SQ4 - a little investigation

 Of course, a broken motorcycle can’t be left for too long, especially when your daily rider has been persuaded from your grasp.

There were a few problems to investigate, possibly associated:

• There was a carburetter leak, or possibly flooding 
• The battery indicator was showing far too much red, considering the 60 miles the bike traveled yesterday 
• The bike refused to start twice yesterday, needing pushing to the top of a hill and roll-starting 

Putting the CTEK on the battery had it charging normally, but it was very flat:

But why so flat? According to the ammeter, the battery does charge and indeed, it was charging on the way home. I did notice the ignition had been left on as Tom had complained the engine wouldn’t idle and it had died when he arrived. I currently believe this has more to do with the battery than anything else.

There is no leakage from the battery with the engine not running and you can measure normal currents with lights on, though with the LEDs fitted to this bike these are minimal. The ignition circuit draws 400 mA without the engine running - I think it would be a good idea to try this with the engine running.

I've also realised that the red spot ammeter doesn't show very much deflection at all - I calibrated the one on the Huntmaster against my bench power supply and there is a lot more movement. I wonder if the Square Four has a 30-0-30 ammeter.

A simple test with a 35 watt bulb revealed that the ammeter appears to be 15-0-15 - while I was doing this test the multimeter was reading 5 amps.

As a first step towards sorting the fuel leak, I removed the drain plug/main jet holder and finding no new fibre washer of the right size, sealed it with a wipe of Threebond 1215. That fixed the leak, and there is no evidence of flooding. 

Amelia started first time like she always does.

As to the idle, I don’t know yet as I haven’t got it warm enough to close the bi-starter.

SQ4 - idle mixture screw

 I recently took a punt on an eBay sale for a mixture screw for a carburettor from a Rolls-Royce B40 engine, the one used in the Austin Champ. It was listed as a Solex part and looked very much like the one on the Square Four.

I struck lucky - the parts are exactly the same.

This is the old one, and measures 4.86 mm at the end of the thread

This is the new one, which measures 4.92 mm at the end of the thread:

So perhaps not a huge amount of wear. It does appear though that the nose of the old screw is notched - I don't know if you can see it in this picture:

I fitted the new screw with a t-bar and put it back in the carburettor with a wrap or two of PTFE tape. 

We'll have to see if it makes a difference.

SQ4 - idling

 The Solex is back on, but I still have a problem with idling - but it's different. The bow in the inlet flange that I removed has changed it's behaviour at idle, as you might expect, but in tuning the idle I've realised that the idle volume screw is very worn. 

I've ordered a new one:

While I wait, I've wrapped the old screw with PTFE tape which should help.

SQ4 - Solex again

 Some while back I retarded the timing on the Square Four and improved starting greatly, but it is still running roughly coming off idle and when coasting. Nothing I do to the carburettor seems to improve this so I have resolved to have another look. 

Stripping off the carburettor is quite easy and another inspection revealed nothing new. However looking at the gasket faces (something which I would do every time with an Amal carburettor) revealed a bit of a surprise: 

You can see here, by the dark line along the edge of the flange that it is by no means flat, which can lead to a weakening of the mixture. A few minutes on some 200 grit wet and dry paper revealed this:

A few minutes more work improve that even further. 

The internal mating face between the throttle body and the float chamber and jet castings was nothing like as bad:

This face did show a little distortion, but it was easily removed. You will note that one of the throttle disc screws is missing - I removed that one but as you can see the other one didn't come out without a fight. The reason I removed it is revealed by this picture: 

See the light around the edges? This is ostensibly completely closed! What I needed to do therefore was to remove or loosen the butterfly screws and realign the disc so that it was properly closed. In reality it's not essential that it closes because the throttle stop is there to provide some idle speed and the butterfly will always be partially open. This kind of leakage will prevent you idling as slowly as you might want to so it pays to reduce the gap as much as possible. I replaced the screws and got a marginally better result:

I will work on this some more when the proper screws arrive. These should be raised countersunk M4 screws, around 10 mm long.

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!

Solex Idle Circuit

I've known for some time that I didn't have a drawing of the Solex 26 AHD on the blog. Today, coincidentally, someone was asking about the jet locations and problems with idling on the AOMCC Forum. I uploaded a drawing and Pete Silson provided some wise words, and an annotated the drawing to suit. Before we get to that, note that the fuel follows the orange highlighted path before it reaches the pilot air bleed jet, when the blue mixture path begins:

I don't believe the notch under the choke tube has any effect on tick-over. The four usual sites for blockage of the pilot mixture path are shown in the diagram below.

Air leaks anywhere in the mixture path (blue in diagram) cause erratic tickover. Blockage of the pilot jet (3) weakens the tickover mix, blockage of the pilot air bleed (4) richens the tickover mix. There are two small holes either side of the butterfly (1 & 2) which can also get gummed up - the size of these are important so best to blow them through with air rather than poking with wire.

The tickover mixture is governed by the ratio of the Pilot Jet 070 (0.7mm) and Pilot Air Bleed 150 (1.5mm). The tickover mixture is further adjustable using the Volume Control Screw - clockwise for weaker, anti-clockwise for richer. The tickover speed is adjusted by the throttle stop on the main butterfly.

