Still wont Start!

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Stu.B
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Still wont Start!

Post by Stu.B »

Following earlier suggestions of why I am having difficulty starting my Arrow sport it is still refusing to start. Just to recap bought about 5 months ago as a runner. It started with no issues when i got it home and all seamed ok so I decided not to rebuild the engine just strip the bike and re spray the metalwork and sort out other issues seat, wiring etc. I did remove the cylinders and all looked good so I have now checked them again ( 20thou. Oversize ) and fitted new rings which the gaps are in spec. Carb cleaned and Aspen fuel In the tank. Timing checked and Drags electronic ignition fitted good spark. The only thing that I found and don’t understand is the low compression I am only getting 60/70 psi on both cylinders. Any ideas?
Stu
will_curry
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Re: Still wont Start!

Post by will_curry »

Lack of compression happens in at least two ways. First, there has to be some gas in the
cylinder to compress - did you open the throttle fully when making the test? Secondly,
what gas there is in the cylinder just leaks out quickly.

I have heard of pistons on back-to-front and home-made gaskets with the wrong shape.

Are you getting petrol to the spark plugs? I remember well the words of a Scott owner
'Two-strokes are like a woman - they need a stiff drink to get them going'.
Stu.B
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Re: Still wont Start!

Post by Stu.B »

Yes I opened the throttle and the pistons are correct plugs wet. New gaskets. Although it ran some months ago I have no idea of the engines history I think it may be lack of crankcase pressure so it looks like covers of and crank out.
Ivor Collins
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Re: Still wont Start!

Post by Ivor Collins »

Hi Stu
Cylinder pressure that low will make it very difficult, if not impossible to start the engine.
What I don’t understand is, if it was fine when you got it, why you would suspect the bottom end needs to be rebuilt?
Also, I very seriously doubt that any deterioration in the crankcase would cause such a loss in cylinder compression.

The disappearing compression is almost certainly due to the recent work undertaken.
Have the rings broken during assembly?
Did you check ring gaps before fitting?
And yes, if you removed the pistons, check for correct orientation.
Is the compression tester accurate?

The good thing is that all this is easy to check and correct.
Good luck with your quest and let us know how you get on/ what you find
Stu.B
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Re: Still wont Start!

Post by Stu.B »

All I did to the engine was remove carb. strip and clean. Removed the heads barrels did not remove pistons and they are the correct way round +20 thou. I removed the rings and the gaps were over spec. So fitted new rings all in spec. Removed barrels rings ok not broken. As I said I have no history but understand if old black seals were fitted they can fail very quickly?
will_curry
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Re: Still wont Start!

Post by will_curry »

The crankcase seals are essential to the correct transfer of mixture from the
carb to the combustion chamber. They had a short life when new, especially if the
engine was left standing. My own 5 year old Arrow needed the seals replacing
after standing for a year. Even if the previous owner replaced the seals if he didn't
use the proper, modern seals the 'new' ones would not necessarily last.

Nearly 60 years on, I have another Arrow waiting in the workshop for new crankcase
seals.
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