8 replies to this topic

[mini danny]

hiya basically my brother brought a bike with a 2 stroke 50cc engine the bloke before rebuilt it. which he did got the old bits in boxes and today I started it for him and then boom really loud knocking which picked up with the revs so I immediately turned it off. first thing I thought it's a 50cc it blew the top end up so I take the engine straight out and stripped it and to my surprise the piston and top end are fine and so is the crank and theres no play in the rod ect ect there's no loose bits inside I've checked everythink over nothing loose ect what else could it be

thanks Dan

[Tomm]

I have had bottom end bearings go in larger (250) 2T engine and there has been no noticeable play in the crank but upon pulling the bottom end apart it is all to clear that is where the problem was.

Was the motor knocking from the moment it started or was it something that developed after starting?

What engine is it? Only 2T engines I really know are the Rotax lumps.

How does the piston look? Any discolouration? Have you measured the piston tollerance? The piston to barrel sizing is vital on a 2T engine, when ever I rebuild a top end I had the barrel honed and then had the machinist match a piston to the bore, incorrect sizing can cause a noticeable piston slap. Possibly the last builder might not have done this.

Does the bike start up with any hesitation? What is the plug colour and piston top like? Is it a Auto-mix or pre-mix?

[mini danny]

hiya thanks for the reply with the piston it comes as a kit piston and barrel so are machined together so are correct. the piston itself is mint along With the bore and it's only a little 50cc engine out of a derbi gpr50 and the bike was fine until I started it this time as soon as it started it began to knock it tits off and I immediately turned it off. I did restart it to investigate the noise and the bike couldn't idle and the knocking increased with revs

[mini danny]

also forgot to say when the knocking started it almost sounded like something fell off inside and was rattling around

Danny

[Carlos W]

timing out?

loose fly wheel?

is it for sale cheap?

[mini danny]

there's is no timing on it lol and no flywheel nice and tight and nope my little bros but maybe depends on the offer


Danny

[Klevdo2202]

When I saw 2T I thought you had a Villiers engined bike! but then noticed it's short for two stroke.

Derbi's a yamaha engine isn't it? like some of the Aprilia's are Rotax engines

Could be timing.

Also, it could be just the way the kit is, some aftermarket kits for the Puch Maxi are known for a knock/tingling noise.

Edited by Klevdo2202, 16 March 2013 - 09:32 PM.

[scrog]

check rings if the explosion is getting into crank case and pre ignighting petrol oil mix
or reed/rotory valve might be out of sink

also make sure transfer ports are not blocked

Edited by scrog, 16 March 2013 - 10:12 PM.

[mini danny]

everythink fine inside there is no engine timing as such just a crank sensor for the spark and engine has been fine after rebuild this knocking just started yesterday everythink I can see inside is fine only think to check is the crank bearings which I'll do at work on monday

Dan