Modern Vespa

hi I'm new here. First, I would like to stay, I am a seasoned mechanic. I work on diesels currently but I was a motorcycle mechanic for about 7 years, three of those years it was nothing but scooters. so I iv done this quite a few times, just not so much with the piaggios.
its my buddy scooter LX150 (leader)not sure the year, he wanted to get it faster. it was running pretty slow at the time. he had thrown a new carb on it, didn't help so he had bought 190cc malossi cylinder head & piston and asked me to put it on, i said ok. what he had failed to tell me at that time was, that something had happened to causing it to not run just prier of me working on it.
well to get to the point. the kits on, correctly, i'v got good spark, I know my timing and valves are right . iv checked multiple times. and its still not running.it is back firing a small amount. that why I have checked my timing marks 4 or 5 timed and there dead on. compression is good, intake manifold new. the scooter only has about 6000 miles on it.is it a cdi issue?? I know I'm missing something.iv installed, I dont know how many kit over the years. just never on a leader motor before. any input would be helpful. thanks

Hello and welcome!

If I understood correctly, the bike wasn't running for unknown reasons. Then you replaced the top end and head, and it's still not running. So now you're stuck.

Are there any fault codes on the dash light? Since you say there's good spark, I'm guessing the immobilizer recognizes the key properly - you should see the dash LED turn off after a couple of blinks.

And it's backfiring, so it must be cranking. So it means the brake switches are working and recognized as engaged when you hit the starter.

I guess valve clearances should be good on a new head?

Maybe as simple as a bad spark plug?

What do you have for compression? Does the Malossi head have a decompression mechanism?

I wouldn't normally think the CDI would be bad. But I guess it's possible given the bike's age (carbureted LX) and the fact the bike wasn't running before this job.

yes, please forgive my grammar. Yes , I replaced it, unknowing that there was an issue. The fall codes is somthing I did not know about. The bike is at my work, so I won't be able to check it till tomorrow evening.

The funny thing about the back fire is it stoped doing it after I adjusted the valve clearence to the specs from Malossi. the first time I set the valves, I had the wrong set of feller gauges with me. So, I did it by feel. Which I don't recommend anyone doing. But, I have had to do before in the past and it has always worked.and I thought that the Malossi vales clearance seemed little hight when I was setting it. I don't have it right in front of me but...

intake .010
exhaust .015 if I recall corectly

I grabbed another new plug today thinking the same thing.
95psi is what I got for compression with the new kit.that seemed low to me, and I knew something was up then. Luckily I checked the compression before I put the kit on and was 70psi.the new kit did not have a decompression mechanism.

The bike seemed pretty strong when I had borrowed it for a really last October. the only thing I noticed then was that is was burning little bit of oil. I figured it was the valve seals.

I talked to my buddy today and the few weeks pryer to me putting the kit on, the bike would barely get to 30mph up hill and 45-50mph when going down hill. the original jug and piston didn't look to bad, little bit of scoring. and the valves and cam look ok as well.i puled the cam for inspection, but I didn't pull the valves though.

Welcome. I saw your KCMO location and wondered where that was. Duh. Kansas City, Missouri! Jazz and barbecue. Baseball and football.
Hot summers! I'm sure I left out something important. Like Beer.

What units are you assuming?

The correct figures are:
Inlet: 0.10mm (near enough 4 thou, 0.004 inches)
Exhaust: 0.15mm (or 6 thou)

The pamphlet that came with the kit

You'll note the Malossi numbers agree with Jim's numbers.

And they quote the units - we don't want to lose another Mars probe...

Yes I see that , thank you.

I don't see where a Malossi cylinder kit with a head. What is the part number for it. If it's the stock head it does have a decompression valve on it. the 90PSI is what you get when you check compression with the decompression valve working. Having gone down the rabbit hole of a no start leader engine to discover the problem was the decompression spring being off. Another no start rabbit hole I went down was the relay on the kill switch circuit but you have spark and it kills spark but lets the engine turn over. Just throwing out my leader no start solutions that aren't real common.

It's a barrel and piston in the kit, not a head. So all that should of changed is the displacement increased a little and the compression too. Did you put a new plug in?

Update. Did a leak down test, exhaust valve was the problem.
Thank you all for the insight.

And it runs well now?

Wellll, no not exactly.... with the miles on the on that engine we decided to swap it out with another leader from an wrecked ET4 that was we knew was good... I Did do a little research, From what I discerned ET4 Engine should swop over ..harnesses are the same, engines are identical. I got them swapped over, and it still won't start,Spark is strong. Could it be The immobilizer? Do I need to switch out the CDI's? if so I gotta switch out ignition as well correct?

If there's a strong spark, it's not the immobiliser, but that's a mystery, as you should have had to swap out all the ignition stuff. Perhaps this ET4 engine already had an immobiliser bypass CDI?