Charlie's Shed - Concentric carburettor

 Since yesterday when I managed to accidentally drill out the pilot air screw thread on the 928, I've been thinking about how to approach carburetion on the FH. I fitted the Monobloc, which is a bit worn particularly around the mixing chamber cap screw and I realised that I would need to alter the cable lengths to use it. 

Some more thought had me realise that I could repair the damage to the 928 and have a better solution for my non-standard Huntmaster more quickly than messing about with a Monobloc. 

The first thing I did was to tap out the new pilot circuit access ports. The one in the inlet taps out to M5, but the one opposite the pilot air screw is a bit more tricky in that it's too big to tap to M6 and the wall thickness is too small to tap to M8 - but it is perfect for M7. Try buying an M7 grub screw! However, I do have M7 by 1.0 taps and dies by some streak of good fortune and I have tapped that hole M7. 

I drilled out the pilot air screw thread with I think a 4.5 mm drill. I tried tapping it M5 but I guess the whole was not clean enough for that and I had to go to M6. 

The next go/no-go job was to determine whether I could make a successful pilot air screw with a larger thread. I grabbed a bit of 10 mm brass rod:

That turned out ok. It seals most of the pilot air port (you can see the witness mark) but I suspect the thread is bottoming out somewhere. Blowing past it is quite difficult, so it will probably be ok. I replaced the tired O ring.

The next job is to seal the opposite port. I made a plug, but unfortunately the cheapo M7 die I have had for years just won't cut and is now in the bin.

A couple of days later, equipped with a new M7 die I made a new plug:

This seals the cleaning port nicely:

Now we can put it back on and road test it.

FH - Monobloc Carburetter

 I've been toying with the idea of fitting an original carburetter to the FH for a while, and finally bought on on eBay - I actually ended up with two, having bought one, it turning out to be not as described and the seller not wanting it back. That was a 389; I also have the correct 376/12.

Having messed up the 928 by not paying attention (more of that later) I fitted the 376:

It's not in too bad shape. The mixing chamber cap retaining ring is a bit loose, and of course the slide is a bit worn but otherwise it's sound. It's more or less to spec - I can easily modify the #3 slide to a #3 1/2 with a file; I have a close-enough main jet. The 928 that was fitted had a 260 and was obviously 28mm - that's about 1 1/8" which suited the 'Road Rocket' specification to which the engine is built. 

	Monobloc 1	Huntmaster	Road Rocket	Monobloc 2
Part Number	376/12	376/12	376/36	389/12
Size:	1 1/16”	1 1/16"	1 1/8"	1 3/16”
Main Jet:	260	240	290	300
Pilot Jet:	25	25	35	30
Needle Jet:	106	106	106	106
Position:	3	3	3	3
Slide:	3	3 1/2	3 1/2	3

The plan is to see how this goes before we do anything else - it should give me a better base to get the carburation right. Unfortunately I will have to make the throttle cable shorter and the choke cable longer to make this work...

'Do anything else' could mean buying a 928 Premier; it could mean buying a new Monobloc.

And the 928 that came with the bike? It wasn't the Premier, so it had the built in idle jet. I wanted to change the pilot jet size and to be able to clean the idle circuit so I drilled out both the plugs that seal off the idle drillings, re-tapped one M5 and was considering tapping the other one M7, and making a special plug - unfortunately I took my eye off the ball, and thinking if I put the plug further down I could put an M5 plug in that side as well - and drilled out the pilot air screw thread. One useless 928 body, for which you can't get a spare.

Model A - front wheel

One of my key aims is to get the machine rolling so that I can get it off the bench when I need to, to get the others on the bench for servicing etc. To that end I sorted out headstock bearings and the wear in the forks, but of course we still have no tires and the wheels are not fit to have tires fitted.

I thought I would change away from my normal method of dismantling a wheel, involving bolt cutters, and see how feasible it was to dismantle it nipple by nipple. I sprayed each nipple with plus gas months ago and I was pleased to find them all come apart easily.

Before long I had a fully stripped hub and brake drum a pile of loose spokes, nipples and a rather rusty rim - a WM1-20 which might get reused one day if I can ever buy tires for it.

Despite worrying about whether I'd made the right decision with respect to the rusty nature of the rim, the spoke heads confirmed that rebuilding the wheel was a good thing to do. Several of the spoke heads are about to pull out of the hub:

After a short period of decreasing, scraping of general muck and abrasive removal of rust, I put on a coat of primer

Followed by a couple of coats of black gloss:

The black powder coated rims have been delivered from Devon along with spokes and nipples but we will need to wait something like 5 or 6 weeks for this paint to harden before we can build the wheel. More of that later!

Model A - dynamo drive

 I have been wanting to sort out the dynamo drive for a while - not having it in place leaves a lot of parts laying around on the bench that I want to clear up. It was hindered by having to make a special puller for the dynamo bearings, something that I use infrequently but is very necessary if you're going to rebuild one of these dynamos.

