‘85 Vespa Si won’t start after restoration

I have almost completed the restoration of my Vespa Si Monte Carlo, but I can’t get the engine to start.

I have a new case due to the old one had a crack, assembled with new bearings to old crank, new rings on old piston, and old cylinder and head. All stock. All the old ignition parts and coils are reused. Measured the resistance and it checked out fine, only the condencer measured a bit high (around 500nF). 12.12 carb with a 52 jet and a proma mini "lowboy" exhaust.

I have only wired from the internal ignition coil to the ht coil when trying to start it.

The comperession reads around 115PSI, and the spark plug has spark (allthough I think it looks a little weak).

Am i missing something here? All suggestions are highly valued!

Thanks!

Re: ‘85 Vespa Si won’t start after restoration

500nF seems to be about twice the 'normal' @ 22µ .

Maybe try a new condenser and a new or known good spark plug .

Some plugs can produce spark in open air but get 'blown out' when under compression .

Re: ‘85 Vespa Si won’t start after restoration

Thank you for the reply!

Will try new condencer and plug. What plug, brand and type, do you recommend?

Re: ‘85 Vespa Si won’t start after restoration

I'm not sure what spark plug would do best .

Maybe something close to an NGK B7HS .

Hopefully one of the guys that knows for sure will speak up .

Re: ‘85 Vespa Si won’t start after restoration

The stock plug on a super deluxe is a b4h. The NGK part number is 4111. I imagine it'd be the same plug

Re: ‘85 Vespa Si won’t start after restoration

The plug that sits in it now is a NGK BH5S. Could this plug cause it not to start? It is new ofc.

Re: ‘85 Vespa Si won’t start after restoration

> Kristian Lomeland Wrote:

> -------------------------------------------------------

> The plug that sits in it now is a NGK BH5S. Could this plug cause it not

> to start? It is new ofc.

That's pretty close, I don't see that being the issue. I was running a B6HS for a good 150 miles before I fouled it on my super deluxe.

Re: ‘85 Vespa Si won’t start after restoration

if its points system, check that blue wire is grounded. adjust point gap as well.

Re: ‘85 Vespa Si won’t start after restoration

I had to rewire the whole engine, because the old cables were shot to hell. What does this blue wire connects to?

Re: ‘85 Vespa Si won’t start after restoration

All my Vespas use b6hs. But really, anything close is fine. It's not the kind of plug that will make or break running. Put a new one in and it should work.

The clue was given earlier. As was mentioned a weak spark in the open air becomes no spark in the cylinder. The reason is that as the air and fuel are compressed, the molecules are squished into a smaller space, and the spark must be strong in order to jump the gap.

If a new spark plug doesn't solve it, I'm guessing you need a new HT coil. That's been the most common cause of weak spark for me. You can get an Emgo universal coil from Amazon for cheap, and it will work amazing.

Re: ‘85 Vespa Si won’t start after restoration

Re: ‘85 Vespa Si won’t start after restoration

I have researced the problem a bit further, and it is one thing that I find wierd. When trying to start it it blows air out of the carb port. I would think it should be opposite, so that it sucks air (and fuel) into the engine. -not blowing it out.

Any ideas???

Re: ‘85 Vespa Si won’t start after restoration

Dirty30 Dillon /

There will typically be a puff of air out the carb once a cycle. And you probably won't feel any suction from the carb side. Also, you could be rotating it backwards.

Re: ‘85 Vespa Si won’t start after restoration

> Dirty30 Dillon Wrote:

> -------------------------------------------------------

> There will typically be a puff of air out the carb once a cycle. And you

> probably won't feel any suction from the carb side. Also, you could be

> rotating it backwards.

I understand. But it throws a good amount of fuel out of the carb as well. And the plug seems dry i.e no fuel in the cylinder. I suspect that no air is getting into the cylinder as well, as it doesn’t ignite when I pour fuel straight in the plug hole.

I am turning the engine clockwise (viewed straight on the flywheel). Isnt that correct?

