RichardW's Velocette rebuild

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Zelandeth
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Re: RichardW's Velocette rebuild

Post by Zelandeth »

Worth throwing the exhaust on...while I don't have a huge amount of experience with two stroke engines, I've fiddled around with enough from the pile of dead scrap round the back of the farm to have learned that this can be critical.

I'd stick a little dash of fuel straight into the cylinder then start it as if flooded as a starter for ten...that should at least get you a cough or two if you've got spark...if it goes then you know you're looking at fuelling issues. Once you've managed to get even a few seconds of life out of it the thing is far more likely to start on subsequent attempts if my experience is anything to go on.

Also worth noting that given how induction is handled on these engines *any* air leaks on the crankcase, carb etc can cause havoc with the fuelling.

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Re: RichardW's Velocette rebuild

Post by RichardW »

Holiday then a week's decorating intervened, and now I have No 1 Son's back to school cold.... grrr.

I take the point about the exhaust, but on this machine it is just pipes - and given that it was running with a crudely made 2-into-1 rather than the factory fit pair, I don't think they are critical - I think it should run without, even if 'power' may be down (would you notice??). I no longer have the exhaust available to fit as I had to cut it to get the unbroken securing ring off, and then the other side has crumbled....

I got the front brake and decompressor cables back. Yesterday I managed to get the decompressor one fitted and get it outside, and talk to it with some Easy Start. Still no dice... I'm sure it fired once as I had a puff of blue smoke, but that is it. Said cold left me without too much energy or ideas - by the time I had kicked it 4 or 5 times I was done in! I slipped on the main drive chain I have bought which seems to fit, so I could potentially bump it if I rig some brakes up - but then I realised as I have no clutch or gearchange at present it would drive the engine every time I tried to move it, so I had to take the chain off again! It seems to be sucking oil in past the big end as after it has been kicked a few times oil starts to drip out of the bottom of the crankcase where that small brass screw is so it should be sucking in through the carb as well. I tied up he advance mech on the mag to give a bit less advance, but that didn't make any difference. I may try introducing some easy start direct into the cylinder via the exhaust ports and see where that takes it....

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xantia_v6
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Re: RichardW's Velocette rebuild

Post by xantia_v6 »

RichardW wrote:
01 Aug 2019, 21:41

Setting the timing - no computer required here, the essential tools are masking tape, a bit of old wire, and some tissue paper (not having any fag papers!)

Image

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Was unsure what advance to set but I found some info on the web that it should be set to 2.7" on the circumference of the flywheel. This seems to be 35 or so degrees, which sounds like a lot, but it's full advance setting, so we'll see how it goes.
I have a feeling that your ignition timing is wrong, I would expect the timing mark on the flywheel (imaginary in this case, I know) to be much closer to TDC, maybe 0.27"?

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Re: RichardW's Velocette rebuild

Post by RichardW »

This is the info I set it against:
TIMING INFO.JPG
The circumference of the flywheel is about 27" so 2.7" reflects about 36deg advance. The range of the advance mech on the mag seems to be about 30deg, so putting it on full retard would give maybe 5deg of advance. I might try that.

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Re: RichardW's Velocette rebuild

Post by RichardW »

I think the decompressor might be doing too god a job. It vents only into the LH exhaust port and this is chuffing when you turn it over with the plug in and the decompressor not activated, whereas the right hand one is not, so I think I am losing compression here. I was unsure when I assembled it with the head off, as I thought perhaps it was opening, but I thought I had cured it. Perhaps it needs a fibre washer in the back to seal it against the head - I don't remember one coming out, but it may have disintegrated over time!!

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Re: RichardW's Velocette rebuild

Post by RichardW »

Took out and fiddled with the decompressor such that I am pretty sure it is not leaking too much now. Tried again with mag pushed right round to full retard, and there is definitely some life in it! With some easy start sprayed in the cylinder it sometimes fires a couple of strokes, and I had a couple of bangs back through the carb. I will reset the timing to something like 15 deg advance to see if that helps, and get a copper washer under the decompressor. I am wondering about doing a compression test - possibly that bit of wear at the top of the bore is robbing it of compression.

Whilst playing about with it I was watching the interaction between kicker and piston - in fact you only get one compression stroke per kick (it might just be coming up towards the second at the bottom of the kick), so it needs to be pretty spot on if it is going to start - and you also need to confirm that you've got the piston at compression before kicking it to give the best chance of a decent speed and compression on the one firing opportunity per kick!

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Re: RichardW's Velocette rebuild

Post by RichardW »

A little tinkering yesterday... Set the timing back to 15deg, filed down a copper washer for the decompressor then stripped the carb down again. I poked out the idle orifice - it did appear to be blocked. Then had another go at starting it, but still no go on petrol only - only Easy Start it definitely ran a few compression strokes!! But then I put the tin down behind the kick start and stamped on it when kicking and broke the valve, so that stopped play :roll: It might just be possible if it's firing on easy start to catch it on the throttle and open the main venturi to see if that will make it run - I didn't manage this as I was so surprised it appeared to going by itself!! Interestingly on this carb, the choke has no effect on idle running as the air is taken in separately. I still think a compression test is in order, if only to rule it out for peace of mind.

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Zelandeth
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Re: RichardW's Velocette rebuild

Post by Zelandeth »

I assume you've tried directly introducing a little fuel via the spark plug hole?

