smoothbores bog, air screws no effect

[petrolhead]

i?m trying to get my smoothbores to run smooth.
Beacuse carburation still seems a dark art to me i could use some advice.
I have a Suzuki GS750, 1978, Wiseco?s 69mm, original airbox, K & N airfilter, original 4 in 2 exhaust.
VM29, 115 mains, 2,0 cutaway, 25 pilots, 0.9 air, needle clip position 3
it?s okay but it lacks power, bogs from pilot to midrange and the airscrews have no effect whatsoever.
One cold and damp night it suddenly had all the power I hoped for; it run like the fire brigade.
spark plugs are dark
should i lower the needle clip ?
I also stuffed the hole of air screw number 1 one; still no effect turning the screw...
Pilot jet to big ?

 

[zed1015]

The set up you have is for open carbs/velocity stacks and is what the Smoothbores are designed for which is racing/performance situations where an airbox wasn't intended to be used.
The main air jet (air correction jet) is what enables clean low end carburation without hesitation or flat spots when running open and it essentially richens the off idle mix without affecting the fueling further up the rev range.
With the airbox fitted the current main air jet with be causing the bottom end over richness and you will have to fit a larger main air jet or even remove it altogether to lean out the bottom end and cure the bogging.
Before going any further you should try the bike either without the air box, it's connecting tubes of maybe just remove the filter to see if any of those leans the bottom end enough to give a satisfactory result.
If you still want to run with the complete airbox and filter you will have to find the correct size main air jet that cures the bogging without going too lean.
This will mean slowly increasing the main air jet sizes until the bogging is cured .
The main air jet code is BS30/97 and the sizes go from 0.5 to 2.0mm in 0.1 increments
As for the air screw.
This only affects the pilot /idle mix not the off idle transition ( which is what main air jet governs ) which is why it has no effect on your issue and blocking its air passage only richens up the fueling anyway which is the opposite of what you need to achieve..

 

[petrolhead]

Thanks Zed,
This explains a lot; i will go experimenting with the airbox tommorrow.
I like the original airbox for holding the carbs in place and giving a sonore sound when driving down town.
I will try to find a way to make it work.
Maybe something with extra side vents
And i will look into the air jets.

 

[petrolhead]

I got up and drilled 54mm. holes in the lower part of some spare side covers.
It makes a big difference; the bog is almost gone and the engine breathes much better.
Now i am going to find out if i can make a larger inlet in the back, find a way to muffle the intakesound.

 

[petrolhead]

Anyone have experience with enlarging the air intake on a stock airbox ?
I could cut a maximal intake and try to shield of with, gaffer tape for instance, to try to work out the best intake size.
But i am curious if someone else has had good results and how much was cut ?

 

[zed1015]

The carbs will run fine with no airbox at all as they are designed to, so any material you remove won't be too much.
If you want to retain the stock look any cutting will have to be out of sight and if you want to retain the stock filter then only material prior to that but if the filter itself offers a restriction you will have to either find a higher flow version , K&N etc ,fit larger main air jets or both.
Depending on the final solution you should perform a top end plug chop to ensure you have sufficient main jetting to serve the increased air flow.

 

[petrolhead]

Great advice, I will cut the whole lower part of the back side and maybe fit a splash guard or custom metal ?snorkel?
I already have a K&N filter installed.
Thanks Zed !

 

[petrolhead]

I have cut back a rectangular shape and have doubled the size of the intake.
Still want to make a some sort of intake box guard and some dampening material, propably PPI 10/20
The battery box is in the way so i might solve that first.