carb: single mikuni issue (2011)

Status
Not open for further replies.
D

DOXFORD

Joined
Apr 26, 2007
Messages
96
Hello,
Running a single mikuni on my 750 commando. It was a new carb on install no issues. I have put about 10,000 miles on it. Today bike would fire up ok but would not stay running unless choke is on. Turn choke off bike stalls. Even when bike is warm still needs some choke to stay running. I just went 10 thou miles without this problem. Its my first mikuni and first time having to keep choke on to run. 10.000 miles later doesn't seem like much to me but my first thought would be dirty carb. If so is there a specific area of carb I should look out for or is an overall cleaning suffice? Anyone else experience same? Wondering how long other people go between carb cleaning and is the choke a common issue when carb gets dirty? I can't quite remember but I think I went about 60-70 thou on twin amals before a cleaning so this is new to me with a mikuni. Thanks.
Doxford
 
Re: carb

mine will die too until its warmed up - if it would run immediately after starting and then turning the choke off on your mikuni would suggest its not jetted right (to rich). while cleaning won't hurt - there are other variables

things we need to know =
make sure you have good gaskets on the manifold and the 4 allen screws than hold the manifold on the head are secure/tight (some people neglect the two inside screws)
also
what are the jets in it
is your bike a 750 or 850


can you shut the choke off after a minute or so and keep the bike running using the throttle? then you just need to try adjusting your idle/richness screws

oh and also i'm sure someone will tell you to buy a dual carb setup - followed by a trispark, etc etc :roll:
 
Re: carb

Mike - don't forget 90% of carb problems blah blah blah
 
The starter circuit onthe Mikuni is like a little mini-carburetor circuit--if the engine suddenly won't run except on the starter circuit, then something is clogged and not passing gas. It sounds like the pilot jet is clogged, since this occurs only upon low engine rpm with the throttle closed. The very first thing to do is to screw the airscrew in all the way and count the turns (probably 1 1/2 turns) and remember that, then remove the airscrew and shoot a little (very limited -- don't just blow a lot in there, with things still put together) jet of air and or very limited amount of carb cleaner, or both, in the hole, in the hopes of dislodging whatever is blocking things. Try that first, if it is fixed, okey dokey. If not, then next, drop the drain plug (17mm brass nut on bottom of float bowl), and look in it--it usually catches gunk at the bottom, that will tell you if you have rust etc. showing up. Then, take carb off, turn upside down, remove float bowl (4 phillips head screws at corners), look inside, again, this will also tell you if there is gunk floating around. The pilot jet is in its own boss next to the mainjet/needle jet holder, and requires a small screw driver to remove. Unscrew it and pull it out, look in it, blow it out with air and/or carb cleaner (don't run anything through it, like a wire, it will be ruined, and don't get carb cleaner on any o-rings, ditto), and if you cannot get it cleared completely, buy another one, same number, they are cheap. Also inspect the main jet at the same time, if there is crud in the pilot, it is probably also in the main. Then assemble in reverse order, making sure the airscrew is screwed in the amount it was initially, but it should be 1 1/2 turns or so. If you are overcome with initiative, you might pull the petcocks off and clean the screens on them, or depending on your fuel line arrangement, put a couple inline filters on, to keep this from happening. But hey, 10,000 miles before a problem is pretty good.
 
I agree with the previous post about the pilot jet. They often get plugged when a bike sits over the winter if the fuel is not drained. Often if you buy an old dirt bike that is not running well this is the problem.

To clean them often the only way is to poke the blockage out of the jet with a fine wire. This is a lot cheaper than buying a new jet and I have found that despite a lot of warnings from other people and books that it doesn't harm the jet. Just use a very thin wire and don't try to ream the hole out.
 
Very well said linker, and tricatcent, yes I too use a small wire to clean the Pilot jet, usually a brissle from a wire brush.
DOXFORD, I would recomend you clean the entire carb, it doesn't take long, simple to remove and replace, just make sure you get
Floats sitting properly and don't over tighten anything.
When blowing the Jets and passages out, turn the pressure down a bit on the compressor as more than once I've blown a jet out of my
fingers and spent time looking for it on the floor.
Find a nice clean, white Tea towel, from the kitchen and lay everything out on that.
I think by cleaning everything up your problem will be solved, however more than one person has actually created a problem with
gunk in a carby being blown from one place to another.
Pay attention to the two tiny holes in the rear of the throat where the slide would sit when closed. One is inside the line and one is outside the line.
They are bypass ports that allow Air to pass through the carb when the throttle is closed. As your engine was needing the choke to run, then they may
be blocked, as the choke (enricher circuit) has it's own air inlet port. (3 O' clock at the rear).
Anyway, clean them all up and away she'll go. Hopefully.
AC.
 
