C15 Breakdown, Hot Engine

[cdsdorset]

It's the horizontal screw on the carb side. Before you do anything take some time to read up on setting up a amal carb. If you can get a copy of the Rupert Ratio book. Having knowledge beforehand can save a lot of money in the long run.
You really need to know if the piston has "picked up" on the bore. If ithas there is no point going any further till that damage is rectified.

[Sticky]

Well bike won't start at all now.  It fires occasionally and tries, but I guess it then floods and then nothing.  If I take the head and cylinder off I presume I will see something then.  If what you say has happened am I best to get it honed (is that right) some new piston rings and put it all back together.  I should be able to do that but it won't be for a while.  I've lost interest in it now and will get out riding the B King.  Maybe later when I have got over the disappointment.

[Spaceman]

I honestly doubt if there is anything wrong with the bore and, even if there was, it would not explain why the bike is not starting any more assuming it turns over freely.

The C15 is such a simple bike, it's just a question of taking it step-by-step to work out what the fault is. If you have messed about with the carb, it's best to return it back to where you started. To change the mixture, it's normally a question of changing either the needle or needle jet or the main jet but from earlier comments on the plug colour, it doesn't sound as though there is much wrong with the mixture at the moment.

If it's now not starting, I would start by checking you still have a spark with the spark plug earthed through the engine. If you have a good spark then the problem is almost certainly carburation assuming you still have reasonable compression. One of the problems you can get with old British bikes is because of the state of the carb whose internal passages or jets can get blocked with crud over time. The other problem can be worn out needle and needle jet and a worn out throttle slide and bore resulting in air escaping past it.

I would therefore dismantle the carb and give it the best clean you can. Its difficult to tell whether or not the needle jet is worn and its sometimes best just to replace it with a new one. I would inspect the condition of the float and make sure the float needle is in reasonable condition. However, the most important thing is to check the condition of the throttle slide which should be a reasonably snug sliding fit. If there is significant sideways play in the bore then a new slide may cure that - unfortunately, if the bore is badly worn it may be a replacement carb. The last thing to check is the mounting flange and O-ring. The flange should be absolutely flat and, if not, needs to be rubbed down using emery paper on a flat surface.

Once the carb is back on, I would then check the fuel flow by taking off the drain plug and check (carefully) that you have good fuel flow and haven't got, for example, a blocked petrol tap. If none of the above suggestions gets the engine started again then a top-end inspection of the engine might then be necessary.

[Sticky]

Thanks Spaceman

The carb I can do and will start with this.  What I don't understand is why the problems that could occur with it would cause the engine to overheat.  Is it because the mixture becomes lean?

[cdsdorset]

If it has siezed more than once there are many good reasons why it won't start again.

[Sticky]

Yes, but not convinced it siezed.  When it stopped it wouldnt start again, but it would turn over.

[DAVE BRADY]

Hi,

It may be possible to get an idea about nipping up by looking in the plug hole.  In the shade, a small torch should light up the bore quite nicely and with the piston at the bottom most of one side of the bore can be seen.
A lean mixture will cause over heating.  As the fuel goes into the engine it absorbs heat and I think that a small part of the fuel air mix is exhausted taking a some out of the exhaust.  So without this the engine gets hotter.
What colour is the spark plug?

Dave.

[cdsdorset]

A motor can seize,cool down and run again. If you think about it an ignition or carb setting would probably not make the engine clattery (unless you mean it was pinking like hell).
Do you know if it has has a rebore recently? It's amazing how many companies have no idea about piston clearances.

[B Murphy]

At 13.06 look what happens and re-starts a few seconds later. This is nipping up I believe.

https://www.youtube.com/watch?v=_j9mCW6xghw&t=860s

[Spaceman]

I don't think you can compare what might have happened to a 2-stroke 125 cc D1 Bantam being thrashed up a long steep hill, a lot of the time in a low gear, with what might have happened to a 4-stroke C15 being used on ordinary roads. The poor old Bantam might well have over-heated and I'm surprised he didn't seize the engine completely - the D1 wasn't designed for that sort of abuse!

[cdsdorset]

Why can't you compare it?
An engine is an engine. If something is wrong you have the same result. Either could be badly recored, either could have wrong Carburton and either could have incorrect timing.

[DAVE BRADY]

Hi Sticky,

Where about do you live?  There may be someone close who is prepared to come and help.

Dave.

[B Murphy]

Well, the video was just to show what nipping-up is, that was all.

Sticky - if you live near me at Burton On Trent, I would be happy to take a look at your C15, I've had plenty of them but to be honest I haven't had a good one yet!

[Phil C]

Sticky, When you've sorted this out you'll have a great sense of satisfaction, and you'll have learnt a lot about the bike. Stick with it!    Phil

[Sticky]

Many thanks for all the replies.  I'm in Malton by the way.
I stripped the carb yesterday and cleaned it thoroughly, although it was mostly fine.  The pilot jet did appear gummed up so that is now clear.  But to no avail.  There is a very reputable garage in Malton who are going to have a look at it for me as I have exhausted my skill level.  Watch this space.  One thing I did notice is that in my carb parts 1, 2 and 3 are missing.  See photo.