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Carburetor flooding (?)

J

juhosirvio

Joined Aug 2021
27 Posts | 1+
Finland
My carb overflow hose drips fuel after ive tried to kickstart my bike for a few times. Float height should be fine (@45 degrees the float is parallel with the gasket surface and moves effortlessly), jets should all be fine (blew compressed air in to all of them)

My air/fuel screw is turned 1,5 full rotations from the bottom if that information is important


Edit
I also dont get a spark, just changed the plug, still nothing
Edit2
I have a spark, ignition was turned off :D
 
Last edited:
what year and type of carb?

my common problems are dirt between the float valve and seat or an old float valve that has a groove in the rubber seat area.

I have a bikes checked out in the shop the night before, next morning at the track/park, open the peacock valve and have gas running out the overfull tube. Just tap on the float bowl and no more problems. Why ???
 
Opps, sorry your profile states 2007, assume Keihin, same float idea as a Dellorto.
 
Last edited:
Yes it is Keihin fcr mx 39

I tried to open the air/fuel mix screw half rotations at a time all the way to four rotations and it made no difference
 
The float valve has a rubber tip that seals into the seat, that has gone hard with age. Give it a clean up by spinning in a rag with your fingers. Check that first, also the seat may have corroded, they can be polished using small phillips head scewdriver with soft rag wrapped around the end.
 
Is that something that could cause the bike not start all of a sudden? Cuz the bike started REALLY well on friday and rode well and after i shut if off after riding for under 5 minutes it didnt start again.

But i will give that a try
 
Yes, fuel dribbles through float valve and "floods" engine- basically makes super rich mixture to try to start, OK for cold motor but not when it's hot.
Also affects idling/ slow revs running, as the float is not keeping fuel level down, so you get rich mixture that makes it hard to set pilot screw/ idle mixture.
If you can't get it right there are both rebuild kits and replacement float valve sets on ebay for good prices
 
keep the fuel taps off. keep kicking, keep cleaning thew plug, keep adding a little throttle. it'll go in the end and then adjust the idle and Pilot Screw before you stall it.

the fuel will stop leaking due to the vibrartuion of the running engine.

Taffy
 
So the float was leaking just a tiny bit of air. I cleaned the needle and the seat in which the needle sits which stopped the air leakage. Tried kicking the bike but it didnt start. The first few kicks were somewhat promising but then after some time the kicking did nothing. Spark plug was wet
 
What Taffy said- it's loaded up with fuel. You'll need to clear it out- plug out, fuel and ignition off, full throttle, kick over to clear fuel from cylinder and inlet.
Clean plug with contact cleaner and try again. You may have to pull plug and clean it again. When plug is clean, check it's got good spark.
 
A thing that tends to be forgotten is the O-ring that seals the outer diameter of the float valve seat.
You have to undo the screw that fixes the valve seat and then carefully get the seat (which has a little filter sieve on the bottom)out.
Around it you will find an O-ring that hardens over time and the fuel doesn´t leak through the valve itself,but around the valve seat.Both causing the same effect.
 
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I might check that and if the bike doesnt start then i'll take it to a shop and have them figure it out and most likely sell it afterwards.

I've had the bike for almost a year now and there's always been atleast one problem with it. This ownership has confirmed the fact that japs are superior to euros. I kinda wished this bike wouldve proofed me wrong.
 
It just doesnt make sense how the float could be the cause in this case. Cuz if the rubber parts would have gone bad the bike would have said something. But no, it ran like a dream and rode like a dream. It was only after i let it die while in gear (i let the bike roll in gear till it stalled) that it didnt start back up. And this all happened in the span of 5-7 minutes.

I have fuel, i have spark, my timing should be all good (i have checked the timing from the clutch side but not from the head but im quite confident that my timing hasnt jumped all of a sudden), my valve clearance should be fine, it has good compression.
 
the 450s are known to break the heads off the exhaust valves and the valve springs also break. have a look at the valve clearances and see if one valve is a little low?

Taffy
 
I took the bike to a shop and turns out my intake valves are leaking. They set the engine to ignition top dead center so all valves would be closed, correct? And then they put 1 bar of pressure through the spark plug hole and about 80 percent of that air came through the intake side. No wonder it doesnt start :D
 
That'll do it!!!! Make sure the valve seats aren't carboned up. Has it been blowing smoke/ using oil? Hopefully the seats aren't recessing into the head.
If it sounds good and starts easy after valve clearances are done, problem solved. Might be worthwhile to check them again after a couple of rides.
 
juhosirvio said:
I took the bike to a shop and turns out my intake valves are leaking. They set the engine to ignition top dead center so all valves would be closed, correct? And then they put 1 bar of pressure through the spark plug hole and about 80 percent of that air came through the intake side. No wonder it doesnt start :D
Click to expand...

I would think that they will find heavily 'tuliped' inlets on deformed seats.

Taffy
 

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