FH - carburation

 For the record, at the 500 mile service I have changed the carburetter slide on the FH for one with a #4 cutaway.

A #3 1/2 was previously fitted, and the plugs were quite dark.

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 hole 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 - carburetter investigation

In a spare moment while waiting for parts I thought I would have a look at the carburettor provided with the Model A kit. I've not touched it since it arrived all those years ago:

In the last couple of days I have removed it and giving it a good clean so that I can discover what's missing and what's broken.

It's the original carburettor and is pretty worn but it cleans up well:

Standard fitting for the 1930 Model A is a 6/112 carburetter, now called a Pre-Monobloc, which is 1" throat diameter, and here is the evidence:

According to the Amal book, jetting is as follows:

• Size: 1"
• Main Jet: 4/042-160 (still available new)
• Pilot Jet: none
• Needle Jet: 4/061-106 (still available new)
• Needle: 6/065, position not recorded (still available new)
• Slide: 6/0524 - that's a #4 cutaway, or 1/4"
• Float chamber: 14/004

The slide is really bad - probably the worst wear I've ever seen, but it's the original #4 slide. The grooves around the top of the needle are in pretty poor shape too - we should expect this and the needle jet to need replacement

The choke slide spring appears to have been cut short - fortunately I found a spare one in the carburettor bits box.

The float chamber appears to be correct - I didn't expect the bowl to be held by those two brass fittings at the bottom - there must have been a gasket in there

Initial observations:

• The main jet is missing
• The needle looks very tired 
• If the needle is tired the needle jet will need replacing 
• We probably need another cable adjuster 
• The slide is very bad which probably means we need a rebore from Joe the Carb.
• We need to understand how the float chamber fuel feed was sealed - possibly by some sealant in the thread

That's it for now - it's back on the bike and we can start some other jobs.

FH - test ride

 I took the Huntmaster out on a nice little test ride today, 25 miles around North Norfolk to buy samosas from a nice family run department store in Holt.

It went beautifully - no problems other than the usual sweating from the base gasket! I think we also have to admit that the number two exhaust guide is probably leaking - probably too much clearance. There's wet oil at the silencer.

It's idling very nicely - I stopped to reduce the speed at one point, and to take a few pictures, and the idle is certainly more reliable than it was.

W/NG - on the bench

 With the Square Four away it's time for the Huntmaster to step up and become go-to bike. The W/NG is going on the bench for a bit of a look around and some little jobs .

It's quite oily and I know that the decompressor is still leaking. What I didn't expect was to see that the leak appears to come from the main decompressor body and not from the spindle - it appears that the Dowty washer isn't doing its job and perhaps that is because the mating surface isn't as flat as it might be. I've cleaned up the nut and mating surface and replaced the Dowty washer with a new one, as I don't have a fibre washer of the correct size.

I've also replaced the (missing) gasket in the exhaust inspection cap - see that drip?

On a club run a couple of years ago, the horn fell off and it's retaining bolt was replaced by a 5/16 screw from the spares kit. I've replaced this with the proper 3/8 screw and I've also moved the horn bracket a little bit so that the terminals don't touch the engine plates and the horn body doesn't press the cable against the frame. The horn is now free to vibrate properly.

I've taken the opportunity to reroute the cable as well.

There are always tiny little things that bug you about a bike, particularly one which is in patinated condition like this. The washers under the headlamp bolts were very large and unsightly, being chromed. I've replaced these with some rough old washers of the right size which look a lot better.

I've stripped the carburetter and cleaned the idle ports, but I didn't really find anything.

In other news, the Huntmaster has most definitely wet-sumped. There's probably a pint of oil here in this pan which I drained from the sump, being unable to start the bike. You could feel the oil in the crankcase being churned up when you kicked it over.

SQ4 - a useful modification

 I've been fiddling about with the ignition timing this week as there had been some significant hesitation coming off-idle and some poor performance at low speed - what we used to call 'kangaroo petrol'.

I retarded the timing a bit and realised the bike had stopped kicking back on startup, and the slow running was better, and it would fly along like a bat out of hell - but coming off idle still wasn't as smooth as I'd like.

