Still wont Start!

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.
Stu.B
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Re: Still wont Start!

Post by Stu.B »

Having renewed the crank bearings and fitted new viton oil seals, new piston rings (stepped ones from drags for +20 standard pistons) the bike still won’t start. The max. Pressure is only 60psi. I am struggling to find the problem of the low pressure the cylinder bores are not showing any sign of wear and I deglazed the bores with a honing tool. As the new rings are stepped the gaps were 10 thou at the top of the bore and 12 thou on the running area. Any suggestion ?
If you didn’t see my earlier post I bought the bike as a Project about 6 months ago it started and engine seemed ok so all my efforts were spent on a complete refurbishment but left the engine as it appeared ok. Not I put it all back together did I find it wont start.
Help!!
Stu
Ivor Collins
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Re: Still wont Start!

Post by Ivor Collins »

The first puzzle is that low compression figure. It won’t start until you get it closer to 100 psi
But what technique are you using to take the measurement? The reading should be cumulative so, with the throttle open, continue kicking it over until the pressure no longer rises before taking a measurement.
Is the reading the same on both sides?
What was it before being stripped?
Is the tester faulty?
Are the head bolts pulling down on the heads or just binding on the threads giving the impression of being at the correct torque?

A fresh pair of eyes might spot a mistake, where are you in the country?
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Re: Still wont Start!

Post by Trev Sellars »

Also squirt some good old oil down the plug holes and this will help seal the rings a bit, particularly if you have just de glazed them. Ypu might be getting blow by.
Stu.B
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Re: Still wont Start!

Post by Stu.B »

Latest update
I have a spare engine and rebuilt it in the bench and it also has +20 pistons with piston rings within spec. I am using a new Sealy compression tester which gave a reading of 120psi when I tried it on my Moto Guzzi V50. The engine on the bench also gave low compression readings (no carb fitted.)I have been using a. Method Drags recommended which is to spin the engine with a electric drill via the alternator bolt. works well you can spin the engine faster than the kick start. I seem to be making progress just taken the Cylinders and Pistons off the bike and put them on the spare engine. Spun it up and now up to 100psi! Have put oil in the bore and squirted WD40 in the inlet in order to keep the oil seals lubricated. I have checked the bore piston clearance with a bore gauge and vernier both sets of cylinders and pistons just in spec according to the workshop manual.
Next step is to put the pistons and cylinders back on the bike and take more readings. All very difficult as don’t have a bike lift but not giving up! This is the most difficult bike I have restored look forward to a trip down the road. I am in Rainham Kent thanks for your advice and encouragement.
Stu
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