YZF coolant problem

[pursang262]

Hopefully some of you guys can help me figure this out. Yesterday I brought my YZ to a track day at Loudon. The first couple of sessions I played with jetting and got that sorted out quite well.By the way this motor rips,it's geared 15/39 and wants more.back to my problem, about halfway through the afternoon it started to puke coolant.It didn't seem to be overheating.We swapped radiator caps with another bike,both 1.1 and it still blew the coolant out. This morning I brought the bike to the shop and pressure tested the cap and it was good.We used a digital thermometer and found even temps throughout the radiators and hoses.I kind of suspect the head gasket, but wouldn't that also cause coolant to be sucked into the combustion chamber? We found no sign of coolant being burned.Are there any tests or checks I can perform before I just tear this thing apart?It's not that big a deal to change a gasket,but I don't want to do it twice.Any ideas? Thanks in advance.

 

[CRboy167]

There is a tool I have that I bought from Snap On and it test for combustion gases in the coolant system and it works really well.

 

[Rich Rohrich]

You can take the radiator cap test pump and attach it to the radiator (with an adapter) while the engine is running. If combustion pressure is leaking into the cooling system you should see a noticeable rise in pressure recorded on the guage. There are also some coolants (like Race Kool) that will change color in the presence of combustion gases.

YZFs seem really sensitive to cylinder head bolt torque/stretch. Thankfully Honda did it the right way and used studs in the CRF.

 

[pursang262]

Guess I should have added new head bolts to the pile of parts I bought this winter.Is there a stronger/better bolt I can use? I found the adaptor for the pressure tester, I'll try that tonight. Thanks for the replies guys.I've only got about 3 hours on this motor and I love the way it runs,I hate to take it apart again,I just want to ride it. Rich, do you reccomend a torque other than what Yamaha lists in the service manual? Thanks again.

 

[Rich Rohrich]

Originally posted by pursang262
Rich, do you reccomend a torque other than what Yamaha lists in the service manual? Thanks again.

Yamaha didn't pull that number out of their butt so NO, but they did base the value on a known bolt stretch. I've talked to a few people about getting studs made to replace the bolts, but for the moment IMO it's worthwhile to coopercoat the head gasket, use new head bolts, clean the case threads completely, and use a quality assembly lube like the ARP version during a major rebuild, especially in the case of a big bore.

At the PRI show Cometic talked about the work they were doing on a trick new head gasket that should make this less of an issue. Eric Gorr has an inside track with them so I'll see what he knows about it.

 

[SFO]

There is an off the shelf grade 12stud available for this but it would mean converting to SAE.
Rethread the cases :silly:
Then also think use of studs for the head hold down would require engine removal for top end service.
Trying to second guess published torque #'s is pretty dicey.
You could set up a tube and nut in your bench vise and measure bolt stretch in 5ft# increments and actually see where the bolts goes elastic.
I think everyone should do this once in a life time to see what happens. The bolt post elastic is worthless, BTW. :uh:

 

[pursang262]

I wasn't so much trying to second guess Yamaha's torque spec,as thinking that with the amount of experience you guys have with this motor maybe you'd found a foot pound or two more or less was better.I really should have known better than to re-use the head bolts.I've got new ones on order, and I'll also use the copper coat Rich recommended,I put the gasket on dry last time.As far as studs go,would it be worth it to call someplace like APE or Orient Express that sells heavy-duty studs for drag bikes,and seeing if they have the correct size studs for the YZF? With the amount of attention the crank needs in this motor, thaking it out of the frame every once in a while shouldn't be too big of a problem.If this motor were converted to studs and nuts would the torque setting change depending on the stretch of the studs?Sorry so many questions but I'm trying to learn all I can about this motor ,I plan to keep this bike a long time. Thanks again.

 

[DEANSFASTWAY]

Pursang If you think you need a new Headgasket call Jay at Cometic and he can send you a new 4 peice steel preloaded shim head gasket that works really well .Thicker ring around combustion chamber circumference,In a way its like o ringing a block. He can make it in different bore sizes as well as thicknesses for desired deck height /valve clearance issues as well . They also make a really nice rubber laminated steel base gasket that wont blow out .About the only thing they dont have is good copper exh port crush gaskets .

 

[pursang262]

Thanks, just got off the phone with Jay,nice guy.Gaskets should be here in a couple days,hopefully I can have this thing race ready by Sunday.Thanks again to all you guys for the advice,and patience with my endless questions.4-strokes are kinda new to me, and I've learned alot here.