Good day with Petter M

At least it went Mark, well done. I think it could be due to a weak spark, but you will soon find out once you get the mag back.

A weak spark is the most likely cause but it would be worth careful scrutiny of the ball seat on the atomiser. Especially if fitted with a steel ball the seat is hammered until its deeper than the ball radius and then the ball can stick. The only effective cure I've found is to set it up and cut the seat back with a small end mill. hth Roland

I'm surprised it didn't restart. In my experience, the initial start after a long lay off or rebuild is the most difficult to achieve & I applaud your electric drill idea & I've done similar things in the past with little engines & a wire cup brush rammed hard into a rope starter hollow pulley - BRRRM!

Now it's cooled down again, is the compression better than it was before it ran? Have you tried a freshly-dismantled & properly cleaned plug - better still a new one? A friend of mine long ago reckoned that the sideways blast of ported induction blew the spark out on two strokes. Me, I'm always faintly surprised that two strokes run at all.;o)) Regards,

J. Kim Siddorn,

Actually Roland, that is a very good thought. I noticed, but didn't think anything of it at the time, a very slight blow-back of petrol around the drain plunger thingy on the fuel bowl when it was being hand cranked. It was very slight but indicates that the ball is not sealing or seating well. I'll try making up a 'D' bit to recut the seat as I don't have a suitably sized round-nose end mill.

Thanks

Mark

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Kim,

The compression has improved a lot. I suppose the rings have freed up a bit. Overall, the engine seems to be in pretty good shape, it has obviously been thoroughly rebuilt in the dim and distant and probably not run much since. I noticed a slight weep from the cylinder water jacket so it obviously has had some frost damage in the past which I will have to investigate.

I did wonder whether the spark was being blown out and I tried various plugs including an ancient side-electrode jobby which was the most successful, which may be a bit of a clue. My big (comparatively) Petter M is very fastidious about it's plugs and seems to prefer the oldest and rattiest available - there is no accounting for taste.

Whoever came up with the two-stroke idea was either a genius or a mad man. The whole idea of getting the appropriate movement of fuel-air / combustion gasses into and out of the thing through a couple of unmanaged holes just beggars belief! The other thing that beggared belief was my first two-stroke experience as a boy with a BSA Bantam (yuk). I was trying to bump start the thing for the first time after rebuilding it. Ran like hell up the garden and just as I got to the exhaustion stage and gave up the engine fired and ran. I leapt aboard, stuffed it in first, gave it loads of revs, dumped the clutch and shot backwards! The bloody thing had bounced off compression and started backwards. Oddly, it did it a couple of times during my short ownership and always seemed to run much better and sweeter backwards. But then anything has to be better than a BSA Bantam running in it's normal manner!

Anyway, I'm off to get soaked at Barleylands, not sure why - if the place isn't a complete quagmire and there is an engine actually running I'll eat my hat (or brolley).

Mark

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Mark,

You may be interested to know the following about your M type. I spoke to the previous owner about it. We always thought it was rings or mag, or it could be both. Perhaps the following will help.

He did quite a bit of work on it way back, - put it back together and it would not run. He had an identical engine which was a good runner, so he swapped bits over until he got it to run.

What is interesting, is in addition to the magneto the engine would not run unless the piston was changed over too. When I say the piston, I mean the piston with rings. It now started easily and ran well.

So... must need new rings I hear you cry ?

Sounds obvious, so just to make sure, he removed the rigs from the swapped piston and used them with the original piston. Now the engine would not run again. Not even a splutter.

Put the swapped piston back in and away it went.

Sounds strange, but there it is !! I wont pretend I know why, but those are the facts as I have them.

Regards

Chris

Well I certainly can't explain that unless the piston was fitted backwards or so mis-machined as to throw the port timing out. Neither of those is as rare as you might think! ttfn Roland

I wonder if the piston that would not fire had a different height above the gudgeon pin?

If higher, it would have banged on the head, obviously, but if the other way around, it would have reduced the compression ratio - already not very high, I suspect - to a sufficient degree for it not to fire. It might well remain unnoticed unless a gudgeon pin was run between the two.

I only thought of this because I once had a couple of visually identical pistons for a Velocette KTT, but one was the right piston & the other for a different engine altogether.

