Scootling Bulletin Board - scooter discussion New Zealand

Puketaha · kickstarting Joined: 13 Feb 2010 Posts: 8

I have a Tr50 that won't start. I've replaced piston and rings (standard size) and done main gaskets etc. mechanic said it has low compression (90psi). Turns over fine but just won't fire.

I've pulled the engine apart again, cleaned carb, cleaned reed block, new plug, fresh fuel.

I have not looked at any crank seals or anything around that area yet. Anyone else struck this problem? Any suggestions?

Thank you!

avai007 · Posted: Thu Feb 18, 2010 1:44 pm Post subject:

at 90psi it generally won't start... at 100psi it will start but indicates worn/broken rings etc.

ps 15% off big bore kits at Scootling this week Smile

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Puketaha · kickstarting Joined: 13 Feb 2010 Posts: 8

Thanks but it has brand new piston and rings supplied by Scootling. The motor has never run just turned over with new rings

Hybrid · Posted: Wed Feb 24, 2010 12:28 pm Post subject:

Did you get to right parts? Is the bore scored to hell? Its a pretty simple engine, once the piston has past the ports, there is only the -rings - the seal between the block and the head, and the spark plug. - a crack in the block or head.
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Puketaha · kickstarting Joined: 13 Feb 2010 Posts: 8

Rings were supplied via bikes serial number. Bore is standard 41mm and seems tidy enough, no big garks! Piston and rings are a nice snug fit sliding up the bore! I have pulled apart the bottom end and checked seals but everything is almost brand new. It sounds like it wants to fire up but just won't. I've got fresh fuel and plug plus I've replaced the high tension lead.

Im stumped! Sad

Hybrid · Posted: Wed Mar 10, 2010 10:30 am Post subject:

Where is the piston at TDC? perfectly flush with the top the bore?
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honda · Posted: Sat Mar 13, 2010 7:58 pm Post subject:

Hi there,

90psi is heaps and the motor should start easily with that. compression will go up when the motor is run in. 100psi is good compression. 160-170psi indicated carbon build up and that can cause knocking and pre-ignition which will eventually burn a hole in your piston as the crown gets super hot. 90 is considered normal but it is slightly on the low side. It will go up when the motor is run in. Something like 40psi is low and indicates excessive wear or a broken ring.

you should try this first.
This is my most effective method for starting up any engine, new ones can become hard to start sometimes for me. All you need to do is take the air cleaner off your carb and make sure their is fuel in the float bowl. Stick your hand hard up against the air intake and crank the motor over with the throttle closed. you should get petrol on your hand. try starting the motor up normally (without your hand across the air cleaner) and 99% of the time, it will fire straight up. this usually indicates a faulty bystarter/choke, clogged carb or faulty crank seals if you need to do this every time the engine is started cold.

Does the motor have spark? try giving the plug a clean up with some sand paper.

If it has spark, you can check the ignition timing with my favourite timing light, a small fluorescent tube. On the magneto, there are two marks usually. 'T' and 'F'. 'T' means TDC(top dead centre) and 'F' is when the plug should fire. there is an index mark on the crankcase. connect a timing light to the HT lead or connect a small fluro tube in between the HT lead and the chassis and move the light next to the magneto. Crank the engine over and the light should emit a flash of light right when the 'F' mark passes the index mark on the crankcase. If it is out (even by 5 degrees) too advanced(away from the 'T' mark), then the motor may not start or run properly. If it is out, the magneto has broken its woodruff key. It is easy enough to replace if you have a rattle gun and pullers. This is a problem i had with much frustration trying to find out what was wrong.

When you do get the motor to start, the rings should bed in and the motor will run smoother and smoother as you run it in.

Good luck and keep us posted.

Puketaha · kickstarting Joined: 13 Feb 2010 Posts: 8

Problem solved! On closer inspection with some quality veniers Embarassed

it turns out the bore is 0.5 mm oversize. Got new piston kit to suit and honed bore. All started up beautifully first pop! Very Happy

New problem has arose. Giving it a run and all going well but then back wheel suddenly grabs/locks and I pull over to the side. Thinking its siezed but it starts straight away and off i go. happened again a bit later. I think it's something to do with the belt and drive system. The belt is near new and everything looks normal. Hmmm Shocked

honda · Posted: Fri Mar 26, 2010 2:30 pm Post subject:

did you run it in? Tight engines run hotter and then the parts expand and sieze unless you run it in slowly for 200 k's or so. It could be the clutch

Puketaha · kickstarting Joined: 13 Feb 2010 Posts: 8

this was within 5 minutes running another time it took 15 minutes. Only seems to do it when ridden. If i put in on the stand and run it a half to full throttle for 10 mins or so it doesn't do it. I am running the standard CDi until its run in.

Puketaha · kickstarting Joined: 13 Feb 2010 Posts: 8

ARRRR! Stripped off the cylinder and discovered that one of the clips that hold in the gudgeon pin has broken (one of the little tags that you use to install/remove the clips has broken off). Tiny broken piece, approx 4mm long found it's way up the side of the piston ruining the piston, rings and putting a big gark in the cylinder wall.
Not happy!
Evil or Very Mad