Lambretta Overheating

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MightyGem
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Thu Jul 27, 2017 10:14 pm

Hi, I have a 1959 Li Series 2 150. It's bog standard in terms of engine/carburettor set up, and has 12v electronic ignition. It will do 55 on the flat, it's happy at 50, but I generally cruise at 45.

The problem is that in warm weather(mid 20s degrees), on a long run it tends to overheat and seize. It gives enough warning so that I can pull in the clutch and coast to a halt. The engine is solid and you can feel the heat radiating.

A minute or two later, it's cooled enough to restart and I can carry on, but I reduce the speed to 40 and everything's OK.

I tried changing the plug from a B8/N4 to a B9/N3 but it didn't make a difference.

Any advice would be appreciated. I mean, how can an air-cooled engine overheat?

Adam_Winstone
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Thu Jul 27, 2017 10:25 pm

TBH the weather should make little difference as standard Lambrettas sold well and ran reliably in warm climates and we've had some VERY warm weather for the last 2 Euro Lambrettas, with many riders screaming their bikes on motorways for hundreds of miles a day, without seizure. Yes, the weather could be the tipping point, however, it indicates that something else is wrong.

I would start by checking your ignition timing and continue other checks from there. IMO your standard bike should be running no more than 21 degrees ignition advance, would probably be ideal at 19 degrees, but could probably run as little as 17 degrees (probably not necessary though if you still have the standard barrel and cylinder head).

Good luck.

Adam

mick1
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Fri Jul 28, 2017 7:56 am

Crikey where to start !! Adam is right and the ignition is an easy check and probably the first thing to do. Out of interest have you checked the plug when it seizes ? The colour may give an indication of the cause.

After the ignition you could check the exhaust. This is a big factor in removing heat from the engine and a blocked or partially blocked exhaust can cause over heating.

Checking the flow rate from the fuel tank/tap would be advisable, a slow flow rate can cause issues. The carb' may also be worth a clean and check the jetting.

Next would be checking the top end. A leak test is always recommended and checking the compression ratio and correct profiled cylinder head would be advised. Checking the piston/bore clearance and piston ring gap is another check to be done.

Good luck and let us know your findings.

Grumpy225
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Fri Jul 28, 2017 6:22 pm

Jetting is usually the culprit when it comes to heat. Too rich, too lean, too much oil, too little oil all can cause the motor to run excessively hot.

Start with the easy stuff first.

Clean your motor if it's covered in grime.
Mix at 32:1 (3%) EVERYTIME and use the highest grade fuel you can get consistently.
Run the coldest plug you can with out it fouling out, I run NGK B9ES on everything.
As mentioned before, timing is important. Make sure it is set correctly and know where its firing.

MightyGem
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Fri Jul 28, 2017 8:15 pm

Thanks for your replies, guys. Some food for thought in there.
TBH the weather should make little difference as standard Lambrettas sold well and ran reliably in warm climates and we've had some VERY warm weather for the last 2 Euro Lambrettas, with many riders screaming their bikes on motorways for hundreds of miles a day, without seizure.
My thoughts as well, but it's never happened on cooler days.
I would start by checking your ignition timing
I'll get that done. With electronic ignition, is it likely to have changed from the initial setup?
Out of interest have you checked the plug when it seizes
Not at the time of seizure, it's too hot. :D
a blocked or partially blocked exhaust can cause over heating.
I've had a blocked exhaust on two occasions, and in both instances the symptoms have been an inability to go up even the slightest incline without having to change down to 3rd, so I'm happy about that.
Checking the flow rate from the fuel tank/tap would be advisable, a slow flow rate can cause issues. The carb' may also be worth a clean and check the jetting.
The fuel flow is quite normal, and the carb was given a good clean only 80 miles before the last occurrence.
Next would be checking the top end. A leak test is always recommended and checking the compression ratio and correct profiled cylinder head would be advised. Checking the piston/bore clearance and piston ring gap is another check to be done.
Hmmm...have to get a man to do that. I don't have the facilities or knowledge for that. By a "leak test" do you mean the head gasket?
too little oil all can cause the motor to run excessively hot.
Mix at 32:1 (3%) EVERYTIME
I run at 4% so going down to 3 is probably not needed.

It could be quite a while until I post any progress because having had something checked I'll have to find a time on a suitable warm day to do a long run to check it.
Thanks once again.

Meds
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Sat Jul 29, 2017 9:08 am

Have you strobed you electronic ignition, the timing marks can vary a lot.

Chris in Margate
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Sat Jul 29, 2017 9:23 am

And too much oil will weaken the mix and cause overheating !

Adam_Winstone
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Sat Jul 29, 2017 11:43 am

Meds wrote:
Sat Jul 29, 2017 9:08 am
Have you strobed you electronic ignition, the timing marks can vary a lot.
^... so true!

Daggs
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Sat Jul 29, 2017 1:04 pm

If it's siezed. You need to open up and inspect the damage. Doing anything else before is a waste of time.

Muppet
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Sat Jul 29, 2017 10:36 pm

Daggs wrote:
Sat Jul 29, 2017 1:04 pm
If it's siezed. You need to open up and inspect the damage. Doing anything else before is a waste of time.
+1
im not a complete idiot
.
.
s me of the piec s are mis i g

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