Modern Vespa

You seized it.

You can usually survive a few seizures with a cast iron cylinder, but the piston will eventually be trashed along with scrapes in the barrel.

If you pull the plug and look, I expect you'll see some aluminum smeared on the inside of the cylinder above the exhaust port.

You can clean up the cylinder with muriatic acid, which will dissolve the aluminum, but you need to figure out the root cause before you push it hard again or it'll happen again and you may not be so lucky next time.

That's what I thought. I didn't touch the jetting as I was able to read about the previous jetting the PO had done that was discussed on this forum.

I'm not sure what else to check for. I resealed the intake when I rebuilt the carb. Could it be a main seal causing an air leak?

I didn't feel like I was pushing the bike that hard. I am still a fairly timid rider, but maybe not as much as I thought.

I was up and down a fair number of hills and on a long straight stretch I went wide open for a bit. I was just thinking… "I'll call that good for tonight", when the skid happened. I must have had some left over muscle memory from 1990 as I had been covering the clutch the whole time, pulled it, rode it out and carefully got off the road when it freed. I knew what was happening at the time, but it was over before I had time to be scared.

I was thinking, do a leak down test on the engine to check the condition of the seals. Pull the P and C and clean, hone or replace as required. Pull the jets and get da numbas.

I could also split the used Stella bottom end I have sitting on my desk and go through that one. Or maybe throw the 200 engine in there and see if I can break that one too.

Where to start?…

Start by pulling the top end and figuring out how much damage was done and sort that out. Then, pressure test after it's back together, then look at jetting.

You are probably too lean in the mid-range (atomizer) and maybe the idle, so when you roll off the throttle, that's when the seize happens.

Man... sorry. Glad you are in good shape after that!

Are you sure the oil injection is good? I would double check that along with your jetting.
Post some pics when you pull the jug so we can see what the damage looks like.

Again, glad you were able to ride through it.

Hmmm….Pretty sure I was rolling off the throttle. I remember the engine going Bleeeeeeeeeehhhh…and thinking, does that sound normal?

60 was a hoot. At 55+ any loose clothing starts to drag. Pretty sure my 80's 150 never got that fast. I'll have a look at the piston and cylinder tonight.

I was wondering how much the mixture screw would effect this. It's turned out 1-1/2 1-3/4, which is what it was when I rebuilt it.

When I resealed the carb, etc. I made sure that the engine turned the autolube drive rod, but I didn't take apart the parts of the autolube that are in the carb box (the round thing that takes a gasket).

One thing I did notice when bleeding the oil line last night was that when I connected it to the carb box air bubbles would flow into it. The hose wasn't a tight fit on the carb box nipple and was held with a spring clip. It is the correct clear transparent hose. When I got home, I changed it for worm and peg hose clamp. Also the plug for the air screw hole in the carb box was missing.

Whats your jetting.. specifically idle jet. If you seize rolling off throttle its most likely too weak idle jet.

That is if you are sure your auto mix system is working. And your rear airbox plug being out will affect it as well. If you aren't close already then you pull that plug out and run with it out it makes it that much further off from correct jetting.

Ok jets are:

Main: 112
Mixer: BE4
Air: 60
Idle:42-160

The Piston and cylinder both felt smooth. The cylinder looks like it has some aluminum transfer from the piston but I can't feel any ridges. Rings move smoothly. My thought was that it didn't look that serious. I was expecting worse.

All you need to do is clean off the cylinder w/ Muriatic Acid, make sure the rings aren't binding on anything, then reassemble, pressure test, re-jet, and roll.

What's the jug? DR 177? That's too little main and too much atomizer. Screams "air leak" to me.

Thanks. I don't know anything about jetting. Not sure if that was the problem. Going to figure out some kind of pressure test. I know there are a lot of write ups on it.

I suspect that the jetting isn't at fault because of the bikes history unless there's something I listed that screams wrong jet. My other thought was the seals since the bike has been off the road for quite awhile. Lastly, making sure the auto lube is working properly.

