Caussados

Thanks guys Job done with copper hammer in the end. Some crankcase bolts partially re-inserted and with my son holding the engine by the "top" half off the bench, struck the crank downwards and eventually it started to move. Same proczdure with the other half. Slight problem is that the bearings stayed on the shaft, any ideas on how to get them off or do I get new ones anyway as they have been operating in a metal soup?

Thanks for the reply OG. I understand what you say and have chacked that there are no remaining fixing screws. On this engine there is mention of a special tool to remove the crankshaft from the other half, but not the clutch side. The only nut left undone is adjacent to the gearbox input shaft and from the diagrams in the manual I believe it is the selector shaft with no attachment on the other end. I'm reluctant to loosen it incase I release something inside but perhaps I should try it and see if I can get any more movement I will examine carefully for possible threaded holes. I like your special tool, that's my kind of contraption too.

Hi After nearly a year of my son riding his bike, I return with another problem that I hope someone can suggest something. The engine has wrecked the needle roller bearing on the gudgeon pin and damaged the surface of the con rod bearing, plus of course, leaving debris in the crankcase. I need to remove the crankshaft and have stripped down to the point of splitting the crankcase but the crank is holding tight in the bearing on the clutch side. The workshop manual says simply split the two halves by gently tapping! and shows the split halves such that the clutch side comes away from the crank. It is starting to move as oil has leaked oround the gearbox and I can see a slight gap opening, but it appears that the crankshaft is holding the two halves together. It won't take any general purpose puller. I have tried sharp hits on the crankshaft end with a copper hammer whilst prising the two halves (there is gap in the crankcase halves in front of the crank shaft that allows entry of a lever but obviously I don't want to overdo it). Sorry its a bit long but I hope some of you experienced bike mechanics can help.

PROBLEM SOLVED!! The motor had been fitted with a "performance piston" with only one ring (mentioned above) and it turns out that the electric starter will never give sufficient speed to start such a configuration. So, given that the cylinder was scored due to lack of gudgeon pin circlips, I decided to replace both piston and cylinder with a standard type (2 piston rings). It starts and runs well now. Close examination of the old piston also showed that it was cracked, so that wouldn't have helped. We got the information from the local Yamaha agent who immediately spotted the problem when I described the engine to him when I went in to get the gudgeon pin circlips. Many thanks to all who offered suggestions but it goes to show that you never know what you are getting with secondhand stuff. Now to change the gearbox oil, brake fluids and put 2 new tyres on and it may be ready for the road. All my son has to do now is pass his test!

New plug is in place, one of the first things tried. Plug gap is nearer to .6mm than .7 but I don't have a true .6mm feeler gauge. Checked spark while cranking and today removed plug suppressor to give best possible spark. Also tried old plug connected to lead and held to block while new plug in place to simulate real crank speeds, but spark obviously not under compression. Previously tried paralleling with car battery and again today, with car engine running. None of the above will start the engine on the electric start, but a push start and it's away. The electric starter will crank at between 500 and 800rpm, bit I don't know if this is fast enough as the quoted idle speed is 1600 to 1900rpm. I'm running out of options other than to buy a new starter.

Yes, I have reassembled the engine and can confirm the bolts are in fact M7 but with a 10MM A/F nut. Having taken the cylinder and piston apart I am prepared to to repeat this operation when I get some circlips, in the meantime I am still trying to find the reason for the inability to start on the electric starter. Looks like the only step left is to replace the starter. Many thanks for your torque data and valuable input.

looking for possible lack of compression, I took the cylinder off today and all looks fine. Cylinder still has honing marks and piston has machining marks. Only has one ring and that still has an even step around the top edge; so I don't think that is worn. As I mentioned earlier, the engine has only done 2500 miles since a re-bore and new piston. There are however score marks in line with the edge of the gudgeon pin on both the bore and the piston but I can't see they have caused a problem. I have seen diagrams that show circlips to hold the gudgeon pin but this engine does not have them. Anyhow, I will eliminate compression problems. Does anyone know the torque for the head bolts? I've done them up to 10Nm as they are only 6mm bolts; Is this OK? Earlier in this investigation, and partly why I wanted to take head/cylinder off, I found a pipe of around half inch diameter hanging loose under the air box then saw a suitable place for it on the clean side of the air box. It comes from a small rectangularish box fixed to the frame. There was evidence of muddy water having entered the air box via the open hole so possible muck in the engine. Does anyone know what the device is? It has an input pipe fitting of about the same size as that to the air box but I can't see a pipe to fit.

