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 Leak-Down Test vs. Compression Test
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Andy Van H
Green Bay, WI

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USA
110 Posts

Topic started on 04/20/2005 at 06:11:43 AM  Member Profile  Reply with Quote
One reason I try to do my own work on my bikes is that I learn more about engines every time. I recently built a leak-down tester for engines to check the internals (rings, valves, cylinder bore) before I tear into an engine. A leak down test tells you MUCH more than a compression test can tell you. A compression test only tells you what the compression level is. It cannot indicate what causes the problem.

A leak-down test can tell you if the rings are worn, if the cylinder wear is extreme (like at the top of the stroke)or if there is a deep scratch in the cylinder, if a valve seat needs reworking, if a valve is bent (like the intake valve on my XL600R), and other cues as well. A compression test cannot tell you any of this. Let's say a buddy does a compression test for you and it shows low compression. Everyone first suspects the rings, spends the bucks on a ring job, and the bike is only marginally better. Could have been a valve was not fully seating, and it wasn't the rings at all.

To anyone out there suspecting your engine needs internal work, spend the money to have a leak-down test done. If your dealer cannot do the test (any good dealer should be equipped) then find another dealer cause they are only guessing what to do to your bike. A good mechanic will have and use a leak-down tester before doing any work because it tells him/her what to look for when going into the engine.

I recently did a top-end repair on an older Kawasaki 305 twin street bike with only 4200 miles on it. Compression test confirmed both cylinders had low compression. But, a leak-down test indicated one exhaust valve was leaking, and one intake valve wasn't sealing. So before I did the rebuild I could tell what was needed and where to spend the money.

83 XL600
Topic: Leak-Down Test vs Compression Test

Rotorcraft
Lake Jackson, TX

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USA
450 Posts

Posted on 02/13/2006 at 6:42:17 PM  Member Profile  Visit Rotorcraft's Homepage  Reply with Quote
I am currently fitting an old sparkplug so that I can use my compression differential tester to check my engine. I am an aircraft mechanic and to test the top end of aricraft piston engines we use this tool. Like Andy says you can use this tool to be able to further diagnose your engine far better than a comperssion tester. The tester has a fixed orfice and two gages on it along with your common air compressor fittings and cost around 60-90 bucks. On aircraft engines you put the piston at TDC and with 80 PSI on one gage and the other gage should be somewhere close to 80 PSI. If my memory is right anything higher than 68 PSI is passing. Some cylinders may show 80 in one gage and 80 on the other, that means no leakage. I've seen that quite often.

Intake valve leaking = hissing in the intake through carb
Exhause valve leaking = hissing in exhaust muffler
leaking rings = hissing from crankcase breather fitting

There is no guess work in using this kind of tool. It will lead you to the exact problem of your engine.

If anyone needs help locating a compression differential tester e-mail me, there are several on-line venders that carry them.

01 XR650L - 9K miles, custom oil cooler, homemade oil temp dip stick, Dave's carb mods: slide drilled 5/32", needle shimmed .029", 55 short pilot, 158 main, 2 turns out (Thx Dave), K&N filter, removed: snorkle, exhaust baffle, smog stuff. Sea level riding. Valvoline 10W-40 mc oil.
- Past bikes: 74 MR50, 83 XR200, 78 KZ650
- Donated to the support of 4Strokes.com

Andy Van H
Green Bay, WI

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USA
110 Posts

Posted on 03/04/2006 at 12:05:12 PM  Member Profile  Reply with Quote
Thanks Rotorcraft for the confirmation about using a leak-down tester. Who better to know the best test tactics than someone working on aircraft engines.

A leak-down tester in capable hands can even tell you if your engine has a scoured/scrathed cylinder below top dead center. With the tester hooked up, say you're maintaining a good 90% at top dead center, but as you slowly rotate the engine say 45 degrees, suddenly it drops off to about 65%. That may indicate a serious scratch or score on the cylinder wall. When the piston rings get to that point the seal leak will allow air into the crankcase and you may hear an air leak coming from the crankcase breather.

I have some professionally trained mechanic friends that rely heavily on doing a leak-down test before tearing into an engine, and sometimes even before buying a used bike or engine. A leak-down test can even be done on an engine out of the frame.

