Yamaha KT100 Tuning

[aj992]

Before you all crucify me for creating a thread of a already greatly talked about subject please understand that I have read through boatloads of strings about this but cannot find any that pertain to my exact problem.

I have a KT100 with a two hole intake on a Walbro WB3A carb with a four hole can exhaust. I am able to get it to run fine at high speeds and actually idle decently while cold. However, at half throttle it begins to four stroke unless I turn my low speed down quite a bit but if I do this it bogs when I give it gas which isn't a huge deal I just turn my low speed back up. The issue that I am trying to solve is the amount of un-burnt gas/oil it spits out and leaves on the axle, I believe this is referred to as spooge. Now the easy answer to this is I am running too rich, which, as I was trying to point out before, is that I believe I have found that sweet spot between too lean and what would be just rich enough to go full out without bogging. Note: my high speed is around 1/16th to 1/8th of a turn during all of this and I just put a fresh spark plug in.

I have a feeling I may be running a too oil rich mixture of 1:16 as instructed by the previous owner. A friend suggests I need to clean/rebuild my carb which I do not know when the last time that happened was as I have not owned the kart too long. Finally, my father suggests that "this is just what two strokes do".

In conclusion, I am trying to put an end to the gas/oil/spooge that ends up on my can, axle and frame if possible. Thanks for the help.

 

[NewbCarter]

Just my 2 cents , Yamaha factory recommended is 5% ( 20:1 fuel to oil)

If my math is correct , 16:1 is almost 6% , which i have not yet seen on any 2 stroke engine to date .

Id place my bets on the mixture just being too oil heavy ... is the engine modified ?

 

[itsid]

16:1 up to 25:1 is all not unseen
(in fact some even risk the lifetime of the engine to gain a bit snappier reactions with 30:1 on synt oils.. which I wouldn't recommend tbh)

Anyways, the question is WHAT OIL?
there are several good two stroke oils on the market,
personally I felt mineral oils work better than synthetic ones most of the time
(for unknown reasons ..maybe psychosomatic ones )

What you MUST NEVER do is use just any engine oil
(that'll not mix well enough and cause all sorts of problems.. spitting oil on the axle is the least important one)

Castrol R30/40, Shell M..
LiquiMoly has a prokart oil that isn't all that bad for all I know...
But make sure it's two stroke mixing oil and it's not the cheapest crap you could find.
(good oil pays off in lower repair costs!)
You have to test some to see which gives you the best performance
and by that I mean smell/dirt/sound as well as throttle response

different oils can be used at slightly different ratios (depending on their specs)
to keep your engine as safe as possible during testing.. 16:1 is indeed what I'd recommend to stick with for now.

As soon as you settled in on an oil you feel very comfortable with using.. you can lower the ratio to 20:1 or such ...

Keep in mind that the KT needs a tight maintenance schedule (as all 100cc two strokes do)
So get a service manual for your specific engine and learn to inspect it properly.
(that'll also help on deciding which oil works best with your setup)

'sid

PS As James May put it..
a car engine lasts 150000 miles with good oil no problem.. w/o oil it lasts maybe 4minutes...that should give you an idea how important good lubrication is.

 

[aj992]

Thanks for the reply Sid

I am currently using what the previous owner also instructed me to use - https://www.amsoil.com/shop/by-prod...-synthetic-2-stroke-racing-oil/?code=TDRQT-EA with 91 octane non-oxy gas.

 

[itsid]

okay.. my antipathy against synthetic oils aside, that looks like a decent product..
but you read the link you send me, right?

I mean the "50:1" ratio suggestion part especially..

chances are that oil needs a much lower mix ratio in order to not spill out your exhaust unused..
50:1 is something far from what I'd even wanted to try tbh...
BUT I'd mix a batch in 20:1 and see if that reduces the problem without
overheating the engine
(since increased heat means increased friction... and too much friction means too little oil... you can follow that thinking, right?)
if you engine still spills oil.. and the temps are just fine 25...
if that does the trick so be it...
if not... frankly I'd swap the oil first to see if that makes any noteworthy difference.

Also it might be a good idea to check your engine NOW..
remove the intake and spark plug,
make sure the piston is at TDC,
pour a cup of gasoline in.. slosh it around, drain it
repeat until it comes out as clean as you poured it in.
ocasionally turning the engine over by hand doesn't harm here

then put in a cup of your two stroke oil,
slosh that around and drain that as well as good as you can..
the engines crankcase should now be considered clean and free of unwanted oily residue.

reinstall the intake (decide if you want to clean the carb while you're at it)
and then mix a fresh batch of fuel...

see how it goes.

