16H Power Loss

Previously david_roberts wrote:

I have a WD 16H that is not particularly original, having been civilianised by its previous owner in the late 1940's. I bought the bike as a basket case, and have concentrated on getting it to a running condition - being able to ride the bike being more important to me that its original condition.

I had the bottom end rebuilt last year, and have a good standard barrel/piston. Sparks are good from a rebuilt magneto, and I purchased a new carburettor - the one arriving with the bike being in pitiful condition. It starts easily, but after riding for a few miles loses power - and feels as if its seizing. Let it coast for a minute and power returns - disappearing quickly if used for a few mins. The engine has done no more than 50 miles since being built, and I'm at a loss to what is causing the power loss. It was so bad this weekend I was sure the piston had started seizing, so I removed head and barrell. Piston and bore are fine, as are little end and big end - as far as I can determine without splitting the crank cases.

I can see no cracks in head or barrell, and head gasget looked OK. I did notice very slight seepage of gas from around one cylinder head bolt when the engine was running - but can find no evidence where this might have come from (no cracks, head gasget OK etc)

I'd appreciate suggestions as to help identify the problem and to resolve this...

As you have checked that the piston hasn't nipped, the first thing I would do is make sure your valve clearances are correct as side valves tend to run hot over here.

regards

Hi David, to eliminate the possibility of a seizure, when you feel the sudden power loss, close the throttle and simultaneously pull in the clutch, If the engine continues to idle, it's not seizing. Check the petrol tank breather as this will give the same symptoms you are experiencing. This is also easily done by, when you feel the power loss on the road, flip open momentarily the tank cap to let the tank breath. Also, check the fuel flow to carburettor, there may be a partial block in tank filter or tap, though I doubt it if you are travelling a few miles before the symptoms appear.

Classic symptons of too tight valve guides causing compression loss when hot.

There should be no pressure difference when opening the tank cap. Sounds like the engine is burning the fuel faster than the air can enter tank to replace the lost volume. I have been also caught out myself with a blocked vent.

A few weeks ago my 16H stopped suddenly at a junction. One of the valve clearances had closed up. and there was no compression. A few minutes with a pair of spanners sorted it - (followed by panic when the engine stopped again- I hadn't opened the fuel tap again!).

So if you lost compression, it's the valve clearance - asIan says above. - but if (as you hint) the tank pressure changes when you remove the cap then the hole on the cap needs clearing.

There isn't a lot else to cause the problem if the mag is OK unless it is seizing. But you'd know about that...

Have fun with it. I enjoy mine ...

Incidentally - it's a funny old world when we worry about a bike 'not being original' when it turns out that the loss of originality took place about 70 years ago! That isn't 'non original", is it? It sounds more like history!