Mikuni 38MM Round Slide Carburetor
#1
02-02-2014, 10:11 PM
Mikuni 38MM Round Slide Carburetor
Okay on my bike I have a Mikuni 38MM Round Slide Carburetor. I need to know which position you have the lever set to enrich the fuel to have it in run mode. I have to have mine in the down position to have the bike running good. As soon as I place this lever in the up position my engine bog's out and shuts off. If I push down on this lever the bike starts okay and runs fine. Is the engine being enriched to much when this lever is in the up position or is one of the jets plugged up in this carburetor or is it running as it should with this lever in the down position. It throttles up fine and runs okay when this lever is in the down position.
#2
02-03-2014, 07:10 AM
I have had the Mikuni VM38-9 carb on my IronHead for a full year.
The lever needs to be in the down position to start the engine, and if it is very cold, for only the first minute or so of running. If the warmed up engine will not run with the lever in the up position then there are plugged jets or fuel/air passages that need to be cleaned.
Here is a stand for working on the carb on the bench - a piece of angle aluminum and a bench vice. You need two free hands when working on this carb so a stand is necessary. I never work on a carb while it is installed on the bike ...
Be very careful, with the bowl off the carb, to not disturb the level of the two floats. It is said to be critical and very difficult to re-set [unlike other carbs]. I use this setup to check the level first thing when i have the bowl off, and last thing before i re-install. Measure from the blade to the moving part, for both floats ...
When mine was new out of the box one of the floats would stick as in this pic. I had to carefully bend the rod until it would no longer stick. Do not remove the part that keeps the float from coming off as it is virtually impossible to re-install ...
For removing/reinstalling the jets use exact fitting flat blade screwdrivers. You may have to grind them to fit. They do not have to be re-installed "gorilla tight"; that only wrecks them.
The best [only!] tool i found for cleaning the tiny passages is the one used to air up footballs. I had to scramble to find a way to hook it up to my air compressor, but i did find the parts at Canadian Tire. Probably find them at any Harbor Freight of sporting goods store. Small holes like these ...
The smallest and most difficult to reach holes are in the venturi, at the engine end. Sorry, no pics of those. But they are very tiny, very difficult to reach [need that football tool], and very important that you be completely confident that they are completely clear. It took me a few tries before i was confident.
Here is the tuning manual ...
http://www.mikuni.com/pdf/vmmanual.pdf
Note the 3 air jets that you can see in the front cover pic. These must be clear. You will need to be certain that these are also all clear [see diagram on third page] ...
starter jet
air screw passage
needle jet
main jet
pilot jet
pilot outlet
by-pass
The last two are two of the tiny holes at the back end of the venturi.
After doing all this, do not re-install the carb unless you are completely confident that the fuel tank is free of all rust, particles, etc; and that the fuel filter and fuel line are clear - this is most likely the real problem. A good idea to install a new fuel line as they may break down internally when old.
Take your time, have fun, it is the best carb for an IronHead.
The lever needs to be in the down position to start the engine, and if it is very cold, for only the first minute or so of running. If the warmed up engine will not run with the lever in the up position then there are plugged jets or fuel/air passages that need to be cleaned.
Here is a stand for working on the carb on the bench - a piece of angle aluminum and a bench vice. You need two free hands when working on this carb so a stand is necessary. I never work on a carb while it is installed on the bike ...
Be very careful, with the bowl off the carb, to not disturb the level of the two floats. It is said to be critical and very difficult to re-set [unlike other carbs]. I use this setup to check the level first thing when i have the bowl off, and last thing before i re-install. Measure from the blade to the moving part, for both floats ...
When mine was new out of the box one of the floats would stick as in this pic. I had to carefully bend the rod until it would no longer stick. Do not remove the part that keeps the float from coming off as it is virtually impossible to re-install ...
For removing/reinstalling the jets use exact fitting flat blade screwdrivers. You may have to grind them to fit. They do not have to be re-installed "gorilla tight"; that only wrecks them.
The best [only!] tool i found for cleaning the tiny passages is the one used to air up footballs. I had to scramble to find a way to hook it up to my air compressor, but i did find the parts at Canadian Tire. Probably find them at any Harbor Freight of sporting goods store. Small holes like these ...
The smallest and most difficult to reach holes are in the venturi, at the engine end. Sorry, no pics of those. But they are very tiny, very difficult to reach [need that football tool], and very important that you be completely confident that they are completely clear. It took me a few tries before i was confident.
Here is the tuning manual ...
http://www.mikuni.com/pdf/vmmanual.pdf
Note the 3 air jets that you can see in the front cover pic. These must be clear. You will need to be certain that these are also all clear [see diagram on third page] ...
starter jet
air screw passage
needle jet
main jet
pilot jet
pilot outlet
by-pass
The last two are two of the tiny holes at the back end of the venturi.
After doing all this, do not re-install the carb unless you are completely confident that the fuel tank is free of all rust, particles, etc; and that the fuel filter and fuel line are clear - this is most likely the real problem. A good idea to install a new fuel line as they may break down internally when old.
Take your time, have fun, it is the best carb for an IronHead.
Last edited by IronMick; 02-03-2014 at 07:19 AM.
