Need help with compression test

[Murat]

My 400 has been hard to start for a while and now it doesn't start anymore. I'm thinking the intake valves are shot and I may have over revved it a few times. I cant start the bike to do a proper compression test, but I did one with the engine cold and got some very bad numbers 64,38,41,42. Any advise?

[yamaha_george]

My 400 has been hard to start for a while and now it doesn't start anymore. I'm thinking the intake valves are shot and I may have over revved it a few times. I cant start the bike to do a proper compression test, but I did one with the engine cold and got some very bad numbers 64,38,41,42. Any advise?

Hi,
put a little oil in each bore, not too much or you will blow the oil into the comp gauge.
The oil will help the piston ring seal and show up if you have valve sealing issues.

[Murat]

With oil, the numbers only wet up by about 30 psi

[CJD]

OK just to be sure your doing the test correctly (obviously you can't warm the engine up) All 4 plugs removed, Throttle wide open also manually open the throttle slider on the same carb # as to the cylinder being tested. Most importantly disconnect the TCI so none of the plug leads spark/arc out, we don't want any burnt body parts.

30 psi is a significant rise (on the rings) but the readings are low. You should consider getting a leak down test for a full diagnosis, before tearing the motor apart.


HTH

[Murat]

The only thing left to do is to check and see if the cam timing is right. If that is good then it's time for a new head or a 600 swap.

[dru86]

I'd be checking your valve clearances with numbers that low. You'll probably find your intakes have zero clearance.

[pefrey]

Just go ahead and check the valve clearances, it's not that hard. A leakdown test is also a good gauge.

Of course if the valves are really tight and have been for a while, you probably "tuliped" the valves and they are shot. A new head would be cheaper but good luck finding one. Rebuilding it with new valves would cost more than a good 600 engine.

[Murat]

The little fizzer is on the back burner for now. I have 2 extra heads that i can swap. That is a good idea though. If I do have zero valve clearance, then I know for sure that the valves are tuliped.

[reelrazor]

If I do have zero valve clearance, then I know for sure that the valves are tuliped.

No.

YOu only know for sure that the valve lash needs adjusting....a lot easier to adjust lash than to swap heads(cheaper too)

[Murat]

The way I understand it is that valve clearances only increase. It's caused by wear between the camshaft lobes and buckets. A decrease in valve clearance can only mean that the valve spring is pulling the valve up higher than normal, which would indicate a mangled valve seat or a deformed valve. Tomorrow I have to get my Celica winter ready, but i will definitely check out the situation and post the results after i get out of work some time this week.

[DonTZ125]

Increasing valve clearance is indeed a symptom of metal-on-metal wear of the cam and tappet (bucket), which on these engines is both rare and a Bad Thing (tm). Far more common is slight deformation of the valve head and wearing-in of the seat, which allows the valve to sit deeper in the head and reducing the clearance.

[Murat]

I just checked the valve clearances. Each cylinder had at least one intake valve with 0 to .004mm clearance. Cylinder 3 had both intakes with 0-.004mm clearance. It's supposed to be .010-.020. All the exhaust valves were good.

Next step is that I have a spare motor with 3k and a rod through the block. I have to check to see if the head on that is still good. I hope that when the rod broke, the piston didn't smash the head's cylinder 4 area.

[reelrazor]

I'm with Don here...

Normal wear in the valvetrain of these engines (and any other overhead cam direct valve actuation engine{no rocker arms}) is tightening valve clearances.

You are right, it used to be that valves got sloppier and sloppier with time and wear......and that is still true for pushrod and rocker arm setups...more points of contact that wear a few thou..adds up to too many thou.

Do the valves tulip? Yes, on the earlier FZR motors they did. Motors that are setup with lash clearances on the high side will accelerate any tulipping(lots of lash allows the valve to slam closed against the seat)

It is COMPLETELY NORMAL for the clearance to go to zero if/when neglected for long enough. The seats wear-that's NORMAL.....it usually is only a matter of the actual contact angle getting wider on the seat allowing the valve to recess. The REASON that they manufacture and sell varying valve shims is to compensate for that wear.

Forget the other engine/head idea and do the shim calculation and install them. Put it back together and ride it another 10,000 miles. At the very least it will save you the cost of a head gasket