Losing power on the motorway

[wanpo]

Hi, my bike was just returned to me after being in the shop

Got it back and was on the motorway, I noticed that in 5th gear ~7k rpm at ~100-110km/h if I rolled off the throttle so it was closed, then rolled it back on as necessary to stay up to speed, there would be a sort of lag until the engine kicked in. This didn't worry me too much until I was riding along a few ks down the road and at constant throttle the bike was spluttering and giving any power until it had slowed itself down to ~85km/h, no matter how much throttle was applied or even if I changed down to 4th. It came back after maybe 10 seconds and on the way to my destination didn't occur again, on the way back, happened once more, soon after, and was fine after that.

I don't know about bikes in detail, but I have a Scorpion muffler, would it be anything to do with the stock jetting of the carbs?

Cheers

[awayatc]

sounds more like bit of dirt in fuel maybe( fuel tank or carb?)
or a dodgy sparkplug.
Pays to check basics first, and see if that cures it.
good luck

[slofox]

Hi, my bike was just returned to me after being in the shop


Assuming it was OK before its visit to the shop, I would take it straight back to the shop and ask them what they have done to it...politely of course!

[wanpo]

Yep, dropping it back to the shop on wednesday, hopefully he'll pick up what's wrong.

[tri boy]

Kinked tank vent tube maybe.
Also, Hyosungs did have issues with their vacuum fuel taps, (not delivering quite enough fuel to maintain correct fuel level in the carb bowls).
To eliminate the theory on kinked vent tube, when it's playing up, pull up, open tank cap for a few seconds (to allow atmospheric pressure to stabilise) and close cap. Ride off. If it makes the problem dissapear, then you have found the fault.

[kwaka_crasher]

Kinked tank vent tube maybe.
Also, Hyosungs did have issues with their vacuum fuel taps, (not delivering quite enough fuel to maintain correct fuel level in the carb bowls).
To eliminate the theory on kinked vent tube, when it's playing up, pull up, open tank cap for a few seconds (to allow atmospheric pressure to stabilise) and close cap. Ride off. If it makes the problem dissapear, then you have found the fault.

Vent (tank breather) tubes are rare on a road bike. Most generally ventilate slowly through the cap as a plain tube would needs to terminate higher than the full level of fuel (although this can be mitigated with a good length above then back down, but not too far down or it may syphon). Flush filler caps cause the need for drain tubes around the filler depression though, but that's for rain water so it doesn't fill to the top then spill into the tank when you open the cap - nothing to do with pressure change in the fuel tank. You sure that's not what you meant?

Here's a pic of a 2008 Hyosung GT250R.

[Hamish Carlson]

Any possibility of water in the carbs? Might have blocked the jet going around a corner, then dropped to the bottom of the bowl when you the bike settled a bit, as in if you went over bumps or a corner and then smooth road or straight? Just an idea, one of my old bikes used to do that when water entered carbs.

[Flip]

I would start with these:
Kink in the fuel hose (it was in the shop)
Kink in the vac tube to the auto fuel tap. (it was in the shop)

then check:
Blocked fuel filter (Inspect)
Water in the fuel (Drain Carbs and check what comes out)
Blocked tank cap-vent.( loosen the filler cap when it is running badly)

[CookMySock]

What was it in the shop for?

Steve

[wanpo]

http://www.kiwibiker.co.nz/forums/showthread.php/116255-Seized-cylinder
That's what it was in for.

Wasn't doing it before I dropped it off to them, but I'm wondering if it's because the engine is running healthier than when the chain cam tensioner was loose. More air through the cams/running lean or something.

Tends to happen after accelerating for a bit then closing the throttle, only in fifth gear ~100-110 (7-7.5kish rpm) then if I try opening the throttle again, there's a couple of seconds of lag until the power comes back in, then after maybe a 30 seconds or so of that at constant rpm/throttle, the engine will splutter and slow down to ~70 with no power coming through at all at any throttle position, problem remains if I try to change down gears. After about 30 seconds of that, the power will come back in and there will be no problem for the rest of the trip (as far as I've noticed)

Got it back from the shop after I made the last post and it seemed to be fine for a couple of days, but came back as bad as it was before hand. I'm dropping it back into the shop on Tuesday for him to have a better look at it. I'm hoping I get it back before that Thursday (when I've got my Restricted License)

[munter]

Hey wanpo - did you ever find out what was causing this issue? I have the exact same problem.
I have replaced the spark plugs, drained the fuel tanks checked the air filter and replaced the fuel filter and still have this problem.
I am about to replace the fuel tap and vacuum fuel pump.
Its doing my head in!!!!


http://www.kiwibiker.co.nz/forums/showthread.php/116255-Seized-cylinder
That's what it was in for.

Wasn't doing it before I dropped it off to them, but I'm wondering if it's because the engine is running healthier than when the chain cam tensioner was loose. More air through the cams/running lean or something.

Tends to happen after accelerating for a bit then closing the throttle, only in fifth gear ~100-110 (7-7.5kish rpm) then if I try opening the throttle again, there's a couple of seconds of lag until the power comes back in, then after maybe a 30 seconds or so of that at constant rpm/throttle, the engine will splutter and slow down to ~70 with no power coming through at all at any throttle position, problem remains if I try to change down gears. After about 30 seconds of that, the power will come back in and there will be no problem for the rest of the trip (as far as I've noticed)

Got it back from the shop after I made the last post and it seemed to be fine for a couple of days, but came back as bad as it was before hand. I'm dropping it back into the shop on Tuesday for him to have a better look at it. I'm hoping I get it back before that Thursday (when I've got my Restricted License)

[OddDuck]

Hi Munter,

I'd go looking for a vacuum leak between carbs and heads, or an exhaust gasket leak at the head. Go around the engine on the inlet side with a can of easy-start, with the bike warmed up and idling, and see if rpm's pick up. Listen for any tractor-like noises on the exhaust side. Are any of the plugs coming back white-ish?

I ask because I had similar with an old Honda and it turned out to be an O-ring sealing the carburettor inlet insulator plate to the cylinder head. The rubber had crushed over time. It sealed OK cold but when the engine warmed up and the rubber shrank a bit it'd start leaking.