Mikuni flooding when engine reved

[Gippslander]

My '73 CR250 Husky is still being a pain, it fuels up badly and drowns itself.

At the recent Yanake VCM meet a friend suggested I lower the float level, so I did, it was level, so I lowered it 1mm, and then I started the bike and warmed it up, and I sat on the milk crate next to the bike and gave it the occassional twist on the throttle and each time I really reved it it flooded, I can see it flooding because the Mikuni has a float chamber overflow tube -- and sure enough it does not flood when it's idleing, but give it a rev and it floods almost straight away -- fuel running out the overflow tube like the "tap's on".

Can it be that the vibration is opening the needle and seat 

Anybody had this before?

Should I lower the float some more to put "more pressure" on the needle and seat?

How low can you run the float in a Mikuni and not get fuel starvation?

Suggestions welcome 'cause I'm a bit stumped...

 

[dirtrumpy]

I had a similar problem on my Can Am, would pour fuel out of the carb overflow and die on the overun when reved, I traced it to a blocked petrol cap breather!

Mike #278

[Gippslander]

OK, I'll check that out when I get home later...

[Gippslander]

Doing the 'ol google search -- found this:

Adjustment is made by bending the float arms, not by bending the tang. Each arm of the yoke must be measured and adjusted separately, and the arms must be even.

So I had better re-do that -- I bent the tang, rhymes with "dang" 

[Lozza]

Got the right needle in it?

[Gippslander]

Not sure about the needle and seat being the right one -- I did not know there were different ones.

It is a new carby, only recently purchased here in OZ.

When I was setting it up I purchased a set of jets from TD Hatrick and at that time got a spare needle and seat from them so it might be worth a try.

I know it's not a real test but when you put a little air pressure "by mouth" on the needle and seat it is closed the moment the brass tang touches the needle.

At this stage the question I'm looking to have answered is, how low can I go with the float level and not get fuel starvation problems, or is like the jetting where it's just a matter of trying a bit/mm at a time until it starves then coming back up a bit/mm?

[bazza]

slightest spec of dust can hold float needle "open" Is the float needle rubber tip not damaged?Set float height to correct specs and go from there

[Freakshow]

open the float bowl while its full from under neath or undo the bolt and measue the petrol in it out,then undo bowl and pour it back in so you can see the level.

I would think the problem is not in the hieght if it covers the main jet, (whats the problem is if it keeps going and then it has to run out somewhere else like the overflows. the problem must be in the needle not shutting off, if it shuts of the petrol it wont pour anywhere no matter what height its at.  And as its not running while its standing ( unless its running into the cases it has to be in the pick up.

What happens when you take the carby off and leave the tap on ? does it overflow then ?  how far do you have to tilt it before it come out the over flows ? if its not running out of the engine side or when youtip it sideway to 45mm its not your flaot height, unless its drowning your needle tube.......

ID say you have dirt somewhere, maybe even in the pilot. OR the air jets, Like the guys say its only one little grain in the intake side makes it rich as shit, you need to strip it down and undo all the vents, jets to be sure its clean. i spet 2 weeks on a similar thing once and i had pulled it apart a milliomn times in the end i found a small grain of sand had lodged in the intake side air vent and pilot jet. everytime i took them out i could see light, what i diding realise was i was looking through clear glass like sand..........  they had been blocked th ewhole time.......  you need to pull them out , blow the seat out and check you feel air come out the other side ( like insid the carby or wherever it goes) and physically shove a piece of copper wire ( spilt an elec cable) through the holes and make sure you see them pass to the other side.

Ok cut a long story short --> Action --> Main, Pilot and front Air vents unscrew them and pass the wire physically through, blow air thought chamber lines and check you feel it on the otherside.

[Freakshow]

PPS --  on another note i have a briggs and stratton ride on mower i spent all weeekend trying to get it to run. it wouldnt run on, id clean out the bowl the tinyest grains of dust like grains where in it, run a wire up the pilot, it  would fire up.  if i slowed down the throttle it would cough and not restart undo bowl again tiny grains of dust in there again ?? had a nrmal Lawn mower screen filter but not fine enough mesh. 

what seemed to happen over and over again ( i pulled the bowl off at least 20 times during the weekend trying to figure this one out) was the dusk grains would suck up into the pilot on low revs and accumulate till it blocked the jet and then the govener would cut it out.

this went on all day till i went to the shop and bought a paper car filter.

I couldnt believe how small this dust was, but it stopped a 12.5 HP motor.   What it brought to my attentin was how small particles can be to stuff up your carby jets ( the pilot is about a mm on this thing yet the dusk size dirt managed to be drawn up and when together filled the hole.

It wont take much to give you greif....  also seen the lining of old rubber hoses shed off and the particle form a beaver type matt over holes...... how old are your fuel lines ?

[JohnnyO]

Make sure the carb body is not rubbing against anything when you rev it causing the fuel to froth and overflow.

[Gippslander]

OK, some good advice there -- will do the workshop time and look at all of that and will report back -- I'm gonna beat the bastard 

[GMC]

Déjà vu

I had a customer recently that has had similar problems to yours all the way through.
One thing about fitting a Mikuni to that model Husky is that the carby sits at a bit of an angle. I stuffed around for a bit rearranging the inlet manifold and airbox so that the carby could sit a bit flatter.
His main problems turned out to be other things but the extreme angle may be contributing to your problem

[Freakshow]

Name the other things.

[cheapracer]

Some good suggestions here but first you need to replace the needle and seat, don't care if it's new I have seen plenty of faulty new ones that act perfect except when on the bike running.
Also rarer but the occasional float doesn't or doesn't very well, while it's floating in fuel push it down and feel the resistance - you'll know when you feel it.
I have seen the odd float slightly touching the bowl, gasket or body too making them 'sticky'.
Check the angle of the tang, shape it so it offers a flat face aprox. 90 degrees to the needle (when closed), have seen some awful angles on those tangs (actually try that first as it also offers more pressure to the needle).

[Gippslander]

Thanks again, for the further suggestions.

GMC -- yes the angle had me wondering, if I had to I could cut and reweld at a "flatter angle".

C'racer -- yep I will try the new needle and seat and the other points you mention -- I never thought about the tang angle -- makes sense -- and the float that doesn't float so well or is sticking are things I would not have checked. 

I've now got a good range of suggestions to work with, will try and get on to it ASAP, and will let you know what I find, will be a few days though, tied up with the missus for a few days (the list!) and then have a tow vehicle to get roadworthy etc etc,

BTW, the earlier problem I had with the ignition seeming to be weak has I think been solved by BigK's suggestion to use a racing plug with the skinny electrode to 'concentrate' the spark, seems to have worked, it now fires up when stinking hot but as said floods when it's idling.