Troubleshooting a JAP Special

[cardan]

Try this http://classicmotorcycleforum.com/index.php?topic=6299.msg31348#msg31348

It didn't work for the person who needed it...

Leon

[john.k]

A failing mag coil needs to get hot before causing problems.......typically this takes 1/2 hour running.............while one of the film capacitors (and remagnetizing) will improve a failing mag,it will also accelerate its demise with the higher voltage in the coil.

[Rockburner]

A failing mag coil needs to get hot before causing problems.......typically this takes 1/2 hour running.............while one of the film capacitors (and remagnetizing) will improve a failing mag,it will also accelerate its demise with the higher voltage in the coil.

It's a fair point.  Can you suggest any other cause for the bike dying after 5 minutes?  I'm pretty sure it's not fuel flow, but will be checking that again.

What other causes might there be for a JAP engine to die after 5 minutes of running?

[cardan]

Some things are easy to check. Just lift the fuel cap after a few minutes and you'll know it's not that. Check the tappet clearance when the bike stops - you need some. Check that the valves aren't tightening in the guides at things warm up, and so not closing properly - check that there is some compression immediately the bike stops. Beyond that, maybe the magneto is tired.

Leon

[Rockburner]

Checked the fuel flow again last night, it's fine.

Also pulled the rocker cover off and rotated the engine using the kickstart (not ideal), but with that cursory inspection, it looks like the inlet valve has closed up completely.

Hopefully it's just the gap closed up and it's not a case of getting the valve seats redone. I think the head has already had phosphor-bronze seats put in, but not sure.

[iansoady]

From what you say it hasn't run much so I doubt whether valve seat recession is the problem. Either the clearance was incorrect from the start or has somehow tightened up - although they usually slacken rather than tighten. Are you sure you're checking at the right point?

[Rockburner]

From what you say it hasn't run much so I doubt whether valve seat recession is the problem. Either the clearance was incorrect from the start or has somehow tightened up - although they usually slacken rather than tighten. Are you sure you're checking at the right point?

Not yet!

Need to get the chain case off to rotate the engine properly, and then expose the valves fully to see what's going on

I also need to find a clearance measurement.

[R]

Need to get the chain case off to rotate the engine properly, and then expose the valves fully to see what's going on

Just remove the spark plug, put it in gear and either rotate the back wheel or wheel it
along the floor to get the engine to where the valves are closed.

You'd probably want to be seeing 5 or 6 thou.
But don't quote me on that, or hold me to it.
I haven't seen your engine...
Much less than that is likely to be a problem though.

[iansoady]

If you do as suggested by R and use a pencil or such through the plug hole you'll find that there are 2 top dead centres. One of these should have a clearance on both valves (the end of the compression stroke), the other should have no clearance (end of the exhaust stroke). Make sure you're checking on the right one.

[Rockburner]

If you do as suggested by R and use a pencil or such through the plug hole you'll find that there are 2 top dead centres. One of these should have a clearance on both valves (the end of the compression stroke), the other should have no clearance (end of the exhaust stroke). Make sure you're checking on the right one.

Yes - I know how to do valve clearances, (been doing them on my 1150 BMW for 20 years)


I just didn't know 'where' on the JAP engine the clearance was measured, or what the clearances should be.   I also don't know if there's paper gaskets that need to be bought first before I dive in.

Luckily I've got a copy of "The Book of the JAP Engine", and I've now found the info I need.  Apparantly the clearances are 0.002" when cold, measured directly between the tappet and the valve-stem.

Which is what I expected, kinda, but I was confused by the multiple covers over the valve-gear: it would have made sense to me if the measurement was made somewhere in the rocker box because then it's 1 cover to remove and reseal, rather than 3 (or more).

I'm also slightly concerned about re-sealing the covers, the rocker cover contains 2 'bosses', which act on the rocker end-faces to maintain the 'end-float'.  So adding a gasket of some sort would affect that end-float. (the rocker cover plate is also cracked, which doesn't make me happy either!)

[cardan]

It's one thing that a particular JAP engine needed 2 thou of tappet clearance, another to know what is required in a hybrid engine made from an unknown collection of bits half a century on. Who knows what the push rods are made of? The key thing relevant to your problem is that there is some valve clearance when the engine stops running. When the engine stops stand on the kick start lever and see if the compression is still there.

Leon

[Rockburner]

It's one thing that a particular JAP engine needed 2 thou of tappet clearance, another to know what is required in a hybrid engine made from an unknown collection of bits half a century on. Who knows what the push rods are made of? The key thing relevant to your problem is that there is some valve clearance when the engine stops running. When the engine stops stand on the kick start lever and see if the compression is still there.

Leon

There's definitely compression.  If anything I think the exhaust valve clearance has come loose. There was a lot more movement on it when I made my cursory check than there was on the inlet.

You're right of course that it's a 'made-up' engine, but I've found some notations made in my father's inimical manner within the books that make me pretty confident that this was what he understood to be 'right' for the engine.

[john.k]

If its a cast iron engine,then inlet just free and exhaust at 002 will be correct...........I wouldnt assume anything drastic has happened ,but concentrate on the faults likely to develop with out of use .......Fuel and magneto.......The symptoms sound like fuel ........check the monobloc float valve ....there is a small spacer on the float spindle sometimes left out.....or maybe the needle valve is blocking fuel flow ........this was a fault with the nylon ones ..........if thats OK for sure ....then Id take off the mag chain,spin it  and see if it has a fat blue spark.........any trace of red spark will be condenser problem from moisture pickup.

[Rockburner]

If its a cast iron engine,then inlet just free and exhaust at 002 will be correct...........I wouldnt assume anything drastic has happened ,but concentrate on the faults likely to develop with out of use .......Fuel and magneto.......The symptoms sound like fuel ........check the monobloc float valve ....there is a small spacer on the float spindle sometimes left out.....or maybe the needle valve is blocking fuel flow ........this was a fault with the nylon ones ..........if thats OK for sure ....then Id take off the mag chain,spin it  and see if it has a fat blue spark.........any trace of red spark will be condenser problem from moisture pickup.

To the best of my knowledge, the engine is predominantly standard JAP parts, apart from the alloy barrel (as used in Cooper race-cars).

I've done everything I can with the carb, it flows fuel fine, but might be jetted or set slightly off - I'm not sure how to check it further because I can't currently run the engine for more than 5 minutes (ie to get it hot), because it loses all power and will barely run at that point.  Hence moving on to things like valves and timings.

[R]

May we enquire what spark plug it currently has in it ?
(you have tried a NEW plug, haven't you ?)

The wrong heat range may see the plug overheat OR be too cold to keep running,
even in those few minutes.
A bit unlikely, given how general purpose plug types of old were, but you don't know until you try it ...