Last edited by orwell84 on Thu, 12 May 2022 01:19:45 +0000; edited 1 time

42/160 idle is also way too lean. I'm betting that is what caused it.

If you are on a tuned cylinder, I am guessing you should be somewhere richer than a 38/120 at the leanest end.. and maybe a 52/140 at the richer end. Anywhere inbetween those two could be the correct one as well. You would have to test them and confirm.

If you are on a stock 150, or Stock 200 then you should be at stock idle jet.

Range of idle jets I would guess you should be in (from richest to leanest and showing the air fuel ratio in parentheses after each):

52/140 (2.692307692)
55/160 (2.909090909)
48/140 (2.916666667)
40/120 (3)
45/140 (3.111111111)
38/120 (3.157894737)

What you have in there now, 42/160 (3.809) is like 4 or 5 sizes leaner than the leanest I mentioned!

Just as an example.. a stock Stella 150 idle jet is 40/130 (3.25), and a stock P200 idle is 55/160 (2.909).

The air fuel ratio numbers smaller equals more rich. bigger equals more lean.. so regardless if your engine is a stock 150 or stock 200, you are still way leaner than you should be for either of those engines.

Last edited by swiss1939 on Thu, 12 May 2022 01:29:38 +0000; edited 4 times

The most important step in jetting is a good pressure test. Once you have that, the rest is just following the algorithm, which Jack has laid out numerous times, but which I still screw up sometimes.

Also, a CHT is cheap insurance and will pay for itself many times over in top ends not ruined, so order one of those sooner than later.

Back to jetting an si carb...off the top of my head it goes...

Start with a main that's going to be so rich it won't rev out. A 125 is probably a good starting point for this.

Atomizer, I'd say BE5 if you have it, but you can probably get away with a BE3. You shouldn't need a BE4 with a bolt-on DR, though.

Air Correctly, 160 is okay in this case. 140 might be better. It sets the air side of the air:fuel ratio that's getting mixed in the main jet stack, so smaller AC means less air, which effectively richens the entire mixture range.

Idle is probably okay.

You'll want to walk the main down until it revs out cleanly at WOT in 2nd or 3rd gear (I guess you could do 4th, just needs a bit more room) and no more, even if it seems like it's better--it'll get hot under load.

AC, same process, but air correctors are numbered (I read somewhere--maybe it was speculation) in the order they were first used, not based on any sizing or other sane mechanism. BE4 is the richest, and you can google a chart from there.

Idle--lots of good writeups on picking an idle jet and setting the screw. Key to it is making sure the bike is good and warm when you do it. Not ridden around the block, but thrashed for a few miles. It's also a good excuse to make a long donut run so you have something to munch on while you work.

Also, make sure the timing is set at 18 BTDC (I'm assuming static timing) at 4,500 RPM's. Anything less is not a reliable read.

At that point, you might want to tweak it, but you should be in good shape.

Ok my edit and your messages crossed. Thank you. I have reading up to do, but I will find the details.

Thanks!

The seize was memorable. I know I was lucky.

Also, it's a Polini 177

Could be any number of things.

Quick sign of possible trouble to come is when the engine doesn't return to normal idle speed just after you throttle it at a stop on the road.

https://www.amazon.com/gp/product/B092CDSQ33/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

Well, I believe here is the instruction manual for that kit:
https://catalogue.polini.com/dep/PI357.pdf

Kinda hard to decipher what they are saying for jetting. I'm thinking on page 6 under this section: VESPA 125 PX-TS-SPRINT VELOCE Ø63, it is saying to use for standard crankshaft a 118-120 main jet, leaving the rest of the jetting stock. It appears they are talking about a stock carb being a vespa rotary valve engine.. so that would mean going from the stock px150 jetting of 160/be3/116.. to 160/be3/118-120, which is 2-4 main jet sizes richer just for the kit. So if you are on a reed valve stella engine, I would add 4-6 main jet sizes richer to the stock jetting and see where that lands you for the main. Hopefully a little on the richer end and safer.. which you can work down if you are good at checking the plug and know what to look for. They also suggest 19 degree ignition timing.

