1. #111
    Senior Member Waddy's Avatar
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    Compression Depression

    I think I've located my issue, and the possible reason that the guy I bought the bike off looked shifty af. I arrived at the shed yesterday full of the joys of spring. Timing the bike properly was my mission, and I skipped no corners. Checked valve clearance, checked points gap, and then used the rigged up light to ensure that the timing could be as accurate as possible.

    Tried to start it. No luck. I'm going to split this section into two parts. Carbs and head.

    Carbs!

    I thought it wise to check the idle mixture screws, as I've read that on these Mikunis if the idle screw is seated to tightly the tip can snap off. Checked the left one... good'o absolutely fine. Checked the right one. the bastard is snapped. So I've got that issue to resolve. So here come the questions:
    1. Does anyone know how to get the snapped tip out? I've tried heating carb body and tapping it with a hammer.
    2. Does anyone know where I can get a spare idle screw for a Mikuni BS32.
    3. Would it be easier to just buy a new set of carbs, and if so does anyone have any? Long shot I know.

    Compression

    Anyway, so after I had calmed down, I thought well even if the right one is fucked, it should still fire on the left. But it wasn't! It was just turning over. Even with EZ Start in it. Nothing, and then after about 5 kicks, a loud pop.
    So I did the checks. Spark, yes, fuel, yes, timing, yes........ compression.
    Put the old tester on it, and I was getting 75psi on the left, and 85 on the right.
    So to try and eliminate the pistons/rings from the equation I dribbled about a spoonfull of engine oil into each cylinder. After retrying the compression test the left compression went up to 125 and the right went up to 135.
    So whats my likely issue? Are the rings buggered, have the pistons gone oval, is the barrel knackered?

    My mate has got a spare top end from a working engine, would my best bet be to just use his barrels and pistons?

    I just feel like I've taken half a step forward and about 8 steps back.

    Any help much appreciated.
    Last edited by Waddy; 13th-08-2016 at 08:38 AM.

  2. #112
    Sorry to hear about your troubles.

    Not sure about the Mikuni idle screw. My guess is finding a second-hand one online, or through the Yamaha Club and using it for spares.

    It does sound like your rings are leaky. Whatever caused that could've caused other issues. What compression should you be getting? Is 125psi okay, or still a little low? If the piston rings are just worn through use that's not too bad, but if they're knackered through misuse and excess heat, it could be that your valve seats have issues too. If you have strip the top end to deal with the piston rings, I'd say it may be worth replacing the whole lot with another if they're in better knick.

    I'm sure it feels like a step back now, but once it's sorted you'll have gained some experience, have a better understanding of your engine and know that your top ends good.

  3. #113
    Senior Member Waddy's Avatar
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    Yeh I completely took the engine apart, and put it together again. Didn't hone out the barrels or anything. Just checked ring gap and rebuilt.
    I've got a mate with a spare top end from a working engine, so I'm gonna give that a whirl. Pistons, barrel etc, and I'll take my valves out and lap them in the head again.

    Minor delay. I'm keeping positive.

  4. #114
    Actually, I was thinking about your idle screw issue whilst doing the washing up and wondered if you're going to have to get a new carb anyway, you could first try getting it out with a left handed drill bit. I know in a cordless and not a pillar drill they can be a bit hit and miss in terms of success, but if the screw wasn't snapped because it was tightened too far, if the bit bites it could help to turn it out?

  5. #115
    Senior Member Waddy's Avatar
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    Shouldn't need to bite either way, as its not the threaded bit thats snapped, its literally just the tip. So if I can get a normal 1mm bit to bite I could pull it out. Good idea.
    I've bought a new set either way from The Bay. £49 for a rebuilt set, which can't be bad. Here's hoping they are right.

  6. #116
    Senior Member Waddy's Avatar
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    It's done. Very, nearly, practically almost!

    I've just seen that it's nearly a YEAR since I last updated this thread. Embarrasing. I have been busy with one thing or another, BUT, I have been beavering away still on the XS250. It is finished. It is MOTd and I have ridden it!
    Oh and I also had a tib and fib break in Feb whilst flying down an icey hill on two planks, which put be in bed for a while and left me sat on the sofa like a slob for about 3 months.

    Anyway!

    I've had carb issues, timing issues, charging issues, its frankly been an un-enjoyable 12 months chasing demons around the bike. However there is one last one. A very strange one. I'm hopeful that someone on here will be able to have a stab at an answer for me, but I am completely at a loss.
    But first some piccys.
    20170521_153750.jpg

    20170521_153758.jpg

    20170521_153835.jpg

    20170521_153914.jpg

    20170521_153924.jpg

  7. #117
    Nice. What's the problem?
    I'd rather have a bottle in front of me, than a frontal lobotomy.

  8. #118
    Senior Member Waddy's Avatar
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    Ok Chico.
    The bike kicks over great and starts second or third kick and just runs on the battery, when the field coil is disconnected. Once I connect the field coil (i.e. create a magnetic field inside the alternator area), something starts rubbing.
    It because stiff to kick, it will turn over but it doesnt feel nice and you can't get the momentum in the crankshaft to bring the bike to life.

    This is a problem that started whilst riding. It must have started rubbing whilst out on a ride because of the following.
    I kicked it up and it started and I was getting good charge. 13.6v on tickover. Rode it to the petrol station and it got very very hot. It was a bitch to start but after leaving it to cool down a bit and having a fiddle it started and got me home. When I got home I slowed down into the drive, revs slowed, and then clunk.
    Felt the engine. Red hot. I thought, oh fook! Over heated and seized. It wouldnt kick over. I almost had tears in my eyes.

    Let the bike cool down and tried the kickstart. Kicked over no problem with the ignition off. Phew it wasn't seized! So I tried turning the ignition on and kicking it over and the engine was stiff! Its been that way since.

    I assumed it must be an issue in the stator area as it only happened when key was turned in the ignition. So I disconnected the stator and field coil, turned the key so the coils were just on the battery. Kicked over great and started second time.

    So wtf is going on inside that stator assembly when its magnetised to create serious friction in the engine? I've taken the stator and cover off and cant spot anything obvious.

    Now stick that one throught your Problem-Solver-1000

    Eth

  9. #119
    Errrr...well, it does sound like the rotor is getting dragged into contact with the stator when the stator is energised.
    .
    It must be that something (not immediately obvious) is loose, misaligned, or worn.
    .
    Time for a proper investigation, I would say.
    I'd rather have a bottle in front of me, than a frontal lobotomy.

  10. #120
    Much as above really.
    If the set up is as I think it will be, windings attached to the side casing with rotor on the crank end, I would prop the bike up leaning to the right ( this will drain the oil away to the right and hopefully not leak ) and take the side case off. With the case off, check all the screws holding the windings to the case and for any cracks around them that could allow it to move when the magnetic field is energised. The gap between rotor and windings is very small so the slightest movement is enough to allow them to come into contact.

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