Air-mixture screws were set differently at the factory

[Darkstar]

After pulling the brass plugs, I checked the screw settings. Left side is 3/4 out, right is zeroed out, not screwed out at all. Anyone know why? I ask because I'm re-jetting and plan to set 3 turns out on both, does that mean I should set to 3/ 3/4 for left and 3 for right?

[J_Walker]

Set both at 2.5 turns out with your jet upgrade. 3 turns is actually at the top end of the fuel mixture screw, and 3.5 is the MAX you should go, if you're touching 3.5 you should bump your jets up another point or so.

[Watcher]

Not sure about the setting from the factory, but when we say "2 turns out" it's 2 turns from snug.

So you'd "reset" the screws all the way in, then start counting.

[Darkstar]

Set both at 2.5 turns

Thanks Walker. Mind looking at my needle setup?
http://imgur.com/a/3dWQE

From the top, this is:
- Thin washer (stock)
- C clip (stock)
- O ring (added)
- White plastic spacer (stock)
- Thick washer (stock)

[Suzuki Stevo]

Are you sure you didn't bump the mixture screw with the drill bit when you remove the plug, that runs them into zero with a quickness....ask me how I know 

[Darkstar]

Are you sure you didn't bump the mixture screw

AHA! Not the drill bit, but with the drywall screw. Thanks!

[J_Walker]

Set both at 2.5 turns

Thanks Walker. Mind looking at my needle setup?
http://imgur.com/a/3dWQE

From the top, this is:
- Thin washer (stock)
- C clip (stock)
- O ring (added)
- White plastic spacer (stock)
- Thick washer (stock)

WHOOA I don't think that needle setup is gonna even allow you to start the bike.. stick with the stock setup, and add the washer if needed. and if still needed, remove washer, and add the oring. if still needed. add both. but I would see how it ran first before getting too wild with the needle adjustments.

take a look/read here. to get a quick and dirty understanding of how each bit works together.
edit; would help If I posted the link. http://www.iwt.com.au/mikunicarb.htm

[rscottlow]

take a look/read here. to get a quick and dirty understanding of how each bit works together.
edit; would help If I posted the link. http://www.iwt.com.au/mikunicarb.htm

This is a great resource. I'm saving this one...

[The Buddha]

Are you sure you didn't bump the mixture screw

AHA! Not the drill bit, but with the drywall screw. Thanks!

AI-I-I-I-I-I...

Cool.
Buddha.

[Darkstar]

AI-I-I-I-I-I...
Cool.
Buddha.

I just tore it apart to have a look. Here's the deal:
- All hoses and vacuum tubes are routed properly and without pinching.
- I pulled the plugs and they're the same as before: if I pull them after idle, they look lean, ie white ground strap.
- Large idle screw was set to 8 turns. I left it there.
- One air mixture screw was somehow at 3.5 and other at 2.0. Both are now set to 1.0 for starting.
- One float bowl was at 10mm, the other at 13mm. Both are now set to 13.
- New air filter installed

I put it back together for a test and it's worse. It starts up the same way: with no enricher/choke, and as before, slight touch of the choke kills it. Once started, it chugs slowly up to speed and i have to give it a bit of throttle to warm it up. Once warm, I now have a big loss of power, it sounds heavy and bogged down. It didn't do this when I first put the larger jets and needle o-ring in, it ran great then but just stalled when I hit the choke. Now full throttle doesn't wind it up at all. Blipping throttle to 1/3 hangs the idle some, and this wasn't happening when I first put the new jets in either. Setting air screw mixtures to 2.0 helps, but not by much. Setting them to zero or 3 doesn't either.

So the changes that I made today were: gave it a bit more air by replacing air filter and a but less fuel by lowering one of the floats. If it was rich before, it should be running even better now. But this is not the case because it stalls when i use the enricher to start, so this idle circuit is still running rich. But the idle hangs and the plug straps are still white, suggesting it's lean. I'm confused. Im wondering if I set the throttle cables too tight. Any advice would be appreciated.

[Watcher]

If your throttle cables are too tight you'll know immediately by turning the bars to full lock right.

Have you checked for vacuum leaks?


What jetting are you running?


I'd also heed J_Walker's advice and not mess with the slide needle.  Revert that back to factory, at least until you have a reliable engine.
That's, like, troubleshooting 101.

[Darkstar]

I don't think that needle setup is gonna even allow you to start the bike...http://www.iwt.com.au/mikunicarb.htm

Interesting, I thought this was a common rejet configuration. Thanks for the tuning site, I'll dig into and see what I can learn.

[mr72]

But this is not the case because it stalls when i use the enricher to start, so this idle circuit is still running rich. But the idle hangs and the plug straps are still white, suggesting it's lean. I'm confused. Im wondering if I set the throttle cables too tight. Any advice would be appreciated.

It could be running rich at idle due to float height or float needles sticking open, and running lean on main jet due to the slide/needle not rising, which would cause the hanging idle and white plugs.

Vacuum leak, poor seal or torn diaphragm on the top of the carb, etc. could cause the slide/needle to not rise.

Also if the pilot needle (the pilot mixture adjustment "screw") o-rings are bad (or missing!) then it will run horrifically rich right at idle and only at idle and you cannot adjust it to not run super rich because fuel leaks past the o-rings rendering the needle useless.

