Author Topic: Troubleshooting a JAP Special  (Read 3691 times)

Offline Rockburner

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Troubleshooting a JAP Special
« on: April 11, 2023, 12:27:31 PM »
I've inherited my father's JAP engined, Featherbed framed 'special', and I'm trying to get it setup for road-riding, it's registered and has been used in the past for road-riding and display events (eg Kop Hill), but since he died in 2021, it hasn't been used (in fact it wasn't used much before then), and I'm struggling to get it running right.

The engine is a a ~400cc* JAP 4-stud motor, with a Cooper ally small fin barrel (ie, from a Cooper race car), running an AMAL Monobloc, and a BTH Magneto.

Frame is a Slimline Norton Featherbed.

Gearbox is a 1950s Norton 4-speed.

Akront wheels, brakes are Grimeca 4-ls front, Norton rear.

So far I've got it 'running' after cleaning out the Monobloc carb and reassembling.

It starts easily and fires up fine - in the workshop**.  Previous to cleaning up the carb it wouldn't tick over, now it ticks over fine and responds to revving fine when on the stands.

However - I've taken it out on the road (round the block) a couple of times, and it always does the same thing:  It'll run fine for a couple of minutes - up through the gears to about 60mph or so, a couple of times, but then, within 5 minutes of starting, the power just dies away completely and it'll barely run, although it does seem to be able to tickover still.

I thought it might be running rich, so I lowered the needle in the carb (to the lowest option, from the 2nd lowest option), and that didn't seem to make much difference - although it did seem a little 'weak' at full throttle - so I'm slightly concerned I've gone too far the other way and made it lean, OR the problem is something else completely.  It goes to full throttle very easily: fitted witha  Tomaselli Quick Action twist-grip - only about 1/4 turn from closed to WOT.

The first time I took it out, and it started playing up, the plug came out very black and oily (but dry).

Then I checked it two days later prior to my second test-run, and the plug was actually wet with oil.

After the 2nd test-run, the plug was again black and oily, but dry (ie not sooty black, but oily black).

The engine is also pushing about twice as much oil to the head as it should - I've read (somewhere) that the Pilgrim pump should be pushing oil to the head at a rate of 1 drop every 5 seconds at tickover.  Currently it's running about 2 drops every 5 seconds, so I'm going to adjust that, but before I do - would running R30 instead of R40 be the cause of a doubling in the oil feed rate?  IIRC Dad ran it on R40, but I was adviced that R30 might be better during the cooler days at present (and switch to R40 during high summer or for faster riding).

The worry I now have is that oil seemed to be getting into the combustion chamber - would that indicate a leak from a valve-stem?  I'm not even sure if these motors run valve-stem-seals (in the modern sense), or could it just be the 'over-oilling' overwhelming the valve-stem?

Very grateful for any advice that anyone can give!




* It's a mix of 500 and 350 parts which come out at around 400cc.

** Starts fine sitting on a paddock stand.... pull the bike out into the fresh air and it takes 10-15 kicks.... could this be due to the fact that I'm starting it on an angle?  on the paddock stand it's pointing slightly 'downwards' (rear wheel is raised a couple of inches), but outside the uneven ground means it's leaning 'backwards' slightly (pointing 'upwards', rear wheel is a couple of inches below front) ???
« Last Edit: April 11, 2023, 12:34:09 PM by Rockburner »

Offline Vreagh

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Re: Troubleshooting a JAP Special
« Reply #1 on: April 11, 2023, 06:25:09 PM »
I've found monoblocks can cause difficult starting when on side stand. One of my bikes is a pig on the side stand but stand it upright and it's fine. Isn't that one of the reasons Amal went to the concentric carb ?

Offline JFerg

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Re: Troubleshooting a JAP Special
« Reply #2 on: April 11, 2023, 10:57:22 PM »
That it runs fine when it goes vindicates the carb set up.  That it plays up hot suggests the magneto.  An open circuit within the mag windings  arcs to form a carbon track.  This is of low resistance when cold, but high resistance once it warms up, reducing magneto output.

Offline chaterlea25

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Re: Troubleshooting a JAP Special
« Reply #3 on: April 11, 2023, 11:03:12 PM »
Hi
Have you checked the fuel tank vent? / fuel tap flow,  it sounds as though it is running out of fuel after 5 min but still enough flows to keep it ticking over ?

John

Offline Rockburner

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Re: Troubleshooting a JAP Special
« Reply #4 on: April 12, 2023, 09:29:10 AM »
That it runs fine when it goes vindicates the carb set up.  That it plays up hot suggests the magneto.  An open circuit within the mag windings  arcs to form a carbon track.  This is of low resistance when cold, but high resistance once it warms up, reducing magneto output.

