Partial heat seizure gp150

[johnnyXS]

good thinking Chris.
Looks like an undersized main jet may well be he problem.

Cambridge Lambrettas http://www.lambretta.co.uk/downloads/carbjetting.html carb jetting table suggests that a GP150 should have a 118 main jet so your 107 looks to be well undersized which could easily account for the weak mixture at WOT
http://www.ebay.co.uk/itm/Dellorto-DO-1486-style-NOT-OEM-5MM-M5-carb-main-jet-Made-in-UK-sizes-90-/272127813288?var=&hash=item3f5c14e6a8:m:mSVsCX9PxAcMIX_kUjmzvow

your Pilot jet, atomiser and starter jets are correct for a standard Gp150 although your air screw setting at 3x turns out is suspect and would suggest that you need a richer pilot jet like a 48 or 50 .They are cheap enough so worth trying that also .
http://www.ebay.co.uk/itm/Scootopia-Lambretta-48-pilot-jet-for-scootopia-SH-Dellorto-carbs-/121760635163?hash=item1c597fd91b:g:~REAAOSwsB9V-CA1
Check your slide.... it should be a 7895-2 with the cutouts.

you may have an air leak at your choke so you could try screwing the choke in with a little ptfe tape to get a tighter fit.

[CHRIS in MARGATE]

Kris it looks like we are all "singing from the same hymn book here" and yes you need to richen up.
With regards to the damaged choke thread, try as in Sticky's book, to gently, gently I said, to squeeze the choke tube on the main carb body together to see if that can induce more grip. ( Never had a need myself so would be interested to see if it works!)

[Kristof Lammy pie]

I didn't stand a chance with a 107 jet did I.


Pretty hacked off if I have to strip the the top end down and replace it all again.

Cheers guys I will look into it.

I think the carb is knackered because of the choke tube but will see.

[Kristof Lammy pie]

Managed to fix the choke tube by squeezing it slightly. The bolt seems to have nipped up ok.


Does anyone have good slide I could buy? Scooter restorations is the only place I can find and they are out of stock and want £20 plus postage!

[johnnyXS]

can't help with a slide I'm afraid but might I suggest that you buy a complete new jetex carb instead. they are available for £22 and you'll then have a spare carb to bolt on when you want to experiment with rejetting and a source of spare jets and parts
http://www.ebay.co.uk/itm/LAMBRETTA-JETEX-22mm-CARB-CARBURETOR-UNIT-GP-150-S3-SX-TV-150-/252151765263?hash=item3ab56a990f:g:wKYAAOSwI-BWNhLm

[Kristof Lammy pie]

Already got a 22mm Jetex but I think I will just buy the right parts and return it base settings and work from there.

Im going to just leave it now. And concentrate on the lis125 doing both of them is frying my brain a little.

[Warkton Tornado No.1]

I didn't stand a chance with a 107 jet did I.


Pretty hacked off if I have to strip the the top end down and replace it all again.

You may get away with a clean-up of the top end, so don't get too down.

If you don't mind more advice.......

If I were you I'd do as you intend & fit the correct Dell'Orto components to your carburettor.

I would pre-order new rings to suit your piston along with any gaskets.

I would strip the top end & carefully examine what you have.

If the piston has only seized, it will be reusable, in most instances. Scraping any seize marks from the piston is a pain, but necessary.

Borrow/scrounge/buy a flex hone for the bore, (which is kind of enjoyable, or is it just me?......)

Reassemble & I’d be surprised if you don’t achieve some kind of satisfaction from the process.

It would still be interesting to learn what plug that was.....

[Kristof Lammy pie]

I will always take as much advice as I can get all very new to me.

The plug was a ngk B6es again not fitted by me.

[HxPaul]

A B6ES is a hot plug and could have contibuted to your seize.

[Covboy]

Does anyone know the correct dellorto slide part number ? I might have on in my box of bits....

[Kristof Lammy pie]

Pm'd you Steve.

[dickie]

A B6ES is a hot plug and could have contibuted to your seize.

I'm asking s question here rather than making a statement but my understanding was that a b6 simply means that the plug stays hot rather than heating up the head or combustion chamber.

If I'm correct then doesn't that imply that running too hot a plug means that the plug itself would be damaged?

[johnnyXS]

A B6ES is a hot plug and could have contibuted to your seize.

I'm asking s question here rather than making a statement but my understanding was that a b6 simply means that the plug stays hot rather than heating up the head or combustion chamber.

If I'm correct then doesn't that imply that running too hot a plug means that the plug itself would be damaged?

google is your friend Dickie I always have to check as i invariably get them the wrong way round
http://www.ngk.com.au/spark-plugs/technical-information/heat-range-explanation

[CHRIS in MARGATE]

I think the spark plug is a red herring. From memory I think the B6 is equivalent to the originally specified Champion N5.

[Warkton Tornado No.1]

The plug was a ngk B6es again not fitted by me.

Thank you for the information & I would agree with other’s that have responded to your answer in that the plug was wrong but not the cause of your issues.

B7 to B8 if NGK is your plug of choice, or Champion N3 if you want something I’ve found to be more ‘tolerant’ despite my initial cynicism.

[Kristof Lammy pie]

Thanks will update the thread when I take it apart.

[Kristof Lammy pie]

So with my other lambrettas working now I've got time to strip the top end and investigate the damage caused by my the seizure.

I think Im going to upgrade it to a gt186 kit with a 25mm Dellorto and a clubman exhaust.

[johnnyXS]

Things are looking a bit better than last week when all of them were not running
Don't forget to check your spark plug for fuel mixture on the engine you've just finished and lots of pics of the strip down of the GP150.
You must be getting pretty slick at stripping and rebuilding engines by now
GT186 sounds like an excellent choice. I'm currently thinking about converting my spare Li150 to a standard TV175 with 116 conrod for touring or if I can get hold of a cast iron 175 barrel I'll bore to 200

[Kristof Lammy pie]

Im far from Slick Johnny, still make loads of mistake and forget the easiest of things.


Anyway footboards and exhaust off, hopefully have it all part today.

[johnnyXS]

great day for it Kris.
I shouldn't be indoors typing this