Modern Vespa

I went to install my new cylinder and noticed a bit of rocking. Left to right. Since I squared my case deck with a brick of glass I am pretty confident that is square and true. I tested the surface with other cylinders and it seem good.
I assume that a good $100+ caliper will suffice as a straight edge. I noticed the same high spots with the straight edge and the same behavior on the deck of the cases with the cylinder in question. My question to the forum is: I have access to a good Bridgeport mill at work. I am thinking of milling the cylinder mating surface true but it appears that this surface is not milled but squared by other means. This is the 3rd goofy cylinder I have received from the online vendor and I am reluctant to try again. Any ideas.

you certainly could set it up and machine off a gnats cock of material until it's true. Is this just a 2-port, 150cc cylinder? all of those are made in India and very economy level.

or you could just buy something better and not have to go through that process.

Yes, just as stock as it can get 2 port 150. I'm getting them from Scootermercado. Any ideas on an alternate vendor?

You can also get anything from SIP thru Scooter Mercato as well.

run a thick base gasket and throw some RTV at it, problem solved.

Have you spoke with SM about this issue? i can forward you their number.

Thinking the same thing. Maybe file down the high spot or make a sanding jig with a flat piece of countertop, then glue it on. Or just glue it on. It's the cool end.

I mean, all kidding aside I'd just hit with a file and get it close enough for government work, and then gasket and schmoo tag team it.

It's a low hp application and stock pieces, and lord knows I've done equally questionable and it's been fine and run for ever and day.

That's the beauty of the stock stuff. Slap it together in bare feet on the dirt floor of your mud hut and it shouldn't blow up.

So I milled down the surface, just had a few high spots. I put it back together, it's a bit better but still leaks even with the gasket. No idea where to get a better gasket. I have purchased them from a few different vendors. Same style.
My next plan. Whether or not it is a good idea, I coated both surfaces with the Dykem Steel Blue layout fluid. I will rub the 2 surfaces together, look for the high spots and sand with 220 and a glass bar. Then RTV, with a new gasket wait till it dries and test for leaks. As for a conversation with SM. I have talked both with Dave and Matt but for other reasons. It's fair to say they have not nor are not going to test every cylinder they receive. I know they are very busy. I can't be the only person to have this issue a few times. The SIP site seems only to have high performance items.
So the word according to Greasy: RTV it !!!!
That' what I will do.
Thanks everyone. I will keep you posted on the results.

just make a gasket, out of appropriate material and use application appropriate sealant. it doesn't have to be metal, and it doesn't have to be pretty.

This. As noted above, a two-port 150 on a largeframe is going to put out 4.9 HP (i.e. spec) if you're lucky.

Thick gasket material and a generous bead of RTV red around the cylinder base and smush it all together. It'll run forever like that.

I cured the leak, well almost, it took a few minutes to leak down from about 7 psi. I said to hell with it and installed the motor. I am also disappointed again that I had to grind the new cylinder as the flywheel was rubbing on it near the side port.

So I started it today, runs ok but it now has a funny rattle, (and a slight miss at mid throttle) almost like a rod knock. Crank turns without any binding or interference. I have no idea what it could be. I thought for a minute, that I took too much material when milling and now the piston is hitting the head but wouldn't that be evident when turning by hand. I linked 2 videos for your listening pleasure.

{Funny thing on a different note, I replaced a water pump in my car a few weeks ago and they supplied the wrong gasket. The correct gasket was a few days away so I made my own. Didn't even think about making a gasket for the Vespa.}

That's not even close to almost cured buddy. You want it to hold 5 or 6 psi forever. Or at least 30mins. Once it holds for 30 you're good to go. Save yourself the headache and sort that out now.

That sounds like something rubbing in the gearbox. If you rev the motor with the bike in neutral on the stand, does it stay constant or increase along with the RPM's?

It increases with RPM's

First thing I'd check is if the clutch is rubbing on something inside the gearbox.

Have you ridden it around the block?

Yes I have ridden just a bit. I was concerned it might be piston/rod issue so not too far.

Is that the kickstarter's gear teeth clacking against the quadrant?

I don't think it's the kick starter, I thought that initially but when I push the kick starter it adds a different sound to the equation. The sound is coming from the area where the cylinder meets the cases or somewhere near.

Is it the flywheel shroud rubbing?

I thought that too so I took off the shroud and still the noise

that doesn't sound like rod knock to me. more like something in the gearbox.

what does it do when you pull in the clutch?

I guess it could be piston rattle/slap too, but it's hard to tell without being there. did you check the piston to wall clearance?

