Motor 4 Stroking like a son of a Gun!!!

[Rockspider99]

Hello All,

So I have run about 2 tanks of Fuel with the stipulated oil fuel ratio of 40ML oil to one Litre of fuel.
The issue I am having is it is 4 stroking and munching through fuel. I have tried adjusting the Needle to the leanest setting.
Found the Float was sitting very low, 19mm, so set that to 21mm but still seems to be over fueling. I have the HT engine mounted on the back of the bike, from what I understand about 2 stroke engines is the orientation of the motor does not matter as long as the carb is upright, but is there a chance that the motor leaning back like that could be causing the issue.
Ohh I have also modified the Intake manifold to get the carb level and checked that it is Air Tight. Lastly other thought is that the high oil to fuel ratio could be causing it to 4 stroke?

 

[HeadSmess]

who told you that adjusting the needle would make it leaner? so much dis-information on forum boards. hard to tell whats real and whats

you hammered the nail on the head but. its too rich!

just, the needle aint going to do a thing until you get the main-jet sized right for WOT. adjust it til your blue in the face, it still wont cure the four stroking. take the float back to the level it was at. was the jet submerged still? of course. raising the level just makes it richer still... less height to jet orifice, less vacuum needed to lift fuel, or, with same vacuum(no reason why it should have changed)...more fuel lifted. just like a hose will let more water out if you open the tap a bit more and increase the difference between the pressures at either end. (head or water level and atmosphere whilst in a carb its atmosphere and vacuum)

more oil makes it leaner, but its not really the best way to get an ideal mixture, as oil doesnt burn (much) and doesnt do anything for making power, just takes up excess space and makes a mess of everything.

try soldering very fine copper wire in the jet, one piece at a time, making sure you dont bog the hole up, or you will need a new jet or micro/fractional drills. (which are a worthwhile investment anyway...with a pin-vice chuck as well)

standard jet appears to be around 0.75mm... ive found these engines usually prefer something around 0.55, depending on engine, altitude and various other factors... what suits yours, is the part YOU have to work out

 

[HeadSmess]

p.s.

just curious...why rack mount when that frame looks like it would accept an engine quite nicely?

having all that weight up high and far back is just so awkward... especially when you open a gate or move the bike around the back or something...

not too mention the exhaust noise being reflected from the ground straight to your ears...

each to there own, not bagging...just asking?

 

[Rockspider99]

Hey HeadSmess,

thanks for advise, I rack mounted it because the carb was just too long, I have made a change to the intake and will try fit it in the frame this weekend. The bikes balance is not so great with the motor so high up and to the back, not to mention my behind getting a little warm.

Will setting the carb float lower not allow more fuel? The float lifts and shuts off the fuel intake as the bowl gets filled so bending the pins in would keep the intake open for longer?

 

[HeadSmess]

what determines the fuel height in the bowl is those pins, or the fork shaped thing in the carb.

the float pushes them UP, and closes the valve attached to the fuel intake.

so with the pins bent down, the float will lift them up earlier, closing the valve earlier, therefore the fuel in the bowl will be lower.

now, with the fuel lower in the bowl, the distance the vacuum in the throat of the carby has to pull fuel up from the bowl is greater. gravity sucks, remember! fuel has weight! this is basically equivalent to either less vacuum,(which wont change for a given speed anyway so forget about that) or a smaller jet... but note! only a very very very tiny little bit smaller!

the reverse applies if you bend them UP towards the carb...which can lead to incessant flooding if you overdo it!

i prefer having the fuel lower. i can live with a slight "miss" when i hit a big bump (the fuel sloshes around and the jet gets exposed briefly) rather than having to turn the fuel tap off all the time and getting crankcases full of fuel



usually tune with 0.05mm increments, thats what sizes metric micro-drills come in. (theres others but they cost as much singly as a full set of 20 standard ones do...about $20-$30) fractional/number drills are imperial and have slightly different sizes...handy for the ultra fine tune but not really required unless you really get into it. you also need one really good drilling machine to get them accurate enough to warrant the expense! all drills drill slightly oversize. process is simple. fill hole with solder, remove excess solder from inside of jet with drill until it reaches the brass of the jet again, then drill the tiny little jet hole itself.



the super fine copper wire works just as well... try things like the wires in headphones...just one strand of it at a time! if you can solder it at least without blocking the jet!


:wackoh yeah... and it gets a lot hotter if you have to get off in a hurry!

 

[Rockspider99]

Cool, now I just need to read up on how and where to solder on the intake jet.

 

[HeadSmess]

there is no intake jet!

it is sort of confusing at first.

there is a needle and seat. that is controlled by the float and sets the fuel height. the barb that the fuel line pokes onto, is the seat. the needle is the bit inside.

then there is the MAIN JET, which is (usually) what appears to be a small nut screwed into the end of that brass tube that goes through the float in the bowl.

this is the bit you adjust for the mixture, at WOT or wide open throttle. this requires PLUG CHOPS. search that and or just test riding. if it dies with full throttle...too lean, too small. what you have now? too rich, too big.


then you adjust the needle for easy starting and nice acceleration, it only controls (basically) half throttle and below... if its hard to start raise it. if it idles high then slows down and stalls if you dont blip the throttle... its too rich, lower it.

i lost the link... search mikuni tuning guide. its simple

 

[Rockspider99]

he he, thanks, I think my wording is all muddled, I have stripped the carb down about 4 times already in the 2 weeks I have had it.
"more familiar with WEBER/Solex Carbs, these are extremely simple but with the lack of air full mixture adjustment needles makes them pretty difficult to tune."
The bike pretty much 4 strokes through the entire range and I am pretty much Sea Level where I live, Cape Town.
So I will try making the Main Jet smaller with wire first. Also read up that the motors to tend to lean out as they get run in?

 

[HeadSmess]

dont mention the phrase "break in" or "run in" around me

 

[Rockspider99]

Curious now as to why I should not mention those things you told me not to mention.