Re: ‘85 Vespa Si won’t start after restoration

Dirty30 Dillon /

> Kristian Lomeland Wrote:

> -------------------------------------------------------

> > Dirty30 Dillon Wrote:

>

> > -------------------------------------------------------

>

> > There will typically be a puff of air out the carb once a cycle. And

> you

>

> > probably won't feel any suction from the carb side. Also, you could be

>

> > rotating it backwards.

>

> I understand. But it throws a good amount of fuel out of the carb as

> well. And the plug seems dry i.e no fuel in the cylinder. I suspect that

> no air is getting into the cylinder as well, as it doesn’t ignite when I

> pour fuel straight in the plug hole.

>

> I am turning the engine clockwise (viewed straight on the flywheel).

> Isnt that correct?

Clockwise is correct. If you have no fuel on the plug, it seems like you may have a case leak which means you aren't creating vacuum to pull fuel into the cylinder.

I would check to make sure your seal is in place and not popped out, and that you don't have a case leak/cylinder gasket leak. Leakdown test works well to find these issues

Re: ‘85 Vespa Si won’t start after restoration

Baron Von Hamilton /

Try and drill start it.

I was having trouble with my rebuild and found the case gasket I used was leaking. What case gasket did you use? Did the base gasket on the jug line up correctly?

Re: ‘85 Vespa Si won’t start after restoration

Is these gaskets the seal around the crank axle and the gasket where the case is separated when installing the crank? Or is it any more?

The gaskets all lined up pretty good. I am going to pull the flywheel off and check the seal around the crank axle.

The only thing I reacted to was that the long bolts that hold the cylinder and head to the case wasn’t straight when tightend in the case. The holes looked to be a bit crooked. I dont know if it would be enough to create a case leak?

I didnt think of it much more when the compression was good.

Re: ‘85 Vespa Si won’t start after restoration

Right, I performed a leak test. Sat compressed air in to the intake, and ran it on low pressure. Everything seemed sealed and good, except for the decompression valve. Slight hiss through the valve even though it was closed.

I find it wierd that I have compression of 120-125 psi, if the decomp valve is not sealing properly?

Re: ‘85 Vespa Si won’t start after restoration

Baron Von Hamilton /

When you compression test a Vespa kit it will always leak at the decomp, due to design of the valve. That however does not mean it leaks on the compression stroke. I have a head with plugged decomp I use for pressure testing. If you did not lap the Valve it still may be leaking and you don’t know it. Make sure your not filling the case with more than 10 psi. You will blow the seals out. If you fill the cases with light air pressure and plug the exhaust you can, spray soapy water and look for leaks at the base gasket and case gasket to see if it is bubbling.

Also are you looking on flat or threaded view on MA?

Re: ‘85 Vespa Si won’t start after restoration

> Baron Von Hamilton Wrote:

> -------------------------------------------------------

> When you compression test a Vespa kit it will always leak at the decomp,

> due to design of the valve. That however does not mean it leaks on the

> compression stroke. I have a head with plugged decomp I use for

> pressure testing. If you did not lap the Valve it still may be leaking

> and you don’t know it. Make sure your not filling the case with more

> than 10 psi. You will blow the seals out. If you fill the cases with

> light air pressure and plug the exhaust you can, spray soapy water and

> look for leaks at the base gasket and case gasket to see if it is

> bubbling.

>

> Also are you looking on flat or threaded view on MA?

It blew everything out of the decomp. Could not hear, feel or see any other leaks. Wouldn’t it be a little strange if it leaked air with new case and seals?

Tried installing and starting it again now, with same result. Gas gets to the carb, but no further into the engine. No sign of it starting what so ever.

Getting real tired of it, and seriously concidering a new engine of some sort...

I am viewing it flat.

Re: ‘85 Vespa Si won’t start after restoration

Dirty30 Dillon /

And you have a fiber base gasket and aluminum head gasket in there? If you didn't use soapy water to hose the motor down for leaks, you may be completely missing it.

