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degreestudio

Rev 3's Mikuni - My Recent Studies

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Hello all Rev3 guys out there !

My recent experiences and studies....

About last two or three months i have been struggling with my Rev3 (year 2001) engine and carburation.

My first problem was with the left side vent of the Mikuni - fuel came out of the vent when the carb's tank was full....

I read all the MIkuni "tuning" articles, even the hose build to connect two vents.

I made actually a bit funny mistake and closed the flooding vent. Seemed that the engine ran fine, but about few months...

It started to make a bit laboring sound. Like, it didnt got the perfect mixture.

I played a bit with the AIR screw, finally the engine needed it fully closed to run "normal". - it was of course abnormal !

I realised it got to be something more, so i opened the cylinder's head to clean the piston's head. Of course , it was dirty and horrible inside, so i got it very clean.

Now, few days ago i got the engine running, but again....the mysterious laboring sound ....

I realised that its something with the carb, because this bike's engine ran very smootly when i bought it. Actually it was rebuilt by recent owner.

I opened the "flooding" vent of my carb, so two float vents are now opened. Also, i checked the float plate, to get it parallel to the carb's "plane". The left vent doesnt flood anymore !

NOW it runs perfectly, almost like new !

So, my thoughts about all of this ...

Actually, when the flooding vent was closed, the left side float didnt got the atmospheric pressure, so basically it was "stuck". It caused abnormal working of the engine - laboring sound.....

So why the vent started to flood ?

In my theory....

Remeber, I had my piston's head very dirty and usually it makes the engine's pressure to go very high.

So, i assume that some amount of pressure "ran backward" to carburation system. This caused carburation pressure to exceed atmospheric pressure in the float tank.

Basically the atmospheric pressure didnt manage to push the fuel to jets in the float tank of the carb !

The result was that one of the float vent started to flood - fuel in float tank ran backward and out of the left vent

Summary

Now, when .......

my piston head is clean and new head gasket is installed,

my carb is checked and cleaned,

my carb's plates are parallel to over all "plane" of the carb and

my carb' two "atmospheric" vents are fully opened without hose tuning..........

The engine runs like new and my Mikuni does not flood anymore !

Model: Rev 3

Engine: 270 cc

Year: 2001

Pilot jet: 27,5

Main jet: 145

Sorry about my english and i hope this helps !

Cheers from Estonia !

Edited by degreestudio
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Degree studio

You wrote: "So, i assume that some amount of pressure "ran backward" to carburation system. This caused carburation pressure to exceed atmospheric pressure in the float tank.

Basically the atmospheric pressure didnt manage to push the fuel to jets in the float tank of the carb !"

Respectfully, I am not quite sure if I agree with your diagnosis.

The vacuum seen at the carb Venturi is due to the stroke of the piston creating a vacuum and the atmospheric pressure pushing on the fuel surface which in turn pushes the fuel up the jets.

It has nothing to do with pressure at the piston dome/crown or compression. If this was accurate in what you say then a 300cc would have more back pressure on the carb than say 125cc bike?

Also your bike has reeds in it which would take care of any back pressure to carb as the piston descends.

Is the bike running okay now?

Edited by billyt

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Thank you for the correction ! - it was just a theory :)

But what caused the vent to flood ? Do you have ideas ?

I cleaned my carb regularly and checked my float plate on every cleaning process

When the piston got 1mm of dust, my mikuni flooded. Now when the piston is cleaned, everything runs perfect.

Could it be possible that, when the piston had 1mm of dust the vacuum wasn't so effective anymore and this messed up the carburation in some way ?

In short - could the vacuum loss make the mikuni to flood ?

Yes, i returned from ride about few hours ago and bike runs like new !

I have two opened vents, with no pipe connection on them.

I checked for any fuel leaks around carb, but everything seems dry.

Looking forward to start increasing the pilot jet in the future - step by step.

Thanks !

Edited by degreestudio

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possibility 1 - when you put the carby back on you might have aligned it slightly differently with respect to vertical compared to before you took it off

possibility 2 - maybe there was something holding the float valve open and you cleaned it out when you took the carby off

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Thank you for the correction ! - it was just a theory :)

Could it be possible that, when the piston had 1mm of dust the vacuum wasn't so effective anymore and this messed up the carburation in some way ?

In short - could the vacuum loss make the mikuni to flood ?

Degree studio By "DUST" I am assuming you mean Carbon build up ?

No, the carbon build up does not affect vacuum (other than scavenging) the length of the stroke, bore size and altitude effects, determines vacuum seen at the carb Venturi. Flooding is a symptom of a mechanical switch (i.e. floats) not switching off for various reasons, vacuum is an effect of pressure differentials.

Feetupfun is giving you some likely scenarios and I am sure there are many others.

Does this help?

Edited by billyt

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