High speed misfire

[dickie]

Eventually managed to get my lambretta out on the road today after a lot of work to the motor, clutch and set up on Darrel Taylor's dyno.

But there's a but. It pulls great in first and second but when it reaches peak(ish) rpm in third it misfires; it gets worse in 4th gear.

Running fixed 17 degrees with bgm stator and external pick up.

I'd have thought it was coil or pick up but then wouldn't it happen the same in all gears?

[Norfolk]

Might sound a bit silly but to start with I would start with something simple like changing the spark plug. I say this because upon replacing my spark plug the indicators played up, changed the plug again and all was ok ! Scooters are here to try us.....

[dickie]

Might sound a bit silly but to start with I would start with something simple like changing the spark plug. I say this because upon replacing my spark plug the indicators played up, changed the plug again and all was ok ! Scooters are here to try us.....

I agree. So that's the first (and only) thing I've done.

Unfortunately it made no difference

[Scooterdude]

Sounds like you may have to wider air gap on your external pick up something that Anthony Tambs says is critical to correct running...

[Fast n Furious]

Sounds like you may have to wider air gap on your external pick up something that Anthony Tambs says is critical to correct running...

This is true, but it would also affect high revs in 1st and 2nd.
A lack of primary voltage to the CDI can result in this symptom of misfire under heavy loads(amongst others)
Using a digital multimeter set on 200+volts AC scale, connect one lead to the green wire on the CDI and the other to earth. You should see around 50-60v on tick over and around 200V when revved out. A low voltage reading results in a weak spark. The usual suspect here is the stator's LT coil, which are notorious for screwing up.

[Strummer10]

Think I have a similar problem , also have new BGM stator fitted. I thought it was jetting but now convinced its electrical system breaking down. Mainly hesitant in 4th gear until hit between 50 and 60, but previously holding back then going again between 40 and 50

[Raveydavey]

Check your earths. I had exactly the same. Turned out the earth strap had 'sheared off' the ring connecting it to the mag flange. Crimped on a new ring connector, bolted it to the mag flange, problem solved

[ROClarke]

Might sound a bit silly but to start with I would start with something simple like changing the spark plug. I say this because upon replacing my spark plug the indicators played up, changed the plug again and all was ok ! Scooters are here to try us.....

I agree. So that's the first (and only) thing I've done.

Unfortunately it made no difference

Defo electrics fron, what you are saying.

[stephen bebbington]

Sounds like you may have to wider air gap on your external pick up something that Anthony Tambs says is critical to correct running...

This is true, but it would also affect high revs in 1st and 2nd.
A lack of primary voltage to the CDI can result in this symptom of misfire under heavy loads(amongst others)
Using a digital multimeter set on 200+volts AC scale, connect one lead to the green wire on the CDI and the other to earth. You should see around 50-60v on tick over and around 200V when revved out. A low voltage reading results in a weak spark. The usual suspect here is the stator's LT coil, which are notorious for screwing up.

hi my tv 175 has started the same what is the fix Stephen bebbington

[MickYork]

The electrics is your first point of call but.......(there's always a but) I wouldn't rule out a fuel issue.

My mates TV regularly back fires when he goes on reserve......and, I ran out of petrol at the weekend which preceded a couple of backfires then the engine died.

Maybe check you flow rate and give the carb a once over.

[Fast n Furious]

Sounds like you may have to wider air gap on your external pick up something that Anthony Tambs says is critical to correct running...

This is true, but it would also affect high revs in 1st and 2nd.
A lack of primary voltage to the CDI can result in this symptom of misfire under heavy loads(amongst others)
Using a digital multimeter set on 200+volts AC scale, connect one lead to the green wire on the CDI and the other to earth. You should see around 50-60v on tick over and around 200V when revved out. A low voltage reading results in a weak spark. The usual suspect here is the stator's LT coil, which are notorious for screwing up.