Nothing changes with the clutch in or out and it runs through the gears OK.
I am leaning more toward piston slap since the new cylinder and piston have been my problem child for weeks now. If I take a large screwdriver and use it like a stethoscope the sound almost rings from the fins on the cylinder, as I go closer to the gearbox the sound fades. I did not measure piston to wall clearance but it felt ok. I made the assumption since the piston, rings and cylinder were new it would be ok. I'm not sure of my next move, perhaps a new cylinder and piston but from where? I would like it not to be made in India but I am not sure at this point were to purchase said kit and the country of manufacture. I also could just ride it until what ever the problem is will manifest into a larger easier to diagnose issue. Such as BOOM or BANG
Any sound bites or videos of piston slap to compare with. Also, if I remove the head and cylinder what would piston slap damage look like?

scuffs on the skirt of the pistons. it *should* be pretty tell tale.

reboring a stock cylinder with a high quality next over piston is probably the best choice quality wise.

the ZZZzzzzZZZZzzzz points to gearbox, but that might be a red herring...

Try physically pulling up on the kickstart lever with your hand.

Maybe a cracked muffler tab? No, i'm not talking about a cracked and leaky exhaust, but just the mounting bracket.

Also, what is the history on your VBB? I notice that it has whitewalls, glovebox, and rusty chrome...

Greasy125
Since we are leaning toward the gearbox, what parts are you thinking could be the issue?

So I did pull up on the kickstarter, no difference. When I push down, that distinct rattle adds to the sound in question. Muffler tab is good.
As for the history. I bought it a few years ago. A barn find, basket case type scoot in central California.
Cases were completely shot, rotary pad destroyed by bad bearings, crank was ground out to change the rotary valve timing, a botched port job, failed turn signal add on, new stator setup with OEM electrical system and lost of rust from sitting out in the rain. I stripped down to just the frame and worked my way back up. Pretty much a complete rebuild/restore. I know the trim on the front fender, trim on the cowlings, glove box and the 10" wheels are not correct but I decided to go with it. In hindsight, I should have stripped the paint since I had it dissembled that far but I like black even though it is not correct color at all.

Based on that picture of the frame and knowing it was on 10"s, any of them that are original to the motor.

I still go back to the clutch rubbing, given that it rides okay and changes linearly with the engine speed.

What is the best way to test for a clutch rubbing on the bench? Everything felt great before I installed the motor. Not a click, rub, bind or anything. Very, very smooth.

If you run it in gear, holding the clutch in, the clutch is away from its cover and the whole gearbox is stationary. If the noise doesn't change then it's all top end related.
Sounds a bit like piston touching head but hard to tell on video. Definitely more than the usual.

I'd pull the cover and clutch to see if I could find any rubbing. that's quick and easy.

I don't know much about your build: what cases, components, etc but there could be a compatibility issue.

if could be the wrong k/s gear, quad or spring is pushing stuff around in there. could be the primary rubbing the case on either side. could be the that the gears aren't shimmed or a gear or the cross is is backward, or the shifter box or plunger has the cross pushed in a hair and it's just touching a gear.

after the clutch and checking the barrel it's time to open it up so I'd try and eliminate everything before getting to that point.

My guess is piston rattle. All the gearbox noises I've dealt with make noise at a slower tempo and clutch rubbing I felt with the kickstart resistance/ lack of.

You can eliminate the gearbox/ clutch by squeezing and holding the clutch while it's idling. It'll take 10-15 seconds of holding but by disengaging the primary gear, everything down the line slows and eventually stops. If the noise slows and goes away, the gears/ kickstart are your culprit. If not, focus on the top end.

I'm guessing primary rivets rubbing…

Is it smooth when you spin the flywheel by hand with the spark plug out?

Yes very smooth with the plug out. That's what makes this so difficult to figure out.

Measure your squish. If it's .6mm or more, it's not your piston hitting. Unless you have a dangerous account of play in the wrist pin bearing.

I'm still thinking gearbox, with piston slap coming in second, more because the top end has sloppy tolerances in general than any actual reason.

time for a rebore. LOL.

The sound does not change with the clutch in, even waiting for everything to wind down. All the components in the cases have been running good for a while. I changed the cylinder because of a leak with the mating surfaces between the cases and cylinder. I opted to just get a new cylinder kit and that one has issues as well. See further up on this posting. That's when these issues appeared. I will check for clutch rubbing later today. I removed the head last night and the piston has a circle around the top like it might be hitting. See attached pic.
I might have taken too much material from the bottom of the cylinder to make the surface even. I used the stock metal gasket and very, very little silicone (I did not want it to squeeze out and clog the oil holes for the bearings) That could bring the head closer to the piston. I will have to measure the squish later. I need to find a procedure on how to do that exactly.

That ring mark on the piston is not normal. Could easily be touching.
You said you can make a paper gasket. Make a 1mm thick one from quality paper for the base. Put it on the cylinder, existing metal one on case. Refit with both gaskets and start up again. Hopefully fixed.

How about gudgeon pin bearing? You have new gudgeon pin with your cylinder kit but use old bearing?