RichardW's Velocette rebuild

Tell us your ongoing tales and experiences with your French car here. Post pictures of your car here as well.

Moderators: RichardW, myglaren

RichardW
Forum Treasurer
Posts: 9485
Joined: 07 Aug 2002, 17:12
x 435

Re: RichardW's Velocette rebuild

Post by RichardW »

Compression tester arrived, I'll give it a go later and see what we have.

RichardW
Forum Treasurer
Posts: 9485
Joined: 07 Aug 2002, 17:12
x 435

Re: RichardW's Velocette rebuild

Post by RichardW »

Annnnd...
compression.jpg
Not going to start like that... :roll:

On to find someone to re-bore it. I suspect it is going to need a sleeve however - the piston fitted is not not original - it's a Heppolite and the casting mark shows it as a 2223, which from a table I found, is actually for an early GTP, which was the successor 2 stroke design, and should be just under 63mm dia.
2223.JPG
However, I'm sure mine measures nearer 64mm at the base, which would suggest it's already a +40 thou piston - surprisingly these are still available, but I haven't seen anything bigger. So I guess it needs to be sleeved.... Found a Co that specialises in re-bores and sleeving, but don't know if they can do a cylinder that has an integral head. I'll get the barrel off tomorrow and re-measure the piston.

User avatar
moizeau
Donor 2020
Posts: 1083
Joined: 12 Jan 2015, 17:08
x 143

Re: RichardW's Velocette rebuild

Post by moizeau »

Bugger, did you measure the ring gap at top mid and bottom , mid is the one to take as bottom never gets used? So already a mm over, there may be someone who can supply the equivalent of cord rings? Are you in the Velocette Club or similar?

User avatar
Zelandeth
Donor 2016
Posts: 3917
Joined: 17 Nov 2014, 00:36
x 438

Re: RichardW's Velocette rebuild

Post by Zelandeth »

That is rather on the low side...

I'd still stuff some fuel down the bore and get the plug wicked hot to see what happens. Wouldn't be the first time I've seen compression get far better once the engine has run...

User avatar
moizeau
Donor 2020
Posts: 1083
Joined: 12 Jan 2015, 17:08
x 143

Re: RichardW's Velocette rebuild

Post by moizeau »

Richard, could you explain the decompressor please. As I understand it a decompressor reduces the compression within the cylinder when the engine isn't running to assist with starting (making it easier to turn the engine over be it by foot or starter cord). My chainsaw has a small button that you press, when the engine starts the pressure within the cylinder at higher revs pushes the button back out closing the 'leak'. Briggs engines have an automatic decompressor that partially holds a valve open using a centrifugal device that runs off the crank. The reason I mention the Briggs engine is because of the auto decompressor you can't do a compression test on the engine without turning the engine over fast enough to release the decompressor.
If your cylinder was being tested with the decompressor working the figure won't be accurate.

RichardW
Forum Treasurer
Posts: 9485
Joined: 07 Aug 2002, 17:12
x 435

Re: RichardW's Velocette rebuild

Post by RichardW »

Decompressor is manual - here's a picture of the parts.
decompparts.jpg
The bit with the spring has a poppet valve in it. It is clamped into the cylinder head by the pipe with the operating lever on it. When the lever is pulled, the cylindrical block transfers the motion to the end of the poppet valve and opens it:
decompopen.jpg
Which then allows the compression to drop away down this tube to the exhaust port.
decompmount.jpg
If the cylindrical block is not fitted, then the valve cannot open, and you have full compression. I checked the compression both with and without the block fitted, and got the same answer.

I didn't measure the ring gaps - they were new rings, and pretty tight at the bottom. I can't get my hand in the bore to push them up, but perhaps no 1 son could!

Video coming later when it uploads, it's rather big...

User avatar
moizeau
Donor 2020
Posts: 1083
Joined: 12 Jan 2015, 17:08
x 143

Re: RichardW's Velocette rebuild

Post by moizeau »

So the decompressor, when opened, vents to the exhaust port via the pipe. The grub screw(?) above the decompressor port secures the valve assembly. The hole between the shoulders on the valve assy aligns with the venting pipe and allows the valve, when open, to vent internally through the valve assy to the pipe. The cylindrical block then screws in to the port and actuates the valve assembly?
What does seem odd is the result decompressed or not. This surely means that either the cylinder has got such low compression that that the decompressor makes no difference or the decompressor is leaking air when closed. There are two things that come to mind. A leakdown test, with the piston at TDC and the decompressor closed no air should escape through it's fitting port. Also you can hear where the air is escaping from. If you haven't got a leakdown tester you could make a disk (coin perhaps) that could block the internal opening of the port, sealed with a bit of RTV, and possibly refit the cylindrical block with a spring (that you've had in a box for years) to hold the disk in place to take the decompressor out of the equation.

User avatar
xantia_v6
Forum Admin Team
Posts: 7733
Joined: 09 Nov 2005, 23:03
x 448

Re: RichardW's Velocette rebuild

Post by xantia_v6 »

You can pretty easily tell if those simple decompressors are sealing properly be ust putting moderate pressure on the kick starter and see how slowly the air escapes to let the piston rise.

The decompressor is the normal way of stopping the engine (the kill switch). It can also be used to make the bike easier to push when puch starting, I never found them to be very helpful (on this type of engine) when using the kick starter.

