GS 550D Mikuni VM22SS tuning problem.

[Stigy]

Hello. I've gone through entire internet on this one...
Carbs on my GS got air mixture screw + fuel mixture screw. I'm repairing motor bikes for a long time now and never seen something like that. When setting a carb is an easy task, when you got only one "pilot jet factor", two factors in this case is too much for me... How to set it ? Do I use factory settings (i found them on Your page) for first screw and set the other ? Haynes manual is absolutely useless on this bike - this manual is piece of crap on this bike, I regret I've bought one... So, will You help me, dear users of this magnificent bike ? Or is it a task only few magicians of this world can handle and I'm doomed ?

 

[hampshirehog]

Firstly are your carbs clean - really clean (fully dismantled, both pilot jets clean and either dipped in a suitable cleaner or ultra sonically cleaned)? Second, are the fuel mixture screws in good condition - no broken tips (they break really easily) stuck in there? If the answers are yes then for a stock set up gently (very gently) seat the fuel screws then back them out 3/4 of a turn. Then adjust using the airscrews, starting at two full turns out from fully seated as a baseline.

 

[Stigy]

Thanks for advices Hampshirehog. Now I'm preparing for carb operations so i need to know the battleground. First of all, in all service set's I found fuel screw factory setting as 1 full turn, not 3/4. Am I wrong ? Second thing is: What are two settings for anyway ? I understand advantages this solution provides - there are wider range's of sets and conditions you can tune your engine for, without changing the pilot jet. But It could be a killer if You got absolutely no info's about procedures predicted by the producer.

So let's assume i got everything right:
1. I go for one turn out on fuel screw.
2. I set idle rpm with throttle stop screw (1000 rpm as i I remember).
3. I turn the air screw until I find max revs with good throttle response.
4. I set idle rpm at 1000 rpm
5. I go with next carb's with steps 2-5 until mission is complete.

Please correct me if i'm wrong at any of those steps (or every step)

 

[Stigy]

I did not ignore him Don't get me wrong. Just asked if my information and service information's from Haynes are wrong. If they are - 3/4 turn out it is... If You agree on that let's get to tuning procedure. As I understand I don't mess with fuel mixture screw during tuning. I use air screw only. Am I right ? Is the procedure I described earlier solid ?

 

[Nessism]

You may have to mess around with both screws. Depends on how the bike runs. In the past with my old 550, when the fuel screws were set at 1 turn the bike was too rich while riding. It wasn't an idle thing, it was while riding. That's just my bike though, the one you are working on may be different.

 

[hampshirehog]

Thanks for advices Hampshirehog. Now I'm preparing for carb operations so i need to know the battleground. First of all, in all service set's I found fuel screw factory setting as 1 full turn, not 3/4. Am I wrong ? Second thing is: What are two settings for anyway ? I understand advantages this solution provides - there are wider range's of sets and conditions you can tune your engine for, without changing the pilot jet. But It could be a killer if You got absolutely no info's about procedures predicted by the producer.

So let's assume i got everything right:
1. I go for one turn out on fuel screw.
2. I set idle rpm with throttle stop screw (1000 rpm as i I remember).
3. I turn the air screw until I find max revs with good throttle response.
4. I set idle rpm at 1000 rpm
5. I go with next carb's with steps 2-5 until mission is complete.

Please correct me if i'm wrong at any of those steps (or every step)

Pretty much but I bet you'll find 1 turn out on the fuel screw too much and you'll be winding the airscrew out too much to compensate - anything over 3 or less than 1 on the airscrew and you'll have to tweak the fuel screw as it won't run evenly.

And the max revs thing - you need a keen ear to tell and I used to wind the air screw back in 1/4 turn to richen things a tad. Now I just plumb in a colortune and do what the colour tells me - piece of cake if you've got one of them.

Don't forget, the pilot circuit isn't just for setting the idle mixture - you'll be doing a lot of your riding on that circuit unless you cane it everywhere. Plus don't forget to balance your carbs if you've had them apart. Best done at about 2k or so depending on which gauge you're using.

 

[Stigy]

Great thanks Gentleman. You helped me a lot. As I can see fo now - air screw + fuel screw on this carbs give you much more possibilities of setting the mixture in various conditions, without need to switch between pilot jets (normally if You go for example over 3 turns out You should consider pilot rejeting). Here You can compensate with fuel screw setting and start again with air screw. I will consider using colotune for this one. Thanks again.

Last thing. Are all carbs suppose to work on similar settings ? (similar fuel,air screw turns out) or should I not bother other carbs and treat them separately ?

 

[hampshirehog]

Each carb should be similar but probably not exactly the same - tolerances etc. I used to play with the fuel screw settings so that I could set all the air screws at exactly 2 turns out but there's no point. And you burn your hands!

 

[Stigy]

Hey Guys. Still here a couple years latter, finally finishing the project after some bumps and delays.
I got couple more detailed questions as for tuning and pilot screw settings.

1. Should I sync the carbs first and then mess with the pilot screw settings ? Or the other way around ? Does messing with pilot circuit affect the vacuum values on individual carbs ?
2. I've seen different ways of tuning for 4 carbs. Some say to make the same change for every carb at one time, some say to treat each individually. So as I wrote earlier - change to highest rpm, back off 1/8 (to be on a safe rich side) and go for another carb.
3. As I've found on one of the VM22 manuals I've found here, the fuel screw has different setting for every carb from the factory. One of You suggested years ago I should start with 3/4 for fuel screw. Now - can I assume that if air screw is within 1-3 turns - the fuel screw is in good position (like with idle jet size) or is this something totally different, and if air screw is 1-3 turns out, it doesn't mean fuel screw is in correct position ? From the physics point of view - it doesn't work the same way (fuel screw and idle jet size) but in the end you adjust the amount of fuel entering the idle circuit, so as I understand it, it should be similar to changing a idle jet...

 

[Nessism]

Pilot fuel screw = .75 of one turn open
Pilot air screw = 1.5 turns open

Start bike, warm up, and check and adjust sync.

Tune the air screw for highest idle. If the idle doesn't respond, leave it alone. After all is said and done, the screws should be within 1/2 turn of each other.

Double check sync, but it should be same as before.