Keihin is a very good carb manufacturer . However 12 yrs ago the epa stepped in and mandated lean burn carbs . The main jet and secondary circuits are very small . Any residual film will cause hard starting . I use ultrasonic cleaning now on all my small engine carbs . I would suggest using concentrated carb cleaner ( 2 ounces to tank) . This will dissolve any residue over night . Startron is a great product . However if you are using e fuel ensure your fuel is treated ( 1 ounce to fresh fuel ) then run the carb dry prior to long term shutdowns ( over 30 days) . I had the same issue with a 1995 800 watt Honda with the Keihien carb . The Pilot jet ( Starting circuits) are so fine that any film will cause poor starting and initial need for extended choke until the treated fuel removes this secondary circuit waxing . The emulsion tube within the pilot jet orifice has several very minute holes , which can wax over in the blink of an eye and cause fuel starvation upon start up while using efuel . Should the over night soak not dissolve the obstruction , you will need to clean the emulsion tube and pilot jet orifice manually with a tag wire . Your correct this is a simple float and bowl style carb also often you cannot see the transparent white waxing film . Give it a try . P.S. Some carbs had a solenoid operated fuel shutoff on the bowl bottom on electric start models . Worst case scenario new oem units available from $17 - $24 .

What the problem is STABIL knows that the RED STABIL does not work that
well with ethanol gas...have torn apart many carbs that used the RED
STABIL with ethanol gas and it leaves a red dusty residue that clogs all
jets and passages...that is why STABIL has come out with the BLUE
STABIL for ethanol gas...if you use RED STABIL with ethanol gas you'll
be sorry especially if it sits any time period.

Thought I'd come back and probably answer y own question. And thanks ssewalk for your thoughts on fine ports and sonication. I don't know what happened to your post. Houzz is getting a little squirly. I had never noticed a red residue with Stabil, but it might be a good point mule meat.

However, I'm pretty sure that I did myself in and just now solved my no start unless primed with fuel directly into carb. My choke has a complicated linkage that could be hooked up two ways, neither of which appeared to be reasonable and I chose the wrong one years ago after a cleaning. I guess the correct choice left the choke still slightly on even after a vacuum controlled lever pulled it further open. Years ago, I probably didn't even see or understand the vacuum and it appeared more closed while unchoked.

Anyway, this morning upon checking and seeing that the engine wouldn't start, I realized that maybe the choke needed to be fully closed to pull fuel to the carb instead of the 3/4 closed that my choke would allow in the other linkage. I was able to put my hand over the choke port while pulling the starter rope and it fired right up. I reset and rebent linkages so the choke remains mostly closed throughout its movement and it starts fine while fully closed. The open choke linkage plus vac. does get the choke almost completely open and I get no speed up if I push it the last 10% so I guess it is open enough.

So, short point: The choke must be fully closed to get this non diaphram carb to start.

Mule , he stated more recently Startron , solves stabil issues too lol. Chao45 , Yep a choke linkage or auto choke failure ( on mowers) will also cause issues , actually not pulling more fuel but rather drawing less air Bud ! P.S. Have to smirk @ Stabil users green , red , blue . Actually the Blue is the Marine product used for cleaning during use of e fuel not as stabilizer . However , yes I have witnessed the problem with the Red Stabl . Thus , Colour me Startron !

I have used Ethanol Shield for a number of years and like it a lot. It works in both regular and 2 cycle mix gas very well. I run some treated gas through my machines about every three or four fill ups, The 2 cycle mix I mix 2 gallon at a time and treat every other mixing.

My Daughter has a 2008 Honda Rubicon 500 ATV. When I fill her tank with Ethanol Shield treated fuel it seems as if the engine runs cooler and she get a little better mileage.

I pretty well have stayed with Startron since it both cleans and stabilizes , liked Lucas top end treatment previously , however it did not stabilize . Ethanol Shield is advertised as a stabilizer only . Mechanic in a bottle is the sister fuel conditioner cleaner . It,s been over eight yrs with Startron initially for my Diesel application,s Truck and Gen Set . I use Amsoil Sabre Syn Oil in my saws and snowmobiles and trimmers it has stabilizer / conditioner already in it . They have to find more effective efuel treatments since their sneaking more ethanol in every day . 10 % today is likely more accurately 13% . P.S. Startron also advertises gas mileage and hp increases , not sure about that , however the engines start everytime lol . Hell back in the 60,s we used a few ounces of ATF. to a tank of fuel for valve and carb cleaning . Some still preach MMO another solvent based product !