JAPton blog

[Rockburner]

@ZRX61 : You mentioned that you had a JAP at one time ( viewtopic.php?p=338529#p338529 ):

Any thoughts on the correct cam/ignition timings for a 400cc J.A.P. (500c but with a 350 crank), running 98RON petrol (not avgas or meths or any variation thereof)? Currently fitted with an AMAL Concentric.

[ZRX61]

@ZRX61 : You mentioned that you had a JAP at one time ( viewtopic.php?p=338529#p338529 ):

Any thoughts on the correct cam/ignition timings for a 400cc J.A.P. (500c but with a 350 crank), running 98RON petrol (not avgas or meths or any variation thereof)? Currently fitted with an AMAL Concentric.

It's been 40 years...

[Rockburner]

@ZRX61 : You mentioned that you had a JAP at one time ( viewtopic.php?p=338529#p338529 ):

Any thoughts on the correct cam/ignition timings for a 400cc J.A.P. (500c but with a 350 crank), running 98RON petrol (not avgas or meths or any variation thereof)? Currently fitted with an AMAL Concentric.

It's been 40 years...

At this point, anything would be helpful!

[ZRX61]

I'd assume it's still in Suffolk under the barn that collapsed on it...

[Horse]

I'd assume it's still in Suffolk under the barn that collapsed on it...

Ricey/Rockey is too polite to ask: could you pop back, borrow a shovel, and check? TIA

[Rockburner]

17418854592915803773078846332976.jpg (1.86 MiB) Viewed 1292 times

Well, i swear i DID see a spark or two..... But it was the teensiest tiniest spark I have ever seen.....

I checked with 2 different plugs. Each gave a tiny attempt at sparking. Admittedly I'm turning the kickstart by hand rather than by foot, but I'd still expect a bit more...

I guess it may be time to send the mag off for a checkover, and look at maybe a new coil. Anyone know how to check a coil?

[Rockburner]

Well, the coil is showing 0 ohms across the 2 spade connections, which seems correct, and 7,070 ohms from each connector to the HT lead (plug end). Not sure if that's correct or not?

The mag is producing about 6 volts with the best spin i can produce by hand, again, not sure if that's good or not... It seems low...

[Rockburner]

Dropped the ramp so i could kick it more effectively.



Bit more like it.

Not much more in the way of sparks though.



Yes it's a useless video, the bike moves about too much, but I assure you... Sparks have left the building.

[ogri]

Is the gap correct, cant really see from that video.

If i wasnt fekked with arthritis i'd do you a favour and take the old wreck off your hands free of charge

ps check the condensor if it has one?

[Rockburner]

Is the gap correct, cant really see from that video.

If i wasnt fekked with arthritis i'd do you a favour and take the old wreck off your hands free of charge

ps check the condensor if it has one?

If it has one, it's buried in the potted electronics in the mag body. The mag has been modernised: there's no points.

[Rockburner]

Well, the coil is showing 0 ohms across the 2 spade connections, which seems correct, and 7,070 ohms from each connector to the HT lead (plug end). Not sure if that's correct or not?

After some brief research - it seems that the primary winding should be showing "some" resistance: ie more than nothing, but not more than about 5 ohms. I'm getting 0.0 ohms, ie no resistance whatsoever, which seems off to me so I'm going to see about finding a new coil as a first step, and at least eliminating that as an issue.

(I'm just wondering whether I need a 12v or 6v coil now.... there doesn't seem to be any markings on the one I've got! I suspect it's a cheap one off a Kempton auto-jumble stall....)

[ogri]

Should imaging something that old could well be 6 volt.

[Rockburner]

Should imaging something that old could well be 6 volt.

Quite possibly, however that video did show the mag putting out (apparently) 15volts....

[Rockburner]

So, spent a bit of time in the workshop today, while I look into the coil replacement, I still need to investigate the almost complete lack of compression. It's a relatively low comp engine anyway, but I shouldn't be able to rotate the engine completely through multiple cycles by hand on the kickstart.

So, started taking things apart and took a few photos as I went.

This is the primary chain case, homemade by my Dad, from a camping cup and a small fry pan.



And this is where my Dad's genius stands out.....

Yes, that's the fry pan handle mount being used as the lug for the mounting bracket! I hadn't realised this until I spotted it today, and I nearly pissed myself laughing.

This is the plate on the Magneto. Hopefully useful info...



Primary chain off.


There's a fair bit of play on the engine cog...



I also have a feeling the timing chain is a tad loose...

[ChrisW]

And this is where my Dad's genius stands out.....

Yes, that's the fry pan handle mount being used as the lug for the mounting bracket! I hadn't realised this until I spotted it today, and I nearly pissed myself laughing.

Lovely!

[Rockburner]

Well, the coil is showing 0 ohms across the 2 spade connections, which seems correct, and 7,070 ohms from each connector to the HT lead (plug end). Not sure if that's correct or not?

After some brief research - it seems that the primary winding should be showing "some" resistance: ie more than nothing, but not more than about 5 ohms. I'm getting 0.0 ohms, ie no resistance whatsoever, which seems off to me so I'm going to see about finding a new coil as a first step, and at least eliminating that as an issue.

(I'm just wondering whether I need a 12v or 6v coil now.... there doesn't seem to be any markings on the one I've got! I suspect it's a cheap one off a Kempton auto-jumble stall....)

Well - I was right about the coil supposed to be showing "some" resistance.

I spoke to the manufacturers of the mag: bt-h.biz, who were very helpful.

The mag I've got is one of their early ones: from 2003,, when the BTH name was "resurrected" and the current company started reproducing mags for old vehicles. As such, the model name has changed in their current line-up, it's now the PM1R (In the current line-up, "C" would indicate an external condenser).
Looking at their website, and especially their FAQs and instruction documents, I realised that the Coil I have is the same one that they sell, with the same hand-written instructions on it!

So - the advice I got is that the coil "should" be showing about 0.4 ohms across the primary coil (mine is showing sfa)., and the resistance from the HT lead to one of the primary coil spades should be about 4.5K-ohms - I'm getting 7.07K-ohms... but I "think" I was including the 5K-0hm plug cap in that - either way - the reading is way off what it should be.

Further advice is that these mags, with the modern CDI electronics, tend to work better with a low/zero resistance plug cap and plug.

So - I've ordered up a replacement coil and we'll see how we go.... after I've stripped the top end and timing chest down to see if there's any obvious reason for the lack of compression.

[Skub]

Might be as well to stock up on a few non resistors plugs. I'm finding the ones for the H1A are harder to get and priced accordingly if you do find them.

[Rockburner]

Might be as well to stock up on a few non resistors plugs. I'm finding the ones for the H1A are harder to get and priced accordingly if you do find them.

Yeah - I'll check what plugs I have, and then try to find non-resistor versions. I currently have 3, the original that was in the bike when I took over custodianship, and a couple of (iirc) same spec replacements.

Edit: according to my notes, it's a Champion L86C. IIRC when I bought the spares last year (year before?) the only application left for them (according to Champion) is lawnmowers.

[Skub]

I usually buy my plugs from here,but I see yours are out of stock too. It lists alternatives though...
https://www.gsparkplug.com/catalogsearc ... mpion+L86C

[Rockburner]

Did some more stripping down of the JAPton head last week/weekend.

The more I tinker with this thing, the more issues I seem to be finding.

The rocker-arms live in their own little alloy box which is bolted onto the head - they rotate in pin-roller bearings which have their own mount so they're replaceable (I assume). The bearings in turn have spacers and a "distance ring" to keep their positions in the rocker-box, and the "End Washer" locates into a slot in the rocker-box. The trouble is - there's supposed to be a maximum of 1/64" of "end-play" (ie the "left-right" movement) of the rocker arm.... and well.... see the photos...

Photos:
1) Inlet lower half of rocker box


2) Exhaust lower half of rocker box


3) Inlet upper half of rocker box:


4) Exhaust upper half of rocker bx:


5) Right side of upper-half of rocker box (where ther push-rods and adjustment side of the rocker arm live)


6) Inlet rocker-arm in place 1:


7) Inlet rocker-arm in place 2:


8) Exhaust rocker arm in place 1:


9) Exhaust rocker arm in place 2:


10) Rocker box upper half (cleaned up)


11) Rocker box lower half (cleaned up a bit)


12) Rocker arm assembly broken down : (exhaust side)
I cleaned up the roller pins by stirring them in meths just to get the old oil off.


13) Bits :
(dissasembled rocker boxes, push rods, push rod covers)


14) Engine head sans rocker box (but with the last half of the valve covers still in place):


15)


16)



17) This is the diagram of the rocker-arm assembly from "The Book of the J.A.P.":




So - as (I hope) you can see - the rocker-arm assemblies appear to be complete - AND YET - there's enough end-play in BOTH rocker arm assemblies to justify another washer between the (right side) bearing liner and the push-road end of the rocker arm!! See photos 6-7, and 7-8. The rocker arms can't actually fall out - but they're moving so much that.....

If you look at the photo of the upper part of the rocker-box - you can see where the push road has actually worn away the alloy of the rocker-box (photo 5, directly under the "R" in "Noral"). That little ledge is a "pathway" for oil to be drip-fed into the rocker-arm bearings (through the holes in the "distance piece"). That can't be good!

You can see the little bearing pins floating around in photo 8!

What I can't figure out is how such a large discrepency could occur - it's not like the rocker-arms could have stretched nearly 2 millimeter in length!, and similarly - I can't see the spacers or distance ring being worn away that much either: to give that much end-play.

It's "possible" that the distance ring is wrong?? I suppose? The issue is that the engine was built up from "parts" - so I've no history on where each component came from: however to counter that.... these engines didn't change between '38 and '50 something (at all!), as I understand it, so I don't really see how any parts wouldn't be compatible.

Weirdly - the inlet rocker pin-roller bearings have 23 pins each, but the exhaust has only 22 pins each: I've no idea if that's by design (the books I have don't go into that much detail), or if the exhaust has lost a couple of pins.....

I seriously need to talk to a subject matter expert.

I'm going to be continuing to tear down the head: slowly: This is the first time I've delved this deep into this engine, and the last time I stripped a head it was on my first bike: my Mum's V50 Guzzi, and I had my Dad helping me (that was ..... err... IIRC I was 18....). I also like to clean and examine every part as I go - to check it's condition and to fully understand how it fits into the whole: so it tends to be a slow process.....