Wristpin

I thought that I'd replied to this question but if I did it has disappeared - perhaps I dosed off without pressing Submit!
Just seems to be rather an odd collection of parts, most of which I would have thought would be obtainable nearer to home - am I missing the point somewhere?
SUBMIT !!!. 

That's all good news. Interestingly I don't remember the single speed 21s being that fast  - but I was younger and fitter then!!  
 
EDIT. Just looked back over your earlier posts and don't see any images of the transmission gearbox input pulley - is it single or double?  It's just possible that you have one of the not very common two speed machines with   double crank and gearbox pulleys and a thin, toothed (and expensive) belt.  In 30+ years in the trade I only saw a couple of them. One was owned by an old boy with two "tin legs" who used it with a seat to drive the half mile through the woods to the pub having had his licence withdrawn.

Thank you for all your good wishes. Back to normal today!

Thanks, I'll take you up on that.  Not the guilty party!
Done all the mechanics including honing the bore and re-ringing, re-grinding the cylinder and bottom blade and repairs to the cowling and recoil.
 

 

That is just one smart bike!
Dare I post this ?
 

The fact that the "hand over"  treatment worked suggests an air leak somewhere.
The gauze ore cleaner shouldn't cause a problem as it was a standard fitment on Industrial Commercial spec engines. 

The fact that the "hand over"  treatment worked suggests an air leak somewhere.
The gauze ore cleaner shouldn't cause a problem as it was a standard fitment on Industrial Commercial spec engines. 

I do hate these guessing games, for *** sake post an image!
There were two machines known as graveyard mowers , the Hawk 12” and the Hawk Major 19”. Both had a friction disc with a bar blade.
 

I had one of those back in Lambretta days , 1958 ish. Seem to remember that it was a bit hit and miss and rather messy.  Haven’t seen it for a long time - may have gone awol when I had the business. Now days I just get a four inch length of the appropriate diameter clear fuel pipe and push it over the engine end of the cable, hold it up heigher than the lever end and fill it with air tool oil ( nice and thin) and work the lever.  If in a hurry, blow down the tube to help things along .
Back in the days of the original Austin and Morris Mimis the engine earthing wasn’t too good and the choke cables tended to warm up and get sticky when the starter was engaged.  . In extreme cases they did a full met down , glowing red hot.  Just might be worth checking the Black Horse’s engine earthing. An earth bond from the engine side of any mounts back to where the battery is earthed to the chassis can sometimes work wonders for cranking rpms.

I had one of those back in Lambretta days , 1958 ish. Seem to remember that it was a bit hit and miss and rather messy.  Haven’t seen it for a long time - may have gone awol when I had the business. Now days I just get a four inch length of the appropriate diameter clear fuel pipe and push it over the engine end of the cable, hold it up heigher than the lever end and fill it with air tool oil ( nice and thin) and work the lever.  If in a hurry, blow down the tube to help things along .
Back in the days of the original Austin and Morris Mimis the engine earthing wasn’t too good and the choke cables tended to warm up and get sticky when the starter was engaged.  . In extreme cases they did a full met down , glowing red hot.  Just might be worth checking the Black Horse’s engine earthing. An earth bond from the engine side of any mounts back to where the battery is earthed to the chassis can sometimes work wonders for cranking rpms.

I had one of those back in Lambretta days , 1958 ish. Seem to remember that it was a bit hit and miss and rather messy.  Haven’t seen it for a long time - may have gone awol when I had the business. Now days I just get a four inch length of the appropriate diameter clear fuel pipe and push it over the engine end of the cable, hold it up heigher than the lever end and fill it with air tool oil ( nice and thin) and work the lever.  If in a hurry, blow down the tube to help things along .
Back in the days of the original Austin and Morris Mimis the engine earthing wasn’t too good and the choke cables tended to warm up and get sticky when the starter was engaged.  . In extreme cases they did a full met down , glowing red hot.  Just might be worth checking the Black Horse’s engine earthing. An earth bond from the engine side of any mounts back to where the battery is earthed to the chassis can sometimes work wonders for cranking rpms.

If you think that the yellow wire is the kill, check it out . With the ignition OFF you should have continuity to Earth . With it ON , it should be open circuit. When you say "wont start" do you mean totally dead or "cranks but wont run" ? 
The danger is that if you’ve got things wrong it’s possible that you may have put voltage through the ignition coil and cooked it, so the first thing to do is to isolate the engine. Disconnect all wires other than the heavy one from the solenoid to the starter motor and crank it over but be prepared to pull the plug lead off to stop it.  If it starts it at least shows that you have a runner and can relax and sort out the stopping!
I will dig out a wireing diagram but normally the kill wire will go to a terminal on the throttle plate and will have a loop terminal under a nut.
The 8 hp lump may have a single charging wire with a “ tubular” connector or a dual circuit  alternator with two wires and a double pin connector that can only be fitted one way round. 
 
