TT250 vacuum issue? Dies after 10min

[flopsweat]

I seem to have an issue that might be somewhat diagnosed, but have no idea how to resolve it. It was somewhat addressed in an old thread from 2016. Once in a great while... maybe 3-4 times out of the 20 that I've even taken it out... it will randomly start chugging while holding a steady throttle after maybe 10-15min of riding, which feels nearly the same as driving a car when the gas starts to run out. Once that happens, as soon as I pull in the clutch, it will die. The first couple times I couldn't get it restarted, but luckily was still in my neighborhood a few blocks away so my walk of shame wasn't too rough.

I called CSC, and he told me it sounds like a vacuum issue, since these are closed systems due to CA emissions. Check for hose kinks, and if it happens, pop the gas cap and try starting it again. I couldn't see any kinks on any hoses, and when it happened again, I popped the gas cap and it started right up!

So something is still going on, and I need to figure it out before the weather warms up and I end up miles rather than blocks from home. I found their tutorial on the SG250 fuel vent, which sounds identical, and blew out the little hole in the cap with some compressed air while checking the rubber grommet.

Any ideas on what to check to actually fix this so it doesn't keep happening?

[65cabriolet]

You may have a valve in the hose on the gas cap that is malfunctioning, you can try removing it and just run the hose for a bit... see if that helps.

[zero_dgz]

As stated, try reproducing it with the gas cap open, or off (if you're feeling brave) and see if that's the culprit.


I had a similar issue after putting a new carb on my Bashan. That turned out to be the float height in the bowl being set a tad too low. Long stretches of open throttle would drain the bowl to the extent that the pilot jet pickup -- which is shorter than the main jet -- would not actually pull any fuel until the bowl caught up and refilled a little. This was easy enough to figure out because I could reproduce it at any speed by winding it out for a while and pulling the clutch plus chopping the throttle. The telltale heart beating beneath the floorboards was that if I did that at speed I could cost for a few seconds and then bump start the bike and it'd light right back up and keep running like nothing happened. I tweaked the tab on the float to raise the fuel level in the bowl and it's been fine ever since. You could get a similar issue by having some crud stuck around the float pivot or under it or similar preventing it from pivoting open all the way.



If you managed to vacuum lock your fuel system you'd be able to immediately cure it by opening the fuel cap after the engine dies, as you've done. If you do it in a quiet area you might even hear the air rushing back into the tank when you crack the cap. But if you can't identify the inrush of air you have not necessarily ruled out a float issue.



If your issue is the cap, trying to clean the vents tends to have mixed success. There's a little rubber check diaphragm thingy in there that tends not to like to have high pressure air blasted at it. Even so, the air channel is not a straight shot on most caps and you may not be able to blow out the obstruction. I'd try soaking it in some alcohol or mineral spirits or something, maybe even gasoline, to loosen any crud before attempting. Don't use carb cleaner to soak, which will eventually dissolve all the rubber components in there including the cap seal...

[flopsweat]

Ok that's all making sense to me. I'd be surprised if this was an issue with crud in the line, as it first happened with 10mi on the OD and I'm only at 40mi right now, so she hasn't seen any action outside of my hood. But I still won't rule that out.



So there's a valve IN the hose that connects near the hinge on the gas cap? I'm not completely clear on what you mean by "you can try removing it and just run the hose for a bit"... remove what exactly?



Also, when this has happened, I can for sure hear a FOOF rush of air when I pop the gas lid, so it is definitely creating at least some suction.

[Boatguy]

If you hear a rush of air inward, that means the vent on your fuel tank isn’t working properly. And that would cause all of these symptoms.

[flopsweat]

Originally Posted by Boatguy

If you hear a rush of air inward, that means the vent on your fuel tank isn’t working properly. And that would cause all of these symptoms.

Sounds like a Pringles can. Where would said vent be in a TT250?

[DualSportDude]

My friend had the issue on his TT250, the two vacuum lines going to the canister (left front frame next to the head) had to be swapped, they were installed reversed at the factory.

[flopsweat]

Sounds like I need to take a better look at all the hoses for any small kinks. I wouldn't know if the canister lines were reversed just by looking, is this the black charcoal canister you're referring to? I believe I read that is only there due to CA emissions standards.

[DualSportDude]

Originally Posted by flopsweat

Sounds like I need to take a better look at all the hoses for any small kinks. I wouldn't know if the canister lines were reversed just by looking, is this the black charcoal canister you're referring to? I believe I read that is only there due to CA emissions standards.

Correct, unfortunately they are two slightly different sizes but simply swapping the two lines at the canister and putting a zip tie on the one that is a little loose worked for my buddy. It was a known issue with CSC.

[Boatguy]

Originally Posted by flopsweat

Sounds like a Pringles can. Where would said vent be in a TT250?

I shouldn’t venture into a thread about a bike I don’t have. Lol but, if air is rushing into your gas tank, it is not getting enough air from outside of the gas tank to replace the fuel you are using.

I’m not sure where the vent is even in mine. I suspect maybe it’s in the complicated fuel cap.

If there is something more specific to your bike like those crossed vacuum lines that the other poster is mentioning, if they are connected to the tank in someway, that could cause it also.