Re: ‘85 Vespa Si won’t start after restoration

> Dirty30 Dillon Wrote:

> -------------------------------------------------------

> And you have a fiber base gasket and aluminum head gasket in there? If

> you didn't use soapy water to hose the motor down for leaks, you may be

> completely missing it.

I had that gasket setup, yes. But I took it all down now to look for mistakes, and the fiber base gasket did not survive that. So must get a new one.

What i found that made me think was that i greased the crankshaft bearings when i assembled it. Could this hurt? I also found small strips of gasket material near the bearings. Furthermore the seal around the crankshaft was pushed quite far in. I replaced this with a new one, mounting it flush to the case.

Added a picture of the case disassembled. Anything i should do to it before putting it back together?

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Re: ‘85 Vespa Si won’t start after restoration

Very clean. I mean surgically clean!! Wow. Anyway if my Alzheimer's isn't acting up I recall there are washers/spacers between each bearing and the crank. Probably not your no start issue.

Can u show us a picture of the ignition components. I think I saw something kinetic in the title or somewhere in the post. Mixing kinetic with a vespa case has been especially vexing to me. I'm using the white kinetic plastic flywheel with the fan moulded on to it, plus kinetic coils that required much sanding to not rub on the magnets in the flywheel. Plus the woodruff key had to be tossed and the timing set by hand .....yikes I wouldn't wish this on my worst enemy but now it works. So if u r mixing things up , I assume for 12 v goodness and the Parma cdi , which I'm using. Maybe if u have a stock vespa points ignition around get the bike to first run on the points and then jump into the kinetic rabbit hole when u r sure all else is good. Btw Alex t says it's a no brainier to pop in kineric ignition goodies and I'm simply doing it wrong.....maybe. let's see the ignition goodies please.

Re: ‘85 Vespa Si won’t start after restoration

I think I'm mixing up posts but ok please post whatever ignition items u r using.

Re: ‘85 Vespa Si won’t start after restoration

I use stock points ignition. The same components that sat in the old engine case (when it was running). Only soldered on new wires, as seen in the picture.

A picture of the HT coil and plug wire as well. I have the wire from the ignition coil attached to the tab on the HT coil, and the HT coil grounded to the engine case with wire. Shouldn’t that be enough electrics to get it started?

Let me know if you spot something wrong!

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C607F4EA-6D7C-40E3-B848-C2FF677A0C43.jpeg

Re: ‘85 Vespa Si won’t start after restoration

Ground should be the mounting bolts... What's the red crimped wire connected to?

Im not a Vespa dude but I don't see a reason for the red wire unless the bolt isn't making proper ground...like ur jumping something?

Edit, did you scrape the paint Offa that ht coil mounting hole so you make proper ground?(edited)

Re: ‘85 Vespa Si won’t start after restoration

> Aaron Blair Wrote:

> -------------------------------------------------------

> Ground should be the mounting bolts... What's the red crimped wire

> connected to?

>

> Im not a Vespa dude but I don't see a reason for the red wire unless the

> bolt isn't making proper ground...like ur jumping something?

>

> Edit, did you scrape the paint Offa that ht coil mounting hole so you

> make proper ground?

The red crimped wire was connected to the engine case. I started without it, believing the engine bolts provided enough ground. But since then I have tried everything to get it to start.

Yes, the paint has been grinded away behind the ht coil.

However, I have had a spark all along. I therefore find it wierd that the ignition is at fault.

Is it any numbers on the gap beteween the crank and case when assembled? It doesn’t pull any fuel in, and it recon the vacuum happens in there somewhere.

Re: ‘85 Vespa Si won’t start after restoration

something dont look right, too too clean n theres no piaggio markings.

Re: ‘85 Vespa Si won’t start after restoration

> pat splat Wrote:

> -------------------------------------------------------

> something dont look right, too too clean n theres no piaggio markings.

It is brand new, and has never statted in this new case, so not so wierd that is clean.

And the case is not a original piaggio part, but i really can’t tell them apart.

Re: ‘85 Vespa Si won’t start after restoration

Who made the case?

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