In the end, after fiddling with the idle mixture and getting nowhere, I stripped the carburettor.

I didn't find much. Some debris in the float chamber, the flange fixing to the head looser than it should have been, and one of the float chamber screws was loose. I blew out all the jets and ports with carb cleaner and found everything clear.

Putting it back together, I added this little tee to the idle mixture screw to make it easier to adjust (it's a bent bit of brazing rod, soft soldered to the brass screw), and used some Threebond on the flange.

We'll see what happens.

FH - weekly maintenance

 Just a little trip out this morning to pick up a part for the boiler from the Cromer plumbing centre. The bike is running very nicely and I managed to start it without the centre stand.

It's feeling a little bit squirmy so I decided to check the tyre pressures - the rear was a little bit down so we'll see what happens. 

I also put some tape and some marks to the throttle to help me set up the carburetion in the future.

This is the more traditional version of the Amal carburettor tuning chart showing us which elements of the carburettor affects tuning at various throttle openings:

Here is a more modern version, showing us a bit more detail and the overlap between the effect of the different elements:

Aside from practical adjustments like needle positions and the setting of the pilot jet, there are a few things we can change: 

• The main jet - available in a myriad of different sizes.
• The slide cut away - various sizes are available both larger and smaller.
• The needle. Most four strokes used the same needle, but there is a 'lean running' needle (part number 104) which is longer and thicker and clearly allows the engine to run with less fuel at higher RPM.

As the engine gets closer to being run in will be able to do some plug chops to determine the mixture at various throttle openings. At the moment, with only 138 miles on the clock what we can do is quite limited but I think we can look at perhaps the first half of the throttle opening range.

FH - a little more progress, and a big step...

 Well there's been a little bit more progress today. First off, I've finished the second of the finned exhaust clamp bolts:

Secondly I've been talking to people in the club about tyre sizes and whether I can get my desired tyre (Avon RoadRiders) in the size that I want. Having a look at the distance between the tyre and the chain guard reveals around 3/8" to 1/2" but also that this tyre, an Avon SM Mk11, is actually a 325-19. The front tyre, a Dunlop TT100 by the way is a 90/90-19 or 3.60-19. They are both from the 1980's.

I am hoping to buy Avon RoadRider tyres but unfortunately they are not available in the right sizes for this bike. The options appear to be to use a 100/90-19 for the rear (Avon's recommended replacement for a 350-19) and a 325-19 (original size) or a 90/90-19 for the front.

The next bit of progress is very small - it's just a P clip for the front brake cable. I've attempted to adjust both brakes but right now the front tyre is in the wheel clamp.

Lastly and very quietly and secretly I have hooked up the temporary fuel tank to the carburettor and started the bike. Unfortunately it will only start using Easy start, but at least it starts and runs. There may be some problem with the fuel supply or with the float bowl. It runs and returns oil and it runs nice and quietly so now I know that I have the HT needs the right way round, and the oil system is working properly, and the valve timing is correct and the ignition timing is more or less correct though it does spit back through the carburettor.

I think we'll have to wait 'til Monday to have another chance at it, but sleeping on it and looking at some of my pictures suggests that the float bowl gasket is missing. This will give you an air leak in the pilot fuelling circuit...

FH - inching closer...

 The day seems to have disappeared again without too much workshop time but at least I've got a little bit done. 

The choke cable is now terminated to the right length, with a new nipple at the carburettor end. I didn't have the flat nipple that goes into the lever unfortunately and I didn't fancy making one on the lathe but I did have a little tiny one that goes in the choke slide.

There's not much to see here but at least we have a throttle slide and a choke slide that move independently of each other and don't have any slack in the cables.

The next little job is to make a couple of 5/16 CEI bolts to retain the finned exhaust clamps. Here I'm reducing some 0.445" stainless hex bar to the appropriate diameter for the bolts I need.