Nothing about the idle jet in there. So I would start a size or two richer on the idle than whatever was stock for your bike. Is this a reed valve stella engine or a rotary valve P150 engine? My guess is you should be on something like a 48/140 or a 45/140 idle jet if its not crazy ported and more plug and play. Also depends on what exhaust you have on there?

It's a reed valve Stella with a SiP road 2. The engine came with the bike. I don't have much experience with Vespas, so if I were building an engine, it would probably be closer to stock. It's worth trying to get this one running right though.

Then I would go to a 140/be3/118 or so on the 20/20 stock carb. And a 48/140 idle. This should get you closer. But again you should start rich and work slowly down until you find the right jetting. One at a time, main jet then idle jet separately.

Jack221 is the tenure track professor at jetting here. Maybe he will pop in and give some advice as well.

That 42/160 idle jet is the leanest one they make. Kind of bizarre and very likely the cause of your seize. Start over with a 55/160. That is a typical baseline for a 177 kit.

The mainstack is odd too. The BE4 mixer tube is the richest one they make. On the other hand, the AC160/MJ112 combo surely is too lean. A typical baseline for a 177 would be an AC140 air tube, BE5 mixer tube and a 120 main jet. Start with that and then adjust to suit.

This was jetting I did years ago to get the 4-stroking out of the low throttle. Anyway cant remember how I got to it, it's been a long time.

Yet, I've run this bike for many many miles with 2up, even on the highway (went over the Newport Bridge if anyone has been there!) also Florida cruising WOT. Always kept an eye on the CHT and never had a single issue. If CHT got a little high I would roll back the throttle a bit but never to any degree that it would seize.

So, maybe 2k miles on it, all in hot weather with two people! Hard to say why it would have an issue now.

I still think it's picked up an air leak. Bad seal, maybe a head bolt or stud has come loose.

Who know really? I had been doing a lot of hills, some of them pretty long and steep. You could say I live in the foothills of the Adirondacks. I ordered a runleader cht/rpm gauge. I run gauges in my bus for the same reason…to know when to back off or when to gear down for a long hill climb. . It's the nature of air cooled engines. The speedo is reading about 5 mph lower than the gps. I thought they tended read higher not lower.

I will get it sorted out.

I'm happy if they read at all. That's why I usually have an old handheld GPS strapped to the headset.

The mixer BE4 might be too rich down low which is probably why the pilot was reduced. This might also cause the main jet to be taken down too. I'd start over with everything beginning with a 120 air corrector, BE3 mixer and 125 main jet. Pilot start with 55/160. Work the main jet down then work on the pilot.

The 120 ac should enrichen the stack the BE3 should clean up the low range.

Yup, been there with those settings, just made for a horrible low throttle 4-stroke.

BTw, it's a 20/20 carb…

I know I'm probably getting ahead of myself, but have been considering different options for getting a reliable scooter on the road. Last night I started cobbling together a pressure tester using a gauge from an old welding regulator.

Results of the pressure test are probably what will determine my way forward. I understand the jetting issues pointed out but I am still thinking air leak. If it turns out to have leaking seals, I will have to split the cases. The other thing I noticed is a rattle from the engine. It has done it since the first start. I have tried to isolate it from engine tin etc. If I end up splitting the cases, I would consider changing out the LML crank, given it's questionable quality.

I also have a spare LML bottom end sitting on my desk. I believe it has new seals and gaskets, I can find out. It might be quicker to just to go through that one, crank included. I would even consider putting on a stock p and c to keep things simple. I would just like to get on the road with something straightforward and reliable. My enjoyment of riding isn't going to change whether my engine does 50, 60, or 70mph. I was having a blast doing 45-50 on winding backroads. This totally meets my needs for now. Being able to get home without seizing would be nice.