Hmm - yes - that could be the issue, I've seen it before (we had a Comet that would do that - it would run fine when running, but if it stalled then you had to wait for it to cool down before it would fire again! Very annoying if you stalled it on a long rideout!).

What's the best way to test this??

Offline Rockburner

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Re: Troubleshooting a JAP Special
« Reply #5 on: April 12, 2023, 09:29:46 AM »
Hi
Have you checked the fuel tank vent? / fuel tap flow,  it sounds as though it is running out of fuel after 5 min but still enough flows to keep it ticking over ?

John

Good thought - I'll check that as well.

Offline Rockburner

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Re: Troubleshooting a JAP Special
« Reply #6 on: April 12, 2023, 04:44:09 PM »
I've found monoblocks can cause difficult starting when on side stand. One of my bikes is a pig on the side stand but stand it upright and it's fine. Isn't that one of the reasons Amal went to the concentric carb ?

Just on this point - when kicking the bike outside, it was upright, not on the side-stand, although I suppose it be leaning slightly when I kick, but not much.

Offline R

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Re: Troubleshooting a JAP Special
« Reply #7 on: April 12, 2023, 10:50:21 PM »
What's the best way to test this??

Take it on a long rideout - and see if it will restart hot !

I'm going to add that this often indicates a problem with the condensor.
Buried within the armature windings, on many magneto types.
Starts easily when cold, difficult or utterly refuses to start when hot.

The opposite with bad or failing wiring really.

Some such ailments can be cured with those little Brightspark Easycaps,
as a condensor replacement. For some magneto types.

Offline iansoady

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Re: Troubleshooting a JAP Special
« Reply #8 on: April 13, 2023, 10:07:01 AM »
If you look at a monobloc - or the earlier 276 type - you'll see why starting on the prop stand can be problematic. The level of petrol seen at the jets will drop as the float chamber is on the left of the carb thus weakening the mixture.
Ian
1952 Norton ES2
1986 Honda XBR500

Offline Rockburner

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Re: Troubleshooting a JAP Special
« Reply #9 on: April 13, 2023, 11:32:13 AM »
What's the best way to test this??

Take it on a long rideout - and see if it will restart hot !

i would if it ran for more than 3 minutes!

I'm going to add that this often indicates a problem with the condensor.
Buried within the armature windings, on many magneto types.
Starts easily when cold, difficult or utterly refuses to start when hot.

The opposite with bad or failing wiring really.

Some such ailments can be cured with those little Brightspark Easycaps,
as a condensor replacement. For some magneto types.

I'm looking into people for that now

Offline cardan

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Re: Troubleshooting a JAP Special
« Reply #10 on: April 13, 2023, 12:37:41 PM »
In the same vein as blocked petrol tank breather, it's worth checking that you still have tappet clearance as the motor warms up.

Leon

Offline john.k

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Re: Troubleshooting a JAP Special
« Reply #11 on: April 14, 2023, 03:06:19 AM »
Are you sure the carb isnt worn out........I had similar trouble with an Ariel,the monobloc had been fitted new,but the slide was completely worn out ,as was the slide bore.......monoblocs had a bad name for being rubbish alloy,but then so did concentrics.

Offline Rockburner

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Re: Troubleshooting a JAP Special
« Reply #12 on: April 14, 2023, 09:55:07 AM »
Hi
Have you checked the fuel tank vent? / fuel tap flow,  it sounds as though it is running out of fuel after 5 min but still enough flows to keep it ticking over ?

John

Fuel flow looks ok to me:

https://youtu.be/7I_E-CXGeMY

And now I need more fibre washers: the brand new one from Burlen has gone hard already and won't seal.  Might try making one out of Viton....

Offline chaterlea25

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Re: Troubleshooting a JAP Special
« Reply #13 on: April 15, 2023, 11:30:42 PM »
Hi,
I would run the flow test for at least 5 min or as long time wise to simulate when the trouble happens ??

John

Offline Rockburner

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Re: Troubleshooting a JAP Special
« Reply #14 on: April 16, 2023, 01:15:07 PM »
Hi,
I would run the flow test for at least 5 min or as long time wise to simulate when the trouble happens ??

John

I've got to replace the seal anyway so I'll have another go.
But pretty sure it's fine.
The filler cap is Monza type and i stripped it to check the breather bikes and they were fine.

I'm going to get the Mag rewound so will be stripping the bike down.  But irritating as i was hoping to be riding it  this summer.

Dies anyone have a link that explains how to setup timing and contacts for someone who's never seen a cintact breaker before? I've never done it and never seen it done. :(