Hi all, after many hours of engine pressure testing, just about to give up, I’ve finally found an air leak.
Its on the top bolt of the front engine mounting, coming out of the end of the thread.
Does anyone know the fix for this? Is it simply a case of taking the nut and bolt out and applying thread lock/sealant or have I got something more involved on my hands?
Thanks in advance… 
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Gosh, i don't know about all these pressure test. Sounds like you may have caused one where the wasn't one before & have forced out some sealant from the center case joint. Not sure what i'd do now apart from nothing & tighten all case screws behind the magneto & all others then stop testing it. What running issue are you trying to rectify. Could get sealant down the engine mount hole & try to spread it around but i don't know, sort of chasing your tail a bit. If its minor it may not affect running as there's a lot of sucking & blowing going on in quick succession--maybe i should rephrase that lol.

How bad is the leak? 

That's actually a common place on a 360/400...  I think it happens when someone rides with loose engine mount bolt?  Don't know for sure but engine mount bolt wasn't tight to spec on bikes I've seen this on.  Only way to fix it right is to split the case and reseal.

If leak is minor, you can choose to run it...  last one I saw was about 1 psi/minute with engine mount bolt removed.  That's pretty bad.  But with mount bolt in and torqued, it slowed way down.

 

Hi MarkT,
You’re spot on. 1psi/min is the leak.
I’ve cleaned/tightened the bolts and applied some thread sealant.
I’ll give it 24hrs (blue loctite says 24hrs to cure) and will test again.
Will report back.
Hopefully it will do the trick - really don’t want to split the cases….

I kind of agree with RT325, I fully rebuilt my engine and didn’t pressure test, I just assembled by the book, used a high quality sealant, checked all parts were clean and good and torqued every nut and bolt.
Fingers crossed, no noticeable faults.

I have never pressure tested a motorcycle engine in my life! rebuilt and worked on hundreds of 2 stroke motorcycles. In fact the only place i see people doing this is on this forum?    

Short answer:  Serious racers and people who don't like to waste hours and hours of time chasing down rideability issues that might be caused by an air leak spend a few minutes to pressure and vacuum test. 

If the bike is running good, no real need to do it. 

I first read about it in the early 70's in a "Motorcycle Repair Manual" written by Bob Greene and the editors of Motorcyclist magazine.

Never really did it until the 1980's when a defective new crank seal cost me a big race and a freshly rebuilt engine.  (If you want to win at racing the engine is tuned to the edge, air leak = meltdown) 

Once you have the test set up stuff  it's so easy to do at the end of a rebuild that for me it's foolish not to...  countless times I've found air leaks on fresh rebuilds. Some major, most probably wouldn't be noticed.  But leaks tend to get worse, not better.  Even if not rebuilding, if you have the carb off and the pipe off a pressure test takes like 5 minutes. 

On my super DT50 project I used a brand new YZ85 engine and it had a pretty decent intake manifold leak right from Yamaha.

I've also had new seals leak...  like the one in my race engine that made me a believer in testing.  And a couple new seals since then.  So a new seal doesn't always mean no leaks.  I've found porous castings and all sorts of gasket leaks.  Crank spacer/sleeves on the clutch side on the RD's seem to be prone to leaking. Stuff that's nearly impossible to find any other way is easy to find with a pressure/vacuum test.  

One of the most surprising was the bike the first enduro I bought after not riding for decades and then reading an article about the first Dave Meis "Endurofest"....  1979 DT175 in really nice shape that had a classic air leak symptom...  it would rev to the moon on its own and not shut off with the key.  Classic air leak symptom.  Previous owner spent more than I bought the bike for at shops...  including the Yamaha dealer...  and nobody could fix it.  So he gave up on getting it running for his sons to ride and just wanted it sold.  I was a little heartbroken when I looked at the receipts he gave me when I bought it and saw the dealer had pressure tested it and it passed perfectly.  I did my pressure test and sure enough it held pressure perfectly.  I mean held 5psi for overnight with negligible loss.  It was when I tried the vacuum test I found the vacuum bled off faster than I could pump the mityvac!  Had to be a seal so I packed thick grease around the ignition side seal (easiest to get to) and held vacuum.  New dual lip seal from an IT175 and bike ran like new. 

The surprise was that was the first time I came across an engine that held pressure perfectly but not vacuum.  Usually once I've fixed all the pressure leaks, it holds vacuum fine. 

If someone doesn't want to pressure/vacuum test...  the answer is simple:  Don't. 

In my experience it's worth the minimal effort...  No damage can happen unless you go over 5 or 6 psi.  But if I didn't have the set up to do it and the experiences I've had over the years...  including several cases of quickly finding rideability issues caused by air leaks (rather than "guessing" and playing with jetting, etc)...  I probably wouldn't do it either. 



 

 

Good news - I have fixed the leak that I had.
It’s now down to half a psi per minute. Not too sure if that is ‘acceptable’ or not? - please let me know your thoughts (directed to the pro pressure testing members of the forum) 
Thanks again…
 

Starting at 5 or 6 psi, if a used engine loses more than one psi in three minutes, you may start to notice issues. 

If it loses one psi a minute, you'll definitely notice issues and it will be down on power. 

Whatever you did to "fix" it...do more of that. Not there yet.