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n00b extraordinaire
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Discussion Starter · #1 ·
Hi guys, having a weird issue that just started with my bike. I am unfortunately terrible with non-efi motors and I dont know what to adjust - or how to adjust it.

Basically - when I come to a stop - the bike will idle for a little while but, seemingly as soon as the light is about to change with tons of traffic behind me, the engine will stall and cant be restarted without some finessing (pulling the choke out, and sometimes even having to turn the fuel-valve off and crank it for a while). BUT - I cant tell if it is dying because of too much or too little fuel, but it sputters for a second (and I try to correct with throttle), and then it dies regardless of what i do, and takes a solid minute or two to get re-started.

This is my daily driver so you can imagine how aggravating and slightly embarrassing this is :/

My friend told me to adjust the idle air screw, but the only screw I can see on the outside of my Mikuni carb is the idle-adjust screw (throttle stop), and a bolt on the bottom of the carb. I have not taken it off because I want to figure out what to do before fucking with it, but this is what it looks like:




Any ideas? I dont think it is a spark issue, but sometimes it misses if I hold a high RPM in 3rd or 4th for a while as well.
 

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Vacuum leak spray some carb cleaner at it while its idling. If the it revs up there's a vacuum leak. Inspect the boot where it hooks up the engine for cracks and all vacuum lines
 

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Discussion Starter · #3 ·
I dont think there is one - boot looks good.

So I tried to drive it to work just now (fucking horrible idea) and it will run just fine while the throttle is open - runs OK at idle with the choke open, and then just dies as soon as I let the engine spin down.

Pushed it home a mile just now. I think it might be something clogging the jet, or the float being set wrong, or I have no clue :Stupid:
 

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Discussion Starter · #5 ·
No smoke or anything - it runs PERFECTLY if I keep 10-15% throttle, starts up on the first crank when its cold, but it does have a very intermittent misfiring problem.

Found someone with EXACTLY the same issue I am having;

It's a 93 LT-4WD 250 Quadrunner with the Mikuni BST31SS carb. Stock 31MM CV carb. I've had it off and apart every day for the last week and a half. It starts right up in our current 40-60 degree weather. No "choke" or gas, just hit the start button. Then after it starts to warm up, it loads up and stalls. Have to hold the throttle half open and crank for anything from 15 to 60 seconds to clear it out before it will restart. Getting gas in the oil and once it starts stalling, it won't idle unless you set the idle speed screw pretty high. Once it cleans itself out it races and I start turning the idle back down. Then it stalls.
I've checked and cleaned everything inside the carb, to no avail. It only has 1200 miles on it and the carb was spotless inside, the first time I disassembled it.
Being a retired GM tech who was raised on Rochester 2GC's and Quadrajets, I thought i understood carbs pretty well. My guess is that there is a crack or pin hole somewhere in the body of the carb.
One of my other ATV's is an identical 93 LT-4WD that I bought new in 94. It has roughly twice the mileage and has had a couple carb issues since new. That was usually the "choke" or "starter" cable working it's way out and resulting in a similiar situation. I bought this one assuming the problem was the same, but it's not. First thing I did was make sure the starter circuit plunger was fully seated.
Guess I'll either drop $400 for a new carb or part it out. Tough choice on a clean low miles bike. Doesn't even have any cracks in the plastic yet.
http://www.suzukiatvforums.com/forums/repair-maintenance/4470-mikuni-carb-article-3.html

Turned out that he had a bad seal in his carb

I am going to try to clean the carb out tonight, but if I cant get it to run I am going to just bite the bullet and buy a brand new carb - they are pretty cheap on Ebay. Cheap enough that it makes spending $15 on parts seem a little worthless when I can get a whole replacement unit for under $40;

This one is only $35 and looks almost the exact same as what I have now (need to measure when I get home though);
http://www.ebay.com/itm/Carburetor-...Parts_Accessories&hash=item27c9e68551&vxp=mtr

It looks identical to this one that claims to fit the GN250;
http://www.ebay.com/itm/Carburetor-...&fits=Make:Suzuki&hash=item4d00fef9a2&vxp=mtr
 

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If it only runs on choke you're getting to much air into your system causing it to stall turn the pilot screw to whatever spec is for the bike if you have a manual. If you don't have a manual screw it all the way ( not to tight just in ) then turn it out to 2 1/2 turns out ( kind of universal setting and start from there. Loosening=more tightening=less air
 

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Discussion Starter · #7 ·
Sweet, thanks for the response!!

