Stella 4-Stroke stalling

Maximus53 · Post by **Maximus53** » Mon Jun 25, 2012 10:28 pm

Hi all,
I did my first extended county hwy ride this weekend (up to Amerivespa) and on the way back, my Stella twice stopped running. The first time was coming to a stop light after about 20 mins of WOT. When I put the scoot in neutral coming to the stop light the engine died and would not restart. I pushed it into a parking lot and after about a minute the engine fired back up and all seemed fine. Then about another 30 mins of WOT down the road I had a couple of skips (like when running out of gas) then the scooter died while i was riding at WOT. Again i pulled off onto the shoulder and after a minute it started back up and was fine for the rest of my ride back to Chicago at slower speeds since hit civilization.

My Stella does seem to have a small oil leak which I am waiting for parts to arrive at the dealer for repair.

I was wondering what the suspected cause of this behavior is. Overheating? The EVAP system air lock issue i keep hearing about?

Thanks!

Lovelandstella · Post by **Lovelandstella** » Mon Jun 25, 2012 11:08 pm

Maximus53 wrote:Hi all,
I did my first extended county hwy ride this weekend (up to Amerivespa) and on the way back, my Stella twice stopped running. The first time was coming to a stop light after about 20 mins of WOT. When I put the scoot in neutral coming to the stop light the engine died and would not restart. I pushed it into a parking lot and after about a minute the engine fired back up and all seemed fine. Then about another 30 mins of WOT down the road I had a couple of skips (like when running out of gas) then the scooter died while i was riding at WOT. Again i pulled off onto the shoulder and after a minute it started back up and was fine for the rest of my ride back to Chicago at slower speeds since hit civilization.

My Stella does seem to have a small oil leak which I am waiting for parts to arrive at the dealer for repair.

I was wondering what the suspected cause of this behavior is. Overheating? The EVAP system air lock issue i keep hearing about?
Thanks!

mine did the same thing this weekend twice and it was 100% because of the outside heat. my 2 cents anyway. an interesting quick fix in my case was to pull the choke. I don't like to ride like that for long- but it's better than pushing it to a parking lot or getting honked at at a light.

Robbie · Post by **Robbie** » Tue Jun 26, 2012 1:30 am

Truth be told, ambient temperature has little to do with engine operating temperature on a fan cooled engine, sooo, I'm thinking either a fuel tank venting problem for both machines.
In Max's case, sounds like just sitting with the machine off allowed enouph time for the fuel system to return to atmospheric pressure.

On the other machine, running with a bit of choke made enouph carb vacuum to overcome the slow venting of the tank.

The other possibility could be that the high, undershroud heat around the uncooled carburetor could have caused fuel to boil in the carburetor bowl....but I admit that might be a long shot.

I would be inclined to put a earlier, vented fuel cap on each machine.
I'm reasonably certain this would resolve Max's concern simply because the machine ran OK at lower throttle positions....proving the venting is inadequate for WOT operation.

Rob

Maximus53 · Post by **Maximus53** » Tue Jun 26, 2012 1:36 am

Thanks guys for the input. A new gas cap is on the list for when next time I head over to Scooterworks. If I get the vented gas cap, do I have to block the evap tube?

Robbie · Post by **Robbie** » Tue Jun 26, 2012 1:38 am

Just exchange the cap Max....you don't have to mess with anything else.

Rob

Lovelandstella · Post by **Lovelandstella** » Tue Jun 26, 2012 3:15 pm

Robbie wrote:Truth be told, ambient temperature has little to do with engine operating temperature on a fan cooled engine...

When the temp 1 foot above the asphalt isn't over 110 degrees, I'd be inclined to agree with you. but when the air blowing into the fan is already crazy hot, I hope we all can agree - that at some point, there can be "not enough" cooling going on.

Riders in AZ and CA, etc, how long do you ride before you let the scooter take a break?

(P.S. I already have the vented gas cap.)

Scalpel · Post by **Scalpel** » Tue Jun 26, 2012 4:49 pm

I've had the same thing happen, and I just assumed it was because the 4T doesn't like to run that fast for too long. I was restricting my speed to like 45smph (Stella MPH) with short sprints to 50-55 before backing off again.

