Troubleshooting a JAP Special

[Rockburner]

Small update.

I haven't really made much progress, but things have happened.

I found the ONLY piece of JAP hardware at Beaulieu Autojumble: a head, complete with fitted valves! Snapped it up for £130 and kindly refused the same seller's offer of £475 for an AMAL Type 27 twin float-chamber dope-sucker!

I think the Head was made up for use, and then either only used very briefly, or put to one side as a "ready spare".

It holds water :


But I want to be sure that's it's fully-functional condition so I've started stripping it down to check valve-stem rattles etc:


It's pretty clean.


But there's always room for improvement... (that's the exhaust outlet)



The valves springs sit in these cups:


And you have mica washers that sit underneath the cups to adjust the valve-spring compression.  LIttle bit of verdigris here...



I've also arranged to take the crank up to the JAP speedway guru in a few weeks to strip it down, check the bearings and rebuild it (it's beyond my equipment and capabilities to rebuild the crank!)  I'll hopefully be able to take some photos of the process, if I'm allowed.

I've also just put in an order for a few bits..... around £600s worth..... (new main bearings, new big-end bearings, new cam-roller, etc etc).

every step is a step forward.... (even if they're not very often!)

[Rockburner]

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.

[cardan]

Wow, that's quite a nice piece of hardware. Particularly for £130, which probably wouldn't cover the cost of supply/fit new valves.

Re a potential new carb: If you're planning just to ride the bike on the road you might want to try the existing carb first. 1.25" is a seriously large carburettor; for example a 500cc works TT racer used a 1 7/32 carb - 1.2188" - in the 1930s, with a big megaphone, 6000+ rpm, bump start and very little idling. With a muffler and road riding, your existing carb might be fine.

Pleased the crank is going to an expert.

Leon

[murdo]

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.

[Rockburner]

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.

I know it won't make a huge difference - but the better I prepare it and assemble it now, the longer it should go before the next strip down.  Hence new bearings etc going in.

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 that case I just need to source a new manifold adaptor - (makes note to measure external diameter of head 2's mainfold)