FH - update

 The FH has been the go-to bike recently, and it is shaking down again after it's 500 mile service. I'm getting on top of the carburation helped by Hitchcock's (or Amal's?) guide to tuning the Mk1 Concentric, and now that I have it ticking over with a #30 pilot jet and the pilot air screw 1/4 turn out, I might go to a #25!

It's flying along and looks great:

Notice that the rear wheel is nicely off the ground in these pictures!

The only problem is - it's still burning oil. The head is going to have to come off again to inspect the guides which is a shame, because the leaks are cured...

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.

It's still got a size 30 pilot jet, and for the record, the pilot air screw is about one quarter of a turn out. Perhaps I'll go to a size 25 pilot jet? 

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 - 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.

FH - pilot circuit

 No pictures, but this afternoon I've drilled out the fixed size 20 pilot jet in the Huntmaster's MK1 Concentric carburettor.

It's now got a size 30 pilot fitted, which reflects the size 35 specified for the BSA Road Rocket (fitted with a Monobloc) and the fact that Concentrics are supposed to run richer than Monoblocs...

Time for a road test.

FH - getting there

 A few minutes in the workshop moves the project on a little bit: the speedo drive locating pin has arrived from the AOMCC gearbox man:

It fits in this hole:

A look at the carburetter reveals that the float chamber gasket is actually there, but that the drip shield is a little distorted and a bit loose, which probably caused the spitting back. The main issue, staring us in the face (and highlighting the perils of long-term projects) is that the carburetter needle is still in the box of bits!

The engine idles now.

By the way, I have changed the 210 main jet to a 260:

A couple more problems are revealed - the inlet rocker oil banjo is loose, and the magneto earth button doesn’t work. The inlet banjo is an easy spanner job; the cutout requires a bit more investigation.

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:

FH Dry Build - Carburetter

The FH kit arrived all those years ago with a brand new Amal Mk1 Concentric, a 928 and a carburetter that I have no experience with whatever, but I am not going to delve into that now. Today I am only concerned with mounting it to the head.

The standard carburetter for the FH is a Monobloc, retained with two 3/4" hexagon bolts with 5/16" CEI threads. All well and good, but the concentric has a rather large tickler button which prevents you getting the bolts in...

No matter, I can make a couple of studs on the lathe. These will be 5/16" CEI both ends, about 1 1/16" long with about 3/16" plain in the middle. Here they are, part way through manufacture:

I bought a new alloy bell mouth, which is very nice. The rubber fits neatly inside it:

The next problem is that either the carb thread or the bell mouth thread is not to size, or is damaged. This is as far as it will screw on:

FH Dry Build - Barrel and Head

After completing the primary case installation, I moved on to the head and barrel. This was pretty straightforward, and I retained the head and barrel in place with a couple of bolts each, though the threads will need cleaning up when I rebuild the engine.

The main purpose of this step is to establish fixing points for the exhaust, carburetter and head steadies. The head steadies fit to two of the downward facing rocker box studs and its clear that the existing ones are fretted quite badly and will need replacing.

Secondly, the kit of parts I bought all those years ago included a brand new Amal Concentric Mk1 928 carburetter so it will be interesting to see how that performs - it will be a new experience for me. The FH was intended to have two bolts holding the carburetter in place, but unfortunately the large tickler fitted to the 928 prevents one of the bolts going in - it will need a couple of studs making up or I could use the 5/16" CEI x 1 1/16" stud specified by Ariel for the iron head singles under part number 0183.