Mikuni VM - setup - does not take more than 1/4 throttle

I have a 1980 Suzuki TS 125 with a Mikuni VM type 'round' carburetor.
It did not start this spring, so I took the carb out and apart, and
cleaned it complete (never been done).

The bike now starts (after lots of trying, ultimately tow starting it
by pulling it with a car).

But in only takes 1/4 throttle, than chokes and would die if not
closing the throttle immidiately.

I had tightened all interal screws, and no start to think that there
may be a mix screw inside that adjust the air/fuel mix.

a) can someone please let me know if that is the case?
b) what screw it is?
c) how to set it to get the bike going again

Note: it is a huge pain in the butt to take the carb out and get the
boot to the airbox back on, so I only want to do that again only once,
if possible.

Thanks.

 

No shit ;-)

Oh dear, another one.....

A pound to a penny:

(a) that you haven't cleaned out the carb properly - really, the way
modern fuel gus, you need a lot of cleaner and a good source of HP air.
Or an ultrasonic cleaning tank.

(b) That you've screwed the pilot adjuster screw right in. Possibly the
tickover screw too.

Get a manual and read it. Or get someone who knows what they're doing.

 

It's probably something wrong with the needle or needle jet, they are
important at around 1/4 to 3/4 throttle. You need to take the carb
apart again.

Leon

 

I have an old 82 PE 175 with the same type carb. You can leave the
airbox off until you get the bike running properly. My bike didnt even
have one and it ran for years without it. When you get the airbox back
on, you may have to adjust the air screw.

There isnt too much to the carb. Needle, seat, float and that round
brass piece with the holes in it with the hexagon nut (forget what its
called) should be all thats inside. When you took the carb apart, did
you also remove the air intake screw and idle screw? If so than thats
what the problem is if its not the needle and seat. I believe if you
screw them all the way in than back out a 1/4 turn or so, that should
be it.

However, if you had to bump start it, another guess would be the float
is heavier than it used to be causing little gas to get into the
chamber, than when you hit the gas, too little gas flows back into the
bowl causing to choke for lack of fuel. The needle could also be
messed up, shooting way too much gas soaking the plug. When it is
running, see if theres gas dripping from a connection point on the
baffle (if there is a semi-loose connection) just as it comes off the
engine. Kinda like the drain pipe under your sink. That means too
much gas and your needles messed up. Look at the tip of the needle.
If it has a ring around close to the tip, than replace it. In fact
while youre at it, get a new seat (usually comes with the needle) and a
new float.

Hope this answer is better for you than go buy a book or have someone
else do it.....

 

The Mikuni VM tuning manual is available on the Web -- I know because
I grabbed a copy last year while trying to sort a DT175MX. It's a 1.2 MB pdf
file; don't recall how hard it was to find...

Ah, here it is (and many more):

http://www.mikuni.com/c-manuals.html

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD "You Porsche. Me pass!" DoD #484 JKLO#003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

On VM carbs, the the slot-headed idle mixture screw is right there on
the side of the carburetor, just upstream of the slide.

The other screw (it may have a knurled knob so you can turn it with
your thumb and forefinger) will move the slide up or down a little to
adjust the idle speed, but the idle mixture screw is an AIR screw.

It controls the amout of AIR in the idle mixture. So turning it
clockwise decreases the air in the idle mixture and makes the idle
mixture richer.

The correct setting is usually about 1.0 to 1.5 full turns
counterclockwise from the fully seated position.

But you may want to completely remove the idle mixture screw and spray
carburetor cleaner through the passageways to get the gum and varnish
out.

 

Thanks chateau;

your contribution to the group is very helpful (duh!).

--> well, that's what YOU are supposed to be

If we just had more like you (then user groups would be extinct in no
time).

volker

 

Update:
The Suzuki starts at the first kick (just can't use any throttle or
choke).
Keeps taking only 1/4 throttle, then chokes.

Volker

 

Spongehead,

thanks for the constructive suggestions (unlike mr. smartypants).

