classic motorcycle forum

All thanks!

Indeed the bush on the right side is worn.

Any option to buy new ones to be fitted on the polished main shaft?
Or just have a copy made?

That No2 head has had a lot of time sent on it. Polishing ports may look good but does very little extra in an engine for a road bike.
The carb will be fine (unless you are going to run it on methonal and throw it sideways around a dirt oval) and the small step from 1 1/8 to 1 1/4 wont be noticable on the road and the step will actually help with fuel reversion at low revs. Keep up the good work and enjoy the bike.

In the interests of keeping up to date.... I also did some work last night.

Finished stripping down the "new" head.  It's tidy.


The valve stems aren't the same - the inlet especially looks to have been either cut short, or made that way to reduce interference on the flow of the charge?




About an hour or so later after a dubious application of Solvol and a toothbrush...








Bit nicer.

Polished up the back face of the valves too...




and then gave them a bit of a grind.  I only used the "fine" grinding paste because they were sealing nicely before, and I just wanted to ensure the mating face was good after the polish.  Obviously I'll test the seals again after reassembly.



Terrible photo of the excellent valve tool I inherited from my Dad.






That's a big hole....



I also took some measurements of the inlet manifolds on each head.  I'd read something about the evolution of the engine recently (must dig it out again), and that last photo made me curious....

Yup - they're different.

Head 1 (current) is 28.5mm / 1 1/8" internal diameter
Head 2 (new) is 32mm / 1 1/4" internal diameter.

I "believe" I'm right in thinking that if I want to use Head 2 (it's in better nick, has better valve stems and would appear to be "less old"), then I'm going to have to look for a new carb.  IIRC the inlet manifold needs to be a consistent diameter/cross-sectional area for as much as is possible - which would mean that the carb would need to be consistent with that too.  The current carb is an AMAL Monobloc (389/229) which measures 1 1/8" across the "exit" hole (ie where the fuel/air mixture exits the carb).  (Oddly, the AMAL website suggests (although they've taken off the details now) that that model Monoblox is a 1 5/32" which is ... err... 1 2.5/8"  (I think...)). So... I'm a little confused about that ..... 

It does mean I have a reason to potentially get a better carb though.... According to this site (https://surreycycles.com/carburettors/amal-monobloc-376-389/) there is no 1 1/4" Monobloc so I don't believe I can get the same carb and therefore have use of my now extensive collection of jets...) Something different then.... probably something like a Mk1 Concentric or something. I do have to say it's very tempting to look at some throaty Dellorto or something equally esoteric though.... might  suit the "crazy ... like a fox" nature of the bike.

And the good thing about iron pistons in an iron bore is that they'd
have to get REALLY hot to think about nipping up, let alone seizing.

Even without much/any oil being in sight - thats the beauty of cast iron on cast iron.
The carbon in the crystal iron lattice makes this possible.
Without this, the old bike industry, with drip feed lube would never have prospered ?

Is it sucking in false air through the bronze bushings on the crankshaft?

It will if the main shafts or the bushes are worn: the shafts and bushes need to be round with only a thou or two clearance (say 0.025-0.05 mm). You definitely should not feel any movement if you grab the flywheel and try to move it up and down. If it clunks you're in trouble! The other place that wear occurs in early two strokes is in the bore: not so much up the top (as it is in four strokes) but above and below the ports where the rings jump across. And the rings, of course.

If these parts are worn the bike will probably still run, but will get hot an bothered when riding. If you've got good compression - it really should be quite "sharp" as you kick it over - then wear is probably not the problem at the moment.

Leon