Hang on.. you know you must not put oil in the crankcase, right?
I mean if you filled it like a four stroke, no wonder it spoils your axle

Just popped to my mind, forgive me that I point out the obvious, but at times people
just don't know the most basic rules

'sid

 

[Desertduler]

16:1 Ratio is correct for the Yamaha KT100 engine as the engines max rpm range is 15400.
Too little of oil in the fuel at those rpm's and you are risking an engine seizure.
Remember that a two stroke engine air fuel mixture is drawn into the crankcase first and then the air fuel mixture enters the cylinder via transfer ports so what little of oil is deposited on the engine parts that need lubrication is not a lot and the higher the rpm of the engine the more of the oil that is deposited will be lost because of internal engine windage. So when you see spooge coming out of the pipe that is oil that has done it's job and that is good for a two stroke engine. I use 25:1 to 20:1 full synthetic oil in my two cycle engines but they don't run at near the rpm range of the KT100. You had better listen to the original owner as he is correct. I had a buddy that raced karts and he used the Yamaha KT100 on his karts and he mixed his oil fuel mixture at 16:1 and his engines lasted quite awhile before they needed freshening up.
http://www.kartbook.net/forum/topics/kt100s-fuel-to-oil-ratio

 

[itsid]

You had better listen to the original owner as he is correct...

He did listen to PO.. down to the oil he recommended.. so not a problem there

Oh and just so you know I didn't pulled my 20:1 option out of thin air
https://www.yamahamotorsports.com/kart-engine/models/kt100/specifications
20:1 is what Yamaha lists on their site as premix ratio for the KT100

unfortunately that's an engine made for decades and thus some things might have changed over time...
(my guess is premix ratio from 16:1 on mineral oils to 20:1 on synthetic oils but as I said: just a guess)

Oh btw:
yamaha australia still holds the sec-sej handbook
So in case you don't have a manual with your engine, this is at least a fair reference:
http://www.yamahaownershandbook.com.au/lib/download.php?f=7YP-28199-SE.pdf

ideally you'd want the one for your exact model kt100 since there are subtle differences of course.

'sid

 

[Desertduler]

Sorry I missed that in his comments. I better sit back and let the ones with all the knowledge proceed.

 

[itsid]

Nooo nono, do not just sit back please..
you said something very interesting (to me that is)
...
[full synth 20:1]... that kinda peaks my interest, I always had troubles with full synthetic oils
The only engine that kinda ignored that was a self-mixing Honda MBX bike back when I was a kid.
Nearly everything else spooged like crazy (until I reduced the amount of oil slightly )
NOW the question..
You think I'd better adjusted the carb to lean it out a hair instead,
or was it that some engines just don't like synthetic oils?

'sid

 

[NewbCarter]

Nooo nono, do not just sit back please..
you said something very interesting (to me that is)
...
[full synth 20:1]... that kinda peaks my interest, I always had troubles with full synthetic oils
The only engine that kinda ignored that was a self-mixing Honda MBX bike back when I was a kid.
Nearly everythign else spooged like crazy (until I reduced the amount of oil slightly )
NOW the question..
You think I'd better adjusted the carb to lean it out a hair instead,
or was it that some engines just don't like synthetic oils?

'sid

As far as im informed ( dont quote me on this , i asked around on JICA and F100 forums ) .

People say that nowdays it doesnt matter much , unless you run Dykes type piston rings , which can be a bit more prone to gum up from excess oil , in which case high quality castor type oil is advised .

Iv run full synthetic on nearly anything i got my hands on , from chainsaws to the KT100 , and if correctly set up , there shouldnt be any difference ?

Also , question for my own sanity : Why would one run an engine like the KT on 6% if its originally meant for 5% ?

 

[itsid]

See that's what I meant.. it's difficult to tell what it is MEANT for just by a calling it KT100
As I said this engines was made for 40 or so years if I'm not mistaking (70s to 2010s)
and there are several iterations of it, not just cosmetically different but technically different.

And I know that indeed older ones came with 16:1 recommendations (always pointing to a specific oil btw) engines 20 years or so younger with different recommendations (pointing to different oils of course)

And that's where I'm confused as well..

My thinking was that with the "lesser refined" oils from the 70s the engine wants a bit more lubrication in order to maintain a steady oil film.. (that's what we care about the most!)
and with the oils getting better and better at lubricating the engine,
less and less oil is needed to do so,
thus the reduction of oil in the specs (the ones I pointed to are likely from the 2000s),

and my extrapolation of that into nowadays quality of fully synthetic oils.
(recommended under close observation ONLY!)

Especially since I had that exact problem with fully synthetic oils myself,
and that a little less of it solved the issue for me
(without adverse effects as far I was able to notice)
OR what I felt more comfortable with older engines personally
is to just use the mineral oils that were around when the engines in question have been built.
(hence my slight aversion against fully synth oils in engines like these...)