My feeling is that although the compression on this engine seems low, they were never a high compression engine, and it is probably not what is preventing it from starting.

RichardW
Forum Treasurer
Posts: 9485
Joined: 07 Aug 2002, 17:12
x 435

Re: RichardW's Velocette rebuild

Post by RichardW »

If the decompressor is opened there's no compression at all. Taking the cylinder block out was to check that it wasn't being held open. Video of it trying with fuel direct into the cylinder might be instructive, I'll get it up tomorrow.

User avatar
Mandrake
Posts: 8287
Joined: 10 Apr 2005, 17:23
x 357

Re: RichardW's Velocette rebuild

Post by Mandrake »

Was that compression reading a dry reading or wet ?

A wet reading is taken after squirting a bit of oil into the cylinder from an oil can before the test...

https://www.freeasestudyguides.com/comp ... g-oil.html

RichardW
Forum Treasurer
Posts: 9485
Joined: 07 Aug 2002, 17:12
x 435

Re: RichardW's Velocette rebuild

Post by RichardW »

Since it's a two stroke there's oil everywhere, so that's a pretty 'wet' reading!

Videos of start attempts with fuel added direct to the cylinder

Attempt 1, flames and everything!



Attempt 2




My face at the end says it all. It's interesting to note that once it spits back from the carb, it stops making any effort at all, until more fuel is added.

Not sure what to make of that. It seems to 'run' but only really at kicking speed, but there's no response to the throttle. It's like it's not sucking fuel through the carb, but I can't see why that should be - that could be a result of low compression I guess...

Found some pics of my Grandad with various bikes

I'm pretty sure this is the Velocette - the exhaust pipes look right, being ridden through a Lakeland stream.
veloinstream.jpg
This is one of the Douglas's he had in the war (Edit - actually I don't think so - all the Douglas's were horizontal fore-aft twins, whereas this is a vertical single - I think it might be a BSA)
douglas.jpg
No idea what this is, or why it has lost it's rear wheel! Could be a candidate for what is this thread!! (Grandad on the right)
unknown.jpg
Last edited by RichardW on 09 Sep 2019, 07:24, edited 1 time in total.

RichardW
Forum Treasurer
Posts: 9485
Joined: 07 Aug 2002, 17:12
x 435

Re: RichardW's Velocette rebuild

Post by RichardW »

I'm reasonably sure it's the compression that is the problem - if the compression is blowing by the piston then it will end up in the crankcase, and hence pressurise it when it should be at vacuum sucking fuel in through the carb. I can check this I think as when I put a bag over the carb it was blowing it out (but not sucking it in) if I check when it's blowing vs where the piston is, then I should be able to see if it's not doing what is supposed to be when it's supposed to be, IYSWIM!

Anyway, if it can be sleeved, then I've decided to get it done - it looks like I can get a std or +20 piston for not too much, so I will probably get a new piston and get it sleeved, that means if it wears out at some point in the future it will only need a re-bore and oversize piston, rather than another re-sleeve - and the current piston is not in the best shape!

User avatar
moizeau
Donor 2020
Posts: 1083
Joined: 12 Jan 2015, 17:08
x 143

Re: RichardW's Velocette rebuild

Post by moizeau »

Have you access to a leak down tester? Failing that, a converted spark plug and a footpump (or even a tight fitting rubber hose) and you would hear where the air is escaping. Prior to spending more money at it (although a std piston would be good for the future), have you found any documentation on the carb, just to confirm where all the internal passages are and what does what? Have you got any pictures of the carb, or the make / model?
I've just tried to start my TY250 for the first time in 2 years. It ran fine before but is having none of it now. I did a quick carb clean to no avail so it's a proper carb off job. Nothing else has changed, it can only be a blocked passage. If had bought the bike in this condition I could be led down a path of engine rebuilds, coils etc.

User avatar
moizeau
Donor 2020
Posts: 1083
Joined: 12 Jan 2015, 17:08
x 143

Re: RichardW's Velocette rebuild

Post by moizeau »

Just found this, it's on 'barnstormers' site. Although the GTP it may be of use? It does mention care when fitting the piston.
http://www.barnstormers.co.nz/barnstorm ... /GTP1a.pdf
On the same site is a list of part numbers.

RichardW
Forum Treasurer
Posts: 9485
Joined: 07 Aug 2002, 17:12
x 435

Re: RichardW's Velocette rebuild

Post by RichardW »

I could make up something for a leak down test, I will have a go - I am sure the (lack of) compression is the issue.

Carb is an Amal pre-monobloc Type 4. Details are here The only difference between the type 4 and type 74 as far as I can work out is that the 4 is brass and the 74 is nickel plated - in the 274 they moved to the pilot air being drawn in through the carb inlet (presumably so a cleaner can be added) rather than the 4 holes drilled around the body - but otherwise it seems to be the same. I've cleaned out all the jets and orifices, so I'm sure that fuel should be getting through. There was no pick up at all when I opened the throttle when it was coughing (I hesitate to say running!) on the direct added fuel, which makes me think there is limited crankcase vacuum caused by blowby. I said earlier that I thought the choke made no difference to the cold start at idle position, but it looks in fact like there is a bypass hole under the carb slide which sucks in more air - and would be affected by the choke. Again, I'm sure this is clear.