Cactus or anyone, I'm curious about a couple of things, and searching yielded too much and too little info if you know what I mean.
Q: OKO26 mixture screw: AIR screw right?
A: .... YES ..... turning it out lets more air in .... turning it in lets more fuel in ... so the pilot jet is more critical .....
Q: My mixture screw has a very "narrow" sweet spot in terms of idle quality and low throttle response under load. Is this normal/typical?
A .... YES .... That's why I find the "neutral" flame point where the engine responds best under all conditions by screwing the mixture screw in until the engine speed or note drops , then out counting the turns until the note changes again .... then screw back in by half the amount of turns .... Lower the idle speed and repeat the procedure until I get the steadiest and slowest plonking idle . As the idle drops lower the engine becomes more sensitive to movement of the screw .... What you're doing is homing in on the perfect blue flame aka the "sweet" spot .... It's similar to setting up an oxy torch to a nice blue neutral flame to achieve the cleanest burn where it produces smooth controlled heating .... too much fuel and the flame starts to cool and carburize ... too much air creates too much heat and starts to oxidize things ... A neutral mixture is stable and consistent giving the best idle .... no fluttering or stalling .... and the engine stays in tune over varying temperatures and atmospheric conditions from day to day ....
Q: External mixture screws in general: On the bigger FCRs, these are on the bottom. In the simple OKO26 flat has its air screw on the side near the carb outflow/manifold. What's the story behind the diffs? The FCR is obviously more complex and expensive but my little OKO seems to work pretty good.
A: The OKO has an air screw and is basically a 2 stroke carb that has been altered to suit a 4 stroke .... the emulsion tube is perforated to work better in a 4 stroke engine and the jetting has been altered to be 4 stroke friendly .... ie since 2 strokes act as a twin cylinder engine of double the capacity of a 4 stroke of the same bore size , they have a higher air speed and twice the intake pulse frequency of the 4 stroke .....so they require larger jetting ....... Obviously , a 26 mm carb on an 80 cc two stroke will work really well because it's getting fed the airflow and vacuum of a 160 cc twin whereas the 80 cc 4 stroke is only flowing the air of an 80 cc single .... This is why I try to educate people about 2 strokes compared to 4 strokes .... you're looking at 80 cc of airflow every crank turn versus 80 cc of airflow every TWO crank turns ... 80/2 is a lot bigger than 80/4 ....
The FCR is a 4 stroke specific carb so it has a fuel screw at the rear ... the idle mixture circuit bypasses the slide and needle jet and trickle feeds fuel into the manifold in order to get around the 4 strokes poor vacuum and wider spaced intake pulses at lower revs ... The fuel screw carbs are supposed to give better throttle response over an air screw carb .... but IF too much fuel is leaked in they can cause rich running .... which can be lethal to the life of an engines bore due to the excess fuel washing oil off the cylinder wall ....
According to the "fuel screw" carb link on 4 strokes.com you adjust them to achieve the highest idle speed , then lower the idle speed via the idle screw ....
XR Hondas have always ran air screw carbs ... that's why they run so good and the bores last just about forever .... unless the float needle is stuffed and the engine is run too rich for extended periods ....
Q: The choke pull/nob on the OKO26: How does it work? I think my engine needs a little more fuel for cold kick starts, since if I expose the carb intake and get someone to put their hand over it for just one or two kicks (ie. literally choke it), then my engine starts up pretty damn quick.
A: Pull chokes (plunger type) are 2 stroke specific .... they allow raw fuel to bypass the slide once again .... Remember .... 2 strokes use the upstroke of the piston to pull fuel/air into the crank case , the crank churns the mix up and they have a reed valve to prevent any reverse flow back thru the carb .... effectively creating a one way pump .... so they are primed and fed from the cases .... 4 strokes have to create a strong vacuum in the intake manifold to pull fuel up thru the needle jet in order to prime themselves .... so opening the throttle (lifting the slide or having nothing to block off the intake air flow lowers the vacuum to the point where it can't pull fuel up from the float bowl) They need a flapper door type choke or butterfly choke OR a manual primer to squirt fuel into the manifold to get some fuel to the cylinder .... You've answered your own question in stating that the engine starts easily if you get someone to choke the carb with their hand .... Squirting fuel in manually does a similar thing ....
That's why 4 strokes should be kicked without touching the throttle or at least only opening it slightly .... FCR carbs with accelerator pumps should work good if you turn the throttle lightly to squirt a bit of fuel in without flooding the engine .... then kick hard with your hand right off the throttle ...
You can damage engines by running them too lean .... but you can also damage them even MORE by running them too rich ....
I personally don't like using chokes since rich running causes fuel wash ..... but on the other hand kick back isn't good either .....
The problem most people have is that they learnt to ride on a 2 stroke so each time they kick they have a spastic response and twist the throttle which drops the vacuum on the jets ....