Can anyone confirm that this is a venting issue? I'd love to hear that someone had this problem, got a vented gas cap, and repeated the same speed/route without a problem. Being able to cruise at 50 would save me a few minutes here and there.

Maximus53 · Post by **Maximus53** » Tue Jun 26, 2012 5:23 pm

Scalpel wrote:I've had the same thing happen, and I just assumed it was because the 4T doesn't like to run that fast for too long. I was restricting my speed to like 45smph (Stella MPH) with short sprints to 50-55 before backing off again.

Can anyone confirm that this is a venting issue? I'd love to hear that someone had this problem, got a vented gas cap, and repeated the same speed/route without a problem. Being able to cruise at 50 would save me a few minutes here and there.

Glad to hear I am not alone

I am going to try to get a vented gas cap this week then maybe give it a shot this weekend. Will post results when I have them.

Robbie · Post by **Robbie** » Wed Jun 27, 2012 12:04 pm

Lovelandstella wrote:
Robbie wrote:Truth be told, ambient temperature has little to do with engine operating temperature on a fan cooled engine...
When the temp 1 foot above the asphalt isn't over 110 degrees, I'd be inclined to agree with you. but when the air blowing into the fan is already crazy hot, I hope we all can agree - that at some point, there can be "not enough" cooling going on.

Riders in AZ and CA, etc, how long do you ride before you let the scooter take a break?

(P.S. I already have the vented gas cap.)

Well, I'll try to simplify as best I can.

Start by remembering all engines are technically air cooled.....some use a heat exchanger (radiator) ie your automobile.
Through the use of a thermostat, the operating temperature remains fairly constant regardless of ambient temperature, even though the air passing through the radiator is, say, 100 degrees.
Using the example of a 200 degree thermostat, the 100 degree air has no problem drawing heat from a 200 degree radiator.
The only difference between 20 degrees ambient and 100 degrees ambient is the length of time it takes the engine to reach operating temperature.
Next, consider an air cooled motorcycle.....one that has no fan is entirely dependent on the heat that radiates from its cooling fins.
At idle, because the firing events are slow, it is capable of cooling strictly off of the rate the fins radiate heat.......not happy about extended idle time but if there is enouph finning, it is harder on the rider sitting over the engine than the engine itself.
At speed, because of the large amount of air passing over the fins, heat transfer is adequete for all day running, even at 110 ambient.....again because that air temperature is much lower than the fin temperature.
However, this type of engine requires large internal clearances because of the large varience in operating temperatures it may be operated in.
For example, a idleing Harley will run quite hot on a 40 degree day, but be overcooled at speed,,,,,as a result, piston to cylinder clearances must be rather large, much larger than a heat exchanger, or liquid cooled design.

A fan cooled engine is somewhere in the middle regarding cooling.
At idle, with slow firing events, even at 100 degrees ambient, the cylinder and head are being bathed in moving 100 degree air.....more than enouph to meet or exceed the btu's being produced.
At speed, the engine makes more heat, but the fan is moving so much more air across the engine, air that is almost 200 degrees cooler than the cooling fins, its cooling needs are more than satisfactory.
Think of a air cooled generator, running for days at a time with no ill effects regardless of ambient temperature.

So, knowing that ambient has little to do with the engine operating temperature itself, my thoughts turn to a phenomina called 'Dead Air'
I am wondering/speculating, if the carburetor, is heat soaking.
It does not have the benifit of much cooling in the area it is located in.
It sits in a box (cowl) above a hot engine case and we all know that gasoline boils at around 220 degrees.

The 'old guy' term, (I'm an old guy) was 'vapor lock'.
This was when the fuel would boil in the fuel line as it neared any hot engine part.
In a gravity feed system, I wonder/speculate about the possibility of fuel boiling in the fuel line, or carburetor bowl.
Either will disrupt carburetion.

To give the theory merit, you would have to operate your machine with the right side cowl removed when the ambient temperature is high enouph to create the situation you experienced.....might want to give it a go.