--> How can I identify the "air intake screw and idle screw"

--> that was just a means to get it going. It now starts on the first
kick. It does not take ANY choke or throttle (otherwise it fouls up
quickly)

--> The first times I tried to start the bike the normal way (some
choke), but flooded it (spark splug totally wet, smell gas, gas
dripping out of the cylinder/exhaust). Now that I know, it starts well,
no throttle, no choke. Idle is setup well, and runs well at idle. Could
it be it is the opposite, meaning too rich, too much gas?

--> Not sure where to look for that connection point, and the baffle.

--> the needle has no marks

--> You are hit it on the head. Thanks for the help.

 

Dear FB:

--> ok, found it (I took it all the way out once, and there was just a
hole to the slide).

--> got this one. the know is knurled. I use it to set the idle at the
desired speed; the conical portion of that screw moves the slide
up/down.

--> I will report back soon. Is the screw on the slide staying in its
partly loose condition all by itself? (Thats why I tightened it in the
first place, afraid that it falls out).

--> Carb was taken apart completely and washed with lots of cleaner.
All build up, varnish, etc. removed nicely.

 

Maybe that is your whole problem? You tightened the idle mixture screw
and the engine isn't getting idle mix?

There should be a small spring on the idle mixture screw. It maintains
tension on the screw.

I leaned up the idle mixture so much on one motorbike I feared the
screws might fall out, so I put a drop of 3M weatherstripping cement
(gorilla snot) on top of each screw to be sure it wouldn't vibrate out
and get lost.

One weird situation I ran into was when the gasoline evaporated in the
float bowl of a motocrosser. It used pre-mix and the oil had a green
dye to indicate that there was oil
in the premix. The green dye formed a solid clump in the main jet.

It's hard to see gum and varnish inside the internal passages. The VM
carb sucks idle
air in through a pilot air jet in the mouth of the carburetor. It sucks
idle fuel in through the pilot jet in the float bowl. The two flows mix
downstream of the idle mixtue screw and they discharge through about
two small ports just downstream of the throttle slide.

It sounds to me from your description that the engine is starving for
fuel in the transition from idle to acceleration mode. The VM
carburetor uses a cutaway slide to help the engine transition from idle
to acceleration. It probably has the right slide and you can't put the
slide in backwards.

The jet needle is attached to the slide. The jet needle has most of its
effect from half throttle to full throttle, and the idle jet is
supplying about half of the needed fuel to the engine up to half
throttle.

As you open the throttle more and more, vacuum downstream of the slide
drops off and the flow of fuel through the idle jet decreases to
nothing at full throttle.

As the slide opens, more and more fuel flows through the jet
needle/needle jet combination. The JN/NJ is a variable orifice that
works in the mid-range of throttle slide position.

The diameter of the NJ has a lot of effect from 1/8th throttle on to
fully open. There are two things that can go wrong with the NJ. One is
that the JN bangs against the hole in the NJ and makes it egg-shaped.
That causes the mixture to be too rich.

The other thing that can happen is that the cross-drilled air emulsion
holes in the NJ get plugged up with gum and varnish. There are two type
of NJ. One is the air bleed type with the emulsion holes, the other is
the "primary type" NJ, which has no holes.

The NJ can usually be removed from VM carbs by pushing it up through
the hole where the slide goes, or pushing it down towards the float
bowl side. There may be a rubber o-ring on the NJ.

The idle jet probably has air emulsion holes, too. Mikuni was probably
the first motorcycle carburetor company to add emulsion holes to their
NJ's and the idle jets.

The holes help mix fuel and air and provide better low speed throttle
response.

If you have to remove the idle jet for cleaning, be sure to use a slot
screwdriver that fits the jet perfectly, otherwise you can damage it.

 

Well, you evidently don't know what you're doing, and I pointed out that
screwing all the screws in is *not* the way to go about it.

Also, as has been discussed many, many times before, with modern fuel
going 'off', what you think is a good carb clean isn't.

The potential of you getting it wrong again is evidently large, so the
advice to get someone along who knows what they're doing is good advice.

You've been pointed in the direction of a Mikuni manual, and FB has
given some good advice.

You need to know size of main jet, size of pilot jet (both may have been
changed in the last quarter-century), size of needle, needle setting on
the carb clip (there are usually three or four settings), float height,
pilot adjuster screw setting (one and a quarter turns out is usually a
good starting point).....

If all those are correct, and the carb is properly cleaned, then it
should be OK.

