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Post by faversham on Sept 5, 2020 5:36:40 GMT
So a mate has a primmy 125 which has recently had a top end rebuild. To finish it off it needed the carb installing which we did and it started and ran for a bit. However, there was a fuel leak from the carb so I whipped it off for an overhaul before refitting. It started and ran sweetly with only a minor amount of slow running adjustment needed. Low end screw was set to 2.5 turns out. Road test round the block all ok.
So my mate then took it for a ride around town. No problems and only at running-in speed.
Second trip out she started to go a bit further afield and this is where the fun starts. On startup for run #3 she needed a little choke or the engine would die. On taking it for a run when stopping at a junction it was over-revving and needed to be kill-switched. It started up ok afterwards but repeated the over-revving trick again a short while later again necessitating the kill switch to be used to stop the engine which she did. It backfired on killing it and since then it now won’t fire up.
Fuel is getting through to the plug and there is a big fat spark but no sign of it firing.
I’m leaning toward an air leak somewhere as the over-revving seems consistent with the engine leaning out which would also be consistent with the choke being needed. Seems odd though given that after the carb rebuild it started and ran as sweet as a nut. I’m leaning toward a bearing oil seal leak maybe??
Before I drop the engine and check for any obvious signs of an air leak, any suggestions? It seems carb manifold leaks are common but what about bearing oil seals?
I’m very familiar with small 2t engines but this is the first time I’ve had a crack at a Vespa lump so Im Hoping the many gurus here May be able to help.
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Post by boreham on Sept 5, 2020 7:01:40 GMT
Hi, sounds like a typical air leak, the common areas to check first are the manifold joint to the casing (a spray of WD40 whilst running or easy start is a good option), then the base gasket, if not the most common are the crank seals, check the flywheel one is seated square, you can do this without splitting the engine, again next is the common crank seal on the gear side, if not fitted exact and square when building they can fail quite quickly
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Post by faversham on Sept 5, 2020 15:56:26 GMT
Thanks Boreham. That underpins my thoughts too. My money is on the crank seals but we’ll see when we get the patient on the workbench a little later this week.
👍
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Post by tzsteve on Sept 6, 2020 8:24:11 GMT
Is it possible that the fuel pipe has a bit of a kink in it. This would let enough fuel into the carb for it to start while the engine has not been running, But after a little running the fuel level in the carb would fall faster than the fuel getting into the carb could top it up.
A very common problem when the carb has Been in and out or replaced, Answer is a little new fuel pipe.
back fire can be a sign that the flywheel has slipped on the taper, That would give you a spark but not enable the engine to start as it would be trying to fire at the wrong point at least that's easy to check
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Post by faversham on Sept 6, 2020 9:18:15 GMT
Thanks for that. I did wonder if the timing had moved and I’ll check that. The fuel pipe is all new and is too stiff to get a kink in it but thanks for the thought. 👍
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Post by jbcollier on Sept 14, 2020 12:50:22 GMT
Check around the carb by spraying with WD40, ether, or something else flammable.
After that, plug the intake and exhaust and do a pressure/vacuum test.
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Post by faversham on Sept 14, 2020 21:38:45 GMT
Thanks JB
It won’t start so the old WD40 or easy start round the manifold won’t really work
I’ve pulled motor now and on inspection the base gasket looks suspect. It’s a very thin gasket which has had silicon sealant added on one side. I don’t know if it’s the proper stuff but it wasn’t overly stuck. (I’m a bit old school so I use blue hylomar on both side of my gaskets).
The jug has a sheared exhaust stud in it too so I’m not going to muck about and helicoil it (although I may do that later and keep it as a spare) - a new barrel and piston kit is in order.
If you’re going to do a job etc....
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