Thanks Peter T. I think I have eliminated electrical problems as we have a spark when cranking, even tried with old plug held to cylinder while new one was in place therefore testing at real cranking speed (but not under compression). We have connected a car battery straight to the starter while all else was set as for a bump start and still no joy. My final conclusion is that the starter cannot turn the motor fast enough. We reckon that it needs around 700rpm to start on a bump start and the starter can only achieve around 4-500rpm. I believe this bike uses a Minarelli AM6 engine. Can anyone confirm this? Looking on ebay there are numerous sellers of replacement starters for the AM6 engine so I guess they must get weak. The permanent magnets must be losing strength. EDITED to add: Removed starter again and re-oiled, connected it on battery in free spinning state and let it ruin on and off for few minutes. Seems fine so re-installed and connected car battery (car engine running) across bike battery and cranked for some time and it did free up such that I had around 800rpm indicated but it still stubbornly refuses to catch. It sounds promising but just won't fire. Disconnect jump battery and it bump starts immediately. What is going on?????? With the clutch cover off it seems very easy to turn over by hand on the flywheel, but as I have said, I have little experience of small 2 strokes, so I don't know what it should feel like on compression. Any more ideas anyone?

No it didn't. I was hoping it would as well

Thanks again 2 Wheels for you input. Bike has only covered 10,000 miles in total and 2,500 since new crank, oversized cylinder and piston (+0.5mm). The seller told us some tale about the crank getting bent when it was in a minor accident, but on thinking about it, it seems more likely to me that it was run out of oil. Anyhow, unless it has been allowed to run out again , then yes the piston/bore should be fine. I am car man of over 40 years experience, so this sort of mileage is nothing in my view, but I do realise that perhaps these small bike engines have a hard life and i've read somewhere that this TZR50 model is a bit fragile. My son doesn't come of age to take it on the road till next week but he has been running it around our property for half hour sessions to get it well warmed up a few times. We will look again at the clutch drag and carry on running it as you suggest and see what happens. Poor lad was hoping to have it all sorted for his birthday, but on the plus side, we are both learning a lot.

OK, latest status. Before I received your latest message my son and I had whipped the tank, carb and reed valve out. It looks like new, a carbon fibre valve with Conti racing written on the flaps (maybe this is a make local to here in France, and is the same as the silencer). Sucking and blowing through it seems to be fine. Refitted, no change, still won't start on the button, but first turn on a push start. Stripped starter down, all appears fine, clean, no burnt commutator segments and, to be honest, it spins the motor over at a speed I would think is OK. Compression test using a tester designed for car engines, we got 95psi dry and 125 psi with a squirt of oil in the cylinder. How does this sound? Seems a bit low to me on the dry test, maybe worn rings, but would this translate to the starting problem. Clutch drag, tested using your method and there is some, but we are trying to start it in neutral as per the manual so I can't see it would affect the starting. Any ideas gratefully received as I think the next step will be an engine strip down and new rings/piston/cylinder.

Thanks 2 wheels, hadn't thought of clutch drag, not sure how i'll check it. Do you know what type of inlet valve this engine uses? Is it a rotary or reed type? As I said 2 strokes are not my forté and i'm just starting to find out about them. Thought are that it could be a leaking reed valve. The bike had not been used for around 1 year prior to our purchase.

Hi, hope someone can help. Just bought a 2004 TZR50 for my son and it refuses to start on the electric starter, but will bump start instantly. Fitted new plug, cleaned carb. Fuel and spark are present when trying to electric start, but it just refuses to fire. I'm no expert on 2 strokes, but thought it maybe a lack of speed from the starter, so to eliminate poor battery tried operating starter using jump leads from car battery straight to the starter but still no joy. If I do a compression test what pressure should I see? Any ideas on a cure?