For goodness sake. You have only one carb that can be removed in 2 minuites.

Just pull the slide and leave it in place. Yank the carb, pull the bowl off. Unscrew the jetting along with the needle and seat. Spray it with cleaner and blow it out with some air. Re-assemble. Done!

Repeat every 10,000 miles.
 
pvisseriii said:
For goodness sake. You have only one carb that can be removed in 2 minuites.

Just pull the slide and leave it in place. Yank the carb, pull the bowl off. Unscrew the jetting along with the needle and seat. Spray it with cleaner and blow it out with some air. Re-assemble. Done!

Repeat every 10,000 miles.
Click to expand...

yes - sorry - i agree- i was under the assumption that you had just fitted the carb - see what i get when i'm sober and read the postings
 
Thanks for the ideas sounds like a simple enough problem and nothing a good cleaning wouldn't fix
live to ride.
Dox
 
While you have the carb off, put a wrench on the bolts that fasten the rubber intake (carb boot) to the head. I had one come loose and let a little bit extra air in. Would only run with choke on.

Ben
 
mikegray660 said:
yes - sorry - i agree- i was under the assumption that you had just fitted the carb - see what i get when i'm sober and read the postings
Click to expand...
I did that before every season. Bowl off jets out clean well. Put back in float bowl back on, good for another season.
 
linker48x said:
The starter circuit onthe Mikuni is like a little mini-carburetor circuit--if the engine suddenly won't run except on the starter circuit, then something is clogged and not passing gas. It sounds like the pilot jet is clogged, since this occurs only upon low engine rpm with the throttle closed. The very first thing to do is to screw the airscrew in all the way and count the turns (probably 1 1/2 turns) and remember that, then remove the airscrew and shoot a little (very limited -- don't just blow a lot in there, with things still put together) jet of air and or very limited amount of carb cleaner, or both, in the hole, in the hopes of dislodging whatever is blocking things. Try that first, if it is fixed, okey dokey. If not, then next, drop the drain plug (17mm brass nut on bottom of float bowl), and look in it--it usually catches gunk at the bottom, that will tell you if you have rust etc. showing up. Then, take carb off, turn upside down, remove float bowl (4 phillips head screws at corners), look inside, again, this will also tell you if there is gunk floating around. The pilot jet is in its own boss next to the mainjet/needle jet holder, and requires a small screw driver to remove. Unscrew it and pull it out, look in it, blow it out with air and/or carb cleaner (don't run anything through it, like a wire, it will be ruined, and don't get carb cleaner on any o-rings, ditto), and if you cannot get it cleared completely, buy another one, same number, they are cheap. Also inspect the main jet at the same time, if there is crud in the pilot, it is probably also in the main. Then assemble in reverse order, making sure the airscrew is screwed in the amount it was initially, but it should be 1 1/2 turns or so. If you are overcome with initiative, you might pull the petcocks off and clean the screens on them, or depending on your fuel line arrangement, put a couple inline filters on, to keep this from happening. But hey, 10,000 miles before a problem is pretty good.
Click to expand...
Not so much the Pilot Jet, but the passages it serves. The tiny drillings in the carb throat. I use a wire plucked from a steel wire brush, held with needle nose pliers. Also, the air passages in the intake horn must be cleared. The one leading to the main well, and smaller still, the one serving the pilot circuit.
I recently helped the neighbor kid resurrect an RD350 with VM Miks that had been "cleaned" by the DPO. I also recommend removing the needle jet, verifying it's cross passages are clear.
carb: single mikuni issue (2011)

Runs perfect now.
Be sure to VERIFY FLOW with an aerosol solvent (don't shoot your eye out kid).
 
replace with new twin amal 932s i had 34mm mikuni on my bike put the amals on made bike different machine smoother faster by far
 
I believe the rubber manifold is the culprit. Old and cracked or passing air around the joint surface.
 
Mine is around 10 years old with 16,000 miles on it. Never had a problem but I have recently noticed the rubber manifold adapter is starting to show cracks so will be replaced.
 
zotz said:
Mine is around 10 years old with 16,000 miles on it. Never had a problem but I have recently noticed the rubber manifold adapter is starting to show cracks so will be replaced.
Click to expand...
I tend to replace my Mikuni rubbers after 2 or 3 years!

They will last a fair bit longer, but I would rather change one too early!
 
Regarding the rubber mounting flanges, I've seen superficial cracking/checking on the outside stay that way for decades without continuing to degrade to all the way through.
 
Status
Not open for further replies.
Back
Top