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I am stumped. 125cc 4T scooter ills

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Apr 23, 2020
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Cedar Park, Texas, USA
My scooter, with about 850 miles on it, has decided not to come back from a brief winter storage.

Prior to putting it away for winter, I changed the oil, did a tweak or two on the carb, replaced the fuel filters, and rode it probably 50 miles like this over a while. It was dead nuts reliable. Stored with ethanol-free fuel with Star-tron added just in case. Same fuel I have always used in this scooter, as well as my wife's scooter and my GS500.

The problem: when it's cold, been sitting, it will start but requires some throttle to keep it running. This is actually a normal behavior of this scooter. It starts and runs fine, as expected, for like 15 seconds, and then just dies. Attempts to restart it are met with occasional catching like it might start, but invariably it won't restart until I go away and let it sit for a good while. It always behaves this way, whether it's 40 degrees outside or 80 (like today). Fuel filters are full of fuel so it appears unlikely it's a fuel pump problem (my initial thought). And if I attempt to ride it while it's in this 15 second running phase, as soon as I give it throttle under load, it dies and won't start back up until I let it sit a few hours. It's never getting warmed up at all.

One thing I noticed today is a surprising amount of oil smoke while it's running for that 15 seconds. It only has like 300 miles on it, so no burning oil is expected. When I changed the oil, ISTR the manual called for 0.9L of oil but I rounded it to 1.0 qt. I guess it's possible it's overfilled a tiny amount and fouling the spark plug with blow by? I checked all of the usual stuff, vacuum hoses and intake boot and all that, everything looks like it should "just work", and since the last time I rode it before parking it back in about November, it was running prerfectly fine.

Ideas? I plan to dig into it this weekend.
 
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How is the oil level? I have had gas go down into crankcase on several motors and raised oil level not to mention run like trash. Sticky floats are a bad deal
It it is FI then that would not be a problem.
I try to have fuel shut off valves on everything small engine that I can.
 
Impossible to tell what the oil level is. There's no sight glass or disptick. You drain it, refill with the correct amount, done deal. AFAIK. I'll go hunting again for the dipstick. But like I say, manual calls for like 0.9 liters, and I put in 1.0 quarts, which is a hair more than 0.9 liters.

It's carburetor, but that Dellorto computer controlled carb. Should work though, it worked when I parked it.

The fuel outlet in the tank is about a foot below the carb, so it requires fuel to be pumped uphill to get to the carb. No need for a shutoff valve. I'm sure I've run it enough to burn old fuel out of the carb.
 
Oh, I will fix it, eventually, one way or another. But I'd rather know what it could be first, rather than going full on shotgun method by the weekend.
 
I just managed to get one of my two ct90s running as good as when parked just two months ago.

Bike was bogging at full throttle. Fiddled with the needle up and down, no change. Checked float and cleaned jets, no change. Getting angry. Put on a new carb I already owned, no change???

Took out the book and started doing a regular tune up. Found the points gas at about .026 should be .012 to .016. HOW could it have changed sitting there? Dunno, it's running perfect now.

Morale of the story? Start over from the beginning.
 
Of course simple things first.
Check the battery voltage. A small engine doesn't need much CCA to crank over strongly, but low voltage could impede the ignition system or other electronics from working properly. Also ensure the ground paths are clean and tight.
Try a shot of starting fluid with the air filter in place. That will help indicate if it is a fuel/air issue.
Compression good after it cuts off? Could be a valve getting warm and sticking in that initial 15 seconds of runtime. Why it would correct itself later, I don't have an answer for, just speculation.

Open the oil filler and get a whiff of the oil. If gasoline is getting down there you can detect it by smell.
 
I’ve had a spark plug act this way once.
 
The plug might help diagnose. If it is running lean, could be the cold opened an air gap between the carb and the head. That can be tested with spraying the area and seeing if the rpms change (if you can get it to idle).