If you don't know how to check all that - like I said, get a manual and
read it or get somewone who knows what they're doing.

There's no shame in ignorance. But mucking around with something when
you know you're ignorant is stupidity.

 

Well *shame* on Volker, for daring to ask for help, then! ;-)

What do you do when a lost motorist asks for directions, do you tell
him he's a fool for getting out of bed and getting lost in a strange
town?

He doesn't need to know any of that shit, except for the starting
setting of the idle screw. He's not trying to retune the carb for
competition, he's just trying to get the friggin' engine to run when he
opens the throttle past 1/4.

How else does a person learn, if not by his mistakes or by asking
knowledgeable people intelligent questions? You're slamming both doors
in Volker's face, apparently for no other reason than to assert your
own "superiority".

The whole idea of rec.motorcycles.tech is that a newbie asks more
experienced riders technical questions about production motorcycles.

The NG is not supposed to be a forum for debating suitability of this
machine over that machine or establishing a pecking order of who is the
most brilliant being on the planet and who kisses yahoo butt.

If we wanted to read such profitless debate, we'd go
to rec.motorcycles or alt.motorcycle and play with the Canadians.

 

Trying to diagnose a mystery carb problem over the internet is like
asking someone to hold their phone next to a running engine to diagnose
what the rattle is.

Yes, he *does* need to know, because with a 25 year-old two-stroke there
is a fair chance that someone has mucked about with it in the last
couple of decades.

OK, it could be an all-original unmessed-with bike, but the odds are
against it. If it was running perfectly before he took the carb off (the
OP doesn't say), then yeah, maybe you don't need to check the right jets
and needle are in it - but I'd still recommend checking the float height
and needle setting, if he dismantled everything to clean it. If you
disagree, please tell us why.

I've bought bikes with carb needles on the wrong setting, non-standard
jets and incorrect float heights. I've bought bikes that have "just been
serviced" with *everything * wrong with them.

Let me tell you a story: two years ago I bought an old GT380 triple off
Ebay. Looked utterly stock, ran very sweetly, etc etc. But it was
choking badly above about 6500rpm: seemed to be running rich. First port
of call was the air filtration. Stock airbox, stock filter cores, but
wrapped around them was a very strange foam indeed.

It was carpet underlay. Fucking carpet foam. Closed-cell foam. It was a
miracle the engine even ran at all. The butchery people inflict on bikes
defies belief.

Where does it say that in the charter? You're talking utter bilge.

Where does it say that in the charter?

Look, you're knowledgeable - you draw laughingly wrong conclusions from
time to time ('post hoc ergo propter hoc' could be your motto), but
ne'er mind - but AFAIC there is a golden rule with machinery.

If you don't what you're doing, don't do it.

And getting in someone who knows, to show you how it ought to be done,
is the best way of learning. If you think my recommendation to that
effect is wrong - well, there we differ.

There is no substitute for hands-on experience. Asking on usenet is
often a help, but you're the man who gave that CG125 poster reams of
utterly useless advice about a carb that his bike wasn't even fitted
with.....

 

FB: the idle screw has the spring and is working fine. I can adjust the
idle (and it runs well in idle, from very low RPM to high RPM idle
speed.

There is one more screw on the outside of the slide. This one is just a
screw with a seal ring. This is the one I thought you referred to as
the air screw. I tried to back it out one turn, but that does not seem
to change the behaviour of the motor (dying if more than 1/4 throttle
is used). This is also the one I would be concerned to leave in a not
tighten position, because nothing would prevent it from falling out. I
have no idea what that screw is used for. In the parts diagram, it is
described as "plug" and "washer".

In the Mikuni manual is a parts drawing for a VM carb, but it shows
more parts than mine has, particular they show an external screw
labeled as "Air Screw" and "Spring, Air Adjusting Screw". My carb does
not have that. It is shown on the oposing side of the idler adjustment
screw, closer to the air intake flansh.

I was hoping to find that I had just put the jet needle in a wrong
groove (in the old days, the needle had several hight positions to
choose from), but checking it out, it only can be installed in one
position.

So I still don't know how on my carb the air fuel ration is
adjusted/set.

I will try to send you the parts list of the carb, maybe you can point
it out for me.