hi my tv 175 has started the same what is the fix Stephen bebbington

CDI stators from virtually all sources are flawed in the way the LT coils are fitted. So much so, I wouldn't ride any scoot with a standard produced plate any further than pushing distance.
The LT coil is made up from a lot of very fine wire turns around a bobbin. This is done so it generates the required 200+volts to charge the CDI. Your lighting coils are made of much thicker winding's because they only need to produce a low voltage.
There are a number of ways these fragile LT coils can fail but basically the two most common ways are:-
1. Insulation breakdown of the winding's. High voltage coils will fail this way usually as a result of humidity and age.......... An acceptable compromise.
2. Mechanical breakage of the winding due to excessive vibration coupled with improper assembly...............An unacceptable product.
Lambretta's vibrate... That is something we have to live with.
Typical "Off the shelf" produced stuff has this very thin wire pulled tight to its soldered earthing point on the laminate. It will fracture internally with vibration. Sometimes these problems will only show when the engine warms up because temperature is now getting in on the act making it even harder to trace the cause. Winding breakage is something that can be fixed by replacing the LT coil with a new one, ensuring that the connection to earth is terminated onto its own terminal point on the bobbin coil and not directly terminated to the earthing point on the stator's metal laminate.
I make an extra hole in the plastic base of the coil bobbin and insert a small ferrule. The coil's earth wire is now soldered to the ferrule along with a separate earth wire which joins the coil termination point to the laminate earthing point. This secondary earthing wire now takes the brunt of the vibration and not the coil itself.

[dickie]

A lack of primary voltage to the CDI can result in this symptom of misfire under heavy loads(amongst others)
Using a digital multimeter set on 200+volts AC scale, connect one lead to the green wire on the CDI and the other to earth. You should see around 50-60v on tick over and around 200V when revved out. A low voltage reading results in a weak spark. The usual suspect here is the stator's LT coil, which are notorious for screwing up.

I'm getting 110 to 120v from tick over all the way up to full revs.

That's using a true rms meter on 400v scale. What do you use?

[Adam_Winstone]

I had a high rpm misfire on a BGM stator that I was given and found this to be because of a failing pickup. This was not an alignment issue and it gave perfect readings, however, a trusted pal advised me to try another pickup and it instantly cured it, allowing it to rev cleanly through the range.

[dickie]

I had a high rpm misfire on a BGM stator that I was given and found this to be because of a failing pickup. This was not an alignment issue and it gave perfect readings, however, a trusted pal advised me to try another pickup and it instantly cured it, allowing it to rev cleanly through the range.

Ok, I'll give that a try although I'm puzzled as to why it only happens in 3rd and 4th (worse in 4th). But I don't have any better ideas!

What is the current best lt coil to use? Any special tricks to fitting it?

Thanks again

[CHRIS in MARGATE]

Greater load on the engine in higher gears. Imagine your legs pedalling a push bike. Takes some work in top gear.

[dickie]

I don't think I said that it's a bgm stator. I've done a bit nosing around and I think bgm coils are the only ones that are compatible with the bgm stator.

Am I right? I hope not as they have a reputation for being crap and I don't want to buy a complete stator as a guess.

[Adam_Winstone]

Nearly right. The BGM laminate core is the same thickness as genuine Vespa staters, whereas the Indian staters have a thicker laminate core. The coils are wound onto a plastic bobbin that suits the core thickness so you either source one for BGM/genuine Vespa or, alternatively, for the Indian staters.

BGM replacement LT coils are good as they are good quality and come with new fibre spacer and copper plates, however, my favourite replacement LT coil is the Ceab one sold by Cambridge LAmbretta.

Adam

[Adam_Winstone]

Cont... I like the Ceab LT coils so much that I even Drexel off laminates from failed Sil/Indian staters so that I can fit the Ceab coil to these too.

[dickie]

Cont... I like the Ceab LT coils so much that I even Drexel off laminates from failed Sil/Indian staters so that I can fit the Ceab coil to these too.

Thanks Adam. I've ordered one from Anthony tambs for considerably less. Hopefully it's as good!

[Fast n Furious]

A lack of primary voltage to the CDI can result in this symptom of misfire under heavy loads(amongst others)
Using a digital multimeter set on 200+volts AC scale, connect one lead to the green wire on the CDI and the other to earth. You should see around 50-60v on tick over and around 200V when revved out. A low voltage reading results in a weak spark. The usual suspect here is the stator's LT coil, which are notorious for screwing up.

I'm getting 110 to 120v from tick over all the way up to full revs.

That's using a true rms meter on 400v scale. What do you use?

I normally use the first meter in box, which is going to be a generic AC averaging measurement like what most hobbyists mechanics have. I've never tried to measure the true RMS. I have a Fluke which can measure both so I'll give it a measure on both and report back.