#4
02-03-2014, 02:21 PM
Good point. Here are some notes. Do these first ...
Intake And Exhaust Leaks Tests
You cannot get a good tuneup if there are intake leaks or exhaust leaks.
1. Check for Intake Leaks
With the engine idling spray propane gas or WD-40 around the carb/intake/cylinder head joints. Propane is best because it leaves no mess, and is most easily sucked in thru leaks. WD-40 is said to work well because it has very fine spray droplets. Be careful to not allow any of either to get sucked in thru the A/C as this will invalidate the results.
Any change in engine RPM is a leak which must be fixed.
EDIT: Experience is that this procedure will identify larger intake leaks, but it may miss very small leaks that also must be fixed. And that propane is definetly the better choice.
EDIT: You have to do a lot of spraying to detect a small leak. Propane will be really clean. Anything else like WD-40, carb cleaner, etc will make a huge mess.
2. Check for Exhaust Leaks
Pour about 1/2 ounce of oil into each cylinder thru the spark plug holes. Run the engine at idle. You will get lots of smoke for about 5 minutes.
All smoke should come from the ends of the exhaust pipes/mufflers. Any other is a leak which must be fixed.
Intake And Exhaust Leaks Tests
You cannot get a good tuneup if there are intake leaks or exhaust leaks.
1. Check for Intake Leaks
With the engine idling spray propane gas or WD-40 around the carb/intake/cylinder head joints. Propane is best because it leaves no mess, and is most easily sucked in thru leaks. WD-40 is said to work well because it has very fine spray droplets. Be careful to not allow any of either to get sucked in thru the A/C as this will invalidate the results.
Any change in engine RPM is a leak which must be fixed.
EDIT: Experience is that this procedure will identify larger intake leaks, but it may miss very small leaks that also must be fixed. And that propane is definetly the better choice.
EDIT: You have to do a lot of spraying to detect a small leak. Propane will be really clean. Anything else like WD-40, carb cleaner, etc will make a huge mess.
2. Check for Exhaust Leaks
Pour about 1/2 ounce of oil into each cylinder thru the spark plug holes. Run the engine at idle. You will get lots of smoke for about 5 minutes.
All smoke should come from the ends of the exhaust pipes/mufflers. Any other is a leak which must be fixed.
#5
02-05-2014, 04:36 AM
Liquids don't compress
Good point. Here are some notes. Do these first ...
Intake And Exhaust Leaks Tests
You cannot get a good tuneup if there are intake leaks or exhaust leaks.
1. Check for Intake Leaks
With the engine idling spray propane gas or WD-40 around the carb/intake/cylinder head joints. Propane is best because it leaves no mess, and is most easily sucked in thru leaks. WD-40 is said to work well because it has very fine spray droplets. Be careful to not allow any of either to get sucked in thru the A/C as this will invalidate the results.
Any change in engine RPM is a leak which must be fixed.
EDIT: Experience is that this procedure will identify larger intake leaks, but it may miss very small leaks that also must be fixed. And that propane is definetly the better choice.
EDIT: You have to do a lot of spraying to detect a small leak. Propane will be really clean. Anything else like WD-40, carb cleaner, etc will make a huge mess.
2. Check for Exhaust Leaks
Pour about 1/2 ounce of oil into each cylinder thru the spark plug holes. Run the engine at idle. You will get lots of smoke for about 5 minutes.
All smoke should come from the ends of the exhaust pipes/mufflers. Any other is a leak which must be fixed.
Intake And Exhaust Leaks Tests
You cannot get a good tuneup if there are intake leaks or exhaust leaks.
1. Check for Intake Leaks
With the engine idling spray propane gas or WD-40 around the carb/intake/cylinder head joints. Propane is best because it leaves no mess, and is most easily sucked in thru leaks. WD-40 is said to work well because it has very fine spray droplets. Be careful to not allow any of either to get sucked in thru the A/C as this will invalidate the results.
Any change in engine RPM is a leak which must be fixed.
EDIT: Experience is that this procedure will identify larger intake leaks, but it may miss very small leaks that also must be fixed. And that propane is definetly the better choice.
EDIT: You have to do a lot of spraying to detect a small leak. Propane will be really clean. Anything else like WD-40, carb cleaner, etc will make a huge mess.
2. Check for Exhaust Leaks
Pour about 1/2 ounce of oil into each cylinder thru the spark plug holes. Run the engine at idle. You will get lots of smoke for about 5 minutes.
All smoke should come from the ends of the exhaust pipes/mufflers. Any other is a leak which must be fixed.
Really?
Pour oil into the cylinders and run at idle?
The motor will hydro, liquids don't compress and something expensive, will bend or break. A 1/2 ounce isn't much but some might read it as 1-2 ounces( more is better right?) and if it's a high compression motor, at the least it will become a low compression one because of bent rods, worst is the motor is scattered all over the garage.
Maybe 1 more EDIT?
Now I have put oil in the cylinders for extended storage, but I hand cranked the motor, with the spark plugs removed, to make sure nothing is in the cylinders and I removed the porcelain and electrode from the spark plugs put them in the engine with fuel hose from the hollowed out plugs into a bucket(to not make a mess) and cranked the motor with the starter to make sure no fluids are in the cylinders.
