Pressure Testing

[Wilf]

Having done a fair bit of work on my 2MA last winter making quite a few improvements, brakes, wheels carbs etc I have been riding it a fair bit and enjoyed doing so. It has revealed a few issues with performance and seeing as I have done very little to the engine I felt the urge to try and improve things. I have had a set of Martins pipes for quite a while and had plans to use them on my other 2MA which needs a major rebuild but that plan has slipped down the list as I have issues with other bikes that need my limited time. So got the pipes cerakoted and opted to fit them to my more road standard blue 2MA, very nice they looked and sounded too but there was a few issues, a water leak, a fouled plug and general not happy engine in my opinion, time to do something I thought so before I knew it engine was out and top end off.

I dived into you tube and decided to do the following, buy a bore gauge and test the bores, they are definitely round all the way down the bore with less that .001mm difference, the only concern was that the bore measurement was 55.96  which was quite a bit smaller than the 56.40 that the Yamaha manual recommends, any answers to that one? I did check the measurement with a micrometer which confirmed this seems to be correct. The bores were mirror shiny so bought a honing tool and dealt with that issue too.

The other thing I thought was prudent was to pressure test the crank case and this is where the fun and problems really started.
I made up a pressure tester with parts from the local industrial parts supplier, the first test provided very little pressure but a lot of air coming from the power valves, so, head off again, strip out power valves, vapour blast them, fit new O rings and oil seals, rebuild and test again. Still loads of air coming out of the power valves so hit facebook for answers and apparently it is not possible to make powervalves air tight.

So I thought I would return to the true experts here on pure2stroke spirit, facebook is easy but I am not convinced that I am getting the right guidance and I am sure there is people here that really do know what they are talking about.

First issue, leaky power valves, is it possible to seal them? if it isn't how do I manage to pressure test the crankcase seals?

Second issue, I have both exhausts sealed with bungs and the right hand intake manifold sealed with a bit of a heath robinson bung but when I test the left hand cylinder the right and intake bung blows out (sometimes) That  to me is not right as there is a Labyrinth in between the two crank cases, so why is there pressure in the right hand crankcase? or is it likely that this seal has gone which is causing the poor performance issues

As you might gather I am trying to learn a lifetime of two stroke experience in a short time, so any pointers in the right direction would be most appreciated.

[biggz]

First off - my pony isn't a 2MA, but close enough, Secondly, there are loads of members of this forum with more experience than I have.

i) If I read the parts diagram right, there is a link pipe between the two intake rubbers, so at that point you'll be testing both sides. Is that where you've applied the pressure source and the bung, or are you downstream from them?

ii) How much pressure are you applying to blow a bung? Much more than 6psi doesn't seem to be recommended.

Good luck

Laurie

[Wilf]

Your absolutely right! the bloody link pipe! it seemed a lot easier than this on the you tube video.
I need to blank that off as well, not to mention the oil pipe going into the inlet rubber.
Pressure wise I have not seen anything over 3 psi on my pressure gauge and only realistically looking at 5 psi max but seeings it is leaking quicker than I can put it in with a small air source I am doing it as gently as I can with a compressor and listening to air leaks as well as looking for bubbles from detergent spray.
Thanks for the reply this is beginning to be a real pain but I can see the benefit from the results if I actually get any.

Update, after thinking about this in bed last night.
I did take into account the link pipe, which is why I blocked up the right hand intake manifold, the idea was that I would be able to test both crank cases at once. I know that would mean that the centre seal is technically untested but it was at least a step in the right direction. How often do labyrinth seals fail anyway?

Bottom line is that I probably need to find a way to block both power valves as well as the exhaust ports, this is becoming more hassle that it seems to be worth.
I see a lot written about the benefits of a pressure test but in the real world, has anyone actually achieved it and more to the point how?

[thump566]

SouthcoastRich aka "Derestricting my...." thread includes a video link he created about using a leak down kit. It's on a 3XV but could be a useful reference.
Sorry I can't find the link for inclusion but I'm off grid and it's not a good signal here.

