Leak down test: What is considered excessive leakage?

[wilit]

I'm about to do a leak down test on my WR250F and for the life of me, I can't seem to find anything that says what is an acceptable amount of leakage. One vid I ran across says anything more than 10% and it's time to tear it apart. Ten percent just seems kind of low to start thinking about pulling the engine apart.

 

[dmfdmf]

Isn't step one a compression test? If the compression is good and it's not burning oil then ride it till it is. A leakdown test is to troubleshoot weak or unbalanced compression results and will tell you if it is rings or valves.

 

[wilit]

Isn't step one a compression test? If the compression is good and it's not burning oil then ride it till it is. A leakdown test is to troubleshoot weak or unbalanced compression results and will tell you if it is rings or valves.

Not really. The bike has an auto decompression cam, so a compression test would be useless since I can't see how to lock out the cam. What I'm trying to do is troubleshoot an issue I'm having and narrow down the problem. A compression test won't tell me if I'm losing compression through the valves.

 

[dmfdmf]

I see. I think a spec for leakdown percent would mostly apply to the rings or cylinder wall wear. Valves sealing tend to be go or no-go whereas rings/cylinder sealing tend to be more progressive thus the need for the spec.

If you think you've got a bad valve then you'd hear the air escaping out the carbs/TBs if an intake valve is bad or out the tail pipe if an exhaust valve is burned. I suppose if the cylinder walls are really scored or a ring is broken you might hear the air escaping into the crankcase.

 

[OaklandF4i]

10% is the figure I have always used as my line in the sand. I honestly dont remember who told me that or where I learned it. I'm no pro, but its worked for me.

Edit: Alot of bikes manual will also have a compression figure taking the auto decomp into consideration. An example is the XR400R, Honda says 125-140 if I remember correctly. You can usually temporarily disable the auto decomp on many bikes too.

 

[afm199]

Depends on many things. Don't get a HF leak down gauge, get one from Motion Pro.

3-5% is great. 10% is fine. I've run motors half a season with 25-30% leakdown with no problems. You'll be down on power some. Generally speaking if it is under 10% I just forget it.

 

[89fj]

What's the original problem?

 

[stangmx13]

I just checked my R6 engine w the MotionPro tester. It has a year of racing on it. Every cylinder was 6-7%. So I’m now confident it’s good for 2 more club rounds before I rebuild it for MA.

 

[JimE]

I'm more inclined to use the compression test numbers as a condition of health and the leak down tester as a troubleshooting tool. Certainly they can both be used the opposite but the nature of the test seems to weight them that way.

 

[fast4d]

I bought the 'popular' one off amazon.

if you look at the gauges they are off a percent or so. with that put into the equation my ktm 990 had 2% leakdown at 40k miles.

everything on that bike is just 'tight' when new. suspension took a 5 - 6 k to loosen up.

 

[augustiron]

I bought the 'popular' one off amazon.

if you look at the gauges they are off a percent or so. with that put into the equation my ktm 990 had 2% leakdown at 40k miles.

everything on that bike is just 'tight' when new. suspension took a 5 - 6 k to loosen up.

I can't imagine a 40k bike being that low. A freshly broken in race rebuild would be lucky to get that, by my understanding.

 

[wilit]

What's the original problem?

The bike has just never ran 100%. When I bought it, it was near impossible to start. I cleaned and put all new seals and JD jet kit in the carb and adjusted the valves (two of the intake valves were a bit tight). Now I can cold start it in 2 kicks, but it just does not idle for shit. I have an electronic hour meter/tach on it and it pretty much verifies that 2k-4k RPM just is not achievable on the bike. Either it'll idle at 1800 RPM for 30-40 seconds and just shut off, or it'll idle at 4400 RPM. No amount of fiddling with the idle speed knob or mixture knob seems to get it to idle where it needs to be. The carb boot looks to be in great shape, and I've searched for air leaks but can't seem to find one. Putting around at slow speeds, it sounds like it has a miss and bit of a sputter, but at 1/2 throttle to WOT, it runs tits. I'm guessing it has to be something I missed on the carb rebuild (it was my first FCR rebuild), but I wanted to eliminate all other possibilities before going down that road.

