GS250t sudden loss of power, can't idle, stalls when stopped or going downhill

[epytus]

When cold the bike runs fine, but when it warms up there will occasionally be a sudden loss of power when stopped at a light, or going on a steep decline in the road. Makes deeper noises and sounds like the engine is working hard. It'll stay in this state until I start moving enough or if I restart the bike, sometimes just flipping the killswitch on and off, and it'll instantly be back to normal. This started happening after I went over a nasty bump in the road a month ago. When I hit the bump the bike instantly stalled and got into this low power state upon restart. Had to walk it home that night, but it seemed fine next morning. Ran fine for awhile but would occasionally act like crap. It's been more frequently lately & I can barely make a few blocks before it'll knock back into the state of having very little power.

I thought it was running on just one cylinder - that's what it sounds like to me - but replacing the spark plugs didn't help. Still could be that, but I could feel exhaust coming from both pipes when it was in this state. The bike barely runs when it's like this and without throttle it dies immediately. Choke has no effect to keep it alive.

Any clues or ideas of where I could start? Does it sound like the engine is flooding, or running on one cylinder?

1980 gs250t

[fbody_mike]

I'd start by checking the floats and needles on the carburetors. The nasty bump may have changed the float setting or messed up a needle.

[Gorminrider]

I'd start by reading the shop manual available here

http://zeus.mtsac.edu/~cliff/storage/gs/GS250T-300LZ_manual-82.pdf

It could be so many things-you're going to have to narrow it down. but running out of fuel because of a faulty fuel supply is one easy one-try turning your fuel tap to PRIME, Checking the wiring for shorts and bad connections is another easy one...one thing that won't be in the manual is under the small cover, right side, where the ignition sensors are. These can come loose and flop about. Also, the wiring to them can chafe.
Etc Etc Etc

[epytus]

thanks for the input. On Prime it behaves the same, the fuel is new. There probably is a wiring issue because the headlight (and tailight) don't turn on anymore, which is a new problem. I might have to go ahead and pull carbs but have been putting it off. I notice every time I run the bike it gets worse, now the bike pretty much never runs properly when before I could make it down the road.

[Gorminrider]

yes .Your first impression is likely soand by all means, pull the carbs,if you feel you're up to it ..ie:good screw drivers, a caliper gauge etc as the shop manual describes-a decent collection of tools in other words, are required..care and caution when forcing bits or separating from gaskets yada yada

I thought it was running on just one cylinder - that's what it sounds like to me...... Still could be that, but I could feel exhaust coming from both pipes when it was in this state. The bike barely runs when it's like this and without throttle it dies immediately. Choke has no effect to keep it alive.

...I could feel exhaust coming from both pipes e

but of course it will the pistons and valves going up and down ...and both might even feel warmish at the mufflers' ends! albeit not the same as properly running and after a ride.

...one cylinder not running- the time to catch that for sure is when the cold bike starts up and you can feel the headers with your hand..not too long after starting, both will get hot as heat is transferred through the block from the running cyclinder to the cold one. But do clean /check the carbs. including Blocked jets and passages, floats needles and seats...all pretty common

[epytus]

That tip for checking the headers helped out a lot Gorminrider - my left cylinder is definitely out - it was ice cold while the right one was cooking. I pulled the (brand new) plugs and did a test with them outside the cylinder and they both work great in the right hand cylinder, but both produce no spark when connected to the wiring for the left hand cylinder. So I supposed it's a wiring issue, which makes a lot of sense since the wiring in this bike is gnarly as hell and a bump could have knocked them around to short against each other or the frame of the bike.

So at least I know what the problem is now, looking through the wiring is going to be another adventure I'm sure!

[fbody_mike]

Before taking the tank off and checking out the wiring, try swapping the spark plug cap from left to right. The plug cap (the hard plastic thing that goes on to the sparkplug) screws into the sparkplug wire. Just unscrew them and swap them. There is a resistor pellet and spring inside the cap that can go bad. Easy to do.

[Gorminrider]

That tip for checking the headers helped out a lot Gorminrider - my left cylinder is definitely out - it was ice cold while the right one was cooking. I pulled the (brand new) plugs and did a test with them outside the cylinder and they both work great in the right hand cylinder, but both produce no spark when connected to the wiring for the left hand cylinder. So I supposed it's a wiring issue, which makes a lot of sense since the wiring in this bike is gnarly as hell and a bump could have knocked them around to short against each other or the frame of the bike.

So at least I know what the problem is now, looking through the wiring is going to be another adventure I'm sure!

Good. That helps a lot. Yes, I'd start with the easiest-the wires to and from that coil... and the wires that go from the sensors (under the small round cover right side) to the Black box ...and sure, if no problems at the coil's power supply and "signal",you can try cleaning as fbody sez, the cap and its little resistor slug inside-it all comes apart when you unscrew the part that clicks down over the sparkplug tip but be careful-pieces are tiny. Do it over a white cloth... I'd say, that this is less likely than the wiring itself...even when dirty, some spark is likely to get through unless the high tension wire itself is disconnected or arcing through a break to...the gas tank..?
Last thing- the worst-is to suspect the black box. You don't want it to be that because they are hard to find cheap.

[epytus]

Figured out the problem. One of the ignition coils was completely unplugged... probably wiggled loose over time. Plugged it back in and the bike has been good to go ever since. So glad it wasn't something that required a replacement. Thanks for the help on this one guys.

[Gorminrider]

Thanks for reporting back, epytus. So many threads trail off and where nobody ever knows what THE FIX was or if the original poster just gave up.

[John Park]

Thanks for reporting back, epytus. So many threads trail off and where nobody ever knows what THE FIX was or if the original poster just gave up.

Plus it reinforces my assessment that 78% of carburetor problems occur in the ignition; well, they used to when all we had was points. I just finished rewiring a 1949 Velocette and it's amazing how far we've come from the days of five wires, two switches and a magneto with just an HT lead. It all works, such as it is, but then it's Miller and BTH rather than the dreaded Lucas. The magneto is always ON and you pull the compression release to shut down. Primitive elegance, and there's always the suspense of whether it will start when you kick it over.

Manual push/pull fuel taps - one is reserve - and lawnmower simple carb. Continuous running oil change via the primary case. With only four gears - upside down on the right - there's no need for a digital indicator, and the oil pressure can be observed by removing the oil tank cap and watching the return line piddling back in.

And it all still works, 70 years later.