Hello everyone. I want to make this a DIY YZ250 Leak down tester thread. There isa thread on the Honda 2 Stroke forum, but its nottoo specific and doesn't apply to a YZ.Please make this a sticky so others can access it for future use.
There's no question the leak down tester is the most important tool you need for a 2 stroke engine, followed by the compression tester. A compression tester will not show you a air leak and pinpoint the leak.
A leak down tester is simple and can save you from burning down your top end with a air leak. Additionally you will never have to guess about replacing crank seals and try tocover up air leak symptoms with jettingever again. Sounds great doesn't it!
Simply put you take off the pipe, block the exhaust port over the spigot flange with a 1 & 1/2" rubber pipe plug. Next remove your carb and insert a pvc pipe with the same outer diameter of the reed block, and pressurize the system to 6 psi, some say you can go to 8 psi but any higher could blow out a crank seal- so I like to stick with 6 psi and its more then enough to show even the smallest leak.
Here's a pic of my $17 lowes leak down pressure tester. In this picture it is Set up for a YZ85 with a 1" 90 degree bend. You can do this by removing the carb and leaving the subframe in. I prefer to use a 90 degree bend so you don't have to remove the subframe and go from the back with a extension, but either way works.
First you pick up a 0-15 psi pressure gauge used to testing plumbing and natural gas systems, it is in theplumbing section at Lowes, I could not find one at my home depot. It has a 3/4" pipe thread on it. This gauge has a built in a schrader valve so no shut off valve is needed.
Next find the outer diameter of your exhaust spigot in your cylinder and pick up a rubber pipe plug. This Fits over the flange of your exhaust spigot on your cylinder. The perfect size for the YZ250 is a 1 & 1/2" rubber pipe plug. it is a tight fit solube it up before putting on.
Find the size of the inside of your reed block- where the carb Slides into your reeds, for my YZ250 I used a 1"90 degree bend piece of pvc pipe. AdditionallyIf your exhaust spigot or reed inlet is a odd size you can use electrical tape to make a tight fit. It is easiest to take the carb or even better take the reed block adapter with you to Lowes to find a snug fit. This of course is in 3/4" pipe thread- the same as the gauge. Or if you can't find a 3/4" thread 90 degree bendyou can use a 1" thread and then use a 1" to 3/4" adapter as I did.
I like using a 3" section of tube between the reed block adapter and the gauge so you have plenty of room away from the bike, that way I rest the tester on the footpeg. Use needle nose vice grips on your powervalve vent tube to block it off or find a plug for the port- either works. Me personally I like to use needle nose vice grips and fill the rest of the tube with your soapy water, then if it still leaks you can watch the bubbles. I also fill the end of the trans vent tube, you can also listen for the leak- but filling it with soapy water will show you even the smallest of leaks.
Pump up to 6 psi with a bike pump or electric tire pump which I prefer personally. Watch for pressure drop, they say less then 1 psi per minute is good, but ideally I don't like ANY leakage over 10 minutes. The MaximumI like to see is .1 psi per minute, and at that rate you can pinpoint it with soapy water in a spray bottle.My bike held 6 psi for 45 minutes. Didn't even loose .1psi. Look for leaks with soapy water and listen.
Upon pressurizing your system if you show a leakwith aYZ250the first thing I would look atis the powervalve shaft seal- this seal leak won't show any driveability signs because the powervalve is under positive pressure and vented anyways so it won't suck in any unmetered air.But in orderto properly pressurize the engine, this seal can't leak.You can spray soapy water around thePV sidecover and watch for bubbles or remove the cover andspray- it will beobvious. Replace the seal if yours is leaking so you can pressurize the engine. I had to replace mine.
If you hear air out of the Trans vent tube or see bubbles in the soapy water that you put in the end of the tube then you have a wet size crank seal leak. A popular leak is the reed block gasket especially with vforce3 reeds. Use sealer on new gaskets if yours leaks. If you do not see bubbles in any of those places then take off your flywheel cover & remove the flywheel. Spray soapy water around the crank seal but it can be tough to see with the stator installed. I like toremove the Stator so you can see the Whole crank seal and spray the entire area.
If you still cant find the leak spray around where both crank cases meet as well as around the whole base gasket and head gasket. If in doubt spray soapy water everywhere and watch for bubbles.
I do this after every top end or If I'm having any drive ability issues. Simple and you KNOW If you have a air leak. No need to replace crank seals without testing first- save yourmoney until you test.
Iwill upload some more pics from my phone in the next post.