I will try that when I get home.
The only screw that is on the outside (as far as I can see), is the idle/throttle stop screw which hasnt really helped in this situation. Is it on the inside, under the bottom fuel-cup cover thing?

I am actually a little scared of doing MORE damage to the carb by disassembling it, but its not like the bike is running properly now anyway.
 

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It's on the bottom of the carb outside of the bowl no disassembly of the carb required. Just take it off you should see sometimes you need to drill the cap out but being your bike is a old I would bet its been drilled out you can see it in this picture. This is with the carb bowl off but it's outside the bowl. On the top left the tube that is part of the carb. The screw is down in there you'll need a skinny flat head screw driver to get to it.


And remember tighten=less air loosen=more air. But you want screw it all the way then not to tight just snug then turn it out 2 1/2 times and reassemble the bike see how it idles and after that you know where it is so you can adjust it while the carb is on the bike. For the first time I would take it off to set it up.
 

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Or your pilot jet could be plugged, check it once you get the carb off and apart. It's the smaller recessed jet immediately adjacent to the main jet.

Some Mikuni carbs have index marks on the outside of the carb body for checking float level. Hook a small length of clear tubing to the bowl drain and hold the other end up next to the carb's index mark. Open the drain screw and the level in the hose will equalize with the level in the bowl. If it's above the index mark it's too high, below is too low. If you carb doesn't have these, with the carb removed and the bowl off, flip the carb body upside down with the float in place. The float arm should be exactly perpendicular/flat. If it's tilting up or down, gently bend the tab that pushes on the needle valve.

In reality though float height has to be waaaay off before you will notice running issues, especially on a 250. A smidge high or low doesn't make any appreciable difference.
 

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Discussion Starter · #10 ·
So I took the carb off and removed the bowl pretty easily (totally forgot to take pics while it was off). I unfortunately got back from work too late to pick up some carb cleaner.

I removed the large and small jet, and blew them out but they look fairly clear as-is. I cleaned them with rubbing alcohol and a small piece of wire and re-installed them.

Took it for a test drive and the same thing happens. Will run and idle perfectly for a few minutes till it warms up, then if I let it idle or sit at very low throttle it will die, even while in gear and under way. I usually put the clutch in and try to rev it to keep it from stalling, but it seems to be getting progressively worse everytime I drive it. It will "cough" a few times when I am flicking the throttle, and then die, but I can keep it from completely dying for a few strokes before it just dies (sounds like it is just turning off). Giving it the beans soon enough usually remedies this but I obviously cant keep it revved at a stoplight without looking and feeling like a jackass.

NOW, if it is warm it will only start with the choke off, which is the opposite as before.
It also "surges" on idle when it does want to idle after it is hot, which I forgot to mention initially.

I now think it must be the air idle screw, but I did not see anything else under the bowl.
There is a screw on the outside of the carb, on the inlet side, but it is seriously frozen shut. Tried for 20 mins to get it off.
Could that be the idle-air screw?

Also, one of the screws on the top "hat" of the carb is frozen and will absolutely not budge. :doh:



Will try again tomorrow when I can pickup some carb cleaner. Sucks because this is my daily driver and it ran great until this issue. If I cant get it running tomorrow I am going to just buy another carb to swap on since they are cheap enough, and rebuild this one.
I fucking hate taking the bus...
 

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Discussion Starter · #11 ·
I also forgot to mention that the gas cap is broken so I taped it shut, but there are tons of air gaps so I dont think it really has anything to do with it (but I have no idea at the same time).
 