 

[295566]

The bike has just never ran 100%. When I bought it, it was near impossible to start. I cleaned and put all new seals and JD jet kit in the carb and adjusted the valves (two of the intake valves were a bit tight). Now I can cold start it in 2 kicks, but it just does not idle for shit. I have an electronic hour meter/tach on it and it pretty much verifies that 2k-4k RPM just is not achievable on the bike. Either it'll idle at 1800 RPM for 30-40 seconds and just shut off, or it'll idle at 4400 RPM. No amount of fiddling with the idle speed knob or mixture knob seems to get it to idle where it needs to be. The carb boot looks to be in great shape, and I've searched for air leaks but can't seem to find one. Putting around at slow speeds, it sounds like it has a miss and bit of a sputter, but at 1/2 throttle to WOT, it runs tits. I'm guessing it has to be something I missed on the carb rebuild (it was my first FCR rebuild), but I wanted to eliminate all other possibilities before going down that road.

Honestly sounds like a pilot jet problem. Either too lean or too rich, hard to say which. But when you're WOT, or using the main jet, bike runs fine. Have you tried having a shop dyno tune it to see where your AFR is at?

 

[wilit]

Honestly sounds like a pilot jet problem. Either too lean or too rich, hard to say which. But when you're WOT, or using the main jet, bike runs fine. Have you tried having a shop dyno tune it to see where your AFR is at?

Not yet. That's my next step.

I believe stock pilot is a 40. JD calls for a 42 and I ran that with no noticeable difference. I bumped up to a 45 and it made a slight improvement.

 

[89fj]

does the spark plug look rich or lean?

 

[bobl]

Not yet. That's my next step.

I believe stock pilot is a 40. JD calls for a 42 and I ran that with no noticeable difference. I bumped up to a 45 and it made a slight improvement.

I'm not familiar with the FCR, but I remember a friend have that problem, and it turned out that the pilot air jet in the mouth of the carb was way out. Maybe?


Edit: I think that it's adjustable.

 

[295566]

does the spark plug look rich or lean?

^^^ this ^^^

I'd be willing to bet the pilot jet is off.

 

[89fj]

^^yes, but just looking for clues at this point. Hard to troubleshoot without being there

 

[JimE]

Got an FCR? Use this: http://www.factorypro.com/tech/tech_tuning_procedures/tuning_FCR_Burns,Pat.html

I was WAY off the paper on a custom M900 with dual FCR 41's and everything evrybody told me was wrong. I used that to the letter and it worked a charm. Almost too good. Had to get a steering damper.

Later on my DRZ400s setup with a FCR 39 and once again everything I was told was wrong. That thing worked.

Here's an excerpt for the problem your having:
Slow fuel jet:
After setting the fuel screws, if you end up having to turn them in closer than 1 turn out from bottomed, select smaller slow jets. If you end up with the fuel screws turned out further than 2 turns from bottomed, select larger slow jets. Go back and repeat the fuel screw adjustment procedure. You have the correct slow jets when your engine passes the fuel screw adjustment procedure (settles to a steady idle after throttle blip) with a fuel screw setting between one and two turns out from lightly bottomed. A good way to see if the slow fuel jet is too large is to slowly turn the fuel screw closed and see if the bike still idles. If you get the fuel screw down to something like half a turn out, or closed, and that cylinder is still firing, the slow fuel jet is probably too big.

 

[wilit]

K. Did some work on the bike last night. I pulled the plug and apologies for the lighting on the pic, but it's the best I could do. There is a slight tan hue to it, but I'd say it's still a bit on the lean side. Also, I checked the valve timing and clearances, and all is within spec. I attempted to do a compression test on it. With the auto decomp active, it was registering about 50psi. With the auto decomp locked out, I could only kick it to about 65psi before I couldn't kick it any more. Fail.