 
EDIT added
https://www.dropbox.com/s/lgchle1pohn1r1z/Lawnflite 504 wireing diag0001.pdf?dl=0
Yes, the yellow is shown as the kill wire. As well as going to the kill terminal on the ignition switch It is also connected to a couple of safety switches which will also prevent the engine from firing unless in the safe position.

Possibly has a pulse tube from the crankcase.

Little blue granules?

Not quite so, you can retime the ignition using the Before Top Dead Centre figure from the engine spec in relation to the points starting to open. I believe the setting for a JAP80 is 25degrees or 7/64" BTDC. Points gap 18thou and plug 25thou

Exercise caution if you split the box, they are usually held together with steel taptite self-threading screws. Steel into alloy, plus moisture and the passage of time equals seized screws and a possibility of sheared screws; at which point the law od Dr Sodt takes over.  

Exercise caution if you split the box, they are usually held together with steel taptite self-threading screws. Steel into alloy, plus moisture and the passage of time equals seized screws and a possibility of sheared screws; at which point the law od Dr Sodt takes over.  

Setting up those clutches is a bit of a black art. In the 80s the roving Hayter Service man used to have an Osprey / 21 axle assembly set up on the bench in his high top van and ran an impromptu service course on site.
There should be a thick friction lining bonded to the large sprocket. I believe these are still available from an aftermarket supplier of friction materials.
However, from your image, it appears that there is a lot more to be done. Looking at the relationship between the bush and the axle it appears that one or the other - or both, are badly worn. I think that a complete strip and assessment of the axle is required.
 
https://www.dropbox.com/s/i5ys5rh1mdtrwc4/Hayter 21%2C Ospey Clutch adjustment0001.pdf?dl=0
 
https://www.dropbox.com/s/a2gbo2quuaymiea/Hayter Osprey and 21 0001.pdf?dl=0

Setting up those clutches is a bit of a black art. In the 80s the roving Hayter Service man used to have an Osprey / 21 axle assembly set up on the bench in his high top van and ran an impromptu service course on site.
There should be a thick friction lining bonded to the large sprocket. I believe these are still available from an aftermarket supplier of friction materials.
However, from your image, it appears that there is a lot more to be done. Looking at the relationship between the bush and the axle it appears that one or the other - or both, are badly worn. I think that a complete strip and assessment of the axle is required.
 
https://www.dropbox.com/s/i5ys5rh1mdtrwc4/Hayter 21%2C Ospey Clutch adjustment0001.pdf?dl=0
 
https://www.dropbox.com/s/a2gbo2quuaymiea/Hayter Osprey and 21 0001.pdf?dl=0

Another example of different terminology from across the pond!
Not just for concrete or stone, with the correct disc, steel and other metals as well. 

What we call a disc cutter. Take care, they can bite.

When I had the business we used to put the purchaser's postcode on various places on machines - an obvious couple to be found and some not so obvious ones to hopefully remain hidden.
We had a ride-on in for service from a new customer some 30 miles away and one of my mechanics called me into the workshop querying the postcode on the job card with one of the ones he'd found on the machine. We had sold the machine to an address a couple of miles up the road and it had been stolen three years previously. To cut a long story short it turned out the present owner had bought it for cash from "a man in a van" and his father in law had also bought one from the same man  - also stolen!
 

I've always been an advocate of "old oils for old engines" - most of which in the UK are Villiers with plain bush main bearings and no oil seals. The extra "body" of straight 30 grade does appear to be beneficial in sealing the mains. Not sure where the OP was posting from but in the UK there is no problem in obtaining straight SAE 30 or 40. 30 is often sold as Lawnmower oil, locally Tesco, B&Q and Halfords have it. it's also sold as Classic Car Oil. Millers and Morris lubricants both sell a "classic bike mixing oil".
 
On a slightly different track, the manufacturers of Aspen alkylate fuel have been promoting it as having less carcinogenic properties together with resistance to the worst effects of ethanol and reduced shelf life. Then, a couple of weeks ago, the UK distributor of Cobra chainsaws issued a service bulletin that on no account was an alkylate fuel to be used in one Cobra model as it had been found to cause piston and bore failures.  

I've always been an advocate of "old oils for old engines" - most of which in the UK are Villiers with plain bush main bearings and no oil seals. The extra "body" of straight 30 grade does appear to be beneficial in sealing the mains. Not sure where the OP was posting from but in the UK there is no problem in obtaining straight SAE 30 or 40. 30 is often sold as Lawnmower oil, locally Tesco, B&Q and Halfords have it. it's also sold as Classic Car Oil. Millers and Morris lubricants both sell a "classic bike mixing oil".
 
On a slightly different track, the manufacturers of Aspen alkylate fuel have been promoting it as having less carcinogenic properties together with resistance to the worst effects of ethanol and reduced shelf life. Then, a couple of weeks ago, the UK distributor of Cobra chainsaws issued a service bulletin that on no account was an alkylate fuel to be used in one Cobra model as it had been found to cause piston and bore failures.