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

I reed it a lot here, not on other fora. I wonder why you people do that, as to me it makes no sence at all.

Putting preasure in the crankase makes the crank seals seal harder as the preasure presses the rubber onto the shafts. But leaking crank seals appear 99.99% of the time when there is vacuum in the crankcase. To me that means you are leak-testing a situation that is not relevant to the real world.

If not for the crank seals, what else would you be testing ? If the crankcase halfs are air-tight ? I certainly never have had leaks between the metal of the cases, nor have I heard from anyone they had them.

If not that either, am I missing something ?

Probably correct it is not a perfect situation on crank seals but will show any major areas of leakage. Have also not seen the actual cases leak but have seen leaks at intakes (both sides of reeds), PVs, base gaskets, head gaskets etc. If you were dealing with all brand new factory parts may not be a huge reason to do it but mostly dealing with old parts.

As said its usually the red block area that leaks and you really do want to know about that.

ALWAYS PRESSURE TEST. It will save your engine.

JanBros wrote:Putting preasure in the crankase makes the crank seals seal harder as the preasure presses the rubber onto the shafts. But leaking crank seals appear 99.99% of the time when there is vacuum in the crankcase.

I thought a proper test was done with both pressure and vacuum.

Maybe it's best to perform a pressure test, as well as an atmospheric pressure test :D

Pressure and Vac would be ideal, but if all you have is pressure (and everyone has that with a bike pump) then do that. Any leaks will kill your engine very quickly and its very simple to locate them on the outside (reeds, cyls etc) with pressure and a bit of liquid soap in water, or use window cleaner which works well and leaves your engine streak free, lol.

I just use a MityVac to pull a vacuum and a small bike pump for pressure. Most important is an accurate low pressure gauge, trying to see change on 6 psi on a 2" diameter 120psi gauge is pointless.

JonW wrote:As said its usually the red block area that leaks and you really do want to know about that.

ALWAYS PRESSURE TEST. It will save your engine.

never done it, and never will. If you make sure all your surfaces are clean and straight, you will not have a leak.

just out of curiosity, how much preasure do you put on it ?

So, your belief is that only members of this forum do pressure testing?

It is a proven fact that the ONLY way to test cranks seals is to pressure and vacuum test them. Well, I suppose there is one other way, and that is to just keep living in denial and wait for a meltdown.

Indeed John.

For the minimal cost and effort to prove the engine is airtight its a no brainer.

You can lead a horse to water... etc.

JanBros wrote:
never done it, and never will. If you make sure all your surfaces are clean and straight, you will not have a leak.

just out of curiosity, how much preasure do you put on it ?

Here was your basic answer:

RuZty wrote:I just use a MityVac to pull a vacuum and a small bike pump for pressure. Most important is an accurate low pressure gauge, trying to see change on 6 psi on a 2" diameter 120psi gauge is pointless.

Never Too late to learn something.
It's not always clean nor straight, you can have wear, warpage, incorrect tq settings, brand new parts that are out of tolerance.
I can say it's quite likley, if you've never checked your motors, you probably have had this issue, your just lucky...that's all.

I personally know quite a few AMA
& 3 Former Motorcycle
GP race team mechanics and a leak down is just standard even in trade schools it's taught as basic. Many times a fresh rebuild is leaking, and many, many motors won't hold pressure/vac..but they will still run, some for a long time... others not so lucky, you sieze. A air leak is hands down the most common cause of 2 stroke failure, with many incorrectly blaming ,fuel/oil, ignition,jetting etc as the cause...

Here's a good read which can shed some light on the subject for you. . As the saying goes, Harry Klemm has forgotten more than most of us know...lol
read his tech article under Air leaks.

http://www.klemmvintage.com/" onclick="window.open(this.href);return false;

JanBros wrote: never done it, and never will. If you make sure all your surfaces are clean and straight, you will not have a leak.

I would agree with your principle except that after assembling an engine with meticulous care and still finding small leaks, you don't know if you don't check. I will also always measure the piston and bore before I assemble an engine rather than trust the machine shop to have done it right. Luck is not a measurable quantity.
It is also a good metric for the state of your motor through it's life, just as a compression test is. If I have the carbs or exhaust off I will take the time to do a test to make sure I don't have any surprises looming.

Smoker is right! You need both Vacuum and pressure test.
Pressure tests make it easier to find the leak. And, by the way, your crank is pressurized briefly on every stroke so those seals need to work in both directions. For a REAL mechanic, there is no substitute for a good leak down test after a rebuild. I blew up an IT175 dirt bike 4 times under warranty (90 days). The Yamaha dealer never did a leak down until the area-rep came asking about all the claims on my bike (slightly mismatched case-halves). That's when the dealer learned the value of a leak-down or vacuum test.

You can buy a brake vacuum pump with a gauge attached at Amazon or any auto-parts store for about $20 or $30. I see leak/compression style gauges on Amazon starting at $30. You can get some freeze plugs at the hardware store.

On the other hand, you could just spray a little starter fluid around your ignition rotor with the engine running at idle or around the base gaskets and even near the reeds if you have an air-box. The idle will go up noticeably if there is an air leak.

My issue with leaks and spraying starter fluid is the danger of a run away engine. I had this happen to me with an MT250. It was a total rebuild and everything was up spec'd, new of refurbished. I didnt own a tester back then, I wish I had. The bike started and idled ok. I revved it a bit and it was ok too. ticked over some more and then the idle rose a bit. I blipped the throttle and suddenly it took off. keyed it off and it kept going, faster and faster. At this point its revving waaay thru the redline. I couldnt get behind the carb so sat on it, yanked the brakes on and dumped it into gear and let the clutch out. it left a 4ft skid mark across my garage floor but finally it stalled. Scary as hell and im sure it ruined the crank as it never ran quiet after that. Such a shame, and it could have been avoided, there was a leak around where the carb bolted to the CR250 cylinder, that's all.

JonW wrote:My issue with leaks and spraying starter fluid is the danger of a run away engine. I had this happen to me with an MT250. It was a total rebuild and everything was up spec'd, new of refurbished. I didnt own a tester back then, I wish I had. The bike started and idled ok. I revved it a bit and it was ok too. ticked over some more and then the idle rose a bit. I blipped the throttle and suddenly it took off. keyed it off and it kept going, faster and faster. At this point its revving waaay thru the redline. I couldnt get behind the carb so sat on it, yanked the brakes on and dumped it into gear and let the clutch out. it left a 4ft skid mark across my garage floor but finally it stalled. Scary as hell and im sure it ruined the crank as it never ran quiet after that. Such a shame, and it could have been avoided, there was a leak around where the carb bolted to the CR250 cylinder, that's all.

The Spraying starter fluid thing is really one of those tests that I only do to confirm the suspicion of a leak when the bike/sled/lawnmower is all together and starts acting weird. I would not recommend it as a reliable test. Leak testing is a great way to check your work BEFORE you put everything back together and give it a rip.