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The a/f screw is usually located on the left side of the carb body, on the outside. Generally if it's upstream of the throttle(airbox side)it's an air screw, and downstrean (engine side) it's a fuel screw. They do essentially the same thing but it varies by carb. The frozen thing in the venturi is likely an air jet.

If you can pick up a known good carb for cheap that might be your best/quickest solution. Removing frozen screws/jets/needles from carbs can be nigh impossible, even if you get a good bite on them they're soft brass and easily snap off.
 

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Discussion Starter · #13 ·
OK so I tore it back down because there was no change after doing a basic/light cleaning job on it. New carb is ordered and should be here shortly (I hope at least). And I finally made it to the store to get carb cleaner (woot).

I took a few more pics though - so the screw on the front I actually got removed by just soaking it in carb cleaner, and tapping it with a hammer. I didnt remove it all the way but I will do a full teardown tonight. It was fully seated, meaning screwed in all the way, which is why I thought it was seized (it was in there pretty tight).

Anyway, when I sprayed carb cleaner through the idle jet pickup, it came out of the jet to the left of the intake (Upstream/front side).

I also found another screw that I apparently missed before - it is on the top-side of the "outtake" side of the carb, where the actual throttle-body is. The screw is marked in red, and the yellow screw someone previously stripped re-installing the hat so I need to easy-out or dremel a slot in tonight.




And here is the front "intake side" of the carb - the jet on the left is the one that is "connected" to the idle-jet pickup in the bowl.



So I assume the Upstream (front side screw) is the air-idle valve, and the downstream (throttle body side), is the fuel regulation.

I screwed both in till they were fully seated, then backed them out 2.5 turns but I am a dolt and forgot to mark the position of (what I think is) the fuel-mixture screw on the outtake side. Does a 2.5 turns sound OK for that as well?


Thanks so much for the help guys, I really appreciate it. I honestly couldnt tell if it was a carb or a spark/electronic problem but it is pretty clear that its the carb being fucky. Hopefully a new carb solves that issue but I assume I need to tune it regardless.

First time for everything I guess (including making an ass out of myself stalling in a work-zone on the highway) :rock:
 

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The "yellow" screw is one of the four that holds the CV diaphragm in. If you have a dremel, use the cutting wheel to cut a slot into the screw, then use a big flat screwdriver to get it out. Behind this cover is a big(though weak) spring, the diaphragm, and the plastic CV slide that holds the needle. There may be an air jet behind it.

The jet you mentioned on the intake is the idle air jet. At closed throttle vacuum draws air though this bypassing the venturi into the idle circuit. As soon as the throttle is opened it's doing pretty much nothing.

The red arrow I would take to be your a/f screw. If it was fully in/seated, the circuit was closed. The general rule of thumb for those is 1.5 turns out from lightly seated and tune from there.

Did you remove/inspect the choke plunger? If it's not seating fully closed, it's allowing extra fuel in at idle causing it to run poorly. Again, this is a circuit that's really only active when the throttle is closed.
 

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Okay, I'll take a stab at this...

If your bike was running correctly before the problem, then your carburetor was properly adjusted. Making adjustments is the wrong thing to do. You need to look for some problem not an adjustment. Most commonly an air leak in the boot or crap in your gas has plugged something.

All carburetors I know of are two carbs in one. There is an idle carb and an above idle carb. Since you are running okay with some throttle we only need to look at the idle carb.

The idle carb has it's own air and fuel passage ways. There are two types, depending on where the adjustment screw is located. If the screw is on the air filter side of the slide/butterfly it controls the amount of air. If it is on the engine side of the slide/butterfly it controls the amount of fuel. Fuel is metered by the Pilot jet limiting how much is available by the size of the hole.

Okay... identify what you have. Here is a diagram I'm guessing is like the one you have.



