View Full Version : Make a leak down tester for about $20

04-16-2009, 08:56 AM
I found this article for a 250cc 2-stroke leak down tester and it all makes sense. I just need to go buy the stuff and try it.

The parts I used are as follows including the price I paid for each:

* 3/4" Pipe Low Pressure Gauge 10-15psi (used to test natural gas pipes), Home Depot - $12.98
* 3/4" to 3/4" Pipe Coupler, 1-1/2" Long, Home Depot - $0.74 (Should buy 90 degree elbow for better viewing)
* 1-1/4" to 1-3/4" Schedule 40 PVC Pipe Cap with 3/4" internal threads, Home Depot - $0.75 (Goes on intake boot; measure beforehand)
* 1-5/8" to 2-1/4" Expansion Plug, Local Auto Parts store (maybe Home Depot) - $2.49 (Goes on exhaust port; measure beforehand)
* a bicycle pump or hand pump maybe ask for pump for gas line tester at Home Depot.
* pipe dope sealant or teflon tape.

When assembling the Pressure Gauge/Coupler/PVC Cap I would recommend the use of some pipe sealant/dope. I had some laying around so I didn't include this in the cost. My carb is a 38mm Mikuni TMX and the PVC fits very snug. I would imagine this would work for most bikes equipped with a 38mm carb. For the exhaust I realize some of the bikes might be different. Don't worry, you can get the expansion plugs in 1/8" increments and they expand to 1/8" over there 'listed' size (my 1-5/8" will go up to 1-3/4") and the cost is relatively the same.

After you’ve built your leak tester you’ll need to install in onto your motor. The idea is to pressurize the engine and monitor the loss of pressure over time. It has been recommend that you start with 6psi over a six-minute time period. Ideally your pressure loss should be minimal at most, but if you loose more than 1 psi over a one-minute period, you have a serious problem. While you’re loosing pressure you can trace the leak by squiring soapy water, Windex, etc. over all gasket surfaces and/or other suspect areas. If you see bubbles you’ll know that you’ve found your leak.

If your motor has a power valve you usually have to block those off some how to do an accurate test.

Anybody have a home made one? Pictures? Tips?

04-16-2009, 09:22 AM
When you buy the parts and build one, take pics of the process and post them up along with how well it works. I think it would make a great addition to the world class help section if you want to take the time.

04-16-2009, 09:40 AM
I was thinking the same thing. I am going out to start my Tecate soon and if it is smoking real bad then I'll be making one.

04-17-2009, 09:59 AM
Here's a pic submission from someone.


04-21-2009, 12:46 PM
From gaz:
This is how I made the leak tester using just PVC pipe, rubber joiner and pump.
On the carby side glue in a valve off a tube (used RTV compound), plug power valve vent, then plug exhaust port, put stroke at TDC with plug installed then pump her up. You can see the leak when you spray soap and water on the main seal. This can be done with motor in the frame.
Trying to rejet with this problem drove me nuts!!!!! Then I saw a thread on building this. Problem solved.

04-21-2009, 12:52 PM
From erbe:

Took me a few years to build mine (6 Quads) need testing
Pump from a medical supply store, 15 lbs air gouge Mc Master Carr, Rubber block offs Home Depot or Mc Master Carr, Others piece are too much time on my hands some aluminum,band saw and lathe, drill press and some air fittings.


05-10-2009, 03:02 PM
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 (http://www.thumpertalk.com/forum/showthread.php?t=386977). 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 (http://www.permatex.com/products/automotive/automotive_gasketing/gasket_sealants/auto_Permatex_Copper_Spray-A-Gasket_Hi-Temp_Adhesive_Sealant.htm) on the base gasket, PV cover gasket and reed cage gasket. I don't use anything on the head gasket.

05-12-2009, 08:26 PM
Thanks for posting this info.
It got me thinking.
I work on cars.We have to use a pressure activated machine to test for emissions leaks in the EVAP system.

We use a machine like this..

It is pricey for the everyday ATV tech.
But since I have use of it, I am going to use it from now on to Diagnose 2 strokes.
It comes with a assortment of blockoff plugs and adapters, and could be clamped right into the intake boot for the test. The pressure can be regulated. 1-30 PSI.

When you suspect a leak, you hit a button and it forces smoke thru the component you test.
There is a Ultraviolate lamp that you hook to a battery to search for the smoke leakage.
It works really well on cars.

I bet many guys could go to a auto shop and pay to have this done somwhere between 40-70 bucks with the engine on a bench.
I would think it would pinpoint the leaks way before soap and water.
For instance, if you suspect a crank seal, you would see smoke coming from the case vent, or oil fill plug, or thru the stator wire grommets.It would definatley pinpoint base gasket and case leaks.
We use it to detect vacumn leaks also in intake manifolds on cars.
It is the cats ass for finding air leaks.

Just figured I would pass the idea along for other Auto/ATV mechanics, that may have acsess to a machine like the one I posted.

05-12-2009, 09:29 PM
$745-$1095. Wow!

But that is a great idea to take your engine to a friendly mechanic with a vacuum leak tester.

04-01-2010, 11:31 PM
I'm definitely going to build one of these kits. Thank you for all the great ideas.

04-15-2010, 12:12 PM
Nice except youve gone to a HUGE amount of trouble to do it incorrectly.
The intake/crankcase operate in SUCTION (vacuum) why are you testing it under positive pressure?

Yes, 29 inches of mercury is 14 psi, so the magnitude is right, but its not testing the system as it should be.

Just drop a vacuum line from a running car engine into the case and read the pressure drop through an orifice. No extra hardware except a needle valve.

PS and check the transmission case also, i just had to do a complete teardown because the case halves leaked oil everywhere:(

01-25-2011, 01:47 AM
Seems right to me...if you want to check for leaks on something put some pressure to it and spray soapy water on it. If it leaks the soap bubbles so in turn you now know you have a gasket that is SUCKING air when the motor is running. I need to make one of these because I think I have a base gasket or main seal leaking...Thanks for the info and pics guys!