What should I see on a compression test.....

[ElderGeek]

On a stock '06 YFZ 450? I will be testing one, (and maybe mine too while I got everything out) with a compression tester and possibly a leakdown tester. Just wondering what pressure is Excellent, Good, Fair, or time to tear it down.

[Bowtie]

You can't compression test a YFZ because of the decompression pin in the exhaust cam. Grab the shop manual from here http://students.washington.edu/yfz450/yfz450/

[ElderGeek]

I have that downloaded already, I'll dive into it tonight.

[ElderGeek]

Well the manual shows it in the drawings but says nothing about it. Surfing a few other forums I read it disengages above 200rpm. So can I assume that when it is static & timing is at TDC it is also disengaged?

Otherwise the leakdown tester would not work either.

I have a Moroso #8960 leak down tester with a 14mm fitting. Looks like I will need to fab an adaptor because I cannot find one for 10mm. Or go to Harbor Freight to pick up a cheapy LDT.

[l1tech]

ElderGeek

The decompressor only is only active when the engine is cranking over. It is there to bleed off the compression so the starter can turn it over. If you put the engine at tdc you will still be able to do a leak down test.

You can do a compression test but it is only a cranking compression test. I don't know exactly how the Yamaha decompressor works becuase I haven't had one apart but on the Honda it is adjustable adn makes a world of diference on how the bike starts...fwiw my cranking compression is 65 psi

[ElderGeek]

From the diagrams its on the exhaust cam gear. A centrifigal arm and spring set up. I don't know exactly how it works (ie how it releases compression) until I get the cover off and study it a bit more.



Now the mission is to get an adaptor that fits the spark plug threads.

[jeephreak]

Is it different from the 3 in the kit I have? I can bring it tomorrow? Let me know (email me).

[ElderGeek]

You have a compression tester, which we can't use on YFZs. A leak down tester pressurizes the combustion area, usually to 100psi. Warm up the engine to operating temp, shut it down, remove the spark plug, insert the LDT, set the piston at TDC. Hold the crank from turning to keep it at TDC. Check your gauge to see how much pressure is leaking by. For non-race purposes anything under 8 - 10% is good.

If higher than that you listen for were the leak is.

From the exhuast = bad exhaust valves
From the cab/intake = bad intake valves
From the crankcase vent/oil fill = bad rings, maybe cylinder

[Bowtie]

Considering all the work you have to do to just do the leak down, you might as well keep going and tear the head off of it anyway and look at the jug and slug, it's only another 10 minutes and a head gasket is $15 bux. Depending on how long he rode with the filter off, you know it sucked sand, the question is how much.

[jeephreak]

Word! I really dont remember him riding it with it off, I thought he noticed it when he flipped his seat over to dry out. Either way, might as well tear it down and talk him into a 470 bigbore!!!