I had a lawn mower that I resurrected a few years back (one of the huge petrol hover mowers, 150cc two stroke engine) which defied all efforts to get it to start until I resorted to dumping fuel straight into the cylinder...after which it started first pull and never had starting issues ever again


I've always been wary of using easy start on two strokes given the lack of lubricating properties.

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Re: RichardW's Velocette rebuild

Post by RichardW »

I haven't actually tried petrol in the cylinder, need a little dropper, it needs perhaps only a ml or 2.

This one has separate oil supply rather than mixed fuel, so a little easy-start on a few slow kicks shouldn't be too disastrous.

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moizeau
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Re: RichardW's Velocette rebuild

Post by moizeau »

A teaspoon full. Is the plug wet? Have you tried the 'hand over the venturi' method when kicking so that it sucks pure fuel. What are the crank case seals like, very important on a 2 stroke? Have you tried it without the decompressor if you think compression may be low?
Look forward to the video of it running.

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Zelandeth
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Re: RichardW's Velocette rebuild

Post by Zelandeth »

Teaspoon or so should be fine. To be fair I've never measured it...just been a "small scoosh" from the fuel can. I stick a scoosh in, throw the plug back in (as close to red hot as possible if you have the opportunity), then give it a kick.

Good point on the oiling system...had forgotten you mentioning that.

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Re: RichardW's Velocette rebuild

Post by RichardW »

moizeau wrote:
02 Sep 2019, 20:59
A teaspoon full. Is the plug wet? Have you tried the 'hand over the venturi' method when kicking so that it sucks pure fuel. What are the crank case seals like, very important on a 2 stroke? Have you tried it without the decompressor if you think compression may be low?
Look forward to the video of it running.
Plug appears to wet, but it could just be oil (although it smells petrol-y!). You can't easily block off the carb - on this design the pilot air comes in through 4 holes around the body - unless you open the throttle it doesn't suck any air through the main venturi at idle (although actually I think there is a bypass in the main venturi, so maybe the choke does do something!). No crankcase seals - this is an overhung crankshaft design - the big end side is in a chamber behind the engine side plate. The flywheel end is the other side of the 4" main 'bearing' where the oil is sucked along and I guess provides the seal such as it is. It's definitely sucking as there is oil everywhere - it drips out of the bottom of the crankcase (where that small bolt is) once it's been kicked over a few times. The decompressor has to be present, as it seals off a big hole in the head - and there's no way to stop the engine if it's not there! I'm pretty sure it's sealing, as there is a grub screw above it, and with this removed there is no chuffing out of the hole when it's turned over. There's certainly some compression as it pops out of the exhausts when turned over, or if you put your finger over the plug hole - but no idea how much. Seems I can get a compression tester for little more than a tenner from Amazon, so it's not worth not checking - at least to rule it in or out. It's so simple it's really frustrating it WON'T go!!

We used to have a Haward Bantam rotorvator with a Villiers engine, that was always a pig to start when it had been stood a while (not helped by having to manually wind on the starting cord each time you wanted to pull it!). Standard operating procedure for that was to heat the plug before giving it a good pull.

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moizeau
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Re: RichardW's Velocette rebuild

Post by moizeau »

RichardW wrote:
03 Sep 2019, 12:08

It's definitely sucking as there is oil everywhere - it drips out of the bottom of the crankcase (where that small bolt is) once it's been kicked over a few times. The decompressor has to be present, as it seals off a big hole in the head - and there's no way to stop the engine if it's not there!
Could it be too much oil, I get called to friends houses regularly in spring when strimmers and chainsaws won't start, 99% of the time it's because they think they're doing the engine a favour by adding a bit extra 2 stroke oil. I guess yours is pumped?

I got the decompressor thing all wrong I think, I was thinking of single cylinder thumpers or big chainsaws that reduce compression in order to turn it over without breaking your ankle.

I'll have to find a diagram of the engine, I'm intrigued.

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moizeau
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Re: RichardW's Velocette rebuild

Post by moizeau »

Just read through the whole thread again, you haven't mentioned anything about which way the piston went in. On 2 strokes there is normally an arrow that points towards the front. If they're fitted 180 degrees out they give your symptoms. This is on much newer engines (70's onwards) though and may not be relevant.
Just noted the oil lever as well. Where is it set? I'd by inclined to set it minimum and once it's started turn it up a tad.

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Re: RichardW's Velocette rebuild

Post by RichardW »

RichardW wrote:
18 Mar 2019, 20:46
Image
I'm sure I've got the piston in the right way round - this pic shows the piston - there is a substantial cut out on the skirt which aligns with the transfer port on the back of the engine. Actually I think the cut out allows the con rod to pass, and if it was in the wrong way round it wouldn't go round :lol: I looked for a diagram of the engine, but I can't find one.

There's no oil pump - there's a drilling between the oil sump (which is what most of the crankcase is) through the crank shaft bush to the crank shaft. There are then oil ways and drillings in the crank shaft so that the oil is sucked along the crank shaft, and then thrown out along the crankpin to oil the big end before being mixed with the petrol in the crank case, before transferring to the cylinder. The oil lever varies the amount of oil by adjusting a needle valve dropping into the drilling in the crank shaft. Got it about in the middle at the mo, could set it to min I guess.

Ordered a compression tester, will see what that says when it gets here.