Troubleshooting a JAP Special

[chaterlea25]

Hi R,
There is no need to pull off all the valve covers to check the tappets
Just make sure there is a little movement at the pushrod adjusters, pushrod free to rotate and a smidgin more on the exhaust side.
No gasket needed on the inspection cover, a light smear of Hylomar if you must

John

[R]

Not my bike John !

My JAP is of the sidevalve persuasion.
You can SEE the valve clearances !

[Rockburner]

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 ...

It's a Champion L86C.  From I've found to  read online about the plug itself, it seems about right.

But a new one is a good idea.  I meant to order one a week or so ago and forgot, Thanks for the reminder!

[chaterlea25]

Hi,
Sorry R I was abbreviating Rockburner

John

[R]

Heyhey, I resemble that remark .........

[Rockburner]

Hi,
Sorry R I was abbreviating Rockburner

John

I've been called worse.....

[Rockburner]

What sort of compression should i be getting?

I realise that it's a hard question to answer given that the engine is a bitsa with 350 and 500 internals rounding out to about 400cc, but any educated guesses would be appreciated.

Currently getting about 75psi.

https://flic.kr/p/2ozWQPH

[R]

Compressions are ALWAYS tested hot, with the throttle wide open.
Otherwise the results are a little haphazard.

Any idea/inkling of the compression ratio ?

75psi seems a little low - for a healthy ohv hotrod. ?
Tipping a teaspoonful of oil down the plughole, kicking over a few times and retesting is advised.
If the number jumps upwards, then the ring seal is not so good.
Either new rings or more running in is required.
Or testing hot, with that throttle wide open ...

[Rockburner]

So, some health issues have kept me out of the garage recently, but recovery is under way.

I've checked the valve clearances and they seem to be ok, ie there is play at tdc on both rockers when cold!

 I've ultrasonicly cleaned the Monobloc and reassembled it with the upgraded Viton tipped float needle and the rest of the refurb kit from Burlen.

I stuck some petrol in the tank and opened the taps.

No leaks was the first win!

I tickled it up but instead of fuel coming out of the tickler, it eventually started dribbling from the manifold, bit odd, but the fuel is flowing.

Kicked it a few times and eventually it fired up, bit rough at tickover, but some small adjustments fixed that.

Opening the throttle though and there's a lot of spitting and backfiring, flames from the open bell mouth. Not good. I'm now worried that the inlet valve isn't shutting correctly, with the throttle fully open you can look down the manifold and see the flame front!

Nevertheless I took some time to adjust the oil feed to the head which wasn't running, now running at (roughly) a drop every 5 seconds on tickover, just like the book says.

I'm going to try a smaller main jet in case it's just a case of over-fuelling, but if it's still backfiring I'm going to have to have another look at the valves.

[R]

The main jet doesn't come into play until the throttle is more than about 3/4 open,
so you can probably omit that step.
Dropping the needle a notch would achieve less fuelling, at lower engine speeds.

It does sound more like cam timing though.
Unless the inlet valve isn't seating too well ??

[iansoady]

I have never understood why the Amal tuning procedure starts with the main jet, which as you say isn't used until wide open throttle. I always start with a main jet I know to be too big and go through the process of setting up idle mixture screw, cutaway and needle in that order before even considering main jet.

People wlays talk about increasing main jet size when they fit a less restrictive exhaust system but unless they go everywhere with the twistgrip on the stop - and by the sound of it many do round here - that seems a bit pointless to me.

[Rockburner]

The main jet doesn't come into play until the throttle is more than about 3/4 open,
so you can probably omit that step.
Dropping the needle a notch would achieve less fuelling, at lower engine speeds.

It does sound more like cam timing though.
Unless the inlet valve isn't seating too well ??

Good point about the needle - I"ll try that (not sure what it's on at the moment tbh)

[john.k]

It appears to be a race bike ,so its going to have a pretty radical cam in it..........this may make it hard to check the valve  timing ..........and spitting back at slow revs may be a side effect of very early inlet opening.

[Rockburner]

It appears to be a race bike ,so its going to have a pretty radical cam in it..........this may make it hard to check the valve  timing ..........and spitting back at slow revs may be a side effect of very early inlet opening.

Its a fair point, I've no idea what cam is in there.  Just looking in the cam bix is an engineer out of the frame job (i think), and knowing my Dad, it may well have a "hot" cam.

What can be done to alleviate that situation?

[john.k]

What kind of motor is it..........I assume its a 4B or similar .....in which case someone like Cameron Engineering will be able to supply a milder cam......at a price,mind you.