Allen Scythe starts easily, but won't rev

[vhgmcbuddy]

Spent a day getting an old Allen Scythe with Mk25c engine I bought up and running. It must have stood for many years as the petrol had gone solid (looked like clumps of soot). Having thoroughly cleaned out the tank, (copper) fuel pipe, fuel filters, carb, exhaust, air-filter, plug, points, etc. Re-set the points and timing after which it fired on first pull and started on second pull.
Only problem is the engine won’t rev up. When you open the throttle, it just sounds like it is being choked and a little drip of fuel comes out of the carb.
Oil is also leaking from where the engine meets the gearbox (oil seal failed I assume)

I re-checked the points gap and the timing which were fine.
I checked the float which was not full of fuel
I have carefully mixed the correct fuel mixture and shake the tank before starting

Any ideas why the revs don’t increase?

[expeatfarmer]

My guess would be a blocked main jet in the carb.

[wristpin]

My guess would be a blocked main jet in the carb.

Also worth having a look at the “other end” – carboned up exhaust port or even a choked muffler.

[andyfrost]

Take into account that the 25C on Allen Scythes have a form of electronic govenor , basically the points cam opens out centrifugally , this leads to what you could say is erratic running.
I would try it at work , in other words under load , and see if the engine smooths out.

Andy.

[vhgmcbuddy]

Thank you as always for the useful replies.
I have checked the exhaust port which seems fine and I also ran it for a minute with no exhaust – still the problem persists.
I took off the carb and the main jet did indeed look blocked – cleaned it out and I could see light through it and thought that would solve it, but still no change.

Yes, I was going to ask about the govenor, as I read about this and noticed it when setting the points. Will try it under load (I have a big patch of stingers and brambles to try it on) and let you know if there is any change under load.

Thanks, Ben.

[wristpin]

Just to enlarge on the governor situation. When it cuts in it is a very distinct sound as the engine in effect four strokes or may be even eight strokes – once heard, for ever remembered, but not what I would class as rough running, as soon as the load goes on the engine starts to sing.

[vhgmcbuddy]

Just to update.
I took the carb off one more time to thoroughly check everything again and all seems well. Unfortunately, when re-connecting the copper petrol pipe I managed to strip the thread where it connects onto the carb.
I also changed the spark plug (used the once from my mk 1 clifford – has same engine as the allen scythe) and it seemed to run and rev much better despite petrol leaking out of the stripped thread!

So this brings up a few questions.
1. could a wrong or ‘bad’ spark plug stop the engine from revving properly?
2. What spark plug should be used in an Allen scythe – the one that it came with had k-17 on it, but the ground electrode looked very worn and thin.
3. What is the ‘fix’ for a stripped thread in the carburetor?

Many thanks

[stuart]

Have you another small Villiers “Junior” type carburettor to try on it ?.If not you could try the larger “Lightweight” carb from your Clifford rotavator and see if the problem persists.

[vhgmcbuddy]

I tried using the carb from my mk1 clifford rotavator, but it still doesn’t rev properly. I also tried putting the machine under load; it’s like it gets quite close to going into that purring mode, but not quite. It does cut the brambles ok, but just doesn’t quite sound right.
I also looked at the govenor, but that seems to be fine. Would the points gap affect the revs? Does anyone know what the points gap should be? Any other ideas as to what the problem could be would be appreciated.

[wristpin]

This may be of interest, I hope that you can print it out for easier reading!
https://www.dropbox.com/s/33v45tlt239r0ur/Villiers%20Mk25c%20two-stroke0001.pdf?dl=0

I notice that someone has noted an alteration to the BTDC timing dimension – may be a clue there?
I also see that on page 3 the plug gap is given as the “expected” 25thou but on page 7 18-22thou.
Fair amount of suggestions under “engine four or eight strokes” and “engine lacks power”.

[Author]

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