ORIGINAL: Roary m
With my limited computer skills, I can't keep in my head how to post a link; don't laugh, I earned my memory lapse. If you take off the carb and look down the open rotating venturi drum, you will see the little brass rod running across the opening. The slit in that rod needs to be facing straight down the venturi regardless of altitude you are flying at. Now, with the engine running, if you transition the throttle up and small droplets of fuel seem to dance out of the carb, your bottom end needle is too rich. If you transition up and the engine sags and maybe dies in the throttle up without the fuel popping out of the carb, your bottom is too lean. Do turn in two maybe three clicks at the most before trying the fireup again. You will need to click the top end needle out as many clicks as you click the bottom end in since they work inversely proportional to each other. If you are holding the carb in your hand, you might be able to see the bottom end needle in the slit since it has a blunt end; the top end is tapered to a needle point. I apologize here for the wordiness, there are some very good links to ST tuning nomenclature out there. I have run these guys since around 1980 and I recognize they weren't ever meant for easy tuning, but they do run very well when you find the sweet spot; just demanding patience from you, I hear you.
Posting links is easy.. Highlight the link you want to post; click with right mouse button and click copy (or copy link location). In your forum reply box, click the "link" button under the smileys. Left mouse click in the "URL" box and follow that with a Right mouse click, then click paste. In the Link name box, Type the text you want the to show up as. Click "OK" and it will fill put your link into your message. Just be sure to start typing your reply after the very last "]".
On the original subject - I know there are many that have zero problems, as there are many with many problems tuning these carburetors. I am confident I can make it happen, but Getting past the random quitting is a milestone in itself. Based on Pé Reivers information, I looked at the barrel in my S90K carb and this is what it looks like:
Seems to be one of the "corrected" ones, though I did not map which angle the translation groove follows, but it looks far more linear than the "dog-legged" translation groove found in some engines. I dont perceive this to be my problem.
I used RTV silicone on top and bottom of the carb base's O-Ring, putting pressure on the o-ring using a medium sized vice grip fitted with rubber hose on the jaws to prevent marring of the carb surface. Not a huge amount of force applied, just enough to have the carb firmly seated. I doubt there are any air leaks anywhere on my engine. The muffler is of the perhaps "newer" variety; Its rounded off and sort-of funny looking as compared to other mufflers I've seen. It is not the square boxy older-style muffler. I had the muffler installed tight to the engine for break-in and after installing it on my boat, I then moved the muffler out so the end of the header protrudes into the muffler only a fraction of an inch past the flange/clamp area.
My engine retains its original fasteners, which are tight and stay tight when warm. I've used 2 different 16oz Texon tanks with 2 different standard clunks, and a 8 or 10oz Sullivan tank with an OS Sintered Clunk. No difference in running characteristics between the two tanks. I dont think the problems I've had are related to the fuel tank or its plumbing for the most part. There are some bubbles getting through still, so I think a Bladder tank is the answer to the bubbles, however the bubbles are not causing the quitting. The engine is not overheating. Car guys will spit on the cyl. head and depending on how long it takes for the spit to sizzle off will tell you how hot the engine is. Bubbling off immediately means its too hot, taking 3-5 seconds is in the "butter zone" and any longer than that and its not up to operating temp. This is kind of a general idea, and usually isnt too far off. Mine took about 6 seconds to bubble and 10-12 seconds to completely dry up. That tells me the engine is not scorching hot.
I really think there is something amiss with fuel delivery inside the carburetor. I will remove the spraybar and measure the I.D. of the spraybar nozzle, as well as the ID of the inlet nipple on the spraybar. If these are indeed smaller like Pé suggests, then perhaps there just isnt enough fuel flow to get a good mixture setting. It makes sense that if there is a lack of flow, opening the needle too wide to compensate would drown the plug going WOT, and if the idle needle is too rich for much the same reason, its just drowning in fuel.
I'm no expert as you can tell, but I'm trying like hell to learn to be one. If I ever get this carb dialed in A+ perfect, I think I'm going to submit a request to be the youngest person to get an original ST carb dialed in without replacing it with a Perry. I'd like to think its in the near future, but who knows. (I do try to add a small amount of humor into my posts every now and again..)
Happy 4th of July, fellow North Americans!