YZ490 head modification

[merlyn1965]

I finally found the time to make some much needed alteration to the cylinder head of my beast. I made my jigs to hold the head in the lathe and made some improvements afterward. I had bough another head on ebay just in case that I'd make major mistake but it turned out really good. I first made the 8mm cut at 30 degree to reduce the sharp angle in the combustion cone and cut into the surrounding surface that follow the piston outside the cone. Sanded it out on the lathe and cleaned it all out and installed it right away.
first pic stock

003 - All Things Moto! Photo Gallery

then the after shot

007-2 - All Things Moto! Photo Gallery

Well, I didn't had to sacrifice a chicken, pour lots of sweat drop and dance around the bike to get it started. Just went up and down the lane a few time and I felt a major difference in the bike. A 490 is smooth in its powerband, everyone knows that, but now, feel heavy duty diesel like with linear power all the way to the top end.
Going for a longer rip tomorrow to get the jetting right but so far I'm extremely happy. BTW it even started easily when warm.
Sorry about no pics, just cant seem to figure it out.

[D tom]

Sounds like you did good! I just screwed a 14mm bolt into the spark plug hole and turned it on the lathe. But I'm scared to run it untill I can fix the loose swingarm. Any sugesstions? The bolt is a little worn,needle bearings,bushings,and the hole in the frame too a little. but it ads up to some major slop.

[Doughbaby]

I would really like to know if this helps a pinging problem that most of these bikes have when they get hot. I had a 1984 YZ 490 years ago, and have another one now, and have always had a detonation/pinging problem with the bike when they get hot and normally in the midrange of the throttle is the worst. Any help would be greatly appreciated.

Thanks for your time !

[Big Maico 393]

Originally Posted by Doughbaby

I would really like to know if this helps a pinging problem that most of these bikes have when they get hot. I had a 1984 YZ 490 years ago, and have another one now, and have always had a detonation/pinging problem with the bike when they get hot and normally in the midrange of the throttle is the worst. Any help would be greatly appreciated.

Thanks for your time !

Kirk:

"bout time you showed up here, welcome to

Yes the head mod is supposed to help with the detonation/pinging problem by changing the combustion flow. also helps clean up the mid range jetting.

[racerfty]

i had a 97 125 that installed the conecting rod bearings, or piece of it in the dome, i did not have the money at the time to replace it and had a buddy of mine take it to work and he cleaned it up on a lathe, after a cheap repair, the bike hit harder on the mid rpm range, but it lost the quickness of the rev, until i did some carb work.
is there plans out there for this, or just somehting that you came up with? either way, u did a good clean job

[merlyn1965]

Well, I'm please to let you know that the pinging and detonation are a thing of the past. I went today on logging road so was able to test it throughout the full RPM on all gear and it went extremely well.
It never pinged at the regular 3/4 throttle and up. Just revved up and worked very well. I did previously put in some Boyesen reeds, a fatty exhaust pipe and now this mod and I will not go any further than this. Jetting is spot on and no sacrifice anywhere and I am honest on this, I gained everywhere even though I did lower the compression a bit.
I found the pic into a old Motocross action issue (October 85) where they're showing the mod done to Broc Glover YZ490 and though that if it was good for him, it'll be awesome for me and I was right. I'll try to scan the graph and put it on later if you guys want to take a look at it.

[yz490freak]

Here you go, I'll just repost it. Told you that you would love that mod. I'm in the process of buying a new part for mine, an aussie water cooled head, been looking for one for a few years, and finally found one, less than a hundred dollars shipped. It helps keep combustion temps down, and has a slightly diffrent squish band. Will probably chuck it up and do the mod, along with some other work, also have another piston that I'm gonna tig weld a dome onto, then turn down smooth, make it ultra tight tolerance, and run it at 13.5 or 14:1 with that water cooled head on 118 octane fuel. Or methanoal. Hell I'm already making 75 or 80 hp with just a hella lot of port and case and head work, and good tuning. Looking for 100 hp, or 130 hp with a boondockers kit, just for ****s and giggles. This is my bike, older pic, don't have anything fresh. And the head mod.

[merlyn1965]

That mod was just part 1. I didn't think it was what they were using back then and found the answer in an old MX magazine. I uploaded the figure of the mod used by Glover.

head_mod2 - All Things Moto! Photo Gallery

It is just a bit different in that there material removal on the head around the dome. I might do the carb slide and try the jetting setup too but I honestly do not need that much more power. I am just happy with the easier starting and clean running.

I tried to upload the whole article but my limit in size has been reached.