Results of a Leak Down Test


4 replies to this topic
  • Jeepentj

Posted August 12, 2009 - 06:16 PM

#1

I just performed a leakdown on my bike and it was 7.5%. Is this a good number? What should the number (or criteria) be to warrant tearing down the motor?

Thanks,
David

  • FinchFan194

Posted August 12, 2009 - 07:05 PM

#2

Gray may know better buy I think the factory specs are at 8% its at the edge of tolerance. I just had my bike done and I was getting I think 9% blowby and now the thing will hardly start after it warms up(no compression) I had just done a valve adjustment right before hand rode it three times after the leak test which lead upto the little compression once warm.

So I say tear it down and see what you've got.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • grayracer513

Posted August 12, 2009 - 08:30 PM

#3

7.5% is not too bad. The person testing should have noted where the leak(s) were; whether it was all rings, or whether the valves are leaking, too.

The amount of leak that is acceptable depends a lot on the bore size. A locomotive engine which gives a leak-down of 10% on a leak-down tester is virtually perfectly sealed while the same tester giving a 10% reading on a 50cc pit bike engine indicates a rather major problem. Because the 450 has a nearly 4" bore, the test readings considered acceptable in 5-6 liter automotive engines can be considered a fair guide.

Because there is no standard size for the restriction orifice in the testers, the reading will vary from one tester to another by as much as 3% (indicated) or more. What has to be relied on here is how yours compared to known healthy engines tested in the past with the same tool. What was the mechanic's opinion?

  • Jeepentj

Posted August 13, 2009 - 08:41 AM

#4

Because there is no standard size for the restriction orifice in the testers, the reading will vary from one tester to another by as much as 3% (indicated) or more. What has to be relied on here is how yours compared to known healthy engines tested in the past with the same tool. What was the mechanic's opinion?


Good question. I actually own the leakdown tester and have used it over the last several on each of my 4-strokes. After adjusting valves on my bikes I do a leakdown test since the bikes's already at TDC. I then record the number to gauge wear on the motor.

Historically (on the same gauge but a different bike) the best reading I've seen on this guage was 4.5%. This was on my KTM 520SX that had just been rebuilt from the ground up head, valves, piston, cylinder, rings, etc.) and had about 3.5 hours on it. The worst reading I've seen with it was 40% (the bike still seemed to run very well and start easily), which was the cause of the previously mentioned rebuild on the KTM and the first time I used the gauge . To provide some additional context, the same bike and motor at about 40 hours after the rebuild was at about 16% loss. So anyway, I've seen numbers ranging from 4.5% to 40% on the same gauge. I've also seen lots of numbers in between but typically, they range between 10% - 20% (all on the same gauge).

When I took the reading on my YZ and came up with 7.5%, based on other readings I've seen on the same gauge, I was pretty happy. I was talking to a tuner yesterday about the number though and he made 7.5% sound bad so it got me thinking about things...

So there's the long story...

Thoughts?

  • grayracer513

Posted August 13, 2009 - 09:06 AM

#5

Because the gauges are so different from one another, your historical reference using that tool is the best guide to go by. If your best number was 4.5%, and you've had reasonably healthy engines show you 10%, I'd say 7.5% was pretty good, as you originally thought.





Related Content

Forums
Photo

2016 YZ450 by CaptainKnobby


Dirt Bike   Dirt Bike Technical Forums   Suspension
  • Hot  59 replies
Forums
Photo
Supercross

Wilson the Blue Privateer by jetfuel


Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  50 replies
Forums
Photo

James Stewart back on a YZ450F by YamaLink


Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  47 replies
Forums
Photo

BIGBORE FOR YZ450F 2006? by Taattie94


Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • 1 reply
Forums
Photo

yz250x vs 450 4t comparisons (preferably desert riding) needed by JakeNorthrupYZ450F


Dirt Bike   Make / Model Specific   Yamaha   Yamaha 2-Stroke
  • 9 replies
 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.