Troubleshooting a JAP Special

[john.k]

What carb is it ......the pic seems to indicate a Type 27 speedway carb ,or maybe a 10TT

[Rockburner]

What kind of motor is it..........I assume its a 4B or similar .....in which case someone like Cameron Engineering will be able to supply a milder cam......at a price,mind you.

It's a 4-stud, with a Cooper barrel (lightweight, small fins), and the internals are a mix of 350 and 500 that equate to 400cc (approx).  I keep forgetting how Dad actually managed that.

[Rockburner]

What carb is it ......the pic seems to indicate a Type 27 speedway carb ,or maybe a 10TT

Amal Monobloc 389.

[Rockburner]

I tinkered with the JAPton last night.  (Life is busy at the moment, so I don't often get into the workshop).

It's all back together but wasn't running properly - lots of spitting back and backfiring in the exhaust.

Got it running, and spent about an hour going back and forth between the ancient Monobloc "Notes" leaflet (Dad had some photocopies so at least I wasn't getting the original filthy!)*  Eventually got my head around it and realised that the damn thing was running "weak" in the mid-range and so pulled the needle up a notch (the first thing I'd done was lower it a notch to the "top" notch, it refused to run on that setting).  Now it's in the middle of the needle's range its actually running pretty well (once it's hot).  The occasional spit back or backfire, but I think I can live with that for now.

Happy bunny!

During the course of the session I'd spotted a couple of nuts that had fallen on the floor - one was an engine bolt that (in hindsight) I know was loose - the primary chase mounting lug is held in place by it and I'd simply forgotten it was loose.
The other was the nut off the forward connection for the gear linkage.... it's a nyloc, but I discovered that Dad had cut the bolt for it down so short that the threads aren't actually engaging the nylon locking collar! ( ffs).  Couldn't be arsed at that point to root around for a better bolt so I just pushed the nut on upside down.  Hardly ideal but it'll do to hold it all together while I figure out another issue.

I was wiping some oil off the top of the crankcase when I discovered something that terrifies me...



That bolt, the one between the push-rod tubes, is loose, and it won't go tight.   

It just revolves.

Now - I may be wrong.... but, to me, that indicates that there's a friggin NUT on the other side of the case, that is loose, and is hanging on the threads of the bolt.


I may be wrong, and the loose bolt is caused by the threads on either the bolt or the casing being thoroughly knackered.

But - either way, the mininum work needed before I can do anything with the bike is to strip the entire right side of the engine away to check what's going on in the timing box.  :roll:

I can't take a chance on the probability of a nut dropping into the timing box.

So - The JAPton is unlikely to see tarmac again this side of Easter, simply because I know that I've never done any of this stuff before and I'll be taking things very slow and thoroughly checking it all, and researching how it all works.  Oddly, for all the tinkering I've done over the years, this is the first time I've ever had to actually do the timing on anything. 

I also seem to have been thrown off / barred from the JAP Engine Facebook group I was on - so also need to find a J.A.P. specialist source of further information.

Dagnabbit.



* I must get a copy of that leaflet blown up and laminated.  I did the same with the exploded diagram for the carb and it's an invaluable reference sheet).

[iansoady]

Most likely the thread in the crankcase is stripped. I think it unlikely there's a nut underneath. Getting the head & barrel off will only take half an hour or so.

Re the spitting back - I had a similar problem with my ES2 fitted with a monobloc and found that a richer slide cutaway (lower number) sorted it out, as the cutaway controls the mixture on transition between idle and the point that the needle jet takes over. The slide was kindly donated by someone on here but sadly I can't remember who.

[Rockburner]

Most likely the thread in the crankcase is stripped. I think it unlikely there's a nut underneath. Getting the head & barrel off will only take half an hour or so.

Re the spitting back - I had a similar problem with my ES2 fitted with a monobloc and found that a richer slide cutaway (lower number) sorted it out, as the cutaway controls the mixture on transition between idle and the point that the needle jet takes over. The slide was kindly donated by someone on here but sadly I can't remember who.

I "believe" that I need to get into the timing chest, rather than the crank case volume itself - so need to get the timing apparatus off the rhs, rather than the head and barrel off, but need to find some internal schematics first to check that.

It looks to me like the pushrods are actuated by the cam-followers in a side-chest.



Would be seriously useful to have a 2nd engine to play with and see where everything goes!

[cardan]

Yes, it's just like 99% of old motorcycle engines.

I don't know what the problem bolt does: I assume that holding the head and cylinder down is left to the four main studs, which are suitably symmetric, in which case is the problem bolt just filling a hole used for an oil inlet or breather? If it turns, what happens if you turn it and pull it up? There is no way it has a nut underneath inside the timing chest.

Leon

[JFerg]

I'm with Leon.

