Troubleshooting a JAP Special

[Rockburner]

Pride, as they say... Cometh before a fall.


I'd been quite chuffed with myself for the way things were going on the bike, so naturally it couldn't last.

Took it out for a longer ride to see how it behaved over 20 or 30 miles of continuous riding, not straying too far from home, riding around the local roads.   At first everything was tickty-boo, it loped along nicely, albeit the fuelling at low revs wasn't perfect: couple of backfires, but nothing worrying at all. Riding "at speed" was fine (tyre pressures were a tad low) but the engine felt good. Clutch and gearchanges were fine (even getting into first at standstill was a lot easier)

Then, while tazzing along nicely it suddenly just went "bleaerrrrrrrrrrrrrrrrrrrrrrrrrrr" and died.  I was on a quiet country road, and luckily a largeish driveway appeared so I rolled into that, safely off the road and stopped.

There was some swearing.

I pulled out the tools and checked the plug: Bit oily, but not overly so - running a tad rich I'd say. (and rolling off tests were indicating that too)


Yes there's sparks.  Yes there's petrol, yes the petrol is flowing into the carb.  Then when trying to kick it... I realised there was absolutely no compression whatsoever.  There is exactly the resistance on the kickstart that indicates the engine is turning over, (ie, it's not clutch slip) but no compression. In addition the head gasket wasn't "puffing" at all which would indicate failure of the gasket. (and there's no marking or blow-by staining to indicate gasket failure)

There was more swearing.

That's not a roadside fix.

I've broken down in worse places, and in worse conditions:




The fork weep is back...


Attempted artsy photo by headlight!




5 hours later:


Eventually got home at about midnight and threw the bike back in the workshop.

Mileage on the odo when I left:


Odo reading at breakdown:  It did nigh on exactly 30 miles!


Reference odo (from after approx 10-15 miles earlier this year)


so - it's done roughly 150 miles on this engine build...  :wtf: 

Still - more than I've done on either of the BMWs this year! :wtf:


Current thinking is the lack of compression is one of the following:
Burnt out valve - maybe that exhaust valve seat wasn't as ground in as well as possible?
Holed piston - messy
Failed piston ring  - that'll do the bore no end of good.
Valve-springs failure - outsider in terms of likelihood, but it would create the symptom.

I won't know until I get in there properly.   The "silver-lining" is that none of it is scary stuff any more and I feel more confident about figuring it out.

However - there's other projects that need to be done, so I'm going to park the JAPton for a month or so and get one or two other projects worked on.

Well - I needed something to keep me busy over winter....  :roll:

[cardan]

Bummer. Fingers crossed that it's something at the lesser end of bad: maybe a valve stuck in its guide, just off the seat. Good luck.

Leon

[Rex]

Good call. I've had a new valve guide pick up enough to grip the valve stem. Can you hear any chuff-chuff noises through the carb or silencer when it's kicked over?

[Rockburner]

Good call. I've had a new valve guide pick up enough to grip the valve stem. Can you hear any chuff-chuff noises through the carb or silencer when it's kicked over?

Not that I noticed. There's what i can only describe as a "springy" noise happening at some point in the rotation though. I'm assuming it'll be a fairly obvious issue when i uncover it.

[33d6]

Years ago had that issue with my 1926 Matchless. Took months to track down a sticky exhaust valve that would stop the engine as soon as it got faintly sticky then free up when cooled down.
You show a rather black and oily sparkplug so you may have a similar problem, particularly if you have bronze valve guides. Sorry I don't really follow four stroke threads so you may have already gone through this.
Cheers,

[Rockburner]

Years ago had that issue with my 1926 Matchless. Took months to track down a sticky exhaust valve that would stop the engine as soon as it got faintly sticky then free up when cooled down.
You show a rather black and oily sparkplug so you may have a similar problem, particularly if you have bronze valve guides. Sorry I don't really follow four stroke threads so you may have already gone through this.
Cheers,

All information is noted carefully and will be referred to, I can assure you of that!   Thanks! 

However I think this is something else (or possibly a follow on issue from maybe a sticky valve) - the engine cooled down nicely while I was waiting for recovery and there's still no compression.

[33d6]

Thank you for the response. I hope you diagnose the problem quickly.
I had a bit of a thing about lubrication of early valves and guides at one time and found it far more subtle than I expected. Looks simple but not so. The lubrication of inlet valves is entirely different from the exhaust. I'm talking single cylinder engines here.
A sticky inlet shows itself by ferociously sneezing back through the carb immediately before the engine stops as you've lost compression. Any lubricant introduced to solve the problem gets washed off by the incoming fuel so needs to be either continually applied (very messy with exposed valves) or be both a lubricant and fuel resistant (bloody nigh impossible). It took years to stumble upon high temperature silicone grease which combined with the basic design of my Matchless, (fixed head with valve caps and a simple penny in the slot valve cotter) made whipping out the valve for a quick grease every now and then an effortless doddle. Problem minimised if not entirely resolved.
The exhaust valve is different. As the engine has total loss lubrication via a Pilgrim pump and no oil control ring by design, (why do you need oil control when you keep pumping the stuff in anyway. An oil control ring doesn't help), you can appreciate that the exhaust valve has a constant light spray of oil coming past it. No lubrication worries there. Nevertheless I'd have these mysterious stoppages every now and then for which I could find no answer. By the time I rolled to a halt, got the bike off the road, took off my helmet, put it on its stand, blah, blah, blah it had cooled down enough to have compression back and start but immediately nip up again so the cycle repeated. It took a long time and repeatedly checking out possible fuel and ignition issues before the light finally dawned. A sticking exhaust valve. A quick clean of the valve stem and problem solved. And the occasional clean keeps it that way.

Same engine, same lubrication system, same valve set up, same everything but absolutely opposite problems. One valve hopelessly under lubricated and the other over lubricated.

No wonder I enjoy proper vintage motorcycles so much. So much challenge from such idiot simple components. They make you think.

   

[R]

Bronze valve guides are well known for causing sticky valve syndrome in the vintage bike world.
Far better to do them in cast iron, like the motorcycling gods commanded !

I bought an old Enfield, in 'good running condition'.
Unknown to me, it was fitted with a new bronze exhaust valve guide - just before the sale.
Ran fine on a few short runs.
On its 1st long run, on a warmish day, the compression suddenly disappeared.
While we were standing around looking at it, there was a huge CLONK.
I'd like to report it then ran fine again.
But it didn't.
Had shed a pushrod (ohv) and despite having a fair toolkit, wasn't sufficient to refit it.
Trailered back home to the workshop, and that elusive 1/8"W spanner was put to good use.
Bit of a ream out, and was good to go ...