I know I'm crossing bridges. I will start with the pressure test. Cht/rev counter arrives today.

I am curious to see the results because the data so far points both ways. On the one hand, the fact that someone saw the need to run a BE4 mixer tube on a Polini 177 does suggest an air leak. On the other hand, the fact that you could even pull out of your driveway with a 42/160 idle jet suggests no air leak.

Another thing I noticed is that it starts better with the choke but won't idle with the choke out. Not sure that tells me much. I just have to go through the checks. No point in filling pages with guessing and what ifs.

The road to getting on the road has been longer than expected. Sometimes it's a bit discouraging.

As I hunt for info in nsm, I realize that my experience is pretty typical. Learning the machine is mandatory and that partly comes through the experience of fixing stuff when it breaks. Prior experience with other engines helps but doesn't replace this. I have learned through years of working on a bus that simply throwing new parts at an engine without figuring out the underlying problem often doesn't get you any further. There's no avoiding the diagnostification.

So as much as I just really want to go for a ride, I know it will be a better one if I know I have put things right. So I'm building a pressure tester using stuff I have kicking around, a gauge from an old acetylene regulator, kids ball pump, spare bus carb bits and air compressor fittings. Can't beat the price and it's a great first step in testing any unknown engine or one you just put together yourself.

The cht/rpm gauge…I'd never run my bus without them…and on a scooter even more so. You're not going to go skidding across the pavement if you overheat your bus engine. Sorry for the windbaggery.

Did you use sealant under the carb box gasket?

Was there any sign of leaking(oil) at the cylinder head or around the base of the carb?

I used Kuril T on both sides of the carb, box and reed block gaskets. It doesn't harden like RTV. I did have some leakage the first time I started the engine, but tightened it up stopped it. I checked again after the seize and found no gas or 2t leakage. None at the cylinder head or spigot either.

I put the sealant on sparingly so as not to clog the oil passages and used a torque wrench to bolt the carb on.

I am trying to figure out how/where the oil is supposed to flow.

I'm not sure how to test the auto lube other than disconnecting the throttle and opening up the oiler to see if the engine smokes. There is a bubble there that moves but never goes away. If I disconnect it and let the oil drain by gravity, it will still flow a lot of oil around a bubble.

Oh, and my fuel tap doesn't stop fuel flow when it's off.

Last edited by orwell84 on Sat, 14 May 2022 16:34:09 +0000; edited 1 time

One other thing. I still don't get the ham reference. I'm thinking it will be explained at an appropriate time.

During my ride I came upon slices of ham strewn across the road out in the middle of nowhere. I didn't know whether it was a good omen or bad. I imagined myself wiping out on a greasy slice of ham…

I'd double-check that oil channel, then.

What can happen with sealant there is when you torque down the carb the sealant gets forced into that channel, restricting the flow of oil.

I did keep that in mind and left some space for the sealant to squish out. I am wondering where the oil actually goes in. I can see where the channel is. Does it enter the engine where the blue arrow is?

Also, when I do a pressure test, do I have to seal the oil pump drive too or just the intake?

"Does it enter the engine where the blue arrow is?"

No, it gets into the motor at the other end of the channel up by the screw head. There's a mating cross drilled hole in your carb so the oil drips into your venturi.

Ok, thank.

I see it now. Do I have to seal the oil pump drive to pressure test. Im thinking no because it's outside the intake/combustion chamber.

….and I broke a piston ring. First time ever and I have handled daintier ones.

It's officially official. I HATE this fucking thing.

I think the first 3 years of doing your own mechanical work on vintage vespas involves hating them 90% of the time. Either you get through those first 3 years and things start to come together easier and less fighting.. or you give up and sell it all. The level of hate is directly proportional to the level of tuning you attempt.

Keep plugging away!