Mikuni Choke Issue: runs on full choke only

ILoveThumpers

Everywhere is thumpers!
After replacing the fuel petcock earlier this week I finally had a chance to see if my carburetor cleaning adventure went as well as I had hoped... Set the choke, followed the kick procedure in the manual and it started on the first try! (Yes, lucky... no skill involved in this, mind you)

Anyway, I fiddled with idle for a bit to get it settled in at an acceptable rate and after about 5 minutes of idle, I tried to go to half choke and it died. Started it up again on full choke and gave it some throttle as I set the choke to half and it died again.

Basically after quite a bit of fiddling, I've come to the conclusion that any choke position other than all the way out is no bueno at the moment.

I'm certain I got the carb clean (at least all the passages I could see, I could have missed one) and everything else seems to be working properly. (Throttle, idle adjuster, transient enricher) I haven't tried riding to see if there's an obvious rich/lean condition yet.

Does anyone here have experience with Mikuni carbs or have suggestions on what might be causing the issue? Depending on how this evening goes, I may pull the choke assembly and see what I can see...

Thanks in advance.

-Deek
 
Bike runs with choke. Choke circuit is fine.
Bike does not run on slow jet. Slow circuit or mixture is buggered.

Could also be the idle set too low. When you run on choke the RPM's should be above idle.

What model are we talking about here?
 
... Stock airbox with filter?

You sure you know which way is choke on or off?

What bike is this, gn400?
 
The bike is a GN400; the same carb was used on the SP400 as well as an 800cc V-twin (can't recall the model at the moment)

It's a 36mm diaphragm model. The same as this one.

-Deek

P.S. how you healin' up, Red?
 
mlinder said:
... Stock airbox with filter?

You sure you know which way is choke on or off?

What bike is this, gn400?

Yep, all stock at the moment. "Out" is full choke, then there's a stop half way in then all the way in is no choke. (It's sprung "in" so I'm certain that "out" is full choke... plus, it's in the manual ;) )

-Deek
 
I got four bikes out this week, three of which were kick-start. I'm at least back where I started ::) Thanks for asking.

On these, that "choke" isn't a choke. It's an enrichment circuit. When it's open, the carb could completely bypass the idle circuit and run solely on the EC. Most times somebody brings a bike to me and it will only run on EC, it's either clogged jet, clogged circuits, or a vacuum leak.

You said that while rev'ing you can't take the choke off. That would lead me to believe something is clogged or air is leaking in.

Spray test :)
 
Thanks for the tip, Red. I'll go pull the pilot jet out again and see if some crud floated in there... lovely.

Glad to hear you're getting on; I read your crash thread to my wife and we had a good laugh over your pain. ;D (Not your pain, really, mostly the situation at the police station)

-Deek
 
ILoveThumpers said:
Glad to hear you're getting on; I read your crash thread to my wife and we had a good laugh over your pain.

Hah, definitely not enough to keep me from riding. It was a fun day for sure.

Before pulling the thing off, spray with carb cleaner! No use delving into the carb when it could be a leak.
 
Deek, that carb looks like a BS36SS. Suzuki used a pair of them on the GR650, so if you need any parts let me know.
 
Big Rich said:
Deek, that carb looks like a BS36SS. Suzuki used a pair of them on the GR650, so if you need any parts let me know.

One in the same. Couldn't dig up the model number after Red asked but that's the one for sure... Ever done a UNI or K&N filter on one? Or is that the red hot screwdriver in the eye vs. jetting situation? :-\

Redliner said:
Before pulling the thing off, spray with carb cleaner! No use delving into the carb when it could be a leak.

Good point. I just ran out of that stuff; guess I need to make a parts run. Sorting out a sticky front brake at the moment... Luckily those are the last two things to make this beast roadworthy before the part swapping begins!

-Deek
 
I started to, but then found a good deal on some VM34's and did the conversion. I remember seeing a GR that had EMGO pods on it and the owner said it ran great, but I don't believe it.
 
Are brand new VM34's really only $100? That seems reasonable.

As you probably already know, the BS36SS as configured on the GN400 has zero adjustments. Well, an idle screw and that's it. I'm confronted with the notion of keeping the air box and exhaust stock or risk ruining the tune. (Short of just changing the fuel jets)

If I can get a slide carb that I can tune to my heart's contentment for $100. I'll drop a bill on it today.

-Deek
 
I ran UNI filters on my GT750 Suzuki with BS40 carbs and managed to get them working pretty well. They are nowhere near as hard to jet as some Keihin carbs. I raced it like that and ran it on the street without any trouble - just don't tell anyone.

What you have to do first though is to clean out the jets and all the small drillings that air and or fuel go through. Most likely to be a blockage in the bowl or the tiny bypass and idle drillings. Ultrasonics are your friend, but yo can clean them with carb cleaned and a small wire brush if you have to. And grab a can of PC compressed air to blow things dry internally.
 
teazer said:
They are nowhere near as hard to jet as some Keihin carbs.

Who, no need for that kinda language here. ;D

Anyway, I'm almost certain that the only passage in the bowl is the EC, correct?
 
A single carb isn't all that expensive, it's the other stuff that you have to buy that adds up quickly. Possibly a new throttle cable, experiments with jetting, and the big $$$........ slides and needles. Of course, you would only need one of everything instead of 2 like me, but that's beside the point.
 
Fair enough. With the revelation that there is some opportunity for tuning a bit with jets and the air adjust under the plug, I'm feeling better about keeping it for now.

Red, the main jet is in the bowl and the pilot as well but its under a plug. The enrichment circuit is on the side just under the diaphragm. (I think... these carbs are quite different than the GL carbs I'm used to.)

Thanks for the tips everyone! I'll post up a video once it's plugging along as it should.

-Deek
 
ILoveThumpers said:
Red, the main jet is in the bowl and the pilot as well but its under a plug. The enrichment circuit is on the side just under the diaphragm.

I meant that the EC usually has a tube that dips into a separate part of the bowl. That separate part is usually fed through a tiny, TINY hole that when clogged is hell to unclog. I normally have to boil my bowls...several times. It's something easily over-looked, but yours is clearly working.
 
Oh I see. I'll have to look at my spare more closely. Its mostly complete (the spare I mean) but completely filthy inside and out so I haven't looked at it much.

I remember boiling my GL racks on the barbecue grill... ah good times.
 
Redliner said:
Who, no need for that kinda language here. ;D

Anyway, I'm almost certain that the only passage in the bowl is the EC, correct?

Good question. There are several different BS designs and some have the starter jet in the bowl and others have the main jet and pilot jet also screwed into the bowl. The starter jet on a Mikuni is typically at the bottom of that deep hole and it's usually 0.5mm or 0.6mm also called #50 and #60. That's around 20 thousandths of an inch and PIA to get to and to clean. They are pressed into place during manufacturing and are not readily removable without considerable damage.
 
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