Modern Vespa

I recently disassembled and reassemble my motor due to a shifting issue.
It had a hard time starting, finally started but it sounded a bit more raspy. After it warmed up, it seem better but the idle was much lower. When I went to raise the idle with the screw, I felt compression from the air box. Also has about 1/3 less power. My guess is, I did not seat the clutch side bearing all the way in the case and there is a small gap near the rotary pad. I have a hard time with the side to side position of the bearings every time. Perhaps I damaged a seal as well.

A bit of blow-back through the carb is normal. I'd think that if you had a bad seal, the compression would be going elsewhere. Check the timing ?

I thought that in the beginning. I marked the stator plate before removing. A few months back while fine tuning the scoot, I checked the timing, it was between 18` and 20` BTDC. I checked it yesterday. It was the same. I advanced the stator plate as far as I could with the 3 screws restricting it without altering the point gap.
The timing is now about 24`. That was the only way I could get it to idle being the idle screw in maxed out. It is way more blow by than I remember before splitting the cases last week.

The point gap will affect the stator position.

Maybe your exhaust is coked up. A restricted exhaust can cause excessive blow back at the carb.

Hec

I don't think it's the exhaust. The scoot ran well, 2 kicks cold and 1 kick warm and idled well for the past year. I had the wrong model selector rod and the trans was beginning to jump. Took everything off set it in a box, split the cases, changed the selector rod, shimed the gear stack, changed the cruciform, seals and case gasket. Put it back together and the low idle/rough running came to life. I am not sure how an exhaust gets plugged sitting in a box for 2 weeks. But anything is possible. I have a new one in a box I did not install yet.
For some odd reason, I think it might be a slight gap at the crank/rotary pad area. I attached a pic from another posting for an example. What would the symptoms be with a gap like this?

Symptoms will be the one you're describing.

But the chances of starting the engine will be low though.

Won't be a good enough seal to create a vacuum to suck the air/fuel.

You can check by removing just the carb. You might be able to check just removing the throttle slide, but don't hold me on that, but that's what I would try.

yeah seems like your crank isn't centered, that's not going to be helpful at all

SDJohn, I was thinking the same thing. I always have trouble installing the crank bearings and putting the cases together. Takes me a few times after every teardown.
Any ideas on how to center the crank? It's hard for me to see since the seals hide the bearing insertion depth. And I am always concerned that the bearings will go too far onto the crank or not deep enough into the case. I swear I have see some sort of shims between the crank bearings and counter weight.

I don't know what engine you are working on. I can only speak about a P125X engine because that's what I have. With the snap-ring installed, I installed the clutch side bearing by freezing the bearing to -20 F and inserted into the heated engine case. The bearing basically dropped into the bore and only needed a love tap with a hammer to drive it home. I did use a SIP bearing drift.
I would think if your crankshaft wasn't centered, the Connecting rod would be binding as it wouldn't be centered either.

Hope this helps.

Hec

That inlet looks like it was ported for a reedvalve intake, maybe a Malossi by the look of the shape. What carb, intake etc are you using?

That's a huge gap. I think you're right. I'm surprised it runs at all.

OP, do you have any pics of the rotary pad from before you assembled the engine?

My gap is not that big nor is it ported for a reed valve. I used that pic from another post from another person as an example of what my issue might be. My rotary pad is in good shape, I am just concerned that there might be a gap since I had no way of ensuring the bearing was seated all the way in the case. I do not have any pics as that has not been an issue for the last few months. People are pointing toward timing, that is not my issue.

maybe you need to pull your carb and get us a look at the gap (or lack thereof) on your own engine, before we go racing off based on somebody else's picture.

Hec - he's got a VBB, so it's the old style with the brass races in the case, and the non-split bearing setup. At least I assume that it is.

sdjohn, that is exactly what I am going to do. I just wanted to check with the forum for any ideas of my symptoms before tearing apart the motor again. I knew it was not the timing but assumed it might be a gap near the rotary pad since I never can get the bearings correct during re assembly. I guess I wanted clarification from the group before proceeding. Thanks everyone.

I'd think if you had the crank so far out that the web missed the pad, you'd have a hard time closing the case. Is you con rod centered on the seam ?

give us the pics before you go deeper, maybe we will have ideas

Ok People, I removed the carb and looked at the rotary valve area. Looks OK to me. The counter weight is a few millimeters under the rotary pad area. I guess that means the crank is centered and the bearings are seated all the way. The thinest feeler gauge I have is .152MM and I can't fit in anywhere in the area. So that now leaves more questions than answers. At least for me.

What kind of compression should I see?
The scoot has a few hundred miles on a new cylinder, rings & piston. All OEM style.

Position looks great… between rotary and crank too.

Problem is elsewhere.

I'd relook at carb/jetting/ignition.

What engine and parts is it?

Stock VBB 150. As for the jetting, I don't remember what has been in it. The funny thing, this scoot ran amazing. 1 kick warm, 2 kicks cold. Idled great and had enough power. I did not touch a thing except split the cases to change the selector rod. Now it runs awful. Plug looks good as well. Not sure how jetting could change sitting on the workbench for 2 weeks but anything is possible. What should I be looking for? Anything come to mind that could affect spark? What is the best way to tell if I have adequate spark? I am afraid to touch the jetting. Being I would now have 2 issues. The original issue and now an issue I caused with incorrect jetting.

Sorry, was always imagining a PX engine for some reason.

Not that it makes much difference on your side.

If it was running great and nothing else has changed, it's just a case of tracking down the "difference".

In regards to jetting, i should reword and say "fuel being fed"… clogged jet is worth looking into.

It's running on points? Points are in decent condition? Have a spare condenser?

OK Vespa people.
I think I figured it out. I took a step back and thought of the 3 basic things.
Fuel, spark & compression.
I did a compression test with 2 different gauges and have 88 & 90 PSI of compression.
That seems a bit low to me but still runable. That might explain the harder starting, the poor idling, the 1/3 less power & more than usual blowback through the carb.
This big question is, what happened during the re-assembly? One might never know.
So I think I will light hone and re-ring the scoot. I was looking closely and was wondering if I can remove the cylinder without removing the motor. It looks really close but doable.
Any input from the forum on my compression values and cylinder removal or am I totally out there.

When you did your compression check, did you remember to do it with the throttle wide open? If you did it with the carb removed that would be ok too.

You should be able to remove the cylinder with the engine still in the scoot. You will have to remove the cylinder studs to do so. Take note of how much thread length of the cylinder studs is inserted into the case and which end of the stud is inserted into the case so you can replicate this on reassembly.

Before you do any more disassembly I would recommend that you do a pressure test on the engine to rule out any air leaks.

Good luck

Hec

Yes carb was off during the test. Dooley noted on the cylinder studs. As for pressure test, rubber plug in the exhaust port, homemade block-off plate for the carb & some sort of spark plug adapter to pump with gauge. Does anyone sell some sort of kit that you know of? Also, how much pressure and for how long of time?

You should hold 5-6 PSI for at least a day. Here is my set up for pressure testing.

Hec

Dang HEC, that is one clean motor, plus the trick cylinder and head. And that's a 1980?

5 psi for 5 minutes is enough. More is just showing off 😉

Sorry, thought the pics were of your engine. Glad you are getting it figured out.

Thanks orwell84