Smartest thing I did when working on my carbs was to replace ALL of the o-rings and remove EVERY potential vacuum leak.

[mr72]

Interesting, I thought this was a common rejet configuration. Thanks for the tuning site, I'll dig into and see what I can learn.

More common is to just rejet and leave the needles/washers/etc. alone.

[Watcher]

Slightly OT but I hate how many things in a carb can be referred to as "needles".  It can get very confusing without context.
Float needles, slide needles, idle adjustment needles, give it a rest already!

[Darkstar]

just rejet and leave the needles alone.

Good to hear because it's easy to test. Can you adjust needles from the top without removing the carbs?

float height

checked, in spec

float needles sticking open

are you referring to the needle valve connected to the float? if so, I checked when reassembling, and they work as intended

slide/needle not rising

checked when reassembling, works as intended 

poor seal or torn diaphragm on the top of the carb

checked when reassembling, works as intended 

pilot needle o-rings are bad or missing

checked when reassembling, o-rings are they were not bad last week, it was running lean

vacuum leak

This was the only item left your list, so I just tested it. After warm-up, I killed the engine, and decided to check the hose connected to the vacuum damper/chamber. As soon as I pulled it, there was an audible whooshing sound, like it has major back pressure. Is this right? I thought this damper controls only slight vacuum issues. Could it mean that my vacuum lines are connected wrong?

[mr72]

i don't know about whooshing sound.

There are four vacuum ports. One on each plastic carb top which should have a cap on it, these are for balancing the carbs.

Then there's a vacuum port facing the side. On the left carb, that port should be connected to the petcock via a 5mm vacuum hose. On the right carb, that port should be blocked internally but I put a cap on mine anyway.

But there are a bunch of other places for a vacuum leak. Big one is the big o-rings between the intake boots and the cylinder. Then the intake boots themselves can be cracked or deformed, or not making a good seal with the carb. Those are the major vacuum leaks if all o-rings inside the carb are in good shape.

I want to reiterate about the mixture adjustment needle/screw o-ring, because that thing is very hard to remove and inspect, and almost impossible to remove without damaging it. And on both of my carb sets they were hard and flat and leaked. One of them even leaked fuel right out of the adjustment screw head on the bottom of the carb. Anyway, my point here is  that it's easily overlooked and also easily screwed up on reassembly so I just want to make sure I am communicating the potential problem correctly. If you already checked it, then move along. But most folks forget.

[Darkstar]

There are four vacuum ports

Thanks, I'll verify the routing of these against the SM.

o-rings between the intake boots and the cylinder

They were good coming out, but maybe I tore one putting the airbox back in, it's such a tight fit. will check

needle/screw o-ring

When I removed the screws I looked inside and verified that the o-rings were there, but did not remove them. How the hell do you get them out? Im not convinced that this is the main problem though because, aside from running lean, the bike ran great before I increased jet sizes. I suppose the real test would be to go back to stock setup and test again...ugh

[mr72]

o-rings between the intake boots and the cylinder

They were good coming out, but maybe I tore one putting the airbox back in, it's such a tight fit. will check

How would you tear it? They fit in a groove on the intake boot (hard plastic part) that bolts to the cylinder. That thing is firmly planted by the time you begin putting the carbs on.

Other side of them, the rubber hose side, slips over the carbs and is reasonably hard to fit in some cases and you might score or crack them if they are old. They are much easier to manhandle when they are new boots since they are far more supple and fit over the carb throats much easier.

I'd say if anything you might have either cracked one of these, broken the rubber part from the plastic part (happened to me!) or managed to get the carb to come loose from the boot and make a leak when putting the &#@#!@#$! airbox on (also happened to me, many times).

The o-rings are like 1.25" diameter and thick... when they are "bad" they are flat and hard, but they still seem to fit in the groove and look like they are supposed to seal. New ones have a round profile, bad ones are flat.

needle/screw o-ring
When I removed the screws I looked inside and verified that the o-rings were there, but did not remove them. How the hell do you get them out? Im not convinced that this is the main problem though because, aside from running lean, the bike ran great before I increased jet sizes. I suppose the real test would be to go back to stock setup and test again...ugh

Well, I think what happens is these are preset from the factory and they seal when new and the o-ring eventually hardens and flattens but IF YOU NEVER MOVE THE ADJUSTMENT SCREW then the seal never breaks. Once you open the caps and start trying to adjust it, you break the factory seal and since the o-ring can't expand anymore if it's hard and flat it doesn't seal any longer.

So if you didn't remove them, I almost 100% guarantee you they are bad.

They are very hard to remove. I used a .018" guitar string bent into a "L" shape on the end to "pick" them out. It took freakin' forever and I swear I aged a decade doing it. Oh yeah and I did this on TWO sets of carbs.

To get them out almost guarantees you will scrape them along the sides of the threads and screw up or tear what is a tiny o-ring.

I chased my tail a whole lot before finally biting the bullet and replacing these o-rings on my current carbs.

BTW you CAN replace these with the carbs on the bike but it's WAY easier to do it with them off and upside-down on the bench.

[Darkstar]

Have you checked for vacuum leaks?

I'll spray some carb cleaner around while it's running to see

What jetting are you running?

These are my configs that i got from http://beergarage.com/GSJet.aspx, halfway down the page username Scooky is running this on a 2009:

20.0    60.0    132.5     1 o-ring      2 turns out