help, scoot chugging, back firing

[lesfaenaboombu]

scoots running p155, chugging back firing after rebuild, tried cdi,stator,ht lead,plug,
cases got cracked after a mate went ahead and tried to put main bearing in but got a top guy in uni to weld it seemed a good job
fitted a sip fast flow tap to so wondering if its that, dont know what else to try and looks like whitby a no go now
polini 210

[acko]

Timing?
Piston in upside down?

[lesfaenaboombu]

thanks for reply but timing was marked and piston in right way, it runs ok at times then just starts chugging

[mick1]

try another carb'. Poor fuel ?

[lesfaenaboombu]

Forgot to write I've tried another carb and more fuel from different garage

[Yanker]

If the main bearing seat was cracked, it is no good just welding arround it. The whole seat needs to be fully over welded, then re-machined preferably in tandem with the fly case to ensure alignment (but can be datum referenced from other axis machined surfaces). Seen a few of this type repair go wrong: Main oil seal fails to seal or inlet fails to seal or bearing floats and therefore crank resulting in ignition misfires. Yours sounds like the last one with poss any of the others.....

[Wayne Miller]

Woodruff key sheered

[Wayne Miller]

Sorry mate just read that its running OK at times, worth checking the woodruff key though

[acko]

Here are a couple of pictures of one of my engine casing repairs from a few years ago.
First two are of the damage.
Still going strong as well.




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[GLscoot]

Did the other carb, cdi, ht lead, stator & spark plug come out of a known working scooter, I don't mean that they were working before, a scooter that is currently running and working ok. What about the continuity test on the green wires for the ignition or even the ignition switch itself.
Gary