I apologize the epic post and if I need to clarify something please ask.

Rob

rondothemidget · Post by **rondothemidget** » Sat Jun 30, 2012 12:09 am

I'd be interested to see if anyone experienced this problem first-hand, and corrected it with a vented gas cap.

I'm fortunate to live near a scooter shop that sells Vespa parts, primarily online. I don't know how reliable the information was but the guy didn't sell me a vented cap because he said my stock gas cap is vented.

Robbie · Post by **Robbie** » Sat Jun 30, 2012 1:31 am

rondothemidget wrote:I'd be interested to see if anyone experienced this problem first-hand, and corrected it with a vented gas cap.

I'm fortunate to live near a scooter shop that sells Vespa parts, primarily online. I don't know how reliable the information was but the guy didn't sell me a vented cap because he said my stock gas cap is vented.

I suspect your shop was misinformed.
With the 2010 and onward 4t Stella the fuel system became a 'closed' system, same as a current automobile.
Fuel vapors can no longer escape to atmosphere.....instead, they are collected in a carbon cannister.
Upon startup, there is a vacuum valve that opens, allowing the fuel vapors into the intake manifold to be burned with the normal air fuel charge.
This valving only allows air into the tank to take the place of the fuel as it is consumed.
Obviously, this system wouldn't work with a vented fuel cap.....it allows air in either direction.

It seems though, the 4t emission system can easily develop its own venting issues.....some are speculating that tank overfilling could cause raw fuel to enter the carbon cannister, causing it to plug and not allow the fuel tank to breathe properly.
This could create a vacuum in the tank and stop fuel from flowing into the carb.

Rob

Robbie · Post by **Robbie** » Sat Jun 30, 2012 1:33 am

And I meant to finish by saying some have had success by installing the 09 and earlier vented fuel cap.

Rob

rondothemidget · Post by **rondothemidget** » Sat Jun 30, 2012 3:28 am

Thank you Robbie.

I might be sticking my neck out but it seems there are two different stalling issues. The first is related with the evap system where, during general use, it sputters and dies. The second is not enough fuel getting to the carb during extended WOT. The second happens to me only after getting down to a half tank. It is perfect from full to half tank and it won't sputter if the throttle is slightly less than wide open when I get to a half tank or less. A motorcycle mechanic told me the second scenario isn't symptomatic of the evap system. His guess was that not enough fuel was getting to the carb. Again, he might be mis-informed.

I haven't tried the 2T gas cap. And I haven't tried pulling out the choke. I have tried flattening the path of the fuel line. In my mind, it is a design flaw that the fuel line points out to the right at the tank connector but the carburetor is on the opposite side. The hose has to bend into an elbow to make the path to the carb. I posted some pictures in another post. That has seemed to help a little.

I might be completely off-base. I'm just a schmo that likes to ride scooters. I will defer to the mechanics and engineers for suggestions.

Robbie · Post by **Robbie** » Sat Jun 30, 2012 12:05 pm

rondothemidget,

I have read the posts regarding the half tank or less thoughts.
Because the system is gravity dependent, this too could be a venting concern.
When the tank is full, the weight of the fuel could overcome a slow venting situation but once the level is below half the venting could become inadequate.
This line of thinking on my part is based on those who report that waiting a few minutes or operating at a lower speed seems to result in the stalling condition disappearing.

I am at a disadvantage as my Stella is a 09 2t so I am working with my mechanical background and the 4t's I have looked at with the seat up and cowling removed.

In an earlier post I mentioned that there was a valve operated by manold vacuum, that opens at startup.
This valve must be fully open whenever the engine is running to provide the needed venting.

When operating at a WOT condition, there is very little manifold vacuum present.....this because the throttle valve is presenting no restriction and the only restriction is between the venturi itself and the intake port.
I currently suspect the vacuum operated valve is not rated properly and is partially closed during these low vacuum periods.

Time and customer complaints will ultimatly find the culprit but for now the only way to address the concern is provide a alternate method to vent the tank and avoid creating vacuum in the tank, leading to fuel starvation.

