Intake valves mystery

17 replies to this topic
  • Motocrosspartsonline

Posted May 20, 2011 - 11:25 PM


My YZ450 head 2005 model has been ported polished seats cut and brand new steel valves installed - bench vacuum tested great holds fuel 5 hours with no hint of a leak, all work was professionally done by machinist.

Heres the mystery installed on the bike and it fails really badly a leak down test - blows all compressed air straight out the intake port? help!

I have un installed and lapped all valves for the second time - re installed got the same bench test results perfectly! but guess what installed on the bike blows straight out the intake port

all was locked at TDC when leak down tested I even removed intake cam and set to TDC to eliminate possibility of the valves being held open slightly from the cam
But same result this bike will be the death of me any got any ideas
regards Jamie

  • yz450fcranker

Posted May 21, 2011 - 06:31 AM


send the whole engine to a the professional who did your head and let him work it

  • dgcars

Posted May 21, 2011 - 06:47 AM


If it vacuum tests & holds fuel for 5 hours on the bench, but blows straight out the inlet on assembly.......then it is not being put together properly. Sorry I can't help further.

  • clutchkowalski

Posted May 21, 2011 - 07:38 AM


What are your valve clearances on the intake side? Too tight and there you have it.

  • dgcars

Posted May 21, 2011 - 08:10 AM


...I even removed intake cam and set to TDC to eliminate possibility of the valves being held open slightly from the cam...

I think he covered that.....:smirk:

What are your valve clearances on the intake side? Too tight and there you have it.

  • Motocrosspartsonline

Posted May 21, 2011 - 02:34 PM


OK guy's thanks for the input I am open to any angle you got but lets just for a second assume I have it assembled correctly and have rebuilt several times so clearance timing not an issue
Do you have any ideas what could be causing this ??

Clearances were set at 0.004 on the intake when this mystery occurred I then set intake at 0.006 to be sure not to tight I then leak down tested with no intake cam at all still got small amount of air blowing through the intake port

all my fuel holding test and vaccum test done on bench were done with head assembled when tested good was just fitted to the bike so I am confident assembly is pretty good

  • harrperf

Posted May 21, 2011 - 04:22 PM


just put it together...I think with thousands of pounds of combustion force, along with the intense pounding the valve and seat experience - that it will have excellent blow down test results 2 hours of run time later.

  • grayracer513

Posted May 21, 2011 - 04:38 PM


What percentage leak loss is the test showing, and what's the high side pressure on the tester?

It's a little odd that it would pass a solvent test and a vacuum test and ten leak under pressure, but I think harrperf is probably right.

One of the things I always do, more out of habit than anything, is to take teh assemble head and bounce each valve off it's seat a couple of times by tapping the top of the stem. This not only improves the sealing, but also seats the keepers, and in the case one of those isn't quite situated right, it will knock it loose right there on the bench where there's no harm in it.

  • Motocrosspartsonline

Posted May 21, 2011 - 10:39 PM


was testing with firstly 100 psi on high side then was testing with less and less just to see if would hold any but outcome is it's not holding basically any pressure at all the more PSI added faster the leak - I had down as low as 10 psi still had light blow by the intake

I completely blocked the intake port and read about %12 leak down which I thought was almost acceptable for big bore cylinder and piston set up? could be wrong?

Graycer - good advice with the knock the valves out its seat few times whilst still on the bench something I have always done too just to be safe - I hate to think what would happen if one of those keepers popped out at a few thousand RPM!

I don't know guy's starting to think head has a crack in it nothing else makes any sense I mean this may be stupid question but being that we know it holds fluid is water tight not as tight as air tight?

is it even possible to pass the fluid test but fail air test? once again with this bike I am stumped - looks like I will have to accept defeat and take it to the Yamaha dealer

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

Posted May 22, 2011 - 05:24 AM


what would be the downside of putting it together and trying it? as long as its assembled correct

  • lumpy790

Posted May 22, 2011 - 06:58 AM


Break it in and re-test.

  • grayracer513

Posted May 22, 2011 - 08:39 AM


...about %12 leak down which I thought was almost acceptable for big bore cylinder and piston set up? could be wrong?

Depends on the tester. Hard to tell without using the same tester on the same engine or similar previously.

  • jratcliffe3

Posted May 22, 2011 - 01:33 PM


How did you drain the head after the solvent test? Did you work any of the valves whilst there was fluid in? The only thing I can think of is if some dirt somehow got on a valve seat whilst you were leak testing it.

Might be worth putting the air on it, tapping each inlet in turn, give any dirt a chance to blow out, then try the leak test again.

EDIT: just re-read your original post, you already re-lapped and got similar results so this is unlikely. If you can get it to start, I'd run it for a min and repeat the test.

  • Motocrosspartsonline

Posted May 22, 2011 - 02:05 PM


No downside to putting back together - I have even done so - before I begun posting this entire thread. I had assembeled tried it got it started, made some adjustment to fuel air screw got it idling nice, road it maybe about 30 minutes, it was OK but down on power and not smooth, shut it down took carb off and header pipe and done the leak down -

It was also started after second attempt when I relapped the valves but same results

I guess really the only downside is these bikes are pain full to start when you blowing your compression straight out the carbi and when / if you get it started you will only have one chance to start it because once warm you got no chance

  • Motocrosspartsonline

Posted May 22, 2011 - 02:23 PM


Yes hearing you there Graycer - to be honest Graycer I don't really even use leak down to prove good or bad percents they completely unreliable in that regard in most cases?

BUT the leak down is perfect for finding a leak if there is one - saves time tearing motor down just to have a look! it gives great information if you just read the obvious signs such as air blowing out the exhaust or bubbles in the radiator - AS for the percentage loss, I personally have never really been to concerned with that result always been interested but never concerned

Cheers mate

  • grayracer513

Posted May 22, 2011 - 02:32 PM


I don't really even use leak down to prove good or bad percents they completely unreliable in that regard in most cases?

They're quite reliable, but they aren't the least cross-comparable because there is no standard for their construction. Thus, unless you use the same tester on one engine over time, or one tester on several engines of the same type, you can't compare results or arbitrarily say that X% loss is good or bad.

  • Bobbed06

Posted May 22, 2011 - 02:53 PM


Have you narrowed the problem down to a specific intake valve? If its consistently a certain valve not sealing for your leakdown test, it may be a valve guide to valve clearance issue. You may have just got lucky with your liquid test (seat cut properly), and the sloppy valve fitment within the guide is quickly degrading the valve sealing capabilities.

This is just a guess on my part, but it sounds like you have covered all of the standard troubleshooting steps.

I sent some stuff to a "reputable" machine shop once and got back some crappy work.

Did you do any compression testing before teardown and after reassembly?

  • Motocrosspartsonline

Posted May 22, 2011 - 08:49 PM


Well have taken to another shop as complete bike to confirm my findings they did leak down in front of me as we were both curious on this one! it lost 40% straight out the intake port... he took the head off and it held fluid ?? what the! the man is now re machining the seats and valves but we still have no logic to this one

I guess the only lesson is don't trust fluid test and you can't assume a machinest has done it perfectly

But realistically who would Know on this one it looked right and on the bench it seemed right but when mounted 30 - 40 % is a massive leak
Far out how's a backyard mechanic meant to work with these results and no logical cause??
Thanks for all the imput guys much appreciated I felt like I was going crazy with these circumstances
I will give an update when shop re installs it today maybe when he has dis assemble he may find some explanation to this mystery
regards Jamie

Related Content


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.