OKO 26 flat slide - choke and screw controls

[numroe]

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.

Setup: OKO 26 flat slide carb. On a Lifan 150. Running fine. Well except for cold starts. Mainly wish to learn here.

OKO26 mixture screw: AIR screw right? My mixture screw has a very "narrow" sweet spot in terms of idle quality and low throttle response under load. Is this normal/typical?

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.

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.

 

[Cactus Jack]

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 ....

 

[roost666]

dam your good cactus

 

[numroe]

Excellent. Thanks Cactus!

OK so poor cold start performance with the OKO26 flatslide (and similar carbs) seems like a fact of life. No probs.

One thing I've noticed with the choke pull nob, is that when it is up/enabled the idle speed goes up a little but so does the exhaust loudness. It gets a real deep boomy sound. I assume this is the air/fuel mix being overly rich - right?

Regarding my air/fuel mix in general and the possibility of running dangerously "wet" and bore oil washing: With my non-AP carb, I know I am on the rich side of the perfect burn, but wondering (again for learning) how rich is too rich. I shall try to explain...

I find it pretty easy to pick my favored jet (main or slow), or needle position via my 3rd gear rolling bike throttle open-to-some-posn tests. Meaning there is clearly one jet size or needle position where the engine runs way better for my normal on/off throttle riding. Actually I can always go one setting leaner and get a cleaner rev out for that throttle posn, but the throttle response is then poor to very bad (lean bog). At present my engine responds very crisp to any reasonably quick throttle opening. Of course it is not perfect and with certain throttle and RPM variations I can still lean bog it, but I can open it fast enough that there is no real problem. So what I am getting at here, is that since my engine is still capable of revving out reasonably clean and clear, then is my fuel dose under some threshold where the wet bore syndrome is not coming into play?

Another way to phrase my question: Does a clear, clean and crisp exhaust note (albeit a deep note) mean a safe air+fuel vapor burn?

An example with the main jet: If I go one leaner in the current climate and I will have a accel lean bog on anything but the slowest throttle turn to wide open for sure. But if I go one richer then it runs as rough as guts and wont rev out at all. A fine line. Easy choice for me in terms of fun and performance. But for reliability. Fresh engine oil often has to help right?

Edit: The other issue is engine temps. At present my engine never seems to get overly hot. Just nice. Go leaner and run too hot. Go to rich and possibly wear out the bore. As Bart Simpson once said "damned if I, and damned if I don't". It's running great right now, and it's cheap to repair. So I am not worried at all. These topics are just interesting.

 

[Cactus Jack]

You're on the ball when it comes to tuning and maintaining your engines to suit your own riding style and usage so you'll have no problems .

It's best to run your engine on the slightly richer side for general riding where it's copping heavy duty use for extended periods and gets pretty hot . And as you've stated , how you throttle and control the engine also affects how the fuel burns in the cylinder . IF it's running clean and crisp , then it'll last a long time .......

For bore wash damage to occur something fairly drastic would have to be wrong .... ie the float level too high or the jetting way off for an extended period of time ....... Riding with a gunked up air filter is similar to riding with the choke partially on and is another factor that can cause the engine to run too rich .

With pit bike engines it's no big deal since it's mainly the rings that would cop the accelerated wear and complete cylinder and piston kits are fairly cheap ......

You run a fuel star ? , high quality oil and change it regularly ....... you can also use an upper bore lubricant such as valve saver or castor oil in the fuel like they do in speedway race engines ......

 

[numroe]

For bore wash damage to occur something fairly drastic would have to be wrong ....
You run a fuel star ? , high quality oil and change it regularly .......

Thanks for the confirmation. I am not excessively rich by your defn.

Yes, Fuelstar TM-VC in the tank. Haven't taken it out since new. Maybe a fair bit to do with the good performance. Yes good oil and change it for every ride.

 

[Cactus Jack]

I'll be running a fuel star as well ..... but I'm a bit wary of the tank mate ..... I have visions of it rubbing bits of plastic off from the inner tank walls ....... Have you noticed any particles trapped in your fuel filter ????? .....

You can't run too fine a filter with them either as it'll trap the tin particles .... they state for you to remove the built in filter from certain car carbs ........

 

[I_THUMP]

Cactus have u seen this carby? Numroe, Masterspoon or anyone else know anything about it?