Hence, the riders trying the vented cap.

Good luck and keep us informed if you run across something being overlooked.

Rob

rondothemidget · Post by **rondothemidget** » Tue Jul 10, 2012 4:10 am

Would loosening the gas cap so it's not tightened at all give me the same effect as a vented cap? Aside from avoiding gas spilling out, do I have to tighten the gas cap all the way? I don't understand the science or engineering behind this so I'm open to trial and error. Can I eliminate the evap system as the cause of my problem if loosening the gas cap doesn't change anything?

Robbie · Post by **Robbie** » Tue Jul 10, 2012 12:07 pm

rondothemidget wrote:Would loosening the gas cap so it's not tightened at all give me the same effect as a vented cap? Aside from avoiding gas spilling out, do I have to tighten the gas cap all the way? I don't understand the science or engineering behind this so I'm open to trial and error. Can I eliminate the evap system as the cause of my problem if loosening the gas cap doesn't change anything?

Operating with a loose cap would keep a vacuum from occuring but fuel splash is probable.
You really don't want to have a flammable liquid splashing about under the seat now do you?
Bit of levity there, but any fuel splashing will cause paint damage as well.

I guess you could try it with the tank at 1/2 or less just to see if the stalling recurs.

My dealer was telling me the other day they have run into several cases of the fuel line being too long and a kink developed at one of the bends.
I know the 4T has a subframe under the rear section so you may be able to see how the line is running without too much disassembly.
The 2T's are all enclosed in that area, so you have a bit of a advantage from a diagnostic standpoint.

Rob

talindsay · Post by **talindsay** » Tue Jul 10, 2012 3:47 pm

Robbie wrote: My dealer was telling me the other day they have run into several cases of the fuel line being too long and a kink developed at one of the bends.
I know the 4T has a subframe under the rear section so you may be able to see how the line is running without too much disassembly.
The 2T's are all enclosed in that area, so you have a bit of a advantage from a diagnostic standpoint.

Yes, having both a 2005 2t and a 2012 4t I can tell you I was amazed when, this past weekend, I removed the spare and rear wheels and saw how accessible everything is in that subframe. Getting to the fuel tank and lines in the 2t was an incredible ordeal; on the 4t it's simple. I didn't even think to look at the line's condition when I was in there, I'll take a look again tonight or tomorrow.

rondothemidget · Post by **rondothemidget** » Wed Jul 11, 2012 5:33 pm

Robbie wrote:I guess you could try it with the tank at 1/2 or less just to see if the stalling recurs.

My dealer was telling me the other day they have run into several cases of the fuel line being too long and a kink developed at one of the bends.
I know the 4T has a subframe under the rear section so you may be able to see how the line is running without too much disassembly.
The 2T's are all enclosed in that area, so you have a bit of a advantage from a diagnostic standpoint.

Rob

Here is the latest. I filled it up and went 12 straight miles on flat land at 60mph indicated with no problems. 60 is less than wide open. Later the tank got down to about half full (or half empty) and it started crapping out around 60. I pulled over and untwisted the cap, placing it loose in the tank opening. I got back on the road and it cut out again after about a mile stretch at 60. It would run without issue if I slowed down to 50, but stall again if I sped up. After re-tightening the cap, it was fine at WOT but going up a steep hill at 40. And it was fine with the cap tightened, riding the tank down to near empty at slower city driving.

Based on this, can I eliminate the evap system? Should I focus on the carburetor? I've got to get this fixed.

Robbie · Post by **Robbie** » Thu Jul 12, 2012 2:22 am

rondothemidget wrote:
Robbie wrote:I guess you could try it with the tank at 1/2 or less just to see if the stalling recurs.

My dealer was telling me the other day they have run into several cases of the fuel line being too long and a kink developed at one of the bends.
I know the 4T has a subframe under the rear section so you may be able to see how the line is running without too much disassembly.
The 2T's are all enclosed in that area, so you have a bit of a advantage from a diagnostic standpoint.

