the S&S is a butterfly carb. If it is not then ignore me as I am all wet.
Look for the butterfly not fully returning when you give it some gas and then shut off the throttle. It won't take much to make the engine idle faster than it did before you gave it some throttle. Sometimes a spring used to keep the return pressure on the butterfly. Not being familiar with the carb, I cannot say this is the case.
Your throttle grip may be returning fully but it sounds like the inner cable may not be. It does not take much friction to keep a butterfly open just a bit and this makes an overly fast idle. So, from your description I would think there is some drag somewhere in your cable set up.........
pg