LML 150 cc Stella engine - 1963 Vespa VBB Problem! Help!!!

[vespamiami]

I have test driven my Vespa VBB 1963 today.

Everything went well until I was coming off a bridge (coming down on a slope) at the speed around 52mph when all of the sudden my rear wheel blocked. The tire started squeezing loudly and the tire skidded. I didn't panic but freaked it me out! I didn't touch the rear break for sure. It looked like as if I had pressed the rear break hard but I didn't. After 2-3 seconds the wheel started running again. By that time my speed was considerably lower. I have looked at the engine and I didn't see anything out of the ordinary. I was riding the scooter afterwards but didn't dare to push it up to 50mph... What happened???

The engine is a modern LML Stella 150cc port engine with sito plus exhaust.

Do you have any idea what happened? Have you ever experienced this problem?

[az_slynch]

The engine probably soft-seized. The appropriate response is to pull in the clutch lever and the rear wheel will spin free. Much easier to handle than skidding to a stop, or blowing a tire and crashing.

I'm guessing that you are using two-stroke oil pre-mixed into the fuel tank, as the VBB never came with an autolube system. Have you added any gas to the bike since purchase? If so, did you add 2T oil in the corresponding amount (2% or 50:1 ratio) to the fuel?

[vespamiami]

The engine probably soft-seized. The appropriate response is to pull in the clutch lever and the rear wheel will spin free. Much easier to handle than skidding to a stop, or blowing a tire and crashing.

I'm guessing that you are using two-stroke oil pre-mixed into the fuel tank, as the VBB never came with an autolube system. Have you added any gas to the bike since purchase? If so, did you add 2T oil in the corresponding amount (2% or 50:1 ratio) to the fuel?

I think you are right! I didn't pull the clutch!!!!

I didn't add any fuel yet into the fuel tank. It is half full. I assumed the previous owner added the right mixture of fuel and 2T oil.

What should I do? Do you think I should add gas with the proper amount of oil?

[az_slynch]

You stated that you were coming off a bridge, down a slope. Did you have the throttle closed?

On a two-stroke scooter running an oil-gas mix in the tank, never coast down a slope with the throttle closed. The carburetor will not supply enough oil to the crankshaft and piston to keep it lubricated if the engine is turning above idle with the throttle closed. Also, the fuel cools the crankcase and cylinder somewhat. Cutting off the fuel completely negates that added cooling. Be sure to either crack the throttle a little to add more gas-oil in to lubricate things, or "blip" the throttle at intervals.

Note: this is not as much of an issue with auto-lube stwo-stroke scooters. The oil injection pump introduces oil below the carburator and is primarily governed by engine speed. Heat is the bigger problem in this case

[vespamiami]

You stated that you were coming off a bridge, down a slope. Did you have the throttle closed?

On a two-stroke scooter running an oil-gas mix in the tank, never coast down a slope with the throttle closed. The carburetor will not supply enough oil to the crankshaft and piston to keep it lubricated if the engine is turning above idle with the throttle closed. Also, the fuel cools the crankcase and cylinder somewhat. Cutting off the fuel completely negates that added cooling. Be sure to either crack the throttle a little to add more gas-oil in to lubricate things, or "blip" the throttle at intervals.

Note: this is not as much of an issue with auto-lube stwo-stroke scooters. The oil injection pump introduces oil below the carburator and is primarily governed by engine speed. Heat is the bigger problem in this case

I don't remember whether the throttle was entirely closed. I think that was the problem. Should I add a little bit of extra oil to the gas in the gas tank in case the previous owner didn't add enough premix oil to the fuel... ? Thanks so much!!! I will never do this again.

[Anachronism]

1. You probably have engine damage. In most cases, a seize like what you describe has scored the cylinder. While it may still run, once you seize up, you are risking destroying the entire engine by continuing to run it. Most likely you now need a new cylinder and piston.

2. You need to empty the gas in the tank and fill it with fresh, properly mixed gas. Not knowing what is in the tank puts your whole engine at risk. You have no idea how old that gas it, or what it was mixed at.

3. If you are replacing the top end, consider doing yourself a favor and replace the crank. LML cranks are just terrible.

[PeteH]

I'm no 2T expert, so I ask this in all comradely good ignorance: so even a soft seize is grounds for a rebuild? I've heard stories of multiple soft-seizes on long vintage rides, and wondered how bad they were.

Which prompts my next definitional question: a soft-seize is one in which the engine can later be restarted, as opposed to a hard-seize which is a total lock-up even after cooling?

[az_slynch]

don't remember whether the throttle was entirely closed. I think that was the problem. Should I add a little bit of extra oil to the gas in the gas tank in case the previous owner didn't add enough premix oil to the fuel... ? Thanks so much!!! I will never do this again.

I would recommend leaving the fuel as-is. Adding more oil will alter the fuel-oil ratio as-fed through the carb jetting. If you increase the fuel:oil ratio too much, you will cause the engine to run leaner and may require a jetting change to correct it. As previously stated, leave it be.

