SIDE VALVE - STICKY EXHAUST VALVE

[rsawatson]

I have recently rebuilt my side valve MM engine. It's been running so sweetly since getting it fired up and I couldn't have been happier.

However, yesterday, the engine all of a sudden started misfiring when pulling away from a roundabout. A quick roadside test and some spark plug changes ascertained the problem to be due to a lack of compression on cylinders 1 and 2, rather than a lack of spark. Fearing head gasket failure, I had to get the car recovered home, which spoilt an otherwise brilliant day.

On getting home, I tested the compression and it was nil on cylinder 1. I took the cylinder head off and (to my relief) discovered the problem was not the head gasket. However, the lack of compression was due to the fact that the exhaust valve on no. 1 had stuck open on this cylinder. Light pressure with my finger caused it to ping back into place. (Given that the engine seemed to definitely not be firing on cylinder 2 when I tested at the roadside, I wonder whether the valve also stuck open on this cylinder and then snapped shut by itself. I can't prove that for definite though.)

The engineering firm which I have used before fitted the valve guides and valves, and I remember them saying that they had checked and ensured that the clearances between the stems and the valves were adequate/correct. However, I am now concerned that this may not be the case. Could anything else be causing the issue? Was it just bad luck and is it likely to bed in and therefore not repeat? Is there any kind of lubricant I ought to apply to the portions of the valve stem that slide up and down in the guide to help prevent it from happening again? I will be absolutely gutted if the whole lump has to come out again... I have worked so hard and from I can gather it is nothing that I have done.

I would appreciate your advice. I'm trying to remain optimistic.

[Mick Lynch]

It’s binding because of excess heat causing the valve stem to expand in the hot exhaust gas. I’ve noticed that the dreaded E10 gas causes my engine to run slightly hotter. I spoke with an older guy at work who used to be a chief engineer in the merchant navy (and what he don’t know about fuel and bunkering is not worth knowing) and he says the extra oxygen in the ethanol causes a leaning effect on naturally aspirated engines so mix kero or diesel to slow the burn and reduce heat. A lot of people in the pre war motoring community do this and let’s face it you have a pre war engine in a series MM!

There’s not much you can do to lubricate the exhaust valve stems. The oiling is by mist, not pressure fed so not fantastic. Doped fuel with oil or redex burns off during the ignition stroke so never reaches them.

Make sure the valve face is wide enough so the seat dissipates the excess heat.

Were the springs replaced too? Could be a weak spring, also I’d check the springs are centralized in the wells. A good wiggle and rive backwards and forwards will tell you this.

Also check the tappet barrel, I’ve had one of these hang up on a sharp drilling just below the cam. You can fill these with oil as they are hollow.

[philthehill]

I have had the same problem with my 1980s 4 stroke motorcycle. I have added 2 stroke oil to the petrol and that cured the problem.
Unleaded fuel has no lubricating qualities and so needs a helping hand.
I am certain it will get getter with a few miles of use. So in the mean time I would recommend that you use a upper cylinder lubricant.

[rsawatson]

Thanks both for your responses. They are consistent with what other people have told me, which is basically 'don't panic' and just take it easy.

I have been advised that using an artist's brush to paint some Wynns Super Charge onto the upper stems of the valves has proved effective too, doing this several times to allow it to seep down. This makes sense when you read that one of its key properties is that it 'strengthens the oil film under high temperature, high shear conditions'.

[svenedin]

If all else fails one option might be to use Tetraethyl lead additive in the fuel as the engine would have originally had. https://tetraboost.com

I have no great expertise with your engine (or any engines for that matter) but an old friend had 2x Morris 8's which I believe use a similar side valve engine. Lead was a useful lubricant for valves (apparently it stops microwelds forming which can cause the valves to stick as well as pit when the welds tear apart) as well as its octane boosting properties.

Stephen

[oliver90owner]

I will be absolutely gutted if the whole lump has to come out again...

Definitely not such a case. Even if the suggested remedies do not work it will only be a case of cylinder head removal, valve removal and a minimal reaming of the guide - as long as it is the culprit.

[rsawatson]

Thanks again everyone.