Volker

 

Chateau:

I am obviously no expert on Mikuni carbs, that's why I'm here.

The problem only occurred after I took the carb apart for a cleaning
(never been done since I got the bike, and likely not before either).

So I don't have to worry about jet sizes etc (since it was running good
with original jets).

Idle is also fine.

It starts great.

External adjustments on the carb is only the idle screw. The other
screw on the outside of the slide is just that: a screw with a washer.
In the parts diagram named "
plug" and "washer". I can't imagine that this screw is a setup screw
(nothing would hold it in place if not tightened"), when removed there
is just a hole to the slide.

I also noticed that the Mikuni parts list in the manual shows an air
adjustment screw, which my carb does NOT have (the manual shows it on
the opposite site of the idle adjustment screw, close to the air intake
flange).

How is the air/fuel mix adjusted on my carb? There don't seem to be
external adjustment screws (besides the idle screw and the screw on the
slide, there is only what looks like a drain screw in the bottom of the
carb).

Volker

 

Yes, I can see that on the schematic.

That drain screw is just that - for draining the float bowl.

Are you *sure* there isn't an air bleed/pilot screw? There damn well
ought to be.

Just a thought - you have got the needle assembly correctly, er,
assembled?

 

I had a similar experience a year back. ISTR you were on holiday at
the time so you may not have seen the post. A DT175MX I'd bought was having
running problems[1], mainly not returning to idle. Turns out someone had
replaced the stock filter foam with a bit of oiled double-layer cotton knit,
the type you (used to) get smoked hams packed in. Fitting the genuine
article made no odds, however. We then suspected a leaky crank seal, but
the bike was stolen before we could test that hypothesis. :-(

[1] This being the reason I'd originally found that Mikuni VM manual.

--
Ivan Reid, Electronic & Computer Engineering, ___ CMS Collaboration,
Brunel University. Ivan.Reid@[brunel.ac.uk|cern.ch] Room 40-1-B12, CERN
GSX600F, RG250WD "You Porsche. Me pass!" DoD #484 JKLO#003, 005
WP7# 3000 LC Unit #2368 (tinlc) UKMC#00009 BOTAFOT#16 UKRMMA#7 (Hon)
KotPT -- "for stupidity above and beyond the call of duty".

 

The Sudco Mikuni Carburetor Parts and Tuning Manual shows an exploded
view of the
spigot mount type Mikuni carburetor which we all know and love as the
VM series
round slide carb.

The air adjusting screw is # 29 and the spring is # 30. Turning # 29
counterclockwise leans the mixture.

# 27 is a plug on the bottom of the float bowl. # 26 is a copper washer
to seal the plug.

In flange mount float bowls, the plug is # 43, but is called a "Banjo
bolt" for some reason known only to Mikuni-san. It is actually used as
a main jet holder in some VM carbs.

# 11 is the needle jet, and, if it is the air bleed type it would
apparently have to be pushed UP through the slide area to remove it.

Those needle clips only fine tune when the carburetor comes full onto
the needle.
Your problem is occurring at smaller slide opening than that.

It may just be possible that your main jet ( part # 36 ) has fallen out
of the needle jet and is lying in the float bowl.

I actually had this happen once. I was trying to pass an 18-wheeler and
when I tried to use full throttle, the excess full coming through the
needle jet was drowning the engine.

The air screw will to that. It's going into the carb body at a 45
degree angle.

I suppose that carb has the bypass starting enrichener system shown in
the inset. #17 (starter plunger) is actually an air valve in a bypass
passage going around the slide. There is a fuel passage that allows the
bypass starting enrichener to suck gasoline right out of the float bowl
when the starter plunger is raised.

The starting enrichener works worse when you open the throttle. Vacuum
downstream of the slide drops off and less gas is sucked out of the
float bowl.

 

Heh. Yes, I remember it well....

Don't remember the air filter thing, though.

I'm amazed at what people do to bikers - my son's DT50 had a charging
problem, after running fine for a while, Turned out there was no
rectifier in it - it had been bypassed, and there was just the
regulator.

I hadn't checked for the presence of a rectifier because - well, you
don't, do you? I didn't check that the air filter on the DT wasn't made
of smoked ham wrapping because... you don't, do you?