Keep a jerry can in the garage and a Ratio Rite mixture measuring cup. Use decent two-stroke oil, none of that lawn implement stuff. If the prevous owner didn't recommend an oil, try Motul 710. It's a 100% esther synthetic and it's a relatively safe bet as 2T oils go. Mix up a gallon or two at the prescribed ratio and keep in in the garage. Top off the tank before you go riding, and all should be well.

The other option is to carry a small mixing cup with you in the glovebox, along with a pint- or quart-sized bottle of two-stroke oil. When you stop for gas, read the pump for how many gallons you put in the tank, then add gallon-equivalent measured doses to the fuel until you have the appropriate ratio, e.g. 2.5 gallons would mean that you fill the mix cup to the 2% or 50:1 line and dump it in twice, then add half that to the cup and dump it in too.

Either method works, go with what works best for you. My moped collection is all pre-mix, so I keep a can mixed up at home and top off before riding. Saves me from having to haul oil and a mix cup on a bike with no storage.

[az_slynch]

I'm no 2T expert, so I ask this in all comradely good ignorance: so even a soft seize is grounds for a rebuild? I've heard stories of multiple soft-seizes on long vintage rides, and wondered how bad they were.

Which prompts my next definitional question: a soft-seize is one in which the engine can later be restarted, as opposed to a hard-seize which is a total lock-up even after cooling?

I'll preface with answing your last question first. You estimation of seize types is pretty much spot-on. In a soft seize, the piston/rings and cylinder get a bit too friendly with one another due to thermal expansion, clearances get too tight and bits get stuck. As the parts cool and the clearances return, and they are free to move again. In a hard seize, the aforementioned engine bits manage to friction-weld themselves together and become unwilling to go their separate ways without the aid of hammers, torches or cutting implements. Hard seizes can also be caused by failures of the crankshaft bearings and connecting rod bearings.

Generally, a well-built two-stroke can suffer at least one or two soft-seizes before requiring top-end service. The rub is, it really depends on how violent the seizure is. Seizing at higher engine speeds are more likely to cause significant damage (broken piston and/or rings, deeply scored cylinder, bearing damage) than seizures at low speeds.

I've ridden alongside a Stella that soft-seized on a level road, WOT at 55mph. Rear-wheel lockup and a 40ft skidmark before the rider heard me yelling, "Clutch!" and freed the wheel. Once the rider pulled over, I took the bike and ran it up and down the road, still warm, declutching in 3rd to try and pop the piston free. It worked, and we rode on at a lower speed. The root cause was an undersized main jet. Three years later, the engine is still going along just fine, with no top-end rebuild required. Based on this observation, I am hesitant to believe that the Stella top end is made of paper mache' and that they are capable of surviving a soft-seize.

[vespamiami]

I'm no 2T expert, so I ask this in all comradely good ignorance: so even a soft seize is grounds for a rebuild? I've heard stories of multiple soft-seizes on long vintage rides, and wondered how bad they were.

Which prompts my next definitional question: a soft-seize is one in which the engine can later be restarted, as opposed to a hard-seize which is a total lock-up even after cooling?

I'll preface with answing your last question first. You estimation of seize types is pretty much spot-on. In a soft seize, the piston/rings and cylinder get a bit too friendly with one another due to thermal expansion, clearances get too tight and bits get stuck. As the parts cool and the clearances return, and they are free to move again. In a hard seize, the aforementioned engine bits manage to friction-weld themselves together and become unwilling to go their separate ways without the aid of hammers, torches or cutting implements. Hard seizes can also be caused by failures of the crankshaft bearings and connecting rod bearings.

Generally, a well-built two-stroke can suffer at least one or two soft-seizes before requiring top-end service. The rub is, it really depends on how violent the seizure is. Seizing at higher engine speeds are more likely to cause significant damage (broken piston and/or rings, deeply scored cylinder, bearing damage) than seizures at low speeds.

I've ridden alongside a Stella that soft-seized on a level road, WOT at 55mph. Rear-wheel lockup and a 40ft skidmark before the rider heard me yelling, "Clutch!" and freed the wheel. Once the rider pulled over, I took the bike and ran it up and down the road, still warm, declutching in 3rd to try and pop the piston free. It worked, and we rode on at a lower speed. The root cause was an undersized main jet. Three years later, the engine is still going along just fine, with no top-end rebuild required. Based on this observation, I am hesitant to believe that the Stella top end is made of paper mache' and that they are capable of surviving a soft-seize.

Thanks you soooooo much! You can't imagine how happy I am now. The engine runs well. I won't touch it until I hear something weird. I will do my research on how to determine whether I have an undersized jet and how to change it.
I would really appreciate if you point me into the right direction.
THANKS!

[Anachronism]

Just because the engine may continue to run does not mean it is safe to do so.

In a seize, the piston and cylinder jammed hard enough to stop the motor dead in their tracks, and there is almost always visible damage from this. Its not always enough to warrant rebuilding, but the safe thing to do is pull the head and check the bore. If there are any scratches you can feel, you need to do a top end. If you question whether the scuffs are deep enough to warrant a top end, take another ten minutes to pull the cylinder so you can look directly at the piston and see how it looks.

The bad thing here is even in a soft seize, you have scratching that screws up oiling, increases blow bly, and increases friction, all things that make it much more likely it will seize harder, and when you lose a ring, you then need to split cases to pick all the little pieces out.