classic motorcycle forum

Quote from: Rex on August 28, 2023, 10:09:27 AM

Quote from: R on August 27, 2023, 11:46:01 PM

There is some chat about viton tipped fuel needles, or not, in monoblocs.

Yep, some claim that the Viton needles are prone to sticking "up" and shutting the supply of juice off.
Re the pit bike Choagy carbs, an old boy I know paid £14 for one off Ebay, and stuck it on his reluctant Tiger Cub. Apparently it started second kick and even ticks over fresh out the box.

I've heard this myself which is why I'm considering it. Ebay also has sets of main & idle jets for a couple of quid or so. I've never had the experience if viton needles sticking despite using them for decades.

Got any links for the jets?

Well, I've still not figured out what's wrong with the JAPton. Did some checks last week and fuel flow is good ( more than half a litre per minute, which ought to be fine).

Raised the needle by one because i thought it might be a bit weak, and tried starting it up and although it started easily enough, its still spitting back and backfiring.

I'm beginning to think the ignition timing has slipped, so i took a cover over the magneto drive sprocket to check it.
Seemed fine so went to put the little cover back on, and bugger me if the sodding thread didn't strip.

GAH!

So I'm going to put the JAPton aside for a bit and concentrate on the other projects i need to get done before winter, and will very likely spend the winter stripping the thing down and learning more of its foibles.

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....

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!

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....

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).

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.

The main jet doesn't come into play until the throttle is more than about 3/4 open,
so you can probably omit that step.
Dropping the needle a notch would achieve less fuelling, at lower engine speeds.

It does sound more like cam timing though.
Unless the inlet valve isn't seating too well ??

Good point about the needle - I"ll try that (not sure what it's on at the moment tbh)