[Wilf]

Thanks for the reply, it was in fact this video that sent me off down this road. The fact it is V twin might make it easier, I'm not 100% on that though and he does not seem to have any issues with his power valves leaking at all. maybe I need to watch it again but I don't recall it was something he blocked prior to testing

[rz500guy]

Patience grasshopper. I am also a novice but i did check my 3ma when I built it. Sure miss that sexy thing. I would use something with less pressure available than an air compressor. You do not want to blow out the crank seals. A foot pump, blood pressure bellow, or aquarium pump. I would not be too concerned with the PVs leaking. It is on the exhaust side. Are you using OEM seals or aftermarket. My TZ leaks but just makes it hard to keep clean.  The leaks I would be concerned with would be the reed cages, carb rubbers, and base gaskets. My reed valves leaked and I had to use a bit of Honda Bond to get them to seal. Do not over tighten or they will warp. If you put it together well such as cleaning crancases, cylinder, heads etc chance of a leak would be slim. I am not familiar with that bike but would think it uses regular base and head gaskets. I use some grease on the base gasket and take my time tourqueing the cylinders. Head if you lap them using something flat and and lapping compound or sheet of sandpaper to be sure they are flat not much else you can do. Good luck. Do not overthink it.

[Wilf]

Thanks for the reply, is my frustration that obvious?
Air pressure wise I did get ball pump to pressurise the crank cases but the amount of leakage meant that i could could not pump quick enough to get even 1 psi on the gauge, I am being careful with the compressor giving it very short and light bursts, even then it leaks out almost immediately.
As far as cleaning parts is concerned, I am fortunately have a vapour blast cabinet so can get everything off the surfaces that should not be there.
I have changed the inlet ports for genuine, which are not cheap by the way! all the other parts and gaskets come from Yambits so feel reasonably sure that it is all pretty good even if it is not all genuine parts.
Building top ends is not something I am totally alien to, I used to race these bikes back in the day so it was a regular event, especially as I used to race with a single ring so each meeting needed a new set of rings. The main difference is that it was a new bike back then, it is now a 40 year old bike with 40 year old problems.
At least the top end is in better condition than it was, cylinders honed and power valves cleaned and rebuilt with new seals and O rings.
I do wish I could get a good pressure test result because that would tick an awful lot of problem boxes and allow me to look at other areas with some surety.
Oh and by the way, I might be back to you when I tackle my 3MA which is also not running 100% but I thought I would tackle a bike I at least had some knowledge of and see what I learn before pulling an unfamiliar bike apart. Fortunately I only have one bench and like to get a job finished before starting another otherwise I will end up with umpteen bikes laying around in bits with nothing to ride, which is the point of having them isn't it?

[Rasher]

Andy at 2T engineering did a leak down test on my 2MA - just bunged up intakes and exhaust pipes IIRC.

Lowish pressure- 5 or 6 PSI I think.

[Wilf]

That is more or less what I have done, I wonder why mine leaks through the power valves and yours doesn't?

[thump566]

Just a thought, are your blanking plugs sealing 100% ? My first set were plastic and we discovered not sealing properly until replaced with machined ones.

[James P]

That  to me is not right as there is a Labyrinth in between the two crank cases, so why is there pressure in the right hand crankcase? or is it likely that this seal has gone which is causing the poor performance issues

I'm not sure if I have understood your question correctly (apologies if not ), but the labyrinth seal only 'seals' when the engine is running - it provides a tortuous path for the gases between the two halves of the crankcase. When the engine is running, gases from one half of the crankcase don't have enough time to get through to the other half before the pressure reverses. This effectively seals each half of the crankcase from the other, even though there is actually a path between them.
When carrying out a pressure test, you must test both halves of the engine together. As I infer you have found, trying to test one half by itself will allow pressure to leak through the labyrinth seal to the other half. As long as you can seal both intakes and exhausts, the link pipe between inlet manifolds shouldn't cause any difficulty...unless there is a leak where it plugs into the rubber of course.

The 3MA and 3XV models have a 'proper' oil seal on the crankshaft between the two crankcase halves, so each half should theoretically be able to be pressure tested individually (perhaps owners of those models can confirm...?).

Regards,
James