The ratio of the Air Correction Jet and the Main Jet and the Emulsion Tube controls the mixture over the whole rev range. The Air Correction Jet has more influence at higher revs.

The jets are illustrated in this post. Apparently Mikuni were agents for Solex at one time and Mikuni jets fit these carburetters.

Square Four - Improving the mixture

 I learned from the folk on the AOMCC forum that the mixture can be adjusted by changing the float level, as in other carburetters. I hadn't thought of this approach with the Solex because the float acts directly on the valve and there is no arm to bend, for example.

However, the collective consciousness revealed that the float valves could be fitted with several washers if necessary, to lower the valve and hence the fuel level, leaning out the mixture. Like many others, I had replaced the fuel valve fibre washer when I rebuilt the carb, with a single washer of about 1 mm thickness, but apparently there can be up to 3 mm of thickness there.

Since my bike was using quite a lot of fuel and, judging by the state of the silencers was clearly running rich, I added a second washer. 

A plug chop revealed that the change had been a success. Previously, I had been running NGK B5HS plugs as my B6HS, equivalent to standard fitment, had been sooty. After the washer was added, I was able to revert to the B6HS and they were a much better colour.

Haven't you done enough shaking down?

So another day, another road test. After the weekend, where I checked out the B5HS plugs (pale grey centres, all the same - as expected) and fixed a very dodgy crimp in the distributor circuit and filling the tank, I took an early evening bimble along the lovely A149 North Norfolk coast road, into the setting sun. As usual, the SQ4 was fine for the first few miles, though I thought a little more reluctant to rev from a closed throttle than normal. We stopped for a break and a picture at Blakeney:





There's a narrow lane down to the beach here, maybe 300 yards down a hill which you do at 20 mph or less - idling, essentially. At the T junction at the bottom, there was a reluctance to rev and I coaxed the bike to a parking place where I enriched the idle mixture half a turn thinking that it had been reluctant to come off the idle jet since leaving the house.

A few more miles, and once out on the open road again she was flying. A little stumble now and then but generally great, until I got to a little village called West Runton, which has a mile or so of descent down a narrow lane into the village centre - again, 20 mph max, throttle closed, followed by a short hop over a railway bridge. By the time I was at the bottom, opening the throttle would kill the engine and there was no way I was getting over that bridge until the engine had cleared its throat. Once on the open road - more or less OK again. I'm convinced it gets very rich on the overrun, or descending hills on closed throttle.

So, back in the shop I am thinking that I have the idle mixture screw set way too rich - but it's only two turns out. My second thought is that there is an internal leak - possible in the bi-starter, or possibly the bi-starter is not closing fully.

We'll have to pull the carburetter off again. I found a useful site here, which takes a while to load but has some great pictures of a 26 AHD strip, refurbish and rebuild.

Solex 26 AHD data for the record...

For the record, the jets from my Solex 26AHD are recorded in this post. Here's a diagram to help identify them:

Starter Jet

Pilot Jet

Main Jet

Pilot air bleed

Air Correction Jet

Float Valve

Starter Air Jet

You can download a copy of the Solex Instruction Booklet here.

Solex Emulsion Tube Repair

Regular readers will know that I've had a few problems with misfires lately. I've cleaned the carburetter several times, reviewed and serviced the ignition system and fitted a tufnol spacer and a heat shield to reduce the carburetter temperature. This has worked to an extent, but I still have an occasional misfire. Cold, my 1951 SQ4 runs beautifully – it’s smooth and responsive. Hotter, it seems to stumble when opening up again after slowing for a roundabout or some other obstruction. This stumbling clears itself quite quickly, after which the bike will pull as it should. Occasionally you will get a cough or the feeling of holding back.

Stripping the carb revealed that the emulsion tube was not in great shape, it was slightly bent and has a long hairline crack along maybe half of it’s length. The bend is probably due to it's location and the fact that it is possible to remove the float bowl with the carb in the bike, which leaves the emulsion tube vulnerable - if you are not careful removing the float bowl, you can catch it on the emulsion tube and bend it.

One of my theories is that the area of the float bowl casting forming the well around the emulsion tube is still exposed to engine heat, though I believe I can grip this with an ungloved hand after a run. The tufnol spacer and the heat shield have made a huge difference to carburetter temperature.

The first job was to straighten the tube. I put the straight section in the lathe, and brought a dead centre in the tailstock up to the air jet, to indicate the eccentricity. A bit of judicious bending, guided by the centre, had the emulsion tube straight again.

This is the emulsion tube, after my wiped solder repair. That crack was along most of it's length which would clearly upset the mixture.

Now, David Jones has new emulsion tubes for £20 plus postage, so that would be the way to go. However, if this doesn't cure the misfire, and I think the misfire might be down to ambient temperature, I'll be glad I kept that £20 in my pocket.

We'll test it later this week. Ambient here is now anywhere between 10 and 18 C, so we may not be able to give it a proper test until next summer...

