smoke from engine case / dies at idle | GTAMotorcycle.com

smoke from engine case / dies at idle

wearelopey

wearelopey

Well-known member
bike is an 08 cbr 125.

having trouble keeping this guy running...

bike will usually start (sometimes wont catch, but will turn over), but it doesnt like to idle. when in gear and in motion bike runs fine. just idle at a stop it will die consistently. Noticed that giving it a constant throttle I can get it to hover in the 1500rpm range, however the rpms will start to drop off and eventually die...
especially after dying, bike will have trouble starting (have to give it throttle) and will intermittently not start at all

opened the oil filler and noticed smoke coming out. checked oil level and its touching the max fill bar...
bike shifts fine etc.

smelled gas could be old. maybe that's it? maybe oil overfill? bike might have been sitting a while maybe the smoke was water/moisture; maybe water in gas too?

any ideas / suggestions??

Appreciate it!
 
Last edited:
Are you checking the oil level correctly ... Dipstick unthreaded but resting on top of the fill hole, bike held vertical on level ground? Overfilled oil will cause trouble.

If the oil level is correct ... That engine uses a solenoid-operated bypass valve to regulate the amount of air going in at idle, and thus control the idle speed, and it's not unheard of for that valve to stick. If it sticks open, the engine idles too high; if it sticks closed, it stalls and is hard to start (but is okay going down the road).

If you know that the bike has old gas in the gas tank, don't try to fix anything else until it has FRESH fuel in it.

How's the spark plug ... how are the valve clearances?
 
Sounds like bad gas. How old is the fuel? It can skunk up in 3 - 6 months without fuel stabilizer. Drain the tank, add fresh fuel and it wouldn't hurt that if the fuel was old, add a 1/4 can of Seafoam Motor Treatment (best cleaner I know) from Canadian Tire to the fresh gas and try it again.
 
Brian P said:
Are you checking the oil level correctly ... Dipstick unthreaded but resting on top of the fill hole, bike held vertical on level ground? Overfilled oil will cause trouble.

If the oil level is correct ... That engine uses a solenoid-operated bypass valve to regulate the amount of air going in at idle, and thus control the idle speed, and it's not unheard of for that valve to stick. If it sticks open, the engine idles too high; if it sticks closed, it stalls and is hard to start (but is okay going down the road).

If you know that the bike has old gas in the gas tank, don't try to fix anything else until it has FRESH fuel in it.

How's the spark plug ... how are the valve clearances?
Click to expand...

And how would I get to and/or check the bypass valve?

to answer others questions the bike was just bought 3rd owner has about 4000km on her
 
Don't touch the idle control bypass valve unless you check all else first. Make sure the fuel is good. At 4000 km it is due for the initial valve clearance check - do that. Replace the spark plug.

If all that is good then the idle air bypass valve is built into the throttle body. There is no factory procedure for servicing it- you are on your own. Don't say you weren't warned.
 
+1 for checking valve clearances. Symptoms sound like tight exhaust valves.
 
How long is "a while"? How long has it been sitting.

Check the fuel filter. If it's been sitting long enough with old gas in it, it could be suffocating from lack of gas. Try simple solutions before opening it up to more complicated ones.

If you do, take BrianP's advice very seriously.
 
Fuel filter on this bike is only the sock on the pump inlet inside the tank, and is not replaceable independently of the entire fuel pump assembly. If it clogs, the first symptom will be fuel starvation under full throttle at high revs, not what the original poster describes. Look into the other things first.
 
K guys thought I'd give an update.

After about 20 hours of diagnostics figured out the cylinder was binding. Most likely due to oil starvation at some point...

Man oh man was that a douzy!
 
That stinks ... sounds like no one ever changed the oil or bothered to check until too late. That engine is dead simple to rebuild, and the parts are cheap and there's only one each.

45 minutes to get the engine out of the bike (and you can carry it with one hand!), 15 minutes to pull apart the top end.

A new piston is around $11, a head gasket around $6. The piston rings are expensive ... around $40. Get the cylinder honed to remove any scuff marks (or just buy a new one).
 
You must log in or register to reply here.

Back
Top Bottom