Author Topic: Has a penny dropped?  (Read 262 times)

Itsme

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Has a penny dropped?
« on: December 22, 2022, 04:03:55 PM »
As most people know I have been chasing around oil use by the Mighty Midget GN250 for some time. The latest manifestation is that when left on the side stand the MM smokes quite heavily for up to a minute before clearing. Those of you used to air-cooled BMW twins and early BMW Ks might recognise this symptom from their tendency for oil to run down the left hand cylinder valve guides (or in the case of the Ks the exhaust valve guides which were under the engine) when parked on the side stand.

I changed the valve guide oil seals earlier this year so thought I had sorted that. However just recently I have noticed that valve guide oil seals for GNs are advertised as inlet or exhaust. I didn't know this, maybe I should have, but I didn't so I just bought 4 seals. I decided to check the valve stem measurements today and you may have guessed that the exhaust valve stems are slightly smaller in diameter than the inlet valve stems, hence the different seals!

I now think I may have unknowingly put inlet seals on my exhaust guides thus, in effect, putting worn seals on the guides. This might explain why the engine smokes when the bike has been left on the side stand. There is a fair amount of oil sloshing about inside a GN head as the cam runs in its own sump of oil.

What think you brave Thumpeteers, has my ignorance likely caused the smoking issue? Answers on a used £20 note to my home address.

Ian

CrazyFrog

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Re: Has a penny dropped?
« Reply #1 on: December 22, 2022, 05:42:43 PM »
Sounds very plausible to me Ian. Ball ache of a job to have to do again, but worth it if it cures the problem!
2016 Suzuki Gw250 Inazuma
2021 Honda CB125F

Itsme

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Re: Has a penny dropped?
« Reply #2 on: December 22, 2022, 06:19:24 PM »
Thanks Pete, I think it seems like a reasonable explanation for the symptoms I'm getting. As you say it is a ball ache, but as I have Jill's Lexmoto 125 at least I wouldn't feel the need to rush to get mobile again so maybe it would be a good time to try it out.

Of course if it turns out to be not that I might just be leaning towards selling the MM for something a little less environmentally dubious!

Ian

themoudie

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Re: Has a penny dropped?
« Reply #3 on: December 22, 2022, 10:11:07 PM »
Aye Ian,

Continuing to run the MM, (Bantam impersonator!) is probably more environmentally friendly than investing in a velocipede from the CPR. I suppose an electric cycle might be better, but you wouldn't get the same 'thrum' as that from the powerful MM engine!  ;)

Good health, Bill

Itsme

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Re: Has a penny dropped?
« Reply #4 on: December 23, 2022, 07:52:32 AM »
Hi Bill

Absolutely right on all counts. I am lucky that my friend Jill, for various reasons, has lent me her Lexmoto to look after for a while and actually prefers me to ride it to keep it in good running order, but I can't think I would put my money that way if it came to selling the MM.

As for being a bantam impersonator that is also a point that I have been considering recently, if I'm going to smoke how about a good old 2 stroke smoke! Somehow though I don't think the reality of going back to a CZ would live up to the dream. The MM does its best to be a proper thumper by having a lot of torque for a small engine, but if I was going to re-visit a machine from my past it would be an SLR 650. The only problem with that is the angle grinder monkeys seem to love attacking them and then asking 6 times what they are worth because they now have no rear end! Don't get me started on that one.

Ian

iansoady

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Re: Has a penny dropped?
« Reply #5 on: December 23, 2022, 09:53:33 AM »
Sounds like a likely cause of your smoke Ian. Re a new bike - you never know, when I've finished the mighty XBR I may not like it.... Hopefully not. On the other hand I may well become too decerepit to ride it in the next few years.......
Ian.
1952 Norton ES2
1986 Honda XBR500

themoudie

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Re: Has a penny dropped?
« Reply #6 on: December 23, 2022, 10:38:30 AM »
Aye ES2 Ian,

Quote
On the other hand I may well become too decerepit to ride it in the next few years.......

Whilst being a realist I agree, I don't like to dwell on it and have plenty of things that I want to do! Glass half full, rather than half empty, for me.  :)

And the the best possible Highland tradition of bye names, would Itsme be insulted with Ian Smoke (Iain smoc)?  ;)

Slainte, Bill

Itsme

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Re: Has a penny dropped?
« Reply #7 on: December 23, 2022, 04:26:12 PM »
Hi Bill

Ian Smoke is fine by me mate, after all as someone who used to work with young offenders I have been called MUCH worse!

Ian Smoke

Itsme

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Re: Has a penny dropped?
« Reply #8 on: December 23, 2022, 04:51:22 PM »
Hi Ian

Oooo don't tempt me with things I know I can't afford!

Ian

CrazyFrog

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Re: Has a penny dropped?
« Reply #9 on: December 24, 2022, 04:40:57 PM »
Yes, I really enjoyed my Slr650, apart from the stupidly small tank...
2016 Suzuki Gw250 Inazuma
2021 Honda CB125F

iansoady

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Re: Has a penny dropped?
« Reply #10 on: December 25, 2022, 09:51:15 AM »
I liked mine as well but it was a bit tall for me. I raised the forks in the yokes by 25mm and fitted a rear suspension unit from a Suzuki (can't remember what model) which not only lowered it but also gave a much better ride.
Ian.
1952 Norton ES2
1986 Honda XBR500

Itsme

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Re: Has a penny dropped?
« Reply #11 on: December 26, 2022, 11:41:06 AM »
Well I think the search goes on. The Mighty Midget was running a bit rough the other day so I whipped the plug out which looked worn but not destroyed and was mostly a good colour so I put a new one in anyway. I then stripped the carb and found two little splits in the diaphragm so there's a new one on order.

Anyway whilst I'm waiting for parts I took the opportunity to take off the inlet rubber and exhaust. The inlet valve stems are dry as a bone and the exhaust stems, although showing signs of burning oil by being covered in black soot show no signs of wetness, despite being left for the last three days on the side stand. So it would seem no oil is running down the stems at a standstill, whether it gets pulled in as soon as the engine starts and then warms up and makes the tolerances tighter I have no idea.

I am of course now considering the rings as likely culprits, but basically I am just guessing all of the time. The reality is that this is a minor problem and shouldn't spoil my enjoyment of what is a lovely little bike to ride.

Anyway everything smoked on startup a few years ago as nothing had valve stem seals and a lot of bikes and cars were used out of necessity by impecunious owners way beyond being worn out. Environment, what environment?

Ian