Rob
Here is the latest. I filled it up and went 12 straight miles on flat land at 60mph indicated with no problems. 60 is less than wide open. Later the tank got down to about half full (or half empty) and it started crapping out around 60. I pulled over and untwisted the cap, placing it loose in the tank opening. I got back on the road and it cut out again after about a mile stretch at 60. It would run without issue if I slowed down to 50, but stall again if I sped up. After re-tightening the cap, it was fine at WOT but going up a steep hill at 40. And it was fine with the cap tightened, riding the tank down to near empty at slower city driving.

Based on this, can I eliminate the evap system? Should I focus on the carburetor? I've got to get this fixed.

Good basic diagnostics.......you are correct, loosening the cap and reproducing the issue eliminates the possibility of a vacuun interupting fuel flow.
Now, it becomes a fuel line that is slightly obstructed or a fuel valve slightly obstructed.
I say this because with a full tank the stalling at WOT doesn't happen.

I contend the weight of the full tank of fuel is enouph to provide adequete fuel for WOT operation but, when the tank reaches half, the lower fuel volume, due to the lighter weight, is inadequete and the carburetor bowl runs out of fuel.
The couple minute wait allows the bowl to refill but, as you found out, WOT will again make it stall.
The fuel volume requirements are much lower at 50 MPH, so the problem doesn't occur.

Next time you are at half tank, put the fuel valve on reserve and try to reproduce the WOT stall.
If it runs OK....bum fuel valve restricted when in the on position.
When in the reserve position, fuel enters the fuel line from the standpipe in the tank as well as the inlet on the floor of the tank.
So, with two sources of fuel getting to the carb, if the valve is the trouble maker you can now prove it.
If the operation is the same, regardless of fuel valve position, the fuel line is either kinked somewhere or has a obstruction within.

If it was a carburetor issue, the problem would occur at full tank as well as half tank.....currently I very much doubt a carb problem.

Stay with it!.....you'll prevail.

These type of problems are why mechanics (me) have white hair,

Rob

jimmbomb · Post by **jimmbomb** » Thu Jul 12, 2012 3:54 pm

this thread is a great read...
ill be sad when the story is over..
till then.... tune in tomorrow ...

talindsay · Post by **talindsay** » Wed Jul 18, 2012 3:20 am

Well, this happened to me tonight, going up a hill near WOT. The tank was more than 3/4 full and the bike crapped out. I pulled over, opened the seat, took off the gas cap and restarted, and it ran fine. Later, at about half tank I went WOT again for a three mile stretch or so and the same thing happened again. When I got home I put on the vented gas cap from my 05 so I can see if that does it.

jimmbomb · Post by **jimmbomb** » Wed Jul 18, 2012 3:11 pm

im going to 'suggest' good old fashion Vapor Lock could be happening.
Thus is just a suggested idea though.. maybe wrong..
Vapor Lock occurs when gasoline boils.
Yes it does get hot enough to actually boil.
Whenit boils.. it just doesn't mix well with the air and give decent combination.
most gas will start to boil around 104f and could go to 400+ depending on additives.
I'm going to say to look for an exhaust leak that could be shooting hot exhaust on or near your fuel line or Carb or intake manifold.
or your cooling shrowd could be directing hot cylinder air towards any items carrying fuel. check to see if your shrowd is properly fastened and exhaust is right.
with all the wide open spaces under the 4t fender and cowls.. it would be easy for super heated air to get in and heat up your fuel.
just a thought...

Eel · Post by **Eel** » Tue Mar 18, 2014 7:05 pm

Sorry to ressurect the thread but I am having the same problem

It happens with continuous WOT after about a minute. The stella runs perfect. Speedometer should about 65mph but its probably off by 10 percent.

Here's what I'm pretty sure its not

1) Its not vapor lock. I gutted the canister and bought a vented cap. I even drilled the hole a 16th of an inch bigger in the vented cap to ensure no issue. with air flow into the tank.

2) Its not an empty tank issue or a fuel reserve issue as this happens regardless of an empty or full tank.

3) Its not an exhaust leak issue as I just replaced the gasket and installed a new exhaust. Main jet was increased by one

4) Not an airflow issue - I just cleaned out the air filter 250 miles ago. the bike runs perfect in every other condition.