Having finished the puller, I spent a little while turning up a short spacer. For some reason the replacement armature is a little shorter than the original though it fits where it needs to!

And I needed to file up a Woodruff key for the dynamo drive sprocket. It actually took me a long time to get to this place because I had inadvertently assembled the dynamo and fitted it with the dynamo body the wrong way round which caused the sprocket to be something like a half an inch out of line!

Here's the drive in place

That was a chain in amongst the past that came with the bike - is the correct 3/8 inch pitch but the width is way too much. I'll have to look at simply bearings and see if I can find another one, but it really needs to be endless.

You can see here that this over wide chain will rub on the crankcase.

The thickness of the drive sprockets is 1/8" - so that tells us what chain we need.

Mini-lathe - front stay spacers

 The mini lathe proved its worth again today and gave me two useful parts from some scrap.

These are 3/4-in diameter by 3/16-in thick spacers for the Model A and the W/NG, with 6.5 mm holes to clear quarter inch screws.

Here is where they fit - what they do is give you a little bit more room between the fork leg and the mudguard stay to stop the brake rod rubbing.

They will be chemically blacked in the next batch.

FH - insecure centre stand

 Some while back I wrote about adjusting the stops on the centre stand to get the wheel off the ground and to enable the bike to roll on and to roll off the centre stand relatively easily. A few months using the bike has made me realise that it's far too easy for the bike to come off the stand and since my son has had it he's remarked on it a couple of times.

I need to do something about this. 

I recall that when I bought the shock absorbers I was encouraged by fellow riders to go for a longer shock absorber option to make the bike easier to get on and off the stand, which it does. The bike is actually fitted with 310 mm shock absorbers - the standard is 300 mm, or more probably 12 3/4".

Stripping one of the shock absorbers finds an M10 by 1.25 pitch thread on the damper. An option would be to increase the length of this thread and then chop the excess 10 mm of the end of the damper rod. This won't be too much of a problem except that I don't have the suitable die and if I can't remove the rod from the damper I will have to cut the thread with the die but leave no undercut which will be untidy. I can't remove the rod from the damper, because the top of the cylinder is rolled over.

My first step was to ask the supplier if I can buy the damper units without the rest of the shock absorber, and these turned up for about half the cost of a new set:

The next step - and it's never straightforward is it - is to modify the spring compressor. The problem is that the compressor now has to reach so far down inside the spring (because the damper is shorter) that I can't get the pin in through the damper eye. 

I sorted that out by chopping one end of the pin off, trimming the end and welding a washer on the end to act as a head. 

Next I rebuilt the shock absorber and then turned my attention to the other. With that done I could remove the centre stand and adjust the stops. There's not much to see with this operation apart from my grubbing knees elbows and nose. Doing this job on the side stand is ok for the right hand side of the centre stand but getting to the left is very difficult, especially when the temperature is in 30s!

I ended up taking 1/16" off both stops which resulted in a bike that needed more effort to get off the centre stand. I noticed that the stand feet are wearing already and will need some attention in the next year or two.

Square Four - another clutch

 Well, I recorded in my last post that the Square Four clutch was slipping badly, so badly that as I rode it up the hill back to the workshop the engine speed was soaring and it was clear that the clutch was on its last legs.

I've said many times before that these bikes are very easy to work on. One thing I've always done when accessing the clutch is removed the footrest - but actually you don't need to do that. All you have to do is remove the nut and rotate the footrest on its mounting out of the way - you don't have to break the taper at all.

Even before I dismantled the clutch it was clear what happened - you could smell the burnt cork and there were bits of it everywhere. Stripping the clutch down revealed carbonized cork inserts - I could put my finger through these and they would just disintegrate.

Fortunately I had a spare set of 90 corks - that's enough for three plates - sitting in the workshop. Once I'd spent a couple of hours cleaning the mess off the plates I put these in boiling water for a few minutes.

You can push them in with your fingers - or a bit of wood is sometimes useful. It does make your fingers hurt after doing all of them, but it doesn't take long really.

I thought there was one missing - so I rescued a wine cork from the bin.

It's easy to slice these up into the right shape.

As it happened, as soon as I started clearing up the plastic bowl I've used to soak the corks I found the missing one so I didn't end up using my homemade one.

A couple of hours work had all the plates re-lined.

When you put new corks in, I always fit them to the bike wet. When you initially fit the plates they will only just be retained by the basket slots but the next day you will find they have settled a lot. You can see in this picture that the first friction plate is now well into the slots - this is about 24 hours after reassembling the clutch and all the corks are now dry.

I tested out a couple of days later, and readjusted the cable. The bike started straight away though as normal the clutch was stuck - I have always had to free the clutch before starting this one.