Author Topic: vapour lock  (Read 703 times)

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BILLINGTON

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Re: vapour lock
« Reply #15 on: 26 May, 2022, 22:06:54 »
Part of the problem is that the ignition advance/retard lever does not stop in position I have turned it to. It slowly returns to closed which equates to full retarded. The friction available on the lever does not seem able to overcome the spring in the magneto.

LightningAndStarfire

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Re: vapour lock
« Reply #16 on: 27 May, 2022, 16:57:17 »
An infrared thermometer would perhaps be a sensible investment. You could easily measure the carb body temp and much more.

idie

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Re: vapour lock
« Reply #17 on: 27 May, 2022, 18:04:28 »
That is why the magneto was changed to slack wire advance.

BILLINGTON

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Re: vapour lock
« Reply #18 on: 29 May, 2022, 08:42:35 »
Iím a little embarrassed to tell you this, however the reason why the ignition lever would always return to retarded was because the nut had worked lose, after tightening the nut I can run on constant full advance. I had previously identified that this nut would work lose so had put some lock wire around it to stop it turning, however over time the lock wire around the nut was insufficient. When I first realised this I thought that I had finally found the solution, but Iím afraid Iíve only found part of the puzzle, because after riding four miles on full advance at 17 C (air temperature) the bike was so hot that smoke was coming from the cylinder head. So itís not the ignition timing.
Should I take the cylinder head off an look for heavy carbon deposits, could it be that I need to decarbonise my bike, would that make it run so hot? 

Spaceman

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Re: vapour lock
« Reply #19 on: 29 May, 2022, 09:02:50 »
I would still double check the ignition timing is accurately set - if you inadvertently run it retarded, the engine will get very hot.

I would then check the mixture to see whether the engine is running lean. Ideally, you need to run the engine on the road at constant throttle and, if possible, stop the engine and coast to a stop before checking the colour of the plug. A moderate amount of black deposit would be normal but a light brown and very little deposit would signify a lean mixture.

If not ignition timing or mixture, it might be necessary to check valve timing and the condition of the valves. I'm not sure a coked up engine would lead to over heating but it would certainly produce poor running and erratic idling.

BILLINGTON

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Re: vapour lock
« Reply #20 on: 29 May, 2022, 09:14:45 »
Thank you spaceman for your post, I will do what you say and post an image of the spark plug.

BILLINGTON

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Re: vapour lock
« Reply #21 on: 29 May, 2022, 16:46:30 »
My thinking has moved on to the point people have been making  about weak fuel mixture and leaks at the head gasket. I had checked and tighten the cylinder head bolt as part of my spring service notes after taking the bike off the road in the autumn, but I now realised that I had set my torque wrench to 30 newton meters, but my notes state 30 foot pounds, which would equate to 40 newton meters.

When I stopped the bike yesterday smoke was coming out of the head gasket, which would indicate that is not hermetically sealed, there was also a shiny oil mark on the cooling fins that come from the cylinder head. All this would point to the cylinder head not being completely airtight. If smoke can get out air can get in during the intake stroke.

I will set the cylinder head bolts to their correct torque setting and see if this makes a difference. If it does not I will remove the cylinder head anneal the copper gasket again and refit the cylinder head.

Spaceman

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Re: vapour lock
« Reply #22 on: 29 May, 2022, 19:46:36 »
I doubt if a leaking head gasket would make much difference to the mixture - it leaks because the pressure inside the cylinder during combustion is >> 500 psi whereas it is very low during induction where the path of least resistance will be through the carb. Therefore, it is still worth doing the plug colour test.

BILLINGTON

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Re: vapour lock
« Reply #23 on: 31 May, 2022, 12:35:23 »
Yesterday, I tightened the cylinder head bolts to 30 foot pounds, then set the engine at TDC using a gauge I made from a wire coat hanger. After this  I set the contact breaker points gap to 0.003 with the ignition fully retarded, these are the instruction in the BSA book for my model.

Today I went for a half mile ride; I stopped early because it had started to rain hard; however I could see smoke coming from the cylinder head again. Hence the next thing I intend is remove the cylinder head anneal the copper gasket and use Wellseal to improve the seal.

I will take photos of the head and post them.

Thanks again for your advice.

