RZ/RD 350 & Misc. 2-Stroke Tech BBS

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 Post subject: Re: Runaway motor
PostPosted: Thu Jul 12, 2018 10:32 am 
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Joined: Fri Oct 03, 2003 11:11 pm
Posts: 3394
Location: Southern Ontario, Canada
Interesting observation, I always assumed it would pass air relatively quickly (pressure cycles 20 times a second at idle and I'm not pumping that fast) so I just test the whole engine. By "check both cylinders together" I assumed they were both being supplied air, not relying on the lab seal to connect both sides, so that is an important clarification. I plug the exhausts with blanking plates or rubber expansion plugs, depending on whether or not the exhaust spigots are on. I made plugs for both intakes with push style air fittings threaded into them, one for a high resolution low pressure gauge (0-30 psi by tenths) and the other for a schrader valve for a small hand bicycle pump. A pressure drop in either cylinder will show on the gauge since both manifolds are linked by the crossover tube. I also apply vacuum (per your suggestion years ago) to the air line with a Mighty Vac, and prop a reed on each side open with a cable tie while doing so just in case they work better than I expect them to.


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 Post subject: Re: Runaway motor
PostPosted: Thu Jul 12, 2018 2:24 pm 
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Joined: Wed Mar 24, 2010 2:20 am
Posts: 6461
Location: Eastern Ontario, Canada
Air will pass, but not at a sufficient rate to always show a small leak on the opposite side, in my experience. No one was more surprised that me with that discovery. Perhaps 30 years of use has caused an accumulation of now hardened oil in the lab seal, over oiling and so on, not to mention the many other bits and pieces that end up in the crank well over many years.

I now isolate each cylinder and test each independently. If reeds are installed I use a cable tie as you do.

I have had the blanking plug fly out of the opposite cylinder while testing, so there is obviously pressure there, but now I don't take for granted that this will always be the case. If you want to find a small leak on each side you should test each side. I have a good example for you. A few years ago I was testing a 1985 Canadian RZ350 engine that I had mounted in an engine stand on my bench. The left side tested fine and held pressure as required. When I tested the right side it would hold 3.5 PSI forever, but if I went above 3.5 PSI it immediately dropped to 0 PSI. When I tested the left cylinder the right side didn't leak. When I tested the right side it didn't leak until it hit more than 3.5 PSI. Once you got to just past 3.5 PSI it dumped it all though a small right crank seal leak. So 3.5 PSI all day long, but 3.6 PSI and you dropped to 0 PSI right away. My conclusion was that when testing the left cylinder the right side never hit more than 3.5 PSI. If it had done so, it would have indicated a right side leak. The lesson learned was never assume. Take the extra time to check each cylinder independently.

_________________
1985 RZ500
1985 RZ500 Project Bike
1986 RG500 ATR
1986 NS400R
1987 TZR250
1988 Honda RC31
2007 DL650 VStrom
2014 Triumph Thruxton Modified


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