1960 Vespa 152l2 - Rebuilt Engine Loses Power

99% of problems are fuel or spark.
I'd guess fuel...or...maybe...spark.
Will it rev hard in 2nd?
Do the easy stuff first, clean air filter & fuel filter.
Try raising the float needle in the carb by 1 notch.
If that fails strip the carburettor and wash it out with petrol.

Pull the spark plug after it's died...is it wet, turn over the engine is there a spark is there a stink of petrol?

Check the timing, then check the timing, try advancing it a tad.
If it's ignition probs, it could be anything, timing slipping due to no lubrication on the cam follower, but ingition probs often give a missfire rather than just dying....I once had to rewind the coil of the magneto to cause a weird ignition problem which kept re-appearing....I think viration had caused a shorted winding.
Del

styarving for air or fuel- if it is air maybe it is smoky, maybe your fuel filter is plugged

change air filter and fuel filter if you have not already doe that- some cars have another filter in the tank sometimes

Hi, you mention that in third gear. so I presume that no problem in 1st & 2nd. check what the previous commenters mentioned but while doing the air filter cleaning. check the air passages completely... including the snorkle (inlet) for anything that might be restricting the air once speed is 35+ MPH.

Sounds like a rag or some piece of something was used to block dust from falling in during the engine repair.

Bike not moving to fast (RPM'S) or for long and starts to die when air is not flowing sufficiently and in the process the sparkplug gets saturated with fuel and stalls.

As good measure, If this was the case. after fixing the air intake problem and the vespa runs better you might want to purge the carb's float bowl by just lossening the screw on 1 side of the carb that's at the lowest part on the carb. (this is used for letting out particles from fuel deposits which the fuel filter does not catch, including formations from sitting more than a month say.

no matter what the problem was, go ahead and buy another new spark plug. these types of engines suffer premature life cycles due to rough idling and poor oil changes (including using car motor oil)

Note* use SJ oils and recommended spark plug for good engine life. you'll be happy that you did a much longer time.

cheers

Good day guest, just for the hell of it, check if your choke is not pulled. I had the same with my vespa and could have kicked myself in the head when the mechanic showed me the problem. Otherwise good luck. regards, Leon.

Just had a thought...
On a Lambretta I once had, a worn crankshaft oil seal on the drive side, it was letting some gearbox oil get sucked into the crankcase and whiskering the plug causing the engine to die every few miles. I had to keep stopping and cleaning the plug until I could get home and fix it.
Del

Incorrect or blocked air filter can also cause the same problem.....a quick run without one will prove that or not.....but only a quick run as the dust will increase engine wear during that test....

Make sure that the airway is completely free.

The wrong piston could theretically cause such problems too.....leaking seals in the crankcase.....they are both long shots though...

You've got an air leak in the intake tract, leaning the out the mixture when you call for sustained power. Piston swells in overheat and siezes in the bore, losening up again when it cools. Then you begin all over again.

Carefully check the seals and gaskets and indeed the carb itself.

Have had rare cases of crank seals leaking air in giving similar symptoms.

Have had both scenarios a few times, thankfully not after rebuilds.

hope this helps,

Cheers

Stu

I had a similar problem with my old '66 Porsche 912. The holes that mounted the butterfly valves in the carbs had warped some and would let air leak in after the fuel+air mixture was created. This leaned out the fuel+air mix getting to the combustion chambers. I wouldn't notice the loss of power much unless I was traveling uphill or in a higher gear (ie 3rd and 4th).

I miss that old Porsche, reliable. handled better than low end new cars. was fuel efficient and had decent acceleration.

i think we are all forgetting somthing, it could be a fuel delivery prob. or possibly the ignition timeing isnt retarding corectly

i think we are all forgetting somthing,
Eh? I don't think I forgot either of those, try reading the first post.
What we are maybe forgetting was to ask for the details of the rebuild.
If it included a rebore, then has it been run in and is the fuel oil mix correct?
Del

One of the first questions that I always ask is "What has changed since the vehicle worked correctly?"

If the problem has been there since the rebuild, You should check compression. While you are there, look at the spark plug for some clues. There are charts that will show spark plugs that have been run in engines with problems.

If the Vespa ever ran correctly after the rebuild, it would still help to read that spark plug. From that the previous good answers should get that thing humming again. Good luck.