BILLINGTON

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Re: vapour lock
« Reply #24 on: 31 May, 2022, 18:48:40 »
The pictures with this post are as a result of me taking the cylinder head off today, because it was leaking smoke which was not solved by tightening the head bolts. Also the bike is running hot possibly due to the air fuel mixture being lean due to the air being drawn in past the cylinder head gasket.

Pictures

1, The spark plug with a light brown indicating hot or lean
2, Contact breaker when the piston is at TDC. The point gap is 0.003 in this picture. This has been set in accordance with be information given by BSA for this bike.
3, Copper head gasket before cleaning. You can see the black marks at the top and bottom of the gasket where smoke was coming out.
4, The Copper head gasket after being cleaned with a Brillo pad, scotch brite and annealed.
5, Cylinder head before cleaning. You can see the shiny marks where smoke was escaping.
6, Cylinder head after cleaning.
7, Piston and valves before cleaning. Again you can see the shiny marks where smoke was escaping.
8, Piston and valves after being cleaned with a Brillo pad, scotch brite.



BEVANC

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Re: vapour lock
« Reply #25 on: 01 June, 2022, 20:23:59 »
An interesting difference in spark plug position compred to my 600 sv Ariel - my plug is directly above the piston.
 
You are lucky the gasket did not blow out - my Ariel has blown two gaskets over the years. I lapped the head  flat on a surface plate to try to prevent a repeat. Both the heads and the barrels do distort due to the heat.I may yet need to get the barrel face machined flat. A fellow Ariel VB owner has had his head & barrel machined and lapped together and is running without a gasket.

If you haven't already put the head back on, i'd suggest checking it for flatness on a surface plate or sheet of glass.

Initially, I had to tighten up the screw on my advance retard lever a lot to prevent it shifting as i ride - but it was my fault as i had sprayed CRC on the mechanism to inhibit corrosion, making it too free to turn. Took it apart, cleaned it and reassembled and its now ok.

As with your BSA, after a ride, the carb on my Ariel is far too hot to touch. Lack of clearance over the magdyno and from the carb mouth to the oil tank means that it would be impossible to add anything more than a very thin a tufnel spacer, but it never seems to cause any issue. Side valves do run really hot.

I hope your bike runs better once you getthe head back on. I must say, your initial descriptions made it sound like fuel starvation (e.g. blocked breather hole in the petrol cap; clogged fuel tap/filter etc..)

Cheers,
Bevan. 
« Last Edit: 01 June, 2022, 20:32:04 by BEVANC »

Mike Farmer

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Re: vapour lock
« Reply #26 on: 01 June, 2022, 21:08:23 »
 :) :) :)

Wow I'm seriously surprised that you needed to use the valve lifter with that problem. Just dont know.

Mike 8) 8) 8)

Spaceman

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Re: vapour lock
« Reply #27 on: 01 June, 2022, 23:12:12 »
I think you definitely need to check the flatness of the mating surfaces on the barrel and the cylinder head - the head in particular may be distorted giving rise to the leaking head gasket. One way of doing this would be to gently bolt on the head without the head gasket and then use a feeler gauge to check for the size of any gaps. Worst case, you may need the head skimming.

I have to confess I have no experience with side valve motors but the thing that struck me is the lop sided combustion pattern shown on the top of the piston, possibly due to the spark plug being positioned above the valves. I assume there was a reason the plug wasn't positioned above the centre of the piston? From the photos, it looks almost as there is a crack in one of the valves or is that just a surface flaw?

BEVANC

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Re: vapour lock
« Reply #28 on: 02 June, 2022, 02:14:11 »
That will be machined slots in the valve head - my Ariel has those too. Here's a picture of my barrel.

And my previous comment about spark plug position was bollocks - don't know what I was thinking ! Heres a picture of the head with some bluing on it when I was checking for flatness.
« Last Edit: 02 June, 2022, 02:36:55 by BEVANC »

Spaceman

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Re: vapour lock
« Reply #29 on: 02 June, 2022, 06:31:34 »
It wasn't the machined slots I was referring to but the strange feature pointed to in the image below. For the Ariel, it would be interesting to know whether this shows the same combustion pattern on the top of the piston as the BSA? This could be normal for a side valve engine but does not look ideal!