Mikuni VM28-418 flat slide 28mm carb TM28 carburetor TM - eBay Other Makes, Bike Parts, Motorcycle Parts, Accessories, Cars, Bikes, Boats. (end time 26-Jul-08 21:05:00 AEST)

I started a thread asking about it and I thought u might know

http://www.miniriders.com.au/forum/mini-chat/18391-mikuni-28mm-flatslide-carby.html

 

[numroe]

I'll be running a fuel star as well ..... but I'm a bit wary of the tank mate ..... I have visions of it rubbing bits of plastic off from the inner tank walls ....... Have you noticed any particles trapped in your fuel filter ????? .....

You can't run too fine a filter with them either as it'll trap the tin particles .... they state for you to remove the built in filter from certain car carbs ........

I put a little TM-VC in my SR, my KX450F and also my 7 yr old daughter's 70cc ducar. It made a difference in the 450 and 70, so I put it in the SR when new. I carefully sand/linish off all sharp edges before I drop the thing in any tank. Then no way it can chip plastic. No probs with the tin blocking my filters either. On the 450 I inspect the filter (in the fuel tap) and no probs. In the SR I have an inline clear plastic filter of the fine steel mesh variety and it looks clean. Same on the 70cc. The tin particles are I believe super fine.

I have to give the little fuelstar device a wrap. I think it makes my OKO26 seem better than it really is. For $30 it's worth a try!

The 70cc semi-auto runs unreal. Real grunty. I took your advice and set the valve clearances. It idles perfect. She leaves it in 3rd gear all day and just rides. It handles mild hills. It has tonnes of torque for a 70cc and just never gives any trouble. e-start in gear is a 2 sec process. The bike is lowered and softened up, but friends or on lookers often ask why I let such a small/young girl ride a 125 engine. Love it. I think the fuelstar has a bit to do the the result.

In the 450 the TM-VC results really blew me away. Fuelstar said I need a much bigger inline device, but I refused to do that because I don't like the extra weight (200g is pretty significant in my books). So I tried the tiny in-tank unit. I knew my 450 engine very well before trying it. What shocked me was the clean crisp running when riding in slow 2nd gear rut corners and lugging the engine down to idle speeds but using a lot of throttle opening and near zero use of the clutch - and the bike hauling out of there nicely. The 06 KX450F has a bottom end which is so good in my books, it's ridiculous. It makes me very lazy to ever fan the clutch. BTW, the 08 KX is similar. But the TM-VC made my engine even more usable. Pre fuelstar, I had to use the clutch at times. A stall is so harsh and sudden in good traction dirt that I often used to cop the cross bar in the guts. Like I said I got lazy to clutch fan and got caught out a few times. With the fuelstar, i just could not believe it. Admittedly idle RPM is about 1900 so the piston is still going up and down a bit. It just sounds like real low RPM.

Getting way off the OKO26 topic but these days I went one step further with clutch free riding on my 450, and installed the rekluse zstart pro clutch. That thing is unreal when setup right. Mine is setup to engage as smooth as silk (27 balls) from 1950 to 3000 rpm (low rpm spring). So I have true clutch free riding now and never stall unless I do stupid on/off throttle things. At 2500+ rpm I don't really know it's there, except at high RPM the clutch fan bite is so positive. Pretty amazing to have an engine that pulls like a daemon from 1900 to 10500 and ride with no clutch usage except I flick it when shifting gears. I have an hr meter with a peak RPM memory function. Redline is 11250. It can hit 13000 on ugly jump landings. Normally I am under 9500. Anyway, maybe you see why I expect a lot from the throttle of my Lifan 150. For the price, I think some China bikes can be real nice.

 

[Cactus Jack]

Great read .... Tin is a better octane booster than lead .... and if the fuel star keeps carbon from coating the piston then it'll definitely make a noticeable improvement ......

I used to race a worked XR75 in a minicycle club .... I tried every possible mod and one of the main mods I did was to keep the combustion chamber , piston and valves highly polished and carbon free .......

That allowed the engine to run super smooth , crisp , clean and snappy ..... it never lost power or ran differently when hot ie stall , hesitate etc .... As carbon gets hot it starts to glow and alters the ignition timing and fuel mixture requirements ..... if you can keep the carbon off , the engine stays in a more consistent state of tune ..... PLUS makes more power everywhere .... the ONLY thing you want to light the intake charge is the spark plug .... timed to exactly where you want it lit ........ you don't want hot carbon to start the intake air from losing density either ....