I'm thinking the float might be the culprit as the carb is gravity fed. Has anyone else messed with the plastic tabs on the float to see if they can get the more fuel into the float bowl?

The bike literally runs perfectly until it inexplicably shuts off unless I close the throttle and drop down 10mph from top speed.

No pops, no backfires, sparkplug is a nice chocolate brown.

Did anyone resolve this issue prior to this thread going dark?

fisher1 · Post by **fisher1** » Wed Mar 19, 2014 12:47 am

I'm thinking the float might be the culprit as the carb is gravity fed. Has anyone else messed with the plastic tabs on the float to see if they can get the more fuel into the float bowl?

That's possible, but if the float assembly can maintain the level called for by however the tabs open and close the needle at 85% throttle, then it should be able to do it at 100%. Usually, the tab setting just dictates the fuel level to be maintained in the bowl, not whether dropping the needle provides sufficient fuel flow into the bowl to re-fill the loss from engine demand or not.

My suspicion would be a fuel flow issue, like an obstruction in the pinhole the fuel passes thru in the needle's seat (like grit or scarf) - or that the hole wasn't drilled correctly to allow sufficient flow or something else related to actual flow to the bowl.

I think you're on the right track, I just don't know that bending the float tabs would fix it. I'd wait until others respond with their thoughts as well before bending float tabs.

Eel · Post by **Eel** » Wed Mar 19, 2014 3:10 am

fisher1 wrote:
I'm thinking the float might be the culprit as the carb is gravity fed. Has anyone else messed with the plastic tabs on the float to see if they can get the more fuel into the float bowl?
That's possible, but if the float assembly can maintain the level called for by however the tabs open and close the needle at 85% throttle, then it should be able to do it at 100%. Usually, the tab setting just dictates the fuel level to be maintained in the bowl, not whether dropping the needle provides sufficient fuel flow into the bowl to re-fill the loss from engine demand or not.

My suspicion would be a fuel flow issue, like an obstruction in the pinhole the fuel passes thru in the needle's seat (like grit or scarf) - or that the hole wasn't drilled correctly to allow sufficient flow or something else related to actual flow to the bowl.

I think you're on the right track, I just don't know that bending the float tabs would fix it. I'd wait until others respond with their thoughts as well before bending float tabs.

I'm assuming the needle you are referring to is the one attached to the float that brings in the fuel to the bowl. I think you have a good point. I'll check that out when I open the carb. If the fuel is just dribbling out I may not be filling the float before the motor uses up the reserve fuel in the bowl.

The float tabs are plastic so you don't get too much change. If they were metal I'd be more worried.

fisher1 · Post by **fisher1** » Wed Mar 19, 2014 7:36 pm

I'm assuming the needle you are referring to is the one attached to the float that brings in the fuel to the bowl. I think you have a good point. I'll check that out when I open the carb. If the fuel is just dribbling out I may not be filling the float before the motor uses up the reserve fuel in the bowl.

Yes, that's what I meant.

slotrod65 · Post by **slotrod65** » Thu Mar 20, 2014 1:23 am

Wasn't there an issue with some 4Ts having a fuel line that was too long, causing a kink or incorrect running path? Something like that is rattling around in my brain, but refuses to come out.

Eel · Post by **Eel** » Thu Mar 20, 2014 2:58 am

Oddly enough I replaced the fuel filter with one someone recommend off Amazon on another post. It was larger so I cut the fuel line for a more correct path. If there was an issue before, it's not there now.

pesce · Post by **pesce** » Tue Apr 29, 2014 4:35 am

Hey everyone! Sorry to resurrect this...

I had the exact same issue... Always happened on super hot days and WOT for long periods. Resting the bike always "fixed" it.

I tried all the venting fixes to no avail.

I haven't had the issue since I shortened the fuel line from the tank to the carburetor.... made it a more direct path.

Modern Buddy

Stella 4-Stroke stalling

Stella 4-Stroke stalling

Re: Stella 4-Stroke stalling