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:
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