Modern Buddy

I posted a few months ago about a stalling/sputtering issue I had with my '12 150 4t. The bike was acting starved at the end of my 15 minute commute and would occasionally even stall. Not fun. I tried a few things recommended in other posts--the evap had already been disconnected by the previous owner, I tried a buddy's vented 2t gas cap, shortened up the gas line to the carb, added a small one-way valve to the evap outflow tube, drilled out the gas cap--but none of these worked for me.

The trick was noticing that the previous owner had swapped out the OEM seat for a "racing" style one, and that someone else on here with that seat was having similar issues.

The problem was that the seat was blocking off air to the vented gas cap, causing a vacuum, and hence starving the carb. The fix was simple: drilling out the gas cap more, putting a short stint through it (a section of a WD-40 spray nozzle), and drilling a hole in the bottom of the seat.

5 tanks and about 600 miles later, and I haven't had a single splutter.

Nice diagnosis and fix! Nothing better than figuring it out and then fixing it for free!

@johnk here's my solution to the stalling problem. I tried several other solutions - but this definitely did the trick. I've put about 1k miles on it with nary a splutter since. Bypassing the evap (with a one-way valve to avoid leaks) and drilling out the seat and gas cap (with a WD-40 spray nozzle to keep it open) was the key. Let me know if you have any questions, and good luck!

Thanks for the update, NCJ8!

Is installing the one-way valve really as easy as snipping the line and shoving it between the (now) two pieces?

Aside from that, if you (or anyone) can explain in relatively simple terms why these two steps (vented gas cap and one-way valve) fix the problem, I'm interested to understand it better. (I've read as many posts here as I could find, but I'm doing so from the perspective of total automotive ignorance.) I have a vague idea of what vapor lock is (gasoline boiling and evaporating due to heat/pressure, preventing sufficient fuel flow), and I as far as I understand, a vented gas cap will prevent or reduce it. Is that just because vapor lock won't occur in a vented system?

The one-way valve prevents liquid gas from going into the EVAP system, right? Does that help to prevent vapor lock, or is it preventing a different problem?

johnk wrote: I have a vague idea of what vapor lock is (gasoline boiling and evaporating due to heat/pressure, preventing sufficient fuel flow), and I as far as I understand, a vented gas cap will prevent or reduce it. Is that just because vapor lock won't occur in a vented system?

Scooter riders sometimes call a problem with gas tank venting vapor lock, but that is actually not correct. Evaporation systems and vented gas caps both are ways to vent a fuel tank so that air enters as fuel leaves. If air can not enter the tank while the engine is running, it slowly develops a vacuum until the carb or fuel pump can no longer extract fuel from the tank. This venting problem gets worse as you ride the scooter and goes away when the scooter sits for a while. That is exactly the opposite of true vapor lock which occurs when a hot engine sits on a hot day causing gas to boil and vaporize. Small scooter engines without much mass and with fuel systems exposed to the air are really not prone to true vapor lock.

johnk wrote:The one-way valve prevents liquid gas from going into the EVAP system, right? Does that help to prevent vapor lock, or is it preventing a different problem?

This is getting into how an evap system works. The system is supposed to let air in to vent the tank, but is intended to avoid venting gasoline vapor into the atmosphere. The simple systems used in scooters can trap liquid gas in the charcoal cannister or in the associated tubing which partially blocks air from being drawn into the tank. Overfilling the tank is a common way to force liquid gas into the evap system.

Thank you, Dooglas! This really helps.

To ensure my understanding via restatement: the main issue is that fuel flow is inhibited when insufficient air gets into the tank, for the same reason that a beer can's tab needs to be cracked open for someone to shotgun it from a hole punched in the side. With a sealed gas cap and fuel potentially blocking the EVAP's inflow of air, it's easy for the tank to become a vacuum. Moreover, the fuller the tank, the less air is in it to begin with, and the more likely it is for fuel to splash into the EVAP. So the one-way valve lets air into the tank from the EVAP without letting fuel into the EVAP, and a vented cap provides even more air inflow.

@johnk, Dooglas' answer was spot-on (including the mis-use of the term "vapor lock"). It's like pouring milk out of a jug by pouring it straight upside down - it has a hard time coming out because there's a vacuum in the empty part of the jug. Poke a hole in the bottom of the jug (like that little valve on a gas can), and it will come out much more easily. The issue with the Stella 4t fuel system with the evap is that it's almost *completely* sealed. Air can't get in to allow fuel to feed the carb, and, unlike in the milk-jug example, air can't just slosh in on gasps from the bottom. So it starves the engine, causing it to stall. The solution is to allow air into the empty part of the tank. I've opted to do that by disconnecting the evap and venting the gas cap. The one-way valve is really just a way to make sure that gas doesn't leak out after overfill (which, considering how small the tank on the 4t Stella is, is not difficult to do, and is almost necessary in order to not stop by the gas station every week!). With a one-way valve, air can get in, but liquid gas can't get out. The more crucial piece, though, is to vent the gas cap - drill it out, put in some sort of stint (I used a shortened WD-40 nozzle) and, if your seat touches your cap, drill a small hole in the plastic bottom of the seat. Hopefully that does the trick. Good luck!

think of the vacuum issue like if you put a straw in a glass of water and put your thumb over the top of the straw and pull the straw out...the water stays in the straw until you move your thumb and let air in...same with a small engine gravity fuel flow...so if the gas cap isn't venting properly you will have a fuel starved engine and cause this stalling issue

also if you insert a valve make sure you don't just cut the line once...you will need to cut twice to take a piece of the line out to account for the length of the valve...do it a little at a time so you don't cut too much by accident

Hi guys. I've had the issue for years that my Stella 2T runs fine when the gas tank is at least half full, but stalls out after a few miles when not more than half full. I've lived with it, simply by keeping the gas full and adding sea foam. But it prevents me from ever going away, eg. to a rally.
I've assumed it was the routing of the fuel line, which a dealer replaced for me a number of years back. But maybe there's a better solution than buying a tool and trying to wrestle the tank out--something I'll never get to.