My 2 cents
Sledbum
#6
02-05-2014, 07:52 AM
Seasoned HDF Member
Really?
Pour oil into the cylinders and run at idle?
The motor will hydro, liquids don't compress and something expensive, will bend or break. A 1/2 ounce isn't much but some might read it as 1-2 ounces( more is better right?) and if it's a high compression motor, at the least it will become a low compression one because of bent rods, worst is the motor is scattered all over the garage.
Maybe 1 more EDIT?
Now I have put oil in the cylinders for extended storage, but I hand cranked the motor, with the spark plugs removed, to make sure nothing is in the cylinders and I removed the porcelain and electrode from the spark plugs put them in the engine with fuel hose from the hollowed out plugs into a bucket(to not make a mess) and cranked the motor with the starter to make sure no fluids are in the cylinders.
My 2 cents
Sledbum
Pour oil into the cylinders and run at idle?
The motor will hydro, liquids don't compress and something expensive, will bend or break. A 1/2 ounce isn't much but some might read it as 1-2 ounces( more is better right?) and if it's a high compression motor, at the least it will become a low compression one because of bent rods, worst is the motor is scattered all over the garage.
Maybe 1 more EDIT?
Now I have put oil in the cylinders for extended storage, but I hand cranked the motor, with the spark plugs removed, to make sure nothing is in the cylinders and I removed the porcelain and electrode from the spark plugs put them in the engine with fuel hose from the hollowed out plugs into a bucket(to not make a mess) and cranked the motor with the starter to make sure no fluids are in the cylinders.
My 2 cents
Sledbum
the guy in question works on his Iron head and the oil trick is not a new one, and i am sure he gets the drift of what to do, its been done many times by many people - having the correct intake and exhaust track tuning is one way to make sure you are in top tune - Mick has volumes of excellent / to the point - correct advise - all the guys on this iron head thread wrench sooooo
chill dude -- johnjzjz
#7
02-05-2014, 01:51 PM
Hi Sledbum, i understand the principle but it is not an issue with this technique. I have had my IronHead for 10 years and have been an IronHead forum moderator [not this one] for at least 7 years. Myself and others have been using this technique for a long time, many for much longer than myself. No one has ever reported this being a problem.
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#8
02-05-2014, 05:03 PM
Hi Mick and zj
Just watching out for the 'newb's and 'I saw it on the internet' types that just don't have a clue or can't see the 'big picture'.
During high school worked at the corner gas station and in the springtime when a customer brought his original '48 chevy in to have the winter air removed and summer air put in, the laughing shop owners 'ran away' to gofer parts or some other lame excuse and said take care of the it. So I did, and talking with the old timer he said he's been doing it every year since new. I also told the customer I adjusted the 'galloping pin' and flushed the 'trotting valve' and he was tickled pink with the extra service all free gratis because this was back when gas was less than 4 bits a gallon. when the shop owners finally reappeared we all had a good laugh. Too each his own I guess.
I'm seriously thinking of starting up a new company called 'Thick Skin'. Don't sweat the small stuff, get over it, ya know!
Now putting on my PC hat, didn't mean to slam the 'experienced vets' just looking out for the newb's.
That being said, I recently acquired an '80ish sportster that was sitting behind the garage, last registered '89. The 'fluid' drained from a nearly full gas tank could pass for some super paint remover. Changing rest of the fluids, nut and bolt it, battery actually had 6 volts date code was 6. Don't know if it was '86, '96 or ???? Going to freshen up the carb etc. Hope I can 'bend an ear' from some of you vets should I have a question. Just dawned on me, this bike is about as old as my youngest kid.
Take care and Have Fun
s
Just watching out for the 'newb's and 'I saw it on the internet' types that just don't have a clue or can't see the 'big picture'.
During high school worked at the corner gas station and in the springtime when a customer brought his original '48 chevy in to have the winter air removed and summer air put in, the laughing shop owners 'ran away' to gofer parts or some other lame excuse and said take care of the it. So I did, and talking with the old timer he said he's been doing it every year since new. I also told the customer I adjusted the 'galloping pin' and flushed the 'trotting valve' and he was tickled pink with the extra service all free gratis because this was back when gas was less than 4 bits a gallon. when the shop owners finally reappeared we all had a good laugh. Too each his own I guess.
I'm seriously thinking of starting up a new company called 'Thick Skin'. Don't sweat the small stuff, get over it, ya know!
Now putting on my PC hat, didn't mean to slam the 'experienced vets' just looking out for the newb's.
That being said, I recently acquired an '80ish sportster that was sitting behind the garage, last registered '89. The 'fluid' drained from a nearly full gas tank could pass for some super paint remover. Changing rest of the fluids, nut and bolt it, battery actually had 6 volts date code was 6. Don't know if it was '86, '96 or ???? Going to freshen up the carb etc. Hope I can 'bend an ear' from some of you vets should I have a question. Just dawned on me, this bike is about as old as my youngest kid.
Take care and Have Fun
s
#9
02-05-2014, 07:28 PM
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