Now go to this site to see the parts by the number...
http://www.cmsnl.com/suzuki-gn250-1998-ew_model13364/partslist/FIG10.html

With the bowl off you should be able to see the pilot jet (9) and the main jet(13). Remove the Pilot and squirt carb cleaner in the passage. You should see it come out a tiny hole in the carb throat between the slide and the engine. Keep cleaning until it does.

Very gently clean the pilot jet. If you fuck up and change the size of the hole in any way you will need a new one.

You shouldn't have to clean any of the other brass parts but you can if you are gentle. Don't mess with the float adjustment... remember it was running okay to begin with. BUT do clean the little filter above the float seat. On the other hand it is probably fine since the bike runs at throttle it is getting fuel thru the float.

Okay, put it back together. I don't suppose you counted the number of turns to gently seat the idle adjust screw when you were messing with it? If not start with 1 1/2 turns out. Then adjust for best (fastest) idle on a warmed up engine.

If that doesn't help... I give up. :D







.
 

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rstar45 is on the right track. A carb that needs choke to run from off idle has plugged pilot circuits. Your jets operate at various rpm, so if you have an issue off idle, its the jet for that circuit. cleaning a carb is more than spraying some carb cleaner, thats a handicap fix. Strip the carb down, jets, needles, slides, everything comes out if you want to do it right. Dip them/boil them/ultrasonic them/ whatever. But be thorough or you will keep chasing problems. Put new orings in it, balance them according to the manual. And yes you need to check float height anytime you fiddle with float bowls. When you know your carbs are right, if the bike still has issues, then you can move to ignition if needed.
 

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Discussion Starter · #19 ·
Sorry guys - got slammed @ work and forgot to update the thread.

So I had basically tried EVERYTHING I could think of with the carb and there was no change.
I basically ruled out the carb and fuel system at that time, since the bike would idle just fine for 5-10 minutes, at which point it would start having the trouble described (bogging, and just not wanting to run).

I then moved through the rest of the bike, pulled the top-end off and adjusted the valves and timing. No dice as well

--
:mad:
THEN - it hit me. The one thing I was doing was charging the bike between fucking around with it.
So, I let it idle for 5-10 minutes and checked the battery voltage. Sure enough - when the bike wanted to just die, the battery voltage was right on the 12.0v mark, dipping in to 11.9x.

It was then, that I facepalmed and realized I did not check the entire electrical system thoroughly. I checked for shorts, and just general wiring issues but NOT for charging. I assumed it would either be charging very slowly or properly.

I busted out the multimeter and it looks like either the rectifier or the stator (or both) are bad. I am getting like 12.5v while the bike is on, and that voltage creeps down after a few mins regardless of RPM.

--
So - some more basic questions; :confused:

1. Do you guys think it is OK to re-seal the stator cover with RTV sealant in place of a proper gasket? Im gonna order the gasket but dont want to wait a week for it to show up.

2. And secondly - any tricks to narrowing down what is bad (between the stator/generator and the rectifier/regulator)?
The wires going between the two are getting pretty hot, and the rectifier is as well (especially for ~5mins of actually running) so I am PRAYING that its the rectifier but I want to be sure. But its not charging at any RPM so it could still be a burned out stator.

I already ordered the rectifier but I figure if it does turn out to be the stator I could have a head start ordering a new one.

--
And the best part is that I have been waiting 2 weeks for USPS to replace my fucking mailbox key, which the last tenant of my rental house decided to take with him.
I have a brand new gas cap, turn signals, grips, and the new carb I ordered just sitting in there :mad::wtf:
 

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Discussion Starter · #20 ·
And thanks so much for the write up Rstar45 (and the rest of CF that tried to lend some expertise)!! It helped me to double-check what I already tried and keep debugging.

I had a look at the GN250 manual (posting here for reference, hopefully to help someone in the future);
http://dl.dropboxusercontent.com/u/26633588/GN250 Manual.pdf

But I cant find any resistance figures in there. Gonna see if I can figure it out when I get home - will update this thread.
 
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