My bet is that if you can run it long enough to run that gas out (or drain) and refill the tank w fresh, the problem is going to disappear.
 
sounds like the carb flooding , if you can start it, new gas with some seafoam is where I would start before I started turning wrenches.
 
The plug might help diagnose. If it is running lean, could be the cold opened an air gap between the carb and the head. That can be tested with spraying the area and seeing if the rpms change (if you can get it to idle).
Problem is you can't get to the carb boot when it's assembled. Like, engine in the chassis.

My bet is that if you can run it long enough to run that gas out (or drain) and refill the tank w fresh, the problem is going to disappear.

It's got fresh gas, and of course the gas was not that old, and it was treated with Star-tron, and it is also ethanol-free gas. Same fuel as mrs72's scooter has, which works fine.

My guess is that the the float needle is sticking somehow and once it starts and the fuel starts pumping it floods, which may also be fouling the spark plug in the process.

Some of the work I did on it just prior to stopping using it for 2 months is replacing both fuel filters, replacing the vacuum hose running to the fuel pump, and a little bit of idle mixture tweaking. I rode it several times with no issues at all after doing this work, then parked it because we went to Europe for two weeks and then the holidays and cold and I never got back on it until a week ago.
 
Could the gas cap not be venting? An obstruction or the hole in the cap plugged? Maybe take the cap off and test?

Edit: if it was flooding, I think your carb overflow would spill gas. More likely the needle is stuck in the valve shutting off the gas.
 
Could the gas cap not be venting? An obstruction or the hole in the cap plugged? Maybe take the cap off and test?

First thing I tried! :) great minds...

Edit: if it was flooding, I think your carb overflow would spill gas. More likely the needle is stuck in the valve shutting off the gas.

Also thought about that, but then why does it start the first time when cold? I mean, it even starts like INSTANTLY, just barely touch the start button and it's running, even before the fuel pump can do anything. So the float bowl is obviously getting full and staying full.

I'll know the answer today. In fact, I think I'm going to go right now, an hour and half before daylight, and start taking it apart in the garage. Who knows, maybe it's a mouse in the intake.
 
Also thought about that, but then why does it start the first time when cold? I mean, it even starts like INSTANTLY, just barely touch the start button and it's running, even before the fuel pump can do anything. So the float bowl is obviously getting full and staying full.
I've not owned a fuel pump bike, always gravity feed so I'm probably no help. But a stuck needle can pass some gas and I have had them fill a bowl after 15 minutes and start again. That would explain the start and soon dying.

I'm following this closely cause I have the ct but probably will move to a scooter in a couple of years.
 
After fighting a bank of 4 carbs for over a month trying to get them tuned and failing, I eventually bench tested them. Will never reinstall a carb/set of carbs without bench testing the float system again! One little weep of gas will make a bike run like crap. Try and adjust a/f mix screw all you want, you're not going to get it to run right.
 
Yeah I couldn't fix it today. Just verified everything is working correctly and drained about a pint of gas, added a lot of seafoam. I'll try again tomorrow.
 
<Mario voice> She's-a Fixed! </Mario voice>

So I went out this morning to try again, after leaving it on the charger yesterday. Barely touched the start button and it fired up instantly. I had cranked the idle speed up so much while fooling with it yesterday that it revved really high and scooted across the garage on the centerstand and I had to use the kill switch to kill it before it went berserk on its own. I did this about three times, turning down the idle speed in between times, then finally it started and idled correctly. And it ran for about 15 seconds and died.

The theory that it's not getting fuel into the carb bowl and it was running it dry was proven wrong when I opened the drain screw and fuel came out, no problem.

So my other theory then was that it is only running while the choke is on. A little background: most scooters with carbs have an automatic choke. This one has a solenoid-operated auto choke that is turned on and off by the ECU. Remember this is that oddball Dellorto electronic carb system with an ECU that is supposed to control stuff like ignition timing, has a TPS, and also has a air injection solenoid to allow it to lean the mixture. Plus the ECU operates the auto choke. So my theory was that the ECU was just shutting off the choke when the cylinder head temp sensor hit a certain threshold and when it shut the choke, the engine dies.