Although I no longer have a JAP engine, I'm sure that there is no actual function from a bolt there between the pushrods.  I suspect that it is a breather someone has added at some point, and that brass "bolt" is actually a plug.  As Leon suggests, try lifting and un-doing it; it will probably screw out to reveal an ill-fitting plug.

[Rockburner]

Yeah - that's the first thing I'm going to try when I'm next in the workshop.  The engine was very hot when I first observed the issue so I was struggling to get at the thing!

I seriously hope that there isn't a nut underneath it... but knowing my Dad..... there's a possibility....

[Rockburner]

oh yeah - there's one other weird thing...happening:

On the Monobloc carb, although I've cleaned it all up, when I use the tickler to push the float down, it "works" - but the fuel doesn't overflow through the ticker - it eventually overflows out of the bellmouth!   (or possibly a tiny hole in the inlet manifold).

The tickler is imply a metal tube, closed at the top end, but with a small hole on the side about a mm down from the top.  The bottom of the tube is squared off, but not closed up, and it's completely clean - it's not blocked in anyway.

The top of the float - which the tickler pushes down on, is rounded, so there's no "flat plane" on which the end of the tickler could possibly "seal".

So I'm highly confused as to why the fuel isn't exiting out of the tickler!

[Rockburner]

Managed to unscrew the bolt this evening.

The threads on both the bolt and the case are "tired". But i found a bolt in my grab bag of Imperial bolts that actually screws in and will go "tight".

https://flic.kr/p/2oUxRsd

https://flic.kr/p/2oUwLGC
Old bolt on left....
So I'm going to cut it a bit shorter (both bolts are twice as long as they need to be), and smother the top end of the threads with silicon and put it in with an ally crush washer (unless i can find a suitable fibre, but i detest fibre washers).

Before that though, I'm going to stuff an endoscope into the hole!

[john.k]

Ive sort of wanted a copy of "The JAP Engine " by Fenner for years ,but never going to pay a silly price .........then I just happened to see an as new copy with the red dust cover intact for $10 at a flea market .........anyhoo,this book includes a section by Wal Phillips on the speedway engines .........and thats where the photo of the timing gear comes from .

[Rockburner]

Ive sort of wanted a copy of "The JAP Engine " by Fenner for years ,but never going to pay a silly price .........then I just happened to see an as new copy with the red dust cover intact for $10 at a flea market .........anyhoo,this book includes a section by Wal Phillips on the speedway engines .........and thats where the photo of the timing gear comes from .

Wow!  That's an excellent find!

I have both the Fenner, and 2 copies of the Pitman "The Book of the J.A.P. Engine"  (2nd and 3rd editions).  My Dad always like to check the definitive sources as much as  he could.  The problem is that I think a lot of his notations were for his other J.A.P engined bike... a 1930 HRD Vincent, which had an earlier J.A.P. engine in it, than the one in the special I've got.

[Rockburner]

last week I "fixed" the hole in the JAPtons crank cases (timing chest actually).

I did stick an endoscope down it, but there wasn't a lot to see!

I cut down the new bolt so that it was roughly the length required to fill the cases, with a crush washer on top:



Then fitted it in with a good amount of Soudall Fix-All smeared into the threads and around the crushwasher and head.  With any luck that will act as a seal and thread-lock even though the bolt isn't done up that tight.  I tried very hard NOT to strip the threads so only did it up as tight as I dared....



Obviously will be keeping an eye on it, but I can't move it with my fingers... so it's an improvement

I had an idea that the issue with the carb tickler might have been the paper gasket between the jet block and the main body (the surfaces this gasket mates to are not the smoothest....).  So I stripped the carb down and put a small amount of blue-hylomar on each face of the gasket and reassembled it.

No difference.  Whatever is preventing the tickler working correctly is still evading me.  Next attempt will mean going into the float bowl itself and seeing there's something odd about the interface between the tickler and the float.  I've been doing some reading up and it seems that the new nylon floats sometimes cause minor problems so it's worth taking a look.




Took the JAPton out for a spin round the block this evening.

It might run ok at less than 1/4 throttle, but any more than that and it starts bogging down, won't fire at all, then after 2 or 3 deafening explosions from the exhaust it'll pick up and run well for about 10 seconds before it starts bogging down again.

Rinse and repeat again and again if I try to ride it anything more than 1/4 throttle (ish).

And obviously it's still pissing oil....




So... Back to to the tuning leaflets....

[cardan]

I wonder if the bolt should be a breather? Earlier JAPs were very clever in how their engines breathed, and the timing chest was vented directly to the atmosphere. Oil will be forced out randomly if there is no method to relieve excess pressure from the cases/timing chest. I know all about the early engines (where venting the crankcase will cause the entire oiling system to fail), but unfortunately I don't know the details of the later oiling systems, other than to say "be careful of what you vent and what you don't".

Leon