DT 175 erratic Idle

[SteveHow]

Hello and thanks for taking the time to read and hopefully help.

 

I have a Yamaha DT 175 that is running rough and when warmed up has Idle issues.

When the engine is at maximum operating temperature, I have to keep adjusting the Idle from either (at times) being to high, or to low.

It's hard to explain because its so erratic.  For example, again when the engine is well up to operating temp and one backs off the Throttle (where earlier the idle had been set), the engines RPM (Or Idle) doesn't came back down to where it was set.  So, I wind it back down.....then later, during the same ride, the Idle becomes to low when throttling off, and will stall the engine.  (Obviously because its now to low)

The bike model is DT175N  which I think is around 1999-2000 date made.  I have not dismantled the carb yet, I was hoping someone might recognize these symptoms, and I was wondering if it might be a fuel line issue....here is why.   (I have checked and cleaned the Air Filter)

Let me say, I know nothing about how these Energy Injection/Carbs work, but today I checked the fuel lines and found something curious/puzzling.

This carb has 2 fuel lines running to it from the fuel tank. One fuel line obviously thicker than the other.  Clearly these operate with the Prime/On/Reserve.  I wanted to check the Carb is getting enough fuel (and that the filter in the tank wasnt blocked) and found the ONLY the Prime line freely runs fuel from the Tank.  (Tap set to Prime) 

Neither with the fuel line Tap set on either ON or Res will fuel drain from the tank.  Odd I thought.  I topped up the fuel thinking it was low...but again, no fuel drains from the tank on either the ON or Res position, only Prime. 

I thought maybe it was blocked in the tank, so I blew up the line (ON and Res)  just with mouth pressure seemed blocked, but when I sucked on the fuel line, (Tap set on either ON or Res) fuel started freely running out of the Prime line.  (Strangest thing)  I thought, wtf does this work on a Vacuum?

I'm out of my league on this guys.  Should fuel freely drain from the ON and Res as the Prime does or does it work on a vacuum.  (This would mean that the Prime Line is infact the main fuel line)  Doesn't it. ??

On my last couple of rides, the engine has been hard to start (when hot)

Anyone point me in the right direction please.

Thank you  very much

Steve

[KevtheRev]

Hi Steve , the Prime position should flow freely , whereas the on/res positions need a vacuum , usually from the inlet stub , to work . My first guess at your running problems would be an air leak somewhere between the airbox and the cylinder head, so check the sealing at airbox , carb and for cracks in the inlet stub . But before you do that pop into the new member section and say HI , you'll get more answers that way . Best of luck !

[BikerMike92]

the first thing that popped into my head when I read the title of the thread was "air leak" as kev has said, grab a can of easy start, get her ticking over, and give anything that could potentially leak air a spray, it your bike revs higher then chances are you've found your problem bud,

 

personally iv not really encountered "prime" settings on a fuel tap, I don't think id like it either lol every bike iv owned has worked on a vaccum line and its never let me down, maybe worth connecting a vacuum pipe also and letting that pull in the fuel instead of basic gravity but as I said iv never encountered one so I'm not 100% sure it would be any better, if any different at all

sorry I couldn't help more but fingers crossed you resolve the problem