So on that theory I started opening the pilot mixture screw a half turn at a time and attempted to restart the engine. After taking it about an additional two whole turns from where it was, for a total of three turns out, it started and ran just like it's supposed to.

So. The problem all the time was that the pilot mixture was too lean and it was dying once the choke was switched off. Then it had to cool enough for the ECU to turn the choke back on before it would start again.

So why did it run fine last time I used it? Well, that was in November when it was 80 degrees outside and high humidity so the air was much less dense than these latest days when it's been very dry and more like 35-60 degrees. I wonder of my carb motorcycle will do the same thing, since I last tuned it and ran it when it was very hot.
 
I'm glad you got it running, but it sounds to me like the computer is turning off the choke too soon. 1-2 turns out on the idle mixture screw is the normal range. My experience with air cooled carbureted engines is that they needed more like 5 minutes or so of choke, not 15 seconds. Makes me wonder if that temp sensor is supposed to be somewhere other than the head.
 
I have considered hotwiring the choke to a switch so I can keep it on longer when it's cold. The ECU is complaining that the temp sensor is not working anyway, and this could be an effect of that. It's just very difficult to replace it... Step one is remove the engine and transmission from the frame. So I have ignored the blinking light.
 
Alright.

I've been so swamped with work for the past three weeks I have not been able to make time to ride my scooter at all, and I've only ridden any of my 2-wheelers one time since November. No surprise, the one that I got to ride is the one with modern EFI that starts and runs perfectly every single time.

Well yesterday I had a 30 minute opening in my schedule and I thought I'd go take the !Vespa for a spin. Welp, no dice. It did the same thing as before, started and ran for maybe 15-30 seconds, then wouldn't start again until I let it cool for about a minute. I thought I could string these 30-second running intervals together long enough to get it warmed up enough to stay running, but the battery died before I could get it to keep running.

As WW suggested, I am hoping a new temp sensor will get the job done. As a hack, I might also hotwire the choke and find a spot for a switch, since I can see how that may be helpful if ever the new temp sensor fails.

Mostly I realized it's foolish not to get this part replaced while a new one is available. That won't last forever. These scooters are somewhat rare, especially in the USA, and parts have not been in production for years. LML (the actual manufacturer) has been out of business since they filed for bankruptcy in 2017 and had sold off all of their assets and been scrapped completely by 2020.

1674820539439.png
 
I went out in the garage yesterday to catch up on a number of projects, and for a minute looked into adding a switch to turn on the choke on this scooter. Then I had a strong sense of deja-vu like I'd attempted this before, and certainly I did.

I separated the connector for the wire feeding the choke and turned the starter to see if the ECU was supplying +12V on that connector. And it's not. So the ECU is not sending any signal to turn on the choke. Still makes me wonder why the scooter will even start at all when it's cold.

I did not end up hotwiring the choke as planned, but not really because I didn't have faith in my logic that it must be +12V that runs it, but simply because I didn't have a switch laying around that would get the job done. Then there's the matter of where to put the switch. I'm not going to drill or cut the sheet metal of the scooter for this, and it doesn't have conventional handlebars where I could just put a handlebar mounted switch.

So I'll wait for the temp sensor. Scooterworks says they shipped it. It's going to rain most of the week anyway. I haven't ridden this scooter since November. Who knows? Maybe when the temp sensor is fixed, this thing will run way better than ever.
 
You need more guitar picking. Bikes within ear shot always run better after hearing a few tunes.
 
Welp. With great effort, but less than I expected, I got the temp sensor changed.

Just so you can get an idea of the scope of this repair. Here's a picture of the space I had to work in, with the arrow pointing at the part I had to remove. It's screwed into the head next to the spark plug but at a 90 degree angle to the side of the head.

