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Another one with spark problems

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Another one with spark problems

Post  pecon4 on Fri Mar 03, 2017 12:04 pm

When I bought my Falcon 74 it had been laid up for a long time. On checking for a spark I could find none so I ordered a new coil hoping that it might be a quick fix. I replaced the coil, condenser, HT lead and cap and when I had the points out I cleaned them as they appeared in good condition and replaced the LT lead from the coil for good measure. I gapped the points to 12 thou, replaced the plug and set the timing to just before TDC. The strange thing is that while I now have a spark at the plug there is nothing at the points. I double checked that I had not misplaced the insulating washer but all seemed fine. The carb was cleaned out but it is not even trying to fire. This is my first adventure with a Villiers flywheel magneto so I am doing a bit of head scratching trying to figure out its workings. Can anyone see something I might be missing?. Should there be a spark at the points just kicking the bike over?. The flywheel seems to have some magnetism as it picks up metal objects easily enough but is there a possibility that it is too weak and is there any way to do a rough test without specialist equipment?. I'm living in Dublin and I don't know of anyone here who can test it, while I will send it to Villiers Services if necessary I would like to eliminate other possibilities first.

Peter

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Poor spark

Post  kerabo on Fri Mar 03, 2017 1:28 pm

It sounds like you have covered most things. If you have a spark at the plug you can't be far out. A big spark at the points in a normal system would point to a bad condenser.
Have you timed the engine correctly?

Ken
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Re: Another one with spark problems

Post  pecon4 on Fri Mar 03, 2017 1:49 pm

Hi Ken. I did check the timing but I'll redo it next, just in case. I thought that the new coil might have been a dud as there is nothing from the LT side to the points but an ohm meter reading would seem to eliminate that if I am reading things correctly.

Peter

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Re: Another one with spark problems

Post  kerabo on Fri Mar 03, 2017 1:59 pm

pecon4 wrote:Hi Ken. I did check the timing but I'll redo it next, just in case. I thought that the new coil might have been a dud as there is nothing from the LT side to the points but an ohm meter reading would seem to eliminate that if I am reading things correctly.

Peter
When you say nothing from the LT side. This is while you are kicking it over?

You have a spark so so long as it's a good spark at the plug you sound on track

You say you timed it just before tdc. There is a cut in the crank end that I believe it tdc
With that at the top so TDC and the arrow on the flywheel at the cut away on the backplate should time it. When the flywheel is tight the points will open just before tdc depending on your points gap. Be sure you don't have excessive up and down in the main bearings so letting the points flutter.
Someone may correct me but that's how I understand it.

Ken
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Re: Another one with spark problems

Post  pecon4 on Fri Mar 03, 2017 2:33 pm

Well it now appears that my electrical problem may be fuel related Embarassed I retimed it as described but nothing until I sprayed some easy start into the carb as a last resort and it started for a second or two and will only fire when I spray into the carb and continues to cut out immediately. It seems that the carb is not doing its job so I'll strip it again and see if I can sort it. In any case progress of a sort. Thanks again for the help.

Peter

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Re: Another one with spark problems

Post  Kevin B Smith on Fri Mar 03, 2017 6:56 pm

Hi,

Try taking the plug out and squirting in some fuel, plug back in and off you go. If it fires the ignition is OK and it is either fuel or the crankshaft seals - having re-read the thread you say it has been laid up for a while - could be the seals are dry and you are losing it there?
Kevin

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Re: Another one with spark problems

Post  pecon4 on Fri Mar 03, 2017 9:10 pm

Hi Kevin

It does indeed run for a few seconds once petrol is introduced into the plug hole but dies immediately. Can you tell me how the crankshaft seals affect the running because as I mentioned I'm new to two strokes. The bike has indeed been laid up for a long time, perhaps as long as eighteen years but I have no idea if it has been started in that time. I just want to see if the engine will run while I still have it in the frame before continuing the strip down in preparation for painting.

Peter

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Re: Another one with spark problems

Post  Kevin B Smith on Fri Mar 03, 2017 9:27 pm

OK,

So this gets a bit 'techy'.

The way a 2 stroke works is as follows:

With the piston on the way up mixture is drawn into the crankcase through the carb. At the same time charge above the piston is compressed prior to ignition. Just before TDC the spark ignites the mixture.

With the piston on the way down (driven by the ignited mixture the charge in the crankshaft is compressed and then transferred into the combustion chamber, also at the same time the exhaust ports are uncovered allowing the spent gas to exit as the new charge enters.

So, if the seals have dried out, as they surely will have done in 18 years, you will draw air into the crankcases through the seals and less air/fuel mixture through the carb as a result. Also with the piston on the way down the compression can escape via the seals as well as being transferred into the combustion chamber.

Upshot being less mixture into the combustion chamber and also weaker (i.e. more air than their should be) - upshot usually being it will not ignite.

Fix is fairly simple - new seals which are easy enough to fit.

Kevin B Smith

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Testing flywheel magnetism

Post  piston 197 on Sat Mar 04, 2017 6:52 am

From your first post you asked about testing the flywheel.
place the flywheel on a flat metal surface, steel table or similar, attach a spring balance with a piece of wire or string around each web of the flywheel and pull it off the table evenly.
A flywheel in full health should take approx. 30Lbs force to pull it off the table, anything less than half of that and it is unlikely to work at all, and will get worse as it gets hotter .

That apart I would be looking to see whether the rings are stuck into the piston and be replacing the seals, one is behind the magneto back plate and the other is behind the engine sprocket, they cost about 5 each and can be removed by spiking with a screwdriver or similar without removing the engine from the frame.
JH
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Re: Another one with spark problems

Post  pecon4 on Sat Mar 04, 2017 11:42 am

Kevin, that was a brilliantly simple explanation and not at all techy, I understood every word. Very Happy . My intention is to do some work while the engine is out of the frame and the seals will be in that list.

John, I'll check out the condition of the piston and bore during the work and many thanks for the information on checking the flywheel. I have read of cased where they cause breakdowns when hot so I'll check that too as it will be a while before it goes back on the road.

pecon4

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Re: Another one with spark problems

Post  pecon4 on Sat Mar 04, 2017 4:56 pm

I decided to remove the head and barrel and found that the top ring was broken with half missing and the other half stuck in the grove which would account for the lack of compression. There were a ring of hammer marks around the top of the piston, but strangely none in the centre and corresponding marks on the head.

I measured the piston and bore with the piston coming in at 2.3 inches or 59mm and the barrel with similar readings. Is this a standard piston measurement for the 8E?. If I can figure out a way to link photos I will put a couple up.

Peter

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Bore size

Post  piston 197 on Sat Mar 04, 2017 5:08 pm

Yes STD size is 59mm, the ring hammer marks will only be around the edge of the top surface of the piston because that is where the dome of the head is smaller than the diameter of the piston, so ring bits are hammered between top surface of piston and squish area of head, there will be no marks in the centre as this equates to where the dome of the head is !

JH
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Re: Another one with spark problems

Post  pecon4 on Sat Mar 04, 2017 6:24 pm

Thanks John, I did not realise tolerances were so tight that it would not allow a foreign object to enter. So, a set of rings to add to the order.

Peter

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Re: Another one with spark problems

Post  pecon4 on Sat Mar 04, 2017 6:34 pm

I forgot to mention that when I removed the head there was no gasket fitted. Not sure what the PO was thinking when he did this. I once had a BSA A10 with two gaskets fitted, presumably to lower compression?.

Peter

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Re: Another one with spark problems

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