Pressure-testing a 2-stroke

[skipn8r]

Note: This specifically applies to the '92 - '01 CR250 engine, but it should be adaptable to any 2-stroke. The basic idea is to pull the carb and exhaust, plug one and pressurize the engine through the other to look for leaks.

It's very important to pressure test a 2-stroke after it's assembled. The reason is that a 2-stroke gets it's lube through the pre-mix. An air leak means your engine can run lean. Lean means too little fuel. Too little fuel means too little lube. Too little lube means death to your engine. So here we go.

First, you need to pull the seat, tank, shock, carb and pipe. Everything you need (other than a tire pump) is shown below.

The assembly below is basically a 1-3/4" PVC plug (same OD as the intake pipe ID) that is transitioned down to a 0-15 psi gauge, a shut-off valve and a Schrader valve (to connect a tire pump). The 1-3/4" PVC plug is a tight fit in the intake pipe, but it'll fit with enough persuasion. I used yellow heavy duty pipe tape (made for gas systems) to put everything together. I don't have all the technical names for everything, but if you take the pictures to a hardware store, they should be able to help you piece it together.

The item below is an adjustable rubber freeze plug. They are available at automotive parts stores in various sizes, but the closest one I could make work was a 1-3/4" version. You tighten the nut which squeezes two metal plates together which squeezes and expands the rubber piece between the plates. You have to tighten it a lot, but it makes a good seal in the exhaust port.

Before you install everything, make sure that the piston is at BDC (so that all the ports are open), the spark plug is installed and all other components are properly tightened. Below, you can see the gauge/valve assembly installed in the intake pipe. Make sure to tighten your intake pipe hose clamp tightly as the pressure will tend to push the assembly out.

Below, you can see the freeze plug installed in the exhaust port.

Once you have everything installed as shown, use a tire pump to pressurize the engine to 6 - 8 psi, then close the valve and watch the gauge for any pressure drop over a 5 - 10 minute period. A well sealed engine will hold pressure with no observable pressure drop for 10 minutes easily.

One common "leak" in a healthy '92 - '01 CR250 engine (and probably other types) is through the powervalve linkage. It probably doesn't affect performance, but it'll sure throw your leak measurements off. To avoid leaks, make sure the powervalve linkage is correctly lubed (per the Service Manual) with grease and Moly 60 Paste during assembly.

If your engine holds pressure with no observable drop for 10 minutes, you're good to go. If the pressure drops 2 - 3 psi or more over 10 minutes, spray soapy water all around the pressure test parts first to check for leaks there. If you find none on the test setup, start spraying the soapy water around the base gasket, head gasket, center gasket etc. Any pressure drop of less than 2 psi over 10 minutes is your call, but I'd definitely fix it.

I've had really good results by using Permatex Copper Gasket Sealant on the base gasket, PV cover gasket and reed cage gasket. I don't use anything on the head gasket.

[kuritaro9]

i vote this be stickied...thanks skip.

[cr125_killa]

yeah please some one makes this a sticky!! i willl need it later

[Faded]

M,

Great post and great pics, way better than my old school one with crappy photos. This tool is invaluable if you're serious about your 2-stroke.

[skipn8r]

This tool is invaluable if you're serious about your 2-stroke.

It sure is. BTW, you've got more experience in this area, so what's your opinion of acceptable pressure drop?

[Crazyced]

1. Was the PVC/pressure valve adapter (the white thing) hard to find?

2. Can you just send me the whole assembly?

[skipn8r]

1. Was the PVC/pressure valve adapter (the white thing) hard to find?

The gray PVC, white PVC and two left most brass pieces were all at Home Depot (don't know if you have those). I got the brass "T" and everything else to the right at a specialty connector/hose/hydraulics supply store.

2. Can you just send me the whole assembly?

Just send me your bike and I'll test it for free .

[cr2fidy]

Just send me your bike and I'll test it for free .

you'll pay the return shipping?

[Faded]

BTW, you've got more experience in this area, so what's your opinion of acceptable pressure drop?

Using the procedure you outlined;

Once you have everything installed as shown, use a tire pump to pressurize the engine to 6 - 8 psi, then close the valve and watch the gauge for any pressure drop over a 5 - 10 minute period. A well sealed engine will hold pressure with no observable pressure drop for 10 minutes easily...

it has been my experience that 'Perfection' is no observable pressure drop after 10 minutes...few can or will strive to achieve that. A good motor that is acceptable can still 'leak' up to 2-3psi over 10 minutes...more than that and you need to start looking. In regards to holding low pressure over a decent span of time...it doesn't take much of a leak to loose a couple psi. A leaking base gasket or reed cage gasket can cost you as much as 2psi in one minute. A regulated air supply (compressor + regulator) set to 10psi and hooked to your tester makes finding the leak(s) easier as well.