They're going to have a chamfer feature where they mate with the clamp - here is the chamfer on the clamp itself. When I have cut the thread on the bolt I will work out what the height of the bolt head above the clamp face needs to be:

That's all for a bit. I've got a 10-hour turn on the railway tomorrow which won't finish until after 11:00 at night, and then my grandchildren are coming for the weekend. Hopefully we will get the speedometer in the post either tomorrow or Saturday, so they'll be plenty to play with next week. I'll order the V-Reg2b voltage regulator as well, so we can convert the RB108.

FH - various jobs

 There's lots of little jobs around the bike to be getting on with. This one is the dynamo cork gasket which I bought from eBay. It was a little big and needed 3/8" taking out of the circumference.

That's the dynamo chain in place. I had one with a split link which I think is unusual - but I think that this is the original chain. I replaced it with an endless one from Draganfly. It's very difficult to tension because whilst you can turn the dynamo to tension the chain, as soon as you do up the clamp it pulls the dynamo back into the crankcase and loosens the chain again...

These are the clamps for the head steady. I've fitted them in place with some leather shim between the clamp and the frame to protect the paint. I'll leave these here for the moment until I have made the new studs for the rock box, and then I can fit the head steadies.

I bought all the cables as universal types from Draganfly. When I unwrapped them I was pleased to find the air and throttle cables were complete with their nipples fixed in place at both ends. Whilst the throttle cable fits ok, the choke inner cable is much too long so I will have to move the nipple.

These two pictures show the routing that I have adopted for the clutch cable. I've shortened the outer to suit this routing, and when the gearbox is back together I will fit the nipple.

I've made up two gaskets for the carburettor and the heat shield. 

Perhaps inevitably, the throttle stop is missing from the twist grip. As soon as you slacken the cable these loose stops fall out and can be easily lost. I'll make one on the lathe later - perhaps two, one for the toolbox as a spare.

SQ4 - idling and a idle mind

 Some while back I had taken part in a forum discussion about a Square Four with a poor hot idle, in which one of the gurus had drawn attention to the absence of a critical notch in the only gasket specified for the Solex. Yesterday, I was out on a little tootle around Holt and Cley, and boy was it hot. Of course, coming through town on the way home you remember that it doesn't idle very well when it's hot.

A few minutes searching the AOMCC forum archive revealed the post I'd contributed to in 2018, where the notch-less gasket problem had been explained. I pulled the Solex apart this morning and lo & behold, no notch:

Furthermore, you can see how the gasket has been squashed into the slot, obstructing it further. It was a simple matter to cut the notch:

Starting the bike up, it seems to start easily and idles happily - but that might have as much to do with resetting the point gap from 5 thou back to the regulation 15!

FH - carburetter

 If any of you recall this post from last year, you'll know that the Huntmaster kit arrived with an Amal 928 Concentric carburetter:

Now that we are getting around to fitting the cylinder head, we'll need to sort out the jetting for this carburetter. A plea to the AOMCC forum revealed that revered Huntmaster rider Steve's FH is fitted with a 626 Concentric with a 200 main jet and a 3 1/2 slide with the needle clip in the middle position.

The difference between a 928 and a 626 is the body size - the 900 series is the larger. The 600 Series comes in 22mm, 24mm, 26mm and 27mm bore sizes and the 900 Series in 28mm, 30mm, and 32mm bore sizes. Presumably a 626 has a 26 mm throat and a 928 has a 28 mm throat; for the record, 28 mm is just over 1 3/32".

As you'll know from this post, this FH is built to Cyclone spec, more or less equivalent to a Road Rocket. Later Road Rockets use a 1 1/8" Monobloc; earlier ones use a 1 1/16" 10TT9 with a 340 main jet, so this 928 should be there or thereabouts but jetting it might be fun. We learned from the forum that Concentrics run about 10% richer than Monoblocs, so we should fit 10% smaller jets and a slide one size smaller.

Standard fitting for the 1958 FH is a 376/12 Monobloc, which is 1 1/16" throat diameter. Jetting is as follows:

• Size: 1 1/16"
• Main Jet: 240
• Pilot Jet: 25
• Needle Jet: 106
• Needle Position: 3
• Slide: 3 1/2
• The carb parts list indicates it uses ferrule 6/132A, which means it doesn't have cable adjusters.

