OzVMX Forum

Clubroom => Tech Talk => Topic started by: Sammundo on July 12, 2012, 07:20:46 pm

Title: yz490 gone pop
Post by: Sammundo on July 12, 2012, 07:20:46 pm

so i bought a 82 yz 490 with no compression. as expected when i got the head off and piston out theres a hole in the piston on the exhaust side. i know they are prone to this for various reasons but was wondering if where it has blown the hole gives an indication to why it got so hot on the exhaust side?(http://photos-d.ak.fbcdn.net/hphotos-ak-ash3/599307_3866778981541_962441681_a.jpg)

Also can anyone tell me if looking at the head you can tell if it has had the head mod ive read about done?(http://a2.sphotos.ak.fbcdn.net/hphotos-ak-prn1/558367_3866775981466_409256740_n.jpg)

(http://a6.sphotos.ak.fbcdn.net/hphotos-ak-snc7/380668_3866778061518_1034403283_n.jpg)

one last thing. Best place for a rebore and possible head mod in brisbane?

Thanks in advance

Title: Re: yz490 gone pop
Post by: tony27 on July 12, 2012, 07:48:12 pm

Can tell you that the head mod hasn't been done, the mod is a second taper.
Check out tip 3 in this link. A local tuner came across this elsewhere & had me do it to a 490 head, is reputed to make jetting them easier as well
http://www.yz465.info/index.php?p=1_14_Mods

Title: Re: yz490 gone pop
Post by: evo550 on July 12, 2012, 09:21:42 pm

Best place for rebore is Serco Workshop Deshon st Woolongabba.

Title: Re: yz490 gone pop
Post by: Lozza on July 13, 2012, 08:51:59 am

Far better fixes for the head these days, it has just far to much comp and advance which is all that second taper is doing more volume. The reason why the piston burnt/deto'd on the exhaust side is................that's the hot side ;D
Seroiusly the head will require a fair bit of machining (not difficult though) as heads with a lot of prior deto seem to det easy again, something to do with free radicals etc etc blah blah blah

Title: Re: yz490 gone pop
Post by: evo550 on July 13, 2012, 04:49:08 pm

Those old Yamaha engines had/some may still do, they either ran blubbery rich, and when that was jetted out they got hot and ran lean.
Do a google search on YZ490J jetting...see what pops up.

Title: Re: yz490 gone pop
Post by: KTMaico on July 14, 2012, 05:57:15 pm

Sammundo,
                  You live in Annerley/Greenslopes ? As stated by EVP550 go to Deshon Street and Allan at Serco is the man to talk to. I would also drill lubricating holes in the piston for the exhaust bridge and dry flim coat the piston as well. Good luck with the repair.

Kevin

Title: Re: yz490 gone pop
Post by: Sammundo on July 14, 2012, 09:19:18 pm

thanks for the replies guys. im in tarragindi. will be going to serco monday to leave the bits with them. Will they drill the piston and dry film it there?

Title: Re: yz490 gone pop
Post by: Lozza on July 15, 2012, 09:55:32 am

Sorta thought the 490 was a single exhaust port  ???

Title: Re: yz490 gone pop
Post by: slider84 on July 15, 2012, 09:15:03 pm

ring john tate about the head , he knows how to sort the problem out

Title: Re: yz490 gone pop
Post by: holeshot buddy on July 15, 2012, 09:20:45 pm

lozza is right shouldnt need to drill piston ;)

Title: Re: yz490 gone pop
Post by: Davey Crocket on July 15, 2012, 09:39:34 pm

We just did 3 barrells, 2 sleeves and 1 trick new combustion chamber. Need the whole motor to do the combustion chamber.