Wyvern cylinder head sealing

[Andrew EvansParticipant @andrewevans67134]

I am hoping to get some help with an issue I have finishing my Wyvern IC engine. I basically finished it over a year ago but couldn't get it to run, so I put it aside for a bit.

I think that one issue that is stopping it running is that the valve housings are not properly sealing in the cylinder head. With the head, valves and valve housings fitted to the engine if I cover the inlet and exhaust holes with my fingers and rotate the engine I can feel (high and low) air pressure – with the valves fully closed. So air is getting from the head, past either the valve seal with the housing or past the valve housing seal with the head.

The housings should seal at the end. I machined the head and housings carefully and then used a fine grinding paste to get a close fit and the fit seems ok, I have blued the mating face of the housing and it seems to be a good fit with the head. I machined the 45 degree back face on the valves at the same topslide setting as the valve housing and again using grinding paste to get a good fit between the valves and the valve housings.

I would be really grateful if anyone has any advice on this. Can I use some sort of a seal or do I just need a tighter fit?

Thanks in advance. Andy

[Andrew EvansParticipant @andrewevans67134]

[Brian Sweeting 2Participant @briansweeting2]

My thoughts as someone who has not made that engine ….

Although you have ground the valves in was the final grinding paste fine enough to leave a smooth surface in the valve seat and on the valve head?

Are the valves fully closed when the rockers are relaxed?

Are the valve springs strong enough to hold the valves closed on the suction stroke?

[Roderick JenkinsParticipant @roderickjenkins93242]

I used some Loctite 5990 **LINK** to seat the valve inserts and head.

HTH,

Rod

[Andrew EvansParticipant @andrewevans67134]

Brian – thanks for these thoughts. Your first suggestion may have hit the nail on the head and I have to say it's something I hadn't even considered. The stuff I used was part of a kit I had lying around designed for grinding valves in car engines – probably too coarse. After reading your reply I just used some Peek fine metal polish on the valve seats and then twisted the valves by hand for a few minutes until it felt very smooth, cleaned it all up and refitted the valve housings. I am now struggling to feel any air getting past. So I will now fully reassemble the engine and see if I can get it to fire up. If I can I will be extremely happy. If not it will probably be the next issue to do with the carb or the timing or the points

Yes, the valves are closed when the rockers are relaxed, I have been testing this with rockers removed to rule out that possibility. The valve springs feel strong enough – but I don't really know.

Rod – thanks for that. I have some of that product and thought about using it but wasn't sure if it would work at the temperatures next to the exhaust valve. Did you use it on the end of the housing that is inserted into the head or on the flange that is bolted on the surface of the head – or both?

One issue I have found with the head in this engine is that the alloy casting, once machined to drawings, is very thin and weak in places. One tapped hole for a valve housing had to be so shallow that the thread stripped – I have got around this for now by loctiting the stud in place.

Thanks again and I will post an update.

[Andrew EvansParticipant @andrewevans67134]

I am still not getting any ignition with this engine. I am using a small Irvine carb from a model aircraft engine to rule out issues with the carb I made. The timing seems OK, with the inlet and exhaust valves opening and a spark happening at broadly the right times.

It feels like there is some compression from the piston – is it worth me testing this (I don't have a compression tester) and what sort of value do I need as a minimum for this sort of engine?

If anyone has any ideas it would be great, its driving me mad.

[Roger BParticipant @rogerb61624]

The compression does not have to be high but there must be no leaks. The usual description is 'bouncy' or 'springy'. You should be able to flick the flywheel in the direction of the compression stroke and it should bounce back.

Are you using a homemade or commercial sparkplug? Do you have a spare? DOA is not unknown.

Have you checked the complete valve and ignition sequence with the sparkplug resting on top of the engine so you can see the point of firing.

Inlet opens around TDC.

Inlet closes just after BDC

Spark plug fires just before TDC

Exhaust opens just before BDC

Exhaust closes around TDC

What fuel are you using? Old petrol can have lost most of it's volatile components.

Best regards

Roger

[JasonBModerator @jasonb]

This will give you an idea of the "bounce" that Roger mentions, until the rigs bed in compression can be a bit low.

Also most of these engines need a very small needle valve opening otherwise they risk flooding. On check for mixture that I learnt from Graham Corry of Alyn Foundry is to hold a flame a couple of inches from the open exhaust and turn the engine over, a big soft orange flame as the unburnt air/fuel comes out means you are too rich but a small almost blue flame is what you are after.

[Andrew EvansParticipant @andrewevans67134]

Thanks Roger and Jason

The fuel is petrol, fresh and expensive low sulphur stuff for 4 stroke lawn mowers. The spark plug is commercial and does spark at the correct moment. I am getting that bounce albeit not as strongly as in that video – it's a smaller engine though.

The timing seems fine when I move the engine slowly by hand, but something must be wrong. Fuel is dripping from the carb air intake on the intake stroke and I don't think any is making its way into the head.

[Roger BParticipant @rogerb61624]

How are you turning the engine to try and start it, by hand or with an electric drill?

Are you sure you are turning it in the correct direction (ref. fuel coming out of the carb air intake)?

Is the exhaust passage free? Pressure in the cylinder when the inlet valve opens could also cause blowback.

Best regards

Roger

[Ian S CParticipant @iansc]

Is the spark plug wet if you remove it right after cranking the motor? If the fuel is getting to the combustion chamber, and there is a spark, you should at least get a pop or two, or some smoke, even if the timing is not quite right.

Ian S C

[Andrew EvansParticipant @andrewevans67134]

Thanks Ian. I am trying to start with a drill.
The sparkplug isn’t wet, I don’t think there is enough compression to draw fuel in through the carb. I am going to take the engine apart and try to be a bit more careful and maybe remake a few things that I am not happy with. I got excited when finishing the engine and wanted to see it run, so I probably wasn’t as careful as I should of been. I was never that happy with the head, the casting is so thin in places that some of the studs for the valve guides are really weak. The plan is for a cast iron head but the casting is aluminum alloy and it’s not really meaty enough. I also messed up the sparkplug thread initially and had to helicoil them. Ideally I would like to remake the head from bar stock.

[Paul KempParticipant @paulkemp46892]

You could try the dodge the AA seem to recomend to verify timing belts intact and give it a whiff of easy start! If you don't get a pop or a bang with that then you really do have a fundamental problem!

Paul.

[JasonBModerator @jasonb]

Andrew did you try the flame held to the exhaust while turning over by hand? if it is drawing in fuel/air then it should ignite when expelled. If it is drawing in fuel you should see the unburnt fuel working its way out of the open end of teh cylinder and building up in the bottom of the crankcase casting, it will also be black as the rings bed in.

I don't start any of my open crank engines with a drill, mostly just flicking the flywheel over or on the odd one I have used a starting handle to get then set up initially.

What diameter is your aero carb venturi, you could sleeve it down a bit to give more velocity which should draw the fuel. Another test for sealing is will it draw fuel up when you put your finger over the air inlet to prime the engine?

[Roger BParticipant @rogerb61624]

This is the 'bounce' I get on 25cc engine with single 125mm flywheel which is nearer your size.

Do you get suction at the inlet port when you turn the engine by hand?

Looking at the drawing on the Hemingway website the valve cages have ports in the side. Is it possible to assemble them the wrong way round?

Best regards

Roger

 

Edited By Roger B on 08/11/2018 11:02:32

[Andrew EvansParticipant @andrewevans67134]

I am not getting as much bounce as that Roger. I do have the valve cages the correct way round

I am going to spend some more time with it over the next few days and see what happens. Thanks all for the advice, it really is appreciated.

Andy

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