Compression test

[Scooter Paul]

This applies to my 125 converted Vega but I'm posting it here as it's likely to get more attention. The scoot runs well when it starts but it is a pig to start and when I stop it it's difficult to get going again. It appears to have a good spark, there is fuel getting through and the timing is spot on. I have had a problem in the past with a leaking head gasket. This time I lapped both the head and barrel, used a new gasket with a little high temperature paste. I've just run a compression test. With a cold engine, throttle open I'm getting 90 psi. That seems low. What do others think? I'm also going to replace the condenser even though it's still sparking. Any other ideas please?
Paul

[kennystag]

Hi I would have though that 90 psi was fine, my GP150 is about the same, and is running spot on, you can feel on the kick start how tight she is when turning her over. Anybody else want to comment.
Cheers
Stag

[Meds]

i had a smiler problem a few years back and it was the timing.
I set it perfectly on the bench using a dial gauge but when strobed it was 3 degrees different.

[Scooter Paul]

Thanks Meds but it has been strobed. It's bang on.
Paul.

[Warkton Tornado No.1]

As somebody that runs points ignition on my road bike & raced with them before electronic ignitions became essential in that arena, your problem sounds like your condensor.

You can always disconnect the one you have & have an external one to prove the failure. Car condensors worked just as well......

[Scooter Paul]

I am changing the condenser anyway but any thoughts on the compression reading?
Paul

[dave411]

Have you tried starting it with the air filter off?

[Scooter Paul]

Yes, tried that. Thanks.

[Warkton Tornado No.1]

Hi I would have though that 90 psi was fine, my GP150 is about the same, and is running spot on, you can feel on the kick start how tight she is when turning her over. Anybody else want to comment.
Cheers
Stag

There you go. As the man said, your compression seems alright.

So, by a process of elimination, what's next?

[grandpa]

Hi Scooter paul.
I am sure my gp was about 95 psi on compression.
Start with simple things first change plug.do compression test when hot.Is the choke fully off. i.e too rich because of choke or blocked filter or bellows collapsing when hot..remove fuel filler cap in case you have a vacuum. Then clean carb. If plug is grey may indicate weak mixture because of air leak.Could use inner tube for leak test
viewtopic.php?f=10&t=140&p=524&hilit=inner+tube+leak+test#p524
Good luck,
Grandpa

[Scooter Paul]

Ta, done all of the above except leak test. I'll report back when I've replaced the condenser.

[Warkton Tornado No.1]

Hi Paul.

I have found that more than once, when the condensor 'goes' or starts to fail, there can still be a spark with the plug held to earth & kicking over the engine.The condensor is there to stop the points arcing.

However, once back in the cylinder, under pressure, with a mist of petroil for company, that same plug will struggle to spark because now there will be an easier path for the electricity (like water) to take, somewhere within the stator or anywhere else it can return to earth.

Jeez! I hope I'm right!

PS As a postscript, I have experienced troubles starting @ times. Last time it happened, I wanted to make a quick getaway & was embarrassed in having to bump the thing.

I got home & in sheer frustration I slung a 40-odd year old Champion N3 instead of the NGK plugs. It’s been running sweet since then.

[kennystag]

My thoughts it amazes me how many new plugs that are fooked, try another new plug first before you go dissecting your bike. Do what a was forced to do ,fit an electronic ignition kit if you can get one for your Vega.
Cheers
Stag

[Scooter Paul]

My thoughts it amazes me how many new plugs that are fooked, try another new plug first before you go dissecting your bike. Do what a was forced to do ,fit an electronic ignition kit if you can get one for your Vega.
Cheers
Stag

Yes, you can get electronic kits. The joy of the Vega is that getting to the stator is a 10 minute job unlike a 'series' whatever. Flywheel coming off tonight and condenser on its way from Armandos. They appear to have tons of Vega parts and have been helpful.

[kennystag]

Good luck, let us know how it all goes.
Cheers
Stag

[HxPaul]

A friend of mine has an Li 125 with a 175 top end.He went out for a run the other day and his scooter kept cutting out,eventually it cut out completely and wouldn't start again and he had to push it home.We tried the plug and there was a good spark so we fitted a special conection between the plug and HT lead,this lit up every time the plug fired,the spark was really good but once under load in the cylinder the spark was almost non-existent.We fitted an Indian 12v electronic system and the scooter fired up on the first kick.

[kennystag]

Hi,
Yes that's the way forward, electronic ignition kit, it toot me a long time to get it, but worth it.
Cheers
Stag

[kennystag]

Hi scooter Paul
I tried a compression test again last night on my standard GP150, and she is reading just over 90 PSI, that's on the kick start, so on your wee lammy 90 PSI Is good compression.
Cheers
Stag

[Scooter Paul]

Thanks all. As some will know I've been around Lambrettas for a looooong time! Every now and then something comes along that makes you scratch your head. Even though these are about as basic as you can get in standard form all it takes is a little imbalance to cause a problem. Luckily most builds fire right up and with the timing strobed and carb adjusted they run like clockwork. I'll report back.

[Warkton Tornado No.1]

Point ignition gets a ‘bad press’ but set up correctly, they work well.

A magneto delivering AC (that increases voltage with RPM) utilising a contact breaker is a good system & will be reliable. Points hardly wear if the set up is clean & correct. The trouble is that before many of us own a Lambretta, they will have been meddled with.

How many are discarded due to ‘unreliability’ is unknown, but often do we read on this forum the problems & failures associated with electronic ignitions, mostly of Indian origin?

I’ll climb down from my High Horse now…..

Anyway, if you decide to go electronic on your Luna, I’d be interested in your flywheel/stator as they are a lovely ‘conversion’ to bigger Lambrettas in reducing flywheel weight & placing less strain on the bottom end!