And that's where Bret comes in (to my rescue mostly)
he not only owns more two stroke engines today than I owned in my whole life,
he also started to work on them before I even was old enough to have one,
and still works on them today
(a point in time where I may still own one or two but haven't really messed with them for about a decade or so)

So saying he knows more about it than I do is quite an understatement

Seeing him run fully synthetic oil in a 20:1 mix without covering the road in oily residue makes me wonder if I could've just tweaked something else instead of the mixture ratio.

'sid

 

[Desertduler]

Nooo nono, do not just sit back please..
you said something very interesting (to me that is)
...
[full synth 20:1]... that kinda peaks my interest, I always had troubles with full synthetic oils
The only engine that kinda ignored that was a self-mixing Honda MBX bike back when I was a kid.
Nearly everything else spooged like crazy (until I reduced the amount of oil slightly )
NOW the question..
You think I'd better adjusted the carb to lean it out a hair instead,
or was it that some engines just don't like synthetic oils?

'sid

Here is my take on synthetic two cycle oils. After many years of desert racing two stroke powered dirt bikes and using full synthetic two stroke engine oil in all the snowmobiles and chainsaws I have and having more than 40 years of engine repairs and modifications I can tell you that indeed there is a difference in synthetic oils.
I have had a lot of engines apart in my time working on them and rebuilding them etc. One thing about oil mix ratios one has to remember and this is very important that when the oil mix ratio is say 16:1 the fuel is more viscous than it is than if the mix ratio is 20:1 for example. That being said say that you had been running a engine with a 20:1 oil mix ratio and you have set up the air fuel mixture correctly depending on the carburetor that is in use either fixed jets or adjustment screws and then you switch to a 16:1 oil ratio now that the fuel has become more viscous in order to achieve the correct air fuel ratio for the engine the carburetor settings will need to be set richer than the previous settings that were set up for the 20:1 oil ratio as the now thicker fuel resists going through the adjustment needles settings which is just the taper of the adjustment needles changing the size of the fuel circuit delivery or the orifice size in a fixed jet carburetor. This is where some people get confused because air fuel ratio and oil to fuel ratio are different. Some people think that when you mix a heavier oil ratio mix that you are richening up the air fuel ratio when indeed it is being leaned out due to the increased viscosity of the fuel. That's right just what I said increased fuel viscosity. I have seen two cycle engines burnt up and seized because of what I have just stated. So just the opposite will happen if say the engines air fuel mixture was correctly adjusted for a 16:1 oil ratio and then the oil mix ratio was changed to 20:1 then the air fuel ratio would be richer and then for maximum engine output the air fuel ratio would need to be leaned out a bit. That being said take note of this fact and adjust the air fuel ratio accordingly as needed. Spooge is a normal thing for two stroke engines. Run them hard and flat out and the spooge will reduce if not go away.

 

[itsid]

Thank you..
So keeping the fuel mix ratio fixed
decreasing the oil viscosity by changing from one to another flavour of oil
the overall viscosity decreases as well
and the fuel/air mix needs to be adjusted
(leaned out in this case, richened up in the case of swapping to a higher viscosity oil)

That's what I didn't think of too much back then..

my thinking was.. as long as the engine runs nicely
(not too hot, sparkplug looks good, idles fine, revs up as expected etc.)
the air/fuel mix is as it should be (say as you'd think on a four stroke engine)

Therefor excessive spooge to me back then was just incombustable fluid that leaves the engine.
reducing the incombustable part of the fuel/oil mix (i.e. carefully reducing the amount of oil)
did the trick.

I can't say that I remember any of the engines idling or running differently tbh,
but as I said too long ago to remember the details.
it went 'fine' with me
and yes I went from IIRC 20:1 to 24:1 maybe 25:1 *shrugs*,
but nowhere near anything crazier than that,
instead -as said- I swapped back to the previous oil which made me feel more comfortable
in case that slight oil-mix adjustment didn't help too much.

And sure spooge is normal, up to a point..
but if you fling a teaspoon or two of greasy smelly mostly sticky liquids onto the road every second you quickly feel uncomfortable with running that enigne in public...
I painted a silver car charcoal grey at a stoplight once... that wasn't very welcome by it's owner

Anywhoo.. compensation through air adjustments would've worked as well.

GREAT!

So Alex.. you heard the man
if you have excessive spooge adjust your carb and see how that goes.
if that does NOT work out to reduce spooge to acceptable amounts,
either adjust your acceptance levels *giggle*
or try a higher viscosity oil before you play around adjusting the fuel to oil ratio.

The latter did it for me,
but maybe I was just lucky enough to never see the long term adverse effects it may have.
So be carefull .. your engine, your responsibility;
and as far as I'm concerned: I'd rather listen to Bret before I take any chances

'sid

 

[aj992]

Thanks for all the help guys. I ended up ordering some Yamalube 2R and am gonna give that a try... And just for the record, no, I did not put oil in my crankcase but I can't blame you for asking, sid