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Engine Mechanical/Operation Question

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Looks like the ATV forums don't get much action, so I laid it out here to hope for an answer.

 

I bought an '05 Honda TRX250EX yesterday. Kinda know the owner. ATV is in pretty good shape, but it wouldn't stay running. After I gave him $860, he made mention of me needing to buy a cap for the rear crankcase cover where the transmission fluid is added.

 

Well, I got the quad home, sat on it, and tried to drive it ~200 yards to my barn. I felt a puff of air on my left pants' leg, looked down, and saw some oil residue on my leg.

 

So - here's the question: why is the crankcase under pressure when trying to fire the engine? In my mind, it is a separate reservoir to hold gearcase oil for the transmission. So it should be isolated from any compression, right?

 

Anyway, I ordered a $15 cap and $4 o-ring. I ended up towing the TRX down to the barn with my Pioneer...

 

Hopefully, this purchase wasn't a mistake...

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On most Honda quads, the oil is shared with the engine & transmission/gearbox so what you experienced is normal.


I can somewhat understand that...however, if both fill caps call for different types of fluid, then wouldn't that mean that they are independent of one another?

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The engine has a dipstick/fill cap where the oil goes that lubes both the engine and transmission/clutch as far as I know. What other cap did you find, where is it located?

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The fill cap is about 8" to the right of the oil dipstick. It isn't a thumb-turn cap, but uses an Allen key, I believe. On schematics, it is on the drawing called rear crankcase.

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Check the vents for a clogging (like perhaps a mud wasp).
At idle, it is normal for a vent to pull and push some air (fumes) and once rpms rise, for that to go away.
Get a service manual too.


Umm...I think the failure to run is because there is no internal combustion pressure due to that cap being missing. I don't think dirt-dobbers stole it. 🙂

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The missing cap would not affect the ability for the engine to run. It might make a mess with oil misting, it might encourage engine damage from allowing the ingress of dirt and debris.

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The missing cap would not affect the ability for the engine to run. It might make a mess with oil misting, it might encourage engine damage from allowing the ingress of dirt and debris.


Hmm... Gotcha. Just figured it was losing pressure and causing a stall.

It seemed to run fine for about 20 seconds and then it just puttered out. I figured I'd hit it hard with some carb cleaner, some octane boost, a new air filter, an oil change, and ride it like I stole it.

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That is the rear end, not the engine transmission.


Hmm. I thought I remembered that page number from last night. I'm on my phone right now. Hang on. I'll be back with a proper page number.

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