What you have found fits like a glove with what I've found ...... even the low speed chuggability and smoothness is spot on until carbon starts building up ...... I met an old German aircraft Engineer one day while riding my bike and he told me about how water injection was used to keep carbon deposits down in the old dog fight planes ..... he told me that I could use water to avoid having to tear the engine down to manually clean the carbon off ........ which I did once a week a few days before race day ........

I tried what he said then pulled my engine down and bingo , he was spot on ........ the water made the carbon flake off ..... he also put me onto Methyl Benzine (Shell 115 MB) and that stuff really made the engine run well ...... LOL ......

As for slip clutches ...... you don't have to convince me of anything ...... most manual drag bikes and cars run slider clutches and a lot of super sedans etc run high stall autos ..... One of their main features is that they maintain more consistent power to the rear wheels and are easier on parts ...... plus they out launch manuals off the start line ....... but that's whole new subject .......

As for gear ratios ...... they make a night and day difference to the performance of small bore 4 strokes ....... I know first hand since I ran a close ratio racing tranny conversion in my 4 speed XR ....... at a time when there was no such thing as a 5 speed XR75/80 etc ........

Honda had the SL70 first , then they brought out the XR75 ...... Both were 72 cc 4 speed engines but the vertical XR's blew the horizontal SL's into the weeds ...... the XR's 4 speed had far superior gear spacings ....... the SL's had the exact same crap ratios that todays china engines have ...... The XR's had far superior airflow thru the intake , head and exhaust pipe ....... The exhaust was huge compared to the skinny SL pipes ......

It's good to finally see that most bikes now have 32 mm headers ...... SL/XL 125 race pipes are 33 mm ....... a lot of people used to claim that skinny 23 to 25 mm pipes made more power ...... It's amazing how that line of thought has disappeared pretty quickly ......

 

[numroe]

Chuggability - that's the word. Interesting about the combustion metal surface conditions and the WW2 planes with water injection and your XR's results. I think the F1 cars used water injection in the late 80s turbo era to permit usage of higher boost - maybe related to this topic too.

The chamber, crown and exhaust port of my Lifan (140 and now 150) and 450F always have a fine black soot layer on them, but I've not (yet) found any carbon deposits. Nor have I polished anything.

With the 98 fuel and the fuelstar treatment, I guess I am avoiding hot spot induced detonation to some extent, but your comments on polishing some surfaces has got me thinking! I have a dremel and my hands, and can buy whatever. Assuming some gains with just removal of fine soot, what would you suggest - next time I need to fiddle but cannot ride. Summer nights I often enjoy some bike work.

No probs to try it on the Lifan one day. To drop the head off and back on my Lifan, I think the slowest step by far is fiddling with the exhaust header and bashplate. Still in 15 mins I'm staring at the piston. I am keen to polish the piston crown with just the head off. Of course, I'd polish the chamber at the same time. The next question is, do you reckon I could do it without getting any abrasive paste in the piston to bore gap with the piston locked at TDC and flush with the cylinder?

 

[Cactus Jack]

Theoretically the fuel star should be already doing what the polishing does ...... Keeping my XR's chamber carbon free allowed me to run it lean with the ignition advanced without the risk of detonation .... as a result it would also lug taller gears smoothly ......

I run PK or Venolia Hi Dome pistons ....

I mainly radius any sharp edges to eliminate hot spots ...... I do all polishing by hand with fine wet/dry paper .... You could probably poke a strip of oiled card board or string down the side of the piston to prevent grit getting down there but I don't like the idea of doing it on the engine .....

I did some checking and found that Turbo F1 cars of the late 80's ran 86% methyl benzine and it's also added to Optimax and other premium unleaded fuels to raise the octane level ......

http://www.shell.com/static/au-en/downloads/fuels/tds/shell_methyl_benzine_tds.pdf

[Toluene can be used as an octane booster in gasoline fuels used in internal combustion engines. Toluene at 86% by volume fueled all the turbo Formula 1 teams in the 1980s.]

Toluene - Wikipedia, the free encyclopedia

 

[Cactus Jack]

Cactus have u seen this carby? Numroe, Masterspoon or anyone else know anything about it?

I started a thread asking about it and I thought u might know

http://www.miniriders.com.au/forum/mini-chat/18391-mikuni-28mm-flatslide-carby.html

I've never actually used one but they should be at least as good as an OKO but better made with spare parts etc more easily available ........

If they had an accelerator pump they'd be heaps better .....