Debris in the pilot circuit

It's amazing what a small piece of debris can do. Coming back from North Walsham the other day, close to home, the SQ4 decided to die every time I closed the throttle, which made slow speed cornering somewhat unnerving and junctions, especially one on a hill, something of a strain. Ever tried to kick start a heavy bike while holding the throttle open and the front brake on with one hand?

Fortunately, the starter circuit and the main jet were fine, and I got home under my own steam.

Fuel for the pilot system comes from the float bowl and is controlled by a jet screwed into the hole in between the float bowl and the emulsion tube:

It passes into the large hole in the top of the carburetter:

And out of the round hole positioned at 10 o'clock in the square flange:

Then it passes through the gasket into the annular machined groove in the throttle body

The pilot mixture passes into the hole at 6 o'clock. From here, it goes forward to the idle screw, and passes around there into the engine when the throttle is closed; additionally, there is a hole near the butterfly which provides additional mixture from the pilot circuit as the throttle opens, to prevent a flat spot occurring before the main jet is active.

Stripping the carburetter down revealed a little gremlin in the pilot jet:

Poor focus macro picture of a jet with extracted debris

Quite enough to disrupt the flow of fuel into the pilot circuit. All fine now.

Solex again

Time to start finishing stuff off. I have some new carburetter parts from David Jones from Sleaford, who specializes in the repair of the Solex carburetters fitted to the SQ4.

It's the bi-starter control and the air bell:

All in pieces

Assembled. This is the bi-starter in the off position - that knob is a little long I think.

I have an insulator that I will be fitting as well.

It's a 3 mm thick insulator and it is manufactured from Silicon Resin Bonded Fabric. It is impervious to Ethanol and it is 50% more heat resistant than a standard Tufnol insulator and 100% stronger than phenolic. In an ideal world this would reduce the heat going into the carburetter body and reduce the high-temperature misfire.

In the real world it causes the float bowl to foul the distributor cap and is completely useless. But I did discover that when you inevitably drop the carburetter flange washers and they fall behind the dynamo, you can see them sitting under the dynamo from the rear, if you look across the top of the gearbox. I'm sure if I drop enough down there they will start coming out.

Just like the 2p falls at the amusement arcade.

A post from Mark Walsh of the AOMCC shows me that if an insulator won't fit on my machine, there is something wrong: there is a lot of space on Mark's machine, which already has an insulator:

Mark Walsh's SQ4

Here's the Solex back in place:

You can see I have changed the brass tag that is used to mount the throttle return spring. It now has several spring positions so that I can vary the torque the spring exerts on the throttle spindle - I want the throttle to spring shut, but I want a light action. The second spring position is good for that!

Here's a close up of an earlier version:

17 mile test run; fun with brakes

Lovely test run out to Bawburgh today, about 17 miles around the countryside. Bike is definitely running less hot, since after 8 miles I still had maybe 25 psi oil pressure at 30 mph, but that was dropping to 10 psi on the gauge at the end of the trip. I measured the oil temperature - 95 degrees C in the tank (about 200 F), which is much higher than I have seen after, say, a 5 mile trip.

As you can see the bike is on the sidestand - what I noticed later was that the carburettor drips fuel onto the dynamo in this position. I had left the fuel on though. 

The engine sounds happy enough; I'm not too sure about this gauge - I keep wondering how I would feel if it had never been fitted. I'm tempted to buy one of the club reproduction gauges, or to get this one calibrated.

Out on the road though, the Ariel was handling fine for the first few miles until I became aware of a juddering from the front end, which I initially put down to the road surface. On braking however I found no movement in the front brake lever, and realised the brake was seizing on. Near home, I stopped for traffic lights and saw smoke coming from the front brake (I've been here before, with the Bantam)...

Arriving back I was greeted with grease everywhere. As we all know, hot brakes cause the grease in your hubs to melt...

Stripping the brake down revealed some serious damage to the linings, and bits of lining (2-3 mm bits) all over the drum. The linings you see in the pictures are greasy, but those matt grooves in the middle are way below the wearing surface:

Greasy

Cratered Lining

Now, I don't know if this is a result of heat, or the heat is a result of this breaking down. I'll take it back to the supplier tomorrow and see what they say.

Odd, since it had been bedding in nicely and getting quite effective.

Fuel Tank

One thing (there may be others!) that has puzzled me is the absence of a crossover tube on Amelia's fuel tank. It seems to be a fairly well established fact that Solex-equipped Square Fours have one fuel tap, with built in reserve, on the left - because a tap on the right is in the way of the Bi-Starter knob and could drip fuel (they don't leak, do they?) on the distributor.

These first three pictures are all 1951 SQ4's, like Amelia:

Left hand tap (also note the brake lever stop! never seen one of those before) 

No right hand tap, no crossover loop. this bike has a drain point at the RH front of the tank, like mine

Obvious crossover loop

Amelia has two petrol taps, one drain point at the front, just like this 1952 KH

Left hand petrol tap, no crossover loop

Right hand petrol tap, no crossover loop

So... What do y'all think of that?