Standard fitting for the 1958 Road Rocket is a 376/36 Monobloc, which is 1 1/8" throat diameter. Jetting is as follows:

• Size: 1 1/8"
• Main Jet: 290
• Pilot Jet: 35
• Needle Jet: 106
• Needle Position: 3
• Slide: 3 1/2

The kit 928 R300 is set up like this:

• Size: 28 mm or 1 3/32"
• Main Jet: 210 (I've got a 260 to replace this)
• Pilot Jet: None, drilling only; apparently 0.0016 (equivalent to size 20) but I understand these can be drilled out and a jet fitted
• Needle Jet: 106
• Needle Position: 3
• Slide: 3 1/2
• It hasn't got any cable adjusters.

Apparently you can fit a pilot jet in the threaded port shown here, on the right & under the bell mouth:

There's an excellent page on the Amal website that tells you all you need to know. It's here, and it revealed that the needle I have is the correct one for a 4 stroke 600/900 series carburetter:

QR50 - a bit of a service

 After three years in the field, the little beast is back. The boys are complaining the bike has no power after ten minutes running and with busy lives, my son and daughter in law have no time to check it out - so it’s back to Grandad’s workshop.

The Square Four is now off the bench to test the kickstart against the clutch, and then close up the clutch, refit the exhaust and test ride it. All went well with that, so the QR can go on the bench:

The first thing to attend to, and rather alarming, is the fact that the fork top threaded inserts are both loose. Now, these had a ring machined into them which allowed them to sit in the top of the fork leg, which I welded into the fork leg. Now, there is no evidence that the ring was anything other than an interference fit around the insert - indeed, there is no evidence that the insert was anything other than an interference fit in the fork leg.

Now, the inserts are retained in the fork leg by two strong plug welds on each side of the leg:

Next job, the tank. The wrap hasn't survived very well on the tank - the shape is small and complicated. I've pulled it off and treated it with some special plastic primer:

The primer is actually clear - ignore the grey bit, that was an experiment with the ordinary PlastiKote primer to test adhesion: it doesn't work very well, but the special stuff is excellent. This RS Paints Honda Helios Red doesn't come off:

The lack of power appears to be down to the choke - the choke cable goes into the carburetter through a plastic nut which has broken, allowing the choke cable to come out. When it comes out, it appears to operate the choke such that the mixture is rich all the time, even with the choke knob pushed in. That will certainly lead to a lack of power.

The part of the nut that was broken retains the boot, which in turn retains an elbow and the cable.

I turned a new part up from brass and pressed it into the old nut, which I had bored to suit. It starts very well:

As you can see at the end of the video, it dies if you fully open the throttle - the kids were complaining that it didn't have much power. The float height is bang on:

It appears after a lot of searching and cross checking Honda part numbers that a Keihin PA26 carburetter with a #78 main jet is correct for all years of QR50 production. This is the main jet out of mine, which is marked PA 28D but still has the 12.2 mm throat of the correct carburettor:

A #58 is not a #78! It looks like it gets very weak at full throttle, something born out by the colour of the plug. I've made a new carburettor spacer while I wait for the new jet:

A few days later, the new jet arrives as expected - but it's the wrong size. Some of these little carbs use a jet with an M7 x 0.75 mm thread, and these are available; unfortunately mine uses an M6 x 0.75 mm thread and these are not available. So what I did was measure the diameter of the new #78 jet using a range of small drills - I found that a 0.8 mm would not go in, but a 0.75 mm would; the #58 would accept a 0.55 mm, but not a 0.6 mm. I concluded these Keihin jets are identified by their diameter.

One option would be to put the new jet in the lathe and reduce the diameter, and I probably would have gone down this path had I possessed a M6 x 0.75 mm die (it's a fine pitch) but I didn't fancy buying a die or setting up for single point threading, so I did something that ideally you should never do and drilled the old #58 jet out to 0.75 mm, which might leave it a bit lean.

It doesn't run out of breath at high rpm any more:

It's got a new set of tank decals too: