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Series 1 Li125 4 pole points bike won't rev

Technical help for Series one, two and three Lambrettas. Models include the Li, Li Special, TV, SX, GP, Serveta and API/SIL models

Series 1 Li125 4 pole points bike won't rev

Postby Pompeydave » Tue Jun 12, 2018 10:43 pm

Hi all, I'm repairing an early series 1 Li125 fitted with a Filso four pole ignition, BS5 carb, and Filso coil. It came to me as a non-runner and looks like it's a newish restoration that has been jet washed or steam cleaned and then stored for a long time as there are white water chalk marks all over the nuts and bolts and electrical connections. Once I ditched the carb due to the usual flooding issues and fitted an SH1/18 with correct jets for a Li125, and changed the fuel for fresh it ran up sweet as a nut. Only problem is it won't rev. When the throttle is opened the revs try to increase but it just bogs down and won't rev up. Release the throttle again and it settles down into a nice even tickover.

I suspected a dirty carb, so cleaned it all out again, but no joy. There are no air leaks either. A dodgy condenser was the next possible so I changed it for a new one, but to no avail, exactly as before. I've got spare points, flywheels, stators and regulator boxes, but have tried cleaning up all the earthing points first, most of which were painted over. All now cleaned but still as before. Any idea's as to the next move please anyone?
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Re: Series 1 Li125 4 pole points bike won't rev

Postby CHRIS in MARGATE » Wed Jun 13, 2018 8:07 am

Ignition carburation and compression are the 3 key elements. Worth trying a compression test to eliminate one part of it.
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Re: Series 1 Li125 4 pole points bike won't rev

Postby Scooter Paul » Wed Jun 13, 2018 9:25 am

Is the air filter clear? Maybe when washed it became saturated and now won’t breathe beyond tick over.
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Re: Series 1 Li125 4 pole points bike won't rev

Postby Pompeydave » Wed Jun 13, 2018 11:05 am

Hi Paul, the air filter is new. Chris, yep I think I’ll try a new ht lead and cap first. Changed the plug to no avail also. Engine is newly built but I’m not confident as to the quality of the build bearing in mind the other basic errors elsewhere in the bike.
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Re: Series 1 Li125 4 pole points bike won't rev

Postby MickYork » Wed Jun 13, 2018 5:16 pm

have you tried without the filter on ?
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Re: Series 1 Li125 4 pole points bike won't rev

Postby Pompeydave » Wed Jun 13, 2018 5:24 pm

Yep, no difference.
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Re: Series 1 Li125 4 pole points bike won't rev

Postby MickYork » Wed Jun 13, 2018 5:40 pm

blocked exhaust ?

flywheel requires re-magnetising ?
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Re: Series 1 Li125 4 pole points bike won't rev

Postby Pompeydave » Wed Jun 13, 2018 5:45 pm

Will try another flywheel. Hadn’t thought about the exhaust. I’ll have a look. Cheers.
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Re: Series 1 Li125 4 pole points bike won't rev

Postby Pompeydave » Sat Jun 23, 2018 1:21 pm

Hi all, quick update. Cleaned up every electrical joint and run the bike again. It’s starts, runs and revs through the whole Rev range really well with the choke on. Turn the choke off and it won’t rev above 1/4 throttle as before. I’m running a standard Li125 (but series 3) carb, I.e. Dellorto Sh1/18 with a 5914-1 slide, 5899-1 atomiser, 42 pilot jet, 50 starter jet and 99 main jet. The float is the standard 5gm item.

Any suggestions please?
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Re: Series 1 Li125 4 pole points bike won't rev

Postby HxPaul » Sat Jun 23, 2018 3:40 pm

If it runs ok with the choke on but not without,it sounds like its running to weak.
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Re: Series 1 Li125 4 pole points bike won't rev

Postby Pompeydave » Sat Jun 23, 2018 10:15 pm

Something wrong with the SH1/18. Went back to the original carb and it runs fine now.
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Re: Series 1 Li125 4 pole points bike won't rev

Postby Steve J » Fri Jun 29, 2018 9:57 am

The Li Series 1 125, whether it's a framebreather or one with the wide neck airbox, will have a much larger main jet than later machines with the same carb - probably a 92 instead of the later 73. The SH1/18 was only ever used on the Series 3, which had a much narrower airbox neck - maybe there's nothing wrong with the carb, just running very weak with the wrong airbox?
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Re: Series 1 Li125 4 pole points bike won't rev

Postby Pompeydave » Fri Jun 29, 2018 1:47 pm

The SH1/18 was fitted with a 99 main, a 42 pilot and a 50 choke. The carbs in used but unworn condition so it’s a bit of a mystery.
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