View Full Version : Building and operating a leak down tester. My technical process and suggestions

Billy Golightly
08-02-2007, 09:53 AM
If you search around on the old Mac Dizzy site (use Google) you can make a pretty cheap one. I opted to use a radiator cap tester as a base of the tool because it was just much better quality and the dial is not tiny. If you go to an auto parts store and buy a radiator cap tester, have them make a custom hydraulic hose to fit on the end of it that is 1/8th male pipe thread WITH A SWIVEL! If you don't get the swivel you will be cussing yourself trying to get the thing into place and trying to read the gauge. I then turned a piece of solid round aluminum stock on the lathe to fit in my intake boot (I turned mine about 40mm OD if I remember correct. BUT, I've already stretched the boot for a 39mm carb so you might want to go with like 36 or 37mm if you have a smaller regular size carb) drill a hole all the way through it big enough to tap and thread the 1/8th male pipe thread fitting from the new hose onto one end of it. Its a good idea to put a good chamfer or bevel on the end going in the boot, makes it easier. Use a little bit of thread tape or some thread sealer like loctite to ensure theres no leaks there. You'll need to take your pipe off and then remove the ehxaust flange. Make a plate to bolt flat against the cylinder in its place. You'll probably have to add silicone to the back of the plate everytime to put it on to do a test.

Now, for the actual testing. The "Common" test point and procedure is 7lbs of pressure for 7 minutes. Take note though, this does NOT mean it can bleed off 7lbs in 7 minutes. It has to hold the 7lbs entirely without dropping any for the 7 minutes. In my opinion 95% of the machines that complete the 7lbs for 7 minutes test will fail it. I went through a great deal of trouble on the motor in my flat tracker now (The stock one) to make it seal 12lbs over night. This is borderline eccentric, but, its actually a pretty decent power boost once you get the crankcase and the motor COMPLETELY sealed off. It makes jetting infinitely easier to do also.

When :D you do find a leak, the best way if its still in the bike is to use some soapy water to spray around the base gasket, the reed cage/boot area, the plate over the exhaust. The next place if you don't get anything there is to put the transmission vent hose close to your ear and listen to it, or try the soap water there. If anything comes out then you gotta go in deep. Probably a right side crankcase seal, possible a center case gasket (which is what I've been fighting with on another motor because the surface was warped. ) The head gaskets will definitely leak also, but they are pretty much impossible to tell from the outside. I just go by the process of elimination. The intake area don't leak, the exhaust don't leak, the base gasket don't leak, the crank seals and center case gaskets dont leak, then its probably the head gasket and I replace it.

I've got about $90 in the radiator cap tester, a couple bucks in the custom hose, and then just some time machining the round stock. I've used it so much in diagnosing and finding problems. I'd reccomend EVERY 2 stroke owner to build one of these and run the test. If it doesn't meet the standard test, plug the leaks and I GUARANTEE that you will feel a substantial increase in seat of the pants power.

08-02-2007, 11:14 AM
excellent thread Billy.
Also for the exhaust side,,sometimes you can use a rubber expandable freeze plug for car engines to seal the exhuast.

Billy Golightly
08-02-2007, 11:40 AM
Thanks Mosh! The plumbers pipe plug is a good idea, I had heard about that before but had forgotten about it.

Here is also a list I'm compiling of possible places for air leaks to show up. I'm going to break them down into 2 sections. Airleaks that could make the engine run peaky and pipey (actually pulling air in) and airleaks that would make the engine bog, smoke, and generally seem like its impossible for to clean out which would be ones that suck in transmission fluid. This assumes all your nuts and bolts are already tight :) Keep in mind though that the leaks might not always be gaskets, but the actual surface being warped or un level. For your own sake, hope that this is not the case because parts with warped surfaces are a pain in the ass to deal with.

Set #1: Actual air leaks, areas that will pull in outside air.

Base Gasket
Reedcage to cylinder gasket
Reedcage to intake boot gasket
Rubber intake boot
Left crankcase seal
Head gasket*

Set #2: Airleaks that will suck in transmission fluid, oil, or water causing the engine to bog uncontrollably and never "clean out"

Right crankcase seal
Center case gasket***

*This probably would not apply to liquid cooled machines in this section, as they would run into the following problem.

**On liquid cooled machines, normally when you get a headgasket failure they can pressurize the coolant system or even allow coolant into the combustion chamber/crankcase if it leaks bad enough. Anti-freeze has a distinctive smell when it burns, can't miss it.

***On the 250Rs I've found that a common area the center case gasket fails is the small piece of case separating the counter balancer chamber and the crankcase. If you have the right side engine cover off, you can remove the gear set, pull the counter balancer out of this "chamber" and then pressurize the inside of the motor, see if you hear or feel anything coming out of the area where the 2 cases meet together at.

Billy Golightly
08-02-2007, 11:43 AM
Thats a neat setup there Billy. I need to test my 85 250r. I just haven't scratched up the bread to make one. What position should the piston be at when doing this test?

If your concerned about testing the head gasket BDC is probably the best choice since the transfers will be open to the cylinder. Otherwise it doesn't really matter :)

08-02-2007, 12:35 PM
Thanks Billy, that is pretty creative.

08-02-2007, 07:11 PM
Great post Billie

I'll just add that MOST engine leaks are to poor disassembly procedures and not inspecting the surfaces before reassembling.

Screwdrivers ruin everything, I use a plastic hammer to get everything apart or a brass punch on an outside area. I also try to loosen all the bolts in a criss cross pattern and in an even manner, it is aluminum ya know.

I use a countersink to slightly deburr every tapped and clearance hole. I also run a fine stone over every mating surface with WD40 on it until it is all on the same plane. You can see the high spots disappear as you run the WD40 soaked stone over the aluminum.

Proper torquing during assembly is necessary too.

08-03-2007, 05:56 PM
I have to ask. Why not just buy a tester they don't cost that much? I have one & I don't remember what it cost but it wasn't that much.

Billy Golightly
08-03-2007, 06:55 PM
I've never seen one that goes into the intake before? If they are out there thats cool.

08-03-2007, 07:02 PM
AHH Ok...Now I see. I was thinking about one that goes in the plug hole. Why do you test thought the intake and not the plug hole?

Billy Golightly
08-03-2007, 07:53 PM
Well I guess you could test it from the plug hole, but when you go from the intake boot your (IMO) getting a better test since your also checking the boot, the reed cage, etc. I don't know for sure but it seems like if you tested through the plug hole it would push the reeds closed and you wouldn't really be able to test the intake area.

08-03-2007, 10:07 PM
Right on. I just read a write up a on testing 2 stokes, so I have a better idea of what talking about. I really don't know much about 2 strokes. I have only ever owed 2.