83 XL600
Topic: Leak-Down Test vs Compression Test

MotoRacerX36
Hampton, VA

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USA
34 Posts

Posted on 03/04/2006 at 1:10:33 PM  Member Profile  Reply with Quote
My F-15 will shoot your helicopter to hell and back...take that.

Galen
Hazelton, ND

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USA
451 Posts

Posted on 03/04/2006 at 2:10:36 PM  Member Profile  Click to see Galen's MSN Messenger address  Send Galen a Yahoo! Message  Reply with Quote
Yeah thats one of the first things we do in lab before tearing down anything that is for a customer.

one thing I have to say tho, a leak down tester can be dangerous, if you are putting high pressure into an engine just sitting on a bench if it is not properly secured it can and will start moving and if not under control will "fly" violently out of control

86 XR250R
Honda certified technician

Rotorcraft
Lake Jackson, TX

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USA
450 Posts

Posted on 03/05/2006 at 06:10:33 AM  Member Profile  Visit Rotorcraft's Homepage  Reply with Quote
Can you see boobs on the beach in your F-15? When we fly the beach some girls think by flashing us it will get them a ride. We may circle around a few times then it's back to work.

01 XR650L - 9K miles, custom oil cooler, homemade oil temp dip stick, Dave's carb mods: slide drilled 5/32", needle shimmed .029", 55 short pilot, 158 main, 2 turns out (Thx Dave), K&N filter, removed: snorkle, exhaust baffle, smog stuff. Sea level riding. Valvoline 10W-40 mc oil.
- Past bikes: 74 MR50, 83 XR200, 78 KZ650
- Donated to the support of 4Strokes.com

cracked junior
Varmintator

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USA
1469 Posts

Posted on 03/05/2006 at 5:17:44 PM  Member Profile  Visit cracked junior's Homepage  Send cracked junior a Yahoo! Message  Reply with Quote
i got a question for you aero-mechanic peoples. how come fuel on helicopters and air planes is measured in pounds, instead of gallons or litres. when you see a air plane or something on tv or a movie, they say it holds 50,000lbs of fuel

05 XR650L - Dave's carb mods, needle shimmed .024, 160/55, ceramic coated headpipe w/welds ground, fmf quiet exh, snorkle & smog junk gone, tkc80 rear tire, chopped xr400 front fender, 15/45 sprockets.
- Donated to the support of 4Strokes.com

MotoRacerX36
Hampton, VA

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34 Posts

Posted on 03/05/2006 at 8:58:26 PM  Member Profile  Reply with Quote
You always want to know the total weight of the aircraft, so rather than converting gallons to pounds, we just measure fuel in weight.

cracked junior
Varmintator

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USA
1469 Posts

Posted on 03/06/2006 at 5:36:13 PM  Member Profile  Visit cracked junior's Homepage  Send cracked junior a Yahoo! Message  Reply with Quote
thats a very good reason

05 XR650L - Dave's carb mods, needle shimmed .024, 160/55, ceramic coated headpipe w/welds ground, fmf quiet exh, snorkle & smog junk gone, tkc80 rear tire, chopped xr400 front fender, 15/45 sprockets.
- Donated to the support of 4Strokes.com

Rotorcraft
Lake Jackson, TX

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USA
450 Posts

Posted on 03/22/2006 at 5:27:07 PM  Member Profile  Visit Rotorcraft's Homepage  Reply with Quote
Most turbine engine aircraft fuel indicating systems read in pounds. It's more important to know how much weight you have versus how many gallons. On helicopters your always watching the weight, you don't want to over gross it. They rate the consumption of fuel in gallons per hour. Jet-A weighs 6.7 pounds per gallon. Our small helicopters use around 26 gallons per hour. The Bell 230 wich has two engines uses 85 gallons per hour. It's also more accurate to measure it in the weight format.

01 XR650L - 9K miles, custom oil cooler, homemade oil temp dip stick, Dave's carb mods: slide drilled 5/32", needle shimmed .029", 55 short pilot, 158 main, 2 turns out (Thx Dave), K&N filter, removed: snorkle, exhaust baffle, smog stuff. Sea level riding. Valvoline 10W-40 mc oil.
- Past bikes: 74 MR50, 83 XR200, 78 KZ650
- Donated to the support of 4Strokes.com

MotoRacerX36
Hampton, VA

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USA
34 Posts

Posted on 03/22/2006 at 9:39:44 PM  Member Profile  Reply with Quote
Is there an echo in here?
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