But I’d say if you have narrowed it down to air rushing into the tank when you open it, you have found your problem. Something is either not letting enough air in, or something is pulling air out.

[Hunnicutt]

My first long ride on my TT250 was approximately 125 miles one way. January 2021. Florida. Elevation throughout the entire trip did not exceed 120 feet above sea level. My bike was bone stock and had about 100 miles on it. The air temp was 42 degrees. Winds were out of the northwest at 12 MPH, gusting to 25. My entire trip was northwest into the headwind. Needless to say it was a COLD ride, even though I layered up. Anyways, I was about 90 miles into my trip when the bike stalled as I decelerated heading towards a red light. Pulling in the clutch to downshift stalled it, I restarted it, rode a while, decelerated at another red light, and the bike died again. Almost every time I took power off, it died. I kept hearing a lot of popping from the exhaust.



I called CSC and explained my problems. Wade suggested I swap out the carb jet. It comes stock with a 102. He suggested a 112 jet to provide more fuel and prevent the popping. I swapped out the jet at 480 miles. WHAT A DIFFERENCE!!! No more stalling, popping, or anemic acceleration!!



I just bought another TT250 for my daughter as her big Christmas gift. Bone stock, 93 miles on it. When I ride it for about 10-15 minutes, it bogs, pops, coughs, sputters, then smooths out a bit. I have a parts order compiled, which includes a jet kit. I'll place the order soon.



My advice, for what it's worth: Spend the $12.95 for the jet kit, swap the 102 for the 112, and see if that solves your issue. :-)

[flopsweat]

Originally Posted by Hunnicutt

My first long ride on my TT250 was approximately 125 miles one way. January 2021. Florida. Elevation throughout the entire trip did not exceed 120 feet above sea level. My bike was bone stock and had about 100 miles on it. The air temp was 42 degrees. Winds were out of the northwest at 12 MPH, gusting to 25. My entire trip was northwest into the headwind. Needless to say it was a COLD ride, even though I layered up. Anyways, I was about 90 miles into my trip when the bike stalled as I decelerated heading towards a red light. Pulling in the clutch to downshift stalled it, I restarted it, rode a while, decelerated at another red light, and the bike died again. Almost every time I took power off, it died. I kept hearing a lot of popping from the exhaust.



I called CSC and explained my problems. Wade suggested I swap out the carb jet. It comes stock with a 102. He suggested a 112 jet to provide more fuel and prevent the popping. I swapped out the jet at 480 miles. WHAT A DIFFERENCE!!! No more stalling, popping, or anemic acceleration!!



I just bought another TT250 for my daughter as her big Christmas gift. Bone stock, 93 miles on it. When I ride it for about 10-15 minutes, it bogs, pops, coughs, sputters, then smooths out a bit. I have a parts order compiled, which includes a jet kit. I'll place the order soon.



My advice, for what it's worth: Spend the $12.95 for the jet kit, swap the 102 for the 112, and see if that solves your issue. :-)

I appreciate your story! Sometimes context paints a better picture, although my wife says I'm long-winded for being a quiet guy. Anyway, pretty sure you already strongly suggested a jet swap, and I said "nah I'll ride it til break-in just to see how it feels". But I think I might do that sooner now based off what is happening combined with your experienced advice.

I'm in Chicago which is around 700', any idea if that 112 is still good or would the 115 be better? Do you change the idle jet to the #40 that comes in that pack as well? Once I adjusted my idle screw to keep it just below 2k rpm as the CSC guy suggested, it purrs quite smoothly at idle.

I'm just now diving into carburetors, I changed one in my snow blower last year but that was it, so I have been reluctant to crack one open but now might be the time. This guy at Rolling Wrench knows his stuff and this is a fantastic tutorial, . I do get popping in the exhaust when decelerating.

How long does it take you to swap the carb jets? I've watched the csc tutorial numerous times.

[Hunnicutt]

I have spare 115 jets from the kits I got for my TT and my wife's TT. I recently called and asked about installing one of the 115 jets. I was told that the 112 works best with stock exhaust, and the 115 is for modified exhaust. I have no plans to modify any exhaust on my 3 TTs, so I need to order a jet kit and install the 112. I didn't touch the idle jet.



Change the main jet first and see how that works for you. Leave the idle jet alone.


I'm brand new to motorcycles. I paid a mobile motorcycle tech to swap the jets. It took him 10 minutes. He popped the inner bolt out of the carb where it bolts to the head, pulled the air intake, and rotated the carb out a bit to access the bowl. He pulled the bowl, swapped the jet, put the bowl back on, rotated the carb back, hooked up the air intake, and started it up.

[flopsweat]

Awesome. I'll do it in 9 minutes.

I'm also brand new, but have plenty of small tool and repair/troubleshooting experience with industrial electronics so I hope it won't be too big of a feat for me. Even if I'm feeling lazy, my car mechanic always has his KTM's around his garage so I'm sure I could have him do it for next to nothing.

[DualSportDude]

It's the vacuum issue, this was a known defect with some of the TT250's this year. Swapping the lines at the canister is the fix. CSC support told me about this issue, swapping the lines fixed it instantly. Instead of venting the tank, it sucks the air out and gravity flow of fuel stops.