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how sensitive are mikuni carbs 1985 yz250

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1985 yz250 at 10,000-12,000 feet

Trying to figure out why my bike is running rich at 10,000 feet with all my jets replaced. Ill give you my jetting specs and the stock specs.

Im thinking its the reed valves but some people think its crazy that the bikes main jets and everything is so low but maybe theyre used to a different carb with less sensitivity.

Also I just did a top end on it and everything looks good. havent peaked at the reed valves though.

 

main-230 stock 290

pilot-46 stock 60

needle clip position with stock needle- top clip stock is 2nd clip position

running rich and fouling plugs every ride.

REED VALVES?

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Are you running high octane gasoline and/or a 'racing' type premix oil ?

Are you re-using previously fouled plugs that have been cleaned?

 

That's an old engine, when were the crank seals last replaced?

Perhaps a worn RH seal is sucking in gearbox lube. (notice changes in the gearbox oil level?)

 

Despite looking 'okay' (no air gaps around the petals)

reeds are also a wear item needing replacement as they loose tension with use.

 

Carburetor float / fuel level affects the air/fuel ratio, higher than specified level will richen all metering circuits.

A worn / damaged inlet valve seat & needle as well.

 

Inspect the carb for a worn needle jet orifice (hole where the needle slides into)

worn in an oval shape, despite your leaner needle setting it could allow some extra un-metered fuel past the needle.

Edited by mlatour

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1 hour ago, mlatour said:

Are you running high octane gasoline and/or a 'racing' type premix oil ?

Are you re-using previously fouled plugs that have been cleaned?

 

That's an old engine, when were the crank seals last replaced?

Perhaps a worn RH seal is sucking in gearbox lube. (notice changes in the gearbox oil level?)

 

Despite looking 'okay' (no air gaps around the petals)

reeds are also a wear item needing replacement as they loose tension with use.

 

Carburetor float / fuel level affects the air/fuel ratio, higher than specified level will richen all metering circuits.

A worn / damaged inlet valve seat & needle as well.

 

Inspect the carb for a worn needle jet orifice (hole where the needle slides into)

worn in an oval shape, despite your leaner needle setting it could allow some extra un-metered fuel past the needle.

running 91 with lucas
 

These last two rides have been on used plugs.

no change in gear oil levels when i drain it.

My needle clip bent but is still holding the needle in place just fine. dont know if that could be it but it doesnt seem like it. Is the orifice the one closest to the top of needle(thickest end) or the bottom (pilot jet?)

I dont know when the crank seals were replaced but everything else has been gone through semi recently so im guessing it hasnt been 20 years. really clean bike

 

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At 10 000 ft + elevations, since compression is noticeably reduced you might want to consider using lower octane gasoline.

 

The orifice mentionned is located at the bottom of the venturi, it's the hole where the needle slides into.

Also closely inspect the needle itself for wear.

 

Too high a float / fuel level is common for engines with fouling issues.

Unless it's a temporary fix on the trail, avoid re-using previously fouled plugs.

 

Building yourself a 30$ homemade leakdown test fixture can confirm if the crank seals are indeed tight.

Edited by mlatour

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what premix ratio are you using? My '83 CRs (Keihins) specify 20:1 premix ratio. I had to lean out for altitude AND changing the premix ratio to 32:1...changing premix ratio made a significant difference...

I've also had the experience of gapped reeds causing it to miss and feel rich running low rpms at 1/4-1/2 throttle...

 

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mlatour is very correct regarding the low octane gas at altitude. A guru in CO Springs told me about that a long time ago and it is all I use when I go to CO/NM at any location. 

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Humidity is the amount of moisture in the air.  As humidity increases, jetting becomes richer.  An engine that runs fine in the morning’s dry air will begin to run rich as the humidity increases later in the day.  Jetting gets richer as altitude increases because there is less air density the higher you go.  An engine that runs good at sea level will run fat at 10,000 feet due to thinner air.

Because there is less density in the air.  As the air temperature increases, jetting will become richer because the air is less dense as air molecules move farther apart.   Air density increases as air gets colder meaning there are more oxygen molecules in the same space.  When the temperature drops, the engine will run leaner and more fuel is needed to compensate.  As the air temperature gets warm the engine will run richer and less fuel is needed.  An engine jetted for forty degrees Fahrenheit may run poorly at ninety degrees F for that reason.

Humidity is the amount of moisture in the air.  As humidity increases, jetting becomes richer.  An engine that runs fine in the morning’s dry air will begin to run rich as the humidity increases later in the day.  Jetting gets richer as altitude increases because there is less air density the higher you go.  An engine that runs good at sea level will run fat at 10,000 feet due to thinner air.

As the air temperature increases, jetting will become richer because the air is less dense as air molecules move farther apart.   Air density increases as air gets colder meaning there are more oxygen molecules in the same space.  When the temperature drops, the engine will run leaner and more fuel is needed to compensate.  As the air temperature gets warm the engine will run richer and less fuel is needed.  An engine jetted for forty degrees Fahrenheit may run poorly at ninety degrees F for that reason.

 

Edited by Jimmy Pascol

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