1675696248518.png


If you are not familiar with working on air cooled scooters, you might wonder what all this plastic housing is about. Since the engine isn't out in the wind, they usually have forced air in the form of blower fins on the flywheel that work like a squirrel-cage blower to pull air out and a set of plastic covers to completely cover the cylinder and direct airflow over the cooling fins. So that's what this plastic housing is. On most modern scooters, they have a tube frame with plastic cowls and bodywork that can be removed, so you can get the entire engine exposed and work on it. The old Vespa PX like my scooter is based on originally had the engine exposed under the right side cowl and a spare tire under the left side cowl. They are made with a steel monocoque, so you can't remove the bodywork aside from just taking the cowls off. The body is the frame. My scooter is sort of a factory resto-mod of a Vespa PX. It has a modern engine and CVT crammed up in empty space under the seat and the left cowl and cosmetic spare tire cover covering up the fuel tank, which on an old PX would have been under the tank. As a result, to really get to anything to repair the engine, you are supposed to pull the whole swingarm/trans/engine as a unit. I didn't want to do that.

I took out the battery, took off the battery tray, and removed the flywheel cover/vent plastic piece, then with great difficulty removed the screw holding the two sides of this cover together. Then I wedged a 1/2" socket extension in there to get the covers to hold open so I could get to it. Then I managed to pull the old sensor out, believe it or not it was only finger-tight, and a plastic part. The original part had the connector molded into the part. The new part is a metal part with a 6" or so pigtail attached. You can see how I installed it here:

1675696845549.png


Just so you can relate to my suffering, more pics. The new sensor did not come with a compatible connector. But it did come with a mating connector to retrofit onto the scooter. So I had to cut the old connector off and strip and crimp the new connector pins on the old wiring. Here you can get an idea of how much room I had to work, and how much slack in that original wire. This space where that little teeny stub of wiring is, that's where the starter solenoid is, so I had to remove that.

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Believe it or not, I did manage to get the wires cut, stripped, crimped, new connector in place, and got it all hooked up. Since there was a lot more wiring flopping around in there, enough for the wires to maybe run down into the flywheel vent fins, I zip tied it up tidy and also tidied up a lot of the wiring when putting it all back together. So now that annoying job is done.

The ECS light doesn't blink anymore, so that's good. But the scooter still won't run. Nothing Is changed.

I went looking for a carb rebuild kit, figuring I should just bite the bullet and rebuild the carb completely. Tough rocks. No such thing exists. It's a Dell'Orto carb, and while they are pretty common to repair if you're in Italy, it's pretty difficult to get parts or even know what to get here. But I did find a whole brand new NOS carb from an Indian company where I've ordered various parts for these scooters before, for $51. That includes a new electric choke, new TPS already installed, plus of course an entire new carb. Scooterworks here in the USA has the part (they are the official Genuine scooter USA parts source) for $400! Remember these scooters were made in India by LML after being licensed from Piaggio, then LML went bankrupt like in 2018 and sold off all of the parts and tooling under their forced reorganization. So there are caches of parts around in India, plus SIP scooter shop in Germany bought a whole bunch of the stuff including the tooling to manufacture parts like engine parts etc. SIP have 200cc cylinder/piston/head kits that might be calling my name one of these days. This scooter with 65% more displacement would be a blast.

Anyway, I ordered the $51 carb. It could take anywhere from 1-3 months to get here. I guess there's a chance I'll have a scooter to ride before spring is over. I say "chance", because there's also a near 50/50 chance that the part when it gets here is entirely wrong for the scooter. But there's a 50/50 chance it'll just plug right in and fix it 100%. That's the way it is with these Indian scooter parts outfits.

Doesn't matter that much because we have a lot of travel between now and the end of March, so I won't have much need for it. But I would certainly like to get back into the scooter for errands duty since weather is likely to allow it. Putting a lot of miles on the Jeep with 17mpg and $3.50 gas. If the new carb doesn't fix it, I'll punt. Maybe rob parts from the new carb to rebuild the old one, which only has 850 miles on it.
 
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