Stupid thought, but is the piston the right way round - scavenging would be b. awful with the 'slopey' side towards the transfer port.

Thanks for that Chris - useful info.

It sounds like one of a number of things. As Kim says the piston may be the wrong height - does anyone know what the compression ratio should be for a

1.5 without a paraffin ring? I can measure / calculate the compression ratio to verify the piston crown height if I knew what the ratio was supposed to be.

I guess that a closer look at the piston might reveal a crack/hole (seems a bit unlikely) and to verify that it is the right way round (which I didn't notice when I had the head off!).

The other, probably minor thing I noticed when looking at Roland's site was that the exhaust bore should be 1.5". The pipe on mine is thick walled with an internal bore of only about 1". I know that 2-strokes do not tolerate much in the way of back-pressure which, just maybe, the smaller pipe is providing. I suppose that a different shaped piston, as tried by a previous owner, may have overcome any back-pressure issues (clutching at straws here!).

I also noticed, when I had it running, that it seemed a bit under-powered. It was easier to get to full load by wedging a piece of wood against the flywheel rim than other engines of a similar size (my Fairbanks Morse Z 1.5 for instance).

A few more things to investigate when the weekend eventually cometh methinks.

Thanks again,

Mark

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Exactly what I was referring to. I have seen Petter pistons mis-machined in almost every way including pin height and land/groove position. A 1" pipe is fine for a 1 1/2, its all some post would take. CR should be about 4:1 but they will normally run, however badly, with very little compression. As long as we are exploring the obscure you might as well check that its not been badly linered with ports out of position or the plug hole partially covered. However I'm betting its something simple like for example a lack of crankcase compression due to a pinholed reed or a completely coked pot, or a very badly worn butterfly or .......... Have you fired the pot? (Take the washing in first :-) ttfn Roland

Fired the pot? Gosh, that sounds like fun! I used to do this with coked up Japanese two stroke exhausts. I'd start the fire with oxy-acetylene & when the carbon was glowing, I'd turn off the gas & run oxygen into the pipe. The ring of burning carbon heated the silencer red hot in a band that would proceed up the pipe (held vertically in the vice) until there was no fuel left.

Didn't do a lot for the chrome, but any port in a storm if you are a kid with no money & a 250 that is readily beaten by FS1E's!

Regards,

J. Kim Siddorn,

The pot is cast iron. A propane torch is more than adequate! :-) ttfn Roland

Roland,

Another good thought, I did drain the pot when the engine was hot the other day and promptly b******d up my tarmac drive! (why do I do these things on the drive?). Another thing to have bash at over the weekend - sounds like the sort of fun my neighbours might enjoy! :o)

BTW, I'll be checking the other stuff too.

Mark

Is it possible for a 250 that is actually running to be beaten by a FS1E? I luckily never had the pleasure of a FS1E but even my first BSA Bantam (yes, really, I had two!) a 125cc D3? could just about manage to outstrip a FS1E.

Mark

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Leave the exhaust pipe in place, and aim the propane torch into the exhaust pot through the port opening. If it gets going well it will start to sound a bit like the dull roar of a distant jet engine, and the flames and smoke will be very impressive. When I did the 8 hp Atomic, that hadn't been decoked in 40 years, I nearly immolated the fence and one of the sheds :-)

Regards

Philip T-E

Gentlemen,

To all of you without a grasp of the English language IMMOLATE is to sacrifice, kill etc. According to my dictionary:-))

Martin P

Absolutely. I've seen 250's (Suzukis with poorly adjusted pumped lubrication were the worst) that would not run much above tickover because the exhausts were so blocked, the exhaust port having a shiny black fill and a hole through which you could not pass a pencil!

An uptogether Fizzy ought to do around 50 MPH & I've known an exceptional example ridden by a kid that didn't weight seven stone dripping wet get close to seventy.

When you rode a lot of Japanese two strokes, you soon realised that they were far from being all the same & whilst most fell into a pretty narrow performance band, there was the occasional example where all the production variables collected together to give unusually high performance. I owned a GT250 Suzuki that would actually do 100mph against the clock & poo off a warm greased shovel wasn't in it ;o))

Regards,