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British Bikes / Re: Troubleshooting a JAP Special
« Last post by Rockburner on July 16, 2026, 03:48:10 PM »Back in the shed of prevarication....
Well, actually.... progress has been made.
As promised, I tested the cam lobes by the simple mechanism of throwing the cam shaft into the pillar drill and setting up my magnetic stand for a dial gauge.
i tried the digital gauge first, but it was just too hard to follow, so I dug out the needle gauges.
First try, but then I realised that the gauge wasn't being held very well, so there was a hiatus of sorts while I fecked about with the gauge clamp on the stand...
And eventually gave up and used the other needle gauge which isn't as reliable... but should be ok for this.
Here's a couple of lovely little videos...
Ignore what I'm wittering on about in the video re which lobe I'm looking at, I got confused.
This is for the exhaust lobe
https://youtu.be/i2biE-Sa9tY
And this is the inlet.
https://youtu.be/N-WmMQ_Oddc
It might LOOK like the inlet would be the first lobe that's activated when you look at the camshaft... but note that the cam followers are not inline with each other, they're either side of the camshaft, so as the camshaft rotates, the exhaust follower is actually the first one that's actuated.
Anyway - the cam seems good.
I also rotated the timing chest with some oil drizzled over the actual gear-teeth of the camshaft and half-time pinion.... which I realised that I hadn't done yet! the click-clack was definitely reduced when I did that... so I think it's just the backlash that is inherent in the system, and I'm being way too anal about it*.
So i also checked the backlash again (at a point where both valve springs are partially compressed), and it's no more than a degree, if less. Which is inline with expectations, so I pondered while I took a drink, (Fever Tree Mexican Lime Soda if you're interested), and thought.. $%^&& it.
So I assembled the timing gear for the final time and also decided to make a new timing cover gasket, because the one I'd been using had got very oily and I wasn't sure if it would take the 3Bond very well.
So - here we go again:
Cut the a blank bit.
Leave it under something heavy for a short while to ease the bend .
Basic outline and as many screw holes as I could get the pen to work with.
Half way through the dinking.
So - new gasket.
I also remembered a bit of advice I'd seen and found a short length of "wire" to make sure that the oil-feed quill is inline when the timing cover is fitted.
(yes, I spotted that bit of gasket paper...)
Fitted the push rod tubes
Fitting the cover, making sure everything is aligned
Timing cover buttoned up tight. New gasket was fitted with 3Bond. The gasket is so thin, I just spooged out an excessive amount of 3Bond from the tube and brushed it onto the gasket paper thinly with my fingers.
remember to pull out the guide wire.... do NOT want that to wander down the crank shaft....
Then placed the Magneto, fitted the sprockets and chain.
There's no keyways on either of those sprockets, they're held solid by the nut pushing the sprocket onto the taper. I messed around for a bit getting the camshaft sprocket done up - eventually got it done up "tight" by putting the plug into the head and working against the compression, but it wasn't ideal. So once the Mag was tightened down with chain tension, I had another go at it, working against the locked mag spindle.
Mag chain in and tensioned.
With some movement, but not too much.
The combined backlash between the timing disc on the crank output, and the locked magneto spindle is about 10 degrees!!
So ignition timing will be approximately correct at best....
I timed it to 34degrees before TDC, which... according to one of the books, is correct for a 350 at 15:1.... but this bike is about 400cc... running a much lower compression (estimated 9:1) so none of the ignition timings mentioned in any book will be correct, so I've chosen a starting point and will work from there.
This is the "drive plate" for the Pilgrim Pump.
It drives that little thing there.
Don't forget to cover over the Mag's spindle lock hole
Just showing here that even with the chain cover on, I can still get at the mag spindle to make ignition timing adjustments.
And the little cover in place
I didn't bother trying to seal the chain cover, I expect it might come off again reasonably soon....
Fitted the upper valve covers.
And, while I was at it, the return tube for the sump-return pump. (I expect that would be a pain in the arse to fit once the engine is mounted.)
I also, luckily, remembered to fit the mag-platform brace.
The next thing I need to do is sort out the Rocker box oil feed union, which really is absolutely knackered, not only are the hex-flats nearly gone for fitting it, but the thread for the nut that holds the banjo fitting on is absolutely $%^&&ed.
I managed to talk to another JAP expert the other day, and got a bit of a dressing down for not being pro-active enough, eg not having my own lathe, to be working on old bikes and engines. He also said that I was making a mountain out of a mole-hill with this banjo post business.
Which I thought was a tad harsh, but he did have a valid perspective at least, so I need to find localised "mate down the pub" for some of the more engineering-ey work. With any luck....
One other thing he suggested though was taking the 1/8" BSP 28tpi oil union and running a 3/8" BSC 26tpi Die down it. His thinking was that the thead is only about 3/8" (if that) and over that short distance, the thread-form is close enough that the die won't destroy the BSP thread, but will cut a usable BSC thread-form. I'll hold that thought and if I can't find a BSC union then it's an option to try with a new steel post from the suppliers (who are adamant that they only have the BSP unions).
Well, actually.... progress has been made.
As promised, I tested the cam lobes by the simple mechanism of throwing the cam shaft into the pillar drill and setting up my magnetic stand for a dial gauge.
i tried the digital gauge first, but it was just too hard to follow, so I dug out the needle gauges.
First try, but then I realised that the gauge wasn't being held very well, so there was a hiatus of sorts while I fecked about with the gauge clamp on the stand...
And eventually gave up and used the other needle gauge which isn't as reliable... but should be ok for this.
Here's a couple of lovely little videos...
Ignore what I'm wittering on about in the video re which lobe I'm looking at, I got confused.
This is for the exhaust lobe
https://youtu.be/i2biE-Sa9tY
And this is the inlet.
https://youtu.be/N-WmMQ_Oddc
It might LOOK like the inlet would be the first lobe that's activated when you look at the camshaft... but note that the cam followers are not inline with each other, they're either side of the camshaft, so as the camshaft rotates, the exhaust follower is actually the first one that's actuated.
Anyway - the cam seems good.
I also rotated the timing chest with some oil drizzled over the actual gear-teeth of the camshaft and half-time pinion.... which I realised that I hadn't done yet! the click-clack was definitely reduced when I did that... so I think it's just the backlash that is inherent in the system, and I'm being way too anal about it*.
So i also checked the backlash again (at a point where both valve springs are partially compressed), and it's no more than a degree, if less. Which is inline with expectations, so I pondered while I took a drink, (Fever Tree Mexican Lime Soda if you're interested), and thought.. $%^&& it.
So I assembled the timing gear for the final time and also decided to make a new timing cover gasket, because the one I'd been using had got very oily and I wasn't sure if it would take the 3Bond very well.
So - here we go again:
Cut the a blank bit.
Leave it under something heavy for a short while to ease the bend .
Basic outline and as many screw holes as I could get the pen to work with.
Half way through the dinking.
So - new gasket.
I also remembered a bit of advice I'd seen and found a short length of "wire" to make sure that the oil-feed quill is inline when the timing cover is fitted.
(yes, I spotted that bit of gasket paper...)
Fitted the push rod tubes
Fitting the cover, making sure everything is aligned
Timing cover buttoned up tight. New gasket was fitted with 3Bond. The gasket is so thin, I just spooged out an excessive amount of 3Bond from the tube and brushed it onto the gasket paper thinly with my fingers.
remember to pull out the guide wire.... do NOT want that to wander down the crank shaft....
Then placed the Magneto, fitted the sprockets and chain.
There's no keyways on either of those sprockets, they're held solid by the nut pushing the sprocket onto the taper. I messed around for a bit getting the camshaft sprocket done up - eventually got it done up "tight" by putting the plug into the head and working against the compression, but it wasn't ideal. So once the Mag was tightened down with chain tension, I had another go at it, working against the locked mag spindle.
Mag chain in and tensioned.
With some movement, but not too much.
The combined backlash between the timing disc on the crank output, and the locked magneto spindle is about 10 degrees!!
So ignition timing will be approximately correct at best....I timed it to 34degrees before TDC, which... according to one of the books, is correct for a 350 at 15:1.... but this bike is about 400cc... running a much lower compression (estimated 9:1) so none of the ignition timings mentioned in any book will be correct, so I've chosen a starting point and will work from there.
This is the "drive plate" for the Pilgrim Pump.
It drives that little thing there.
Don't forget to cover over the Mag's spindle lock hole
Just showing here that even with the chain cover on, I can still get at the mag spindle to make ignition timing adjustments.
And the little cover in place
I didn't bother trying to seal the chain cover, I expect it might come off again reasonably soon....
Fitted the upper valve covers.
And, while I was at it, the return tube for the sump-return pump. (I expect that would be a pain in the arse to fit once the engine is mounted.)
I also, luckily, remembered to fit the mag-platform brace.
The next thing I need to do is sort out the Rocker box oil feed union, which really is absolutely knackered, not only are the hex-flats nearly gone for fitting it, but the thread for the nut that holds the banjo fitting on is absolutely $%^&&ed.
I managed to talk to another JAP expert the other day, and got a bit of a dressing down for not being pro-active enough, eg not having my own lathe, to be working on old bikes and engines. He also said that I was making a mountain out of a mole-hill with this banjo post business.
Which I thought was a tad harsh, but he did have a valid perspective at least, so I need to find localised "mate down the pub" for some of the more engineering-ey work. With any luck....
One other thing he suggested though was taking the 1/8" BSP 28tpi oil union and running a 3/8" BSC 26tpi Die down it. His thinking was that the thead is only about 3/8" (if that) and over that short distance, the thread-form is close enough that the die won't destroy the BSP thread, but will cut a usable BSC thread-form. I'll hold that thought and if I can't find a BSC union then it's an option to try with a new steel post from the suppliers (who are adamant that they only have the BSP unions).