[Crazyced]

How did you make sure your tester was not leaking? What are the chances.

[Vintage Not Pimpage]

I really do not see this as necessary. If a 2 stroke, or any engine for that matter has a vacumn leak you will know about it by the way it runs.

[skipn8r]

How did you make sure your tester was not leaking? What are the chances.

I didn't try to test it independently, but the fact that I got no leaks with it installed means it's pretty tight. If you use all new parts and assemble them with high quality pipe tape, chances of a leak should be low.

I really do not see this as necessary. If a 2 stroke, or any engine for that matter has a vacumn leak you will know about it by the way it runs.

No, it's not necessary; it's prudent. It's much more convenient to find out there's a problem when you're putting your engine together rather than when you've got it all back together and going out for a ride. Also, when you're starting off with an engine combo for which you don't have a known jetting setup (new porting, new top end on a used bike, etc.), you may damage the engine by the time you figure out that your jetting isn't the problem.

[MikeD94]

Thanks Skipn8r for the writeup! For my '03 I used 1 5/8" dia plug similiar to your 1 3/4" dia plug. It fits perfect. The 1 1/4" PCV is a tight fit (actually too big but made it fit by stretching out the rubber intake). I used a dremel to round off the edges of the PVC and using a "ball" shaped bit on the dremel routed a notch about 3/16" away from the edge all the way around the PVC pipe similiar to what's on the carb. That way it would stay on better when pressurizing. Also, I find that you do not need a shut off valve since the Schrader valve is used (unless the tire pump is screwed on).

As for my test, it held 6.5 psi for about 5 min so the engine checked out good! I wanted to rule out the leak before I start chasing a lean condition with my swapped in Kehein PWK (well, I already did but that's another story).

[skipn8r]

I used a dremel to round off the edges of the PVC and using a "ball" shaped bit on the dremel routed a notch about 3/16" away from the edge all the way around the PVC pipe similiar to what's on the carb. That way it would stay on better when pressurizing.

Excellent idea ?.

Also, I find that you do not need a shut off valve since the Schrader valve is used (unless the tire pump is screwed on).

You're right, it's not necessary with the Schrader. I put it in because I have a variety of input connectors and some (like barb fittings) don't have an internal valve ?.

[Crazyced]

Thanks Skipn8r for the writeup! For my '03 I used 1 5/8" dia plug similiar to your 1 3/4" dia plug. It fits perfect. The 1 1/4" PCV is a tight fit (actually too big but made it fit by stretching out the rubber intake). I used a dremel to round off the edges of the PVC and using a "ball" shaped bit on the dremel routed a notch about 3/16" away from the edge all the way around the PVC pipe similiar to what's on the carb. That way it would stay on better when pressurizing. Also, I find that you do not need a shut off valve since the Schrader valve is used (unless the tire pump is screwed on).

As for my test, it held 6.5 psi for about 5 min so the engine checked out good! I wanted to rule out the leak before I start chasing a lean condition with my swapped in Kehein PWK (well, I already did but that's another story).

So the 3rd generation CR's need 1-1/4 diameter PVC pipe instead of 1-3/4 of the 01'?

On that picture, that's not the stock air boot is it?

[skipn8r]

So the 3rd generation CR's need 1-1/4 diameter PVC pipe instead of 1-3/4 of the 01'?

No, that's a typo. It's correctly listed as 1-3/4" in the original post.

On that picture, that's not the stock air boot is it?

It's a Boyesen Rad Valve.

[Crazyced]

No, that's a typo. It's correctly listed as 1-3/4" in the original post.

So same diameter for both engine generation then.

It's a Boyesen Rad Valve.

I was actually talking about the rubber boot. ? I see that the RAD valve don't use the stock boot like the VForce 3 does.

[skipn8r]

So same diameter for both engine generation then.

I used the original TMX as well as my PWK in the OEM boot and they both fit; although the PWK was a bit of a tight fit. So I assume the 1-3/4" PVC will plug either pipe...

I was actually talking about the rubber boot. ? I see that the RAD valve don't use the stock boot like the VForce 3 does.

The rubber boot shown actually comes as part of the Rad Valve.

[250Nasty]

1. Was the PVC/pressure valve adapter (the white thing) hard to find?

the white thing is just a threaded pvc bushing, i believe

How did you make sure your tester was not leaking? What are the chances.

spray it with soapy water

[ILUVHONDACR250s]

Great help, thanx!