Coolant coming out overflow after rebuild. Help Please.

I just rebuilt my 2001 yz 426f. New trans, piston, valve springs and seals plus all new gaskets and orings. Well after starting it and warming it up at idle for 4 mins with the choke on and another 10 or so without choke it it started pushing antifreeze out of the overflow. I figured this was normal since there was no air moving around the radiators and it got too hot. I took it for a short ride (about 3 miles) at no more then half throttle and it was still pushing antifreeze out of the overflow but not a steady stream. just kinda trickling out. I took it back to the barn and parked it at idle and when I hit the kill switch it stopped coming out of the overflow. After that I checked the radiator and it had little shinny pieces of what looked like aluminum and small black pieces of something in the coolant in the radiator. The black pieces might have been grease that I put on the orings. It didn't look like oil at all.

I called around to different yamaha shops and got a variety of suggestions as to what it might be.

1. Bad head gasket or O-ring that is pressurizing the coolant system.

If this were the case you'd think it would speed up the flow out of the overflow when I'm riding and giving it throttle, which it didn't. It almost stopped the faster I went.

2. Bad radiator cap

Possible I would think with it being 12 years old, but it was good when I took it off.

3. Air pocket in the coolant system

Most likely I would think because the coolant was frothy coming out of the overflow and had some head on it when I opened the radiator cap after it cooled off a little. It also was low on coolant after each ride. This could have obviously been because it's spitting it out too.

As far as the particles in the coolant I'm a little puzzled about that. I still haven't drained the oil to see if there's coolant in the oil. I will probably do that tomorrow morning when I get off work. What color would the oil have if there were anti-freeze in it?

Tell me what you guys think. I would really hate to have to tear the top end apart and spend another $80 for a gasket kit if I don't have to. I'd like to take it on a riding trip the first week of July so I want to get this fixed as soon as possible.


Dennis Hart


A quick look of what I rebuilt. I'll post before and after pics in a new thread when I get it fixed.

Seeing the top of the Radiator fins is normal of the radiator is filled to the top it going to spit out what it don't need.

I normally leave the radiator cap off fill it to the top there is a bleeder air bolt on the side of the right cylinder If I remember right crack that open to make sure there's no air coming out.

I also run the bike with out the radiator cap off to get the air bubbles out before putting the cap back on and run the bike.

If the bike is idling sitting still it going to over heat it don't take long or much fro the bike to over heat this way.

IM me about gaskets,.

I running engine ice in my 426.

I see you got the trans parts.

Well after starting it and warming it up at idle for 4 mins with the choke on and another 10 or so without choke it it started pushing antifreeze out of the overflow.

You're the first person I've ever heard of who got away with idling a 426 for longer than 5 minutes without blowing coolant everywhere. They don't like that. I suggest you refill the cooling system just to the point of covering the tops of the radiator tubes and take it for another 10-20 minute ride. Let it cool a little and see where the coolant level is.

OK, I'll try that tomorrow when I get off work.

I just rebuilt mine and put engine ice in it and I love it

I'm using Preston's 50/50 premix. I figured that would work just fine.

Well the coolant quit coming out the overflow after a short ride but it still has the same problem as before I rebuilt it. When pinned in fifth gear it jerks 3 or 4 times in a row like the engine cuts out. It only does it when its pinned full throttle. Talk to a Yamaha mechanic and he thinks its the jetting. I'm going to take it to him sometime in July when he gets freed up. Also there was about a half dollar size puddle of what looked like milk that came out of the crankcase breather. It was kinda cold when I test drove it and it did that before the rebuild too, but I'd that normal?

Doesn't do the jerking in any gear but fifth? Feels something like the chain jumped up off the sprocket and back on again?

That would be a worn 5th gear pinion and the 3rd gear pinion that locks it, and a bent shift fork to go with it.

I just replaced the transmission and the center shift fork. Could it be that the other shift forks are bent as well. That's what it feels like. Like the chain skips a tooth. It only does it in fifth. Do I need a new shift drum too? Mine looked fine though. There's a yz426 bottom end on eBay now for 300. Wondering if it would be cheaper to buy the complete bottom end. Two shift forks and the drum are $227 alone

Are you hitting the rev limiter.

Do a plug reading After running the bike wide open.

I can't hardly get to the rev limiter in 5th. It jerks so bad. Its gotta be the other two shift forks and shift drum. Its the only thing I can think of. I don't think jetting would cause it to jerk that bad. I'm going to take it to a Yamaha shop this weekend if they aren't too busy. Soo frustrated. This project has officially become a money pit.

Were the parts you used in the trans replacement new or used?

The trans was used but in good condition. The center shift fork was new.

You think I should tear it apart again or take it to a dealer and see what they say?

Yes if your jetting off it will cause the bike to jerk when it rev's out is quick chugs or what size main jet are you running.

Where are you're floats set at is you're vent tube from the gas cap plugged.

Is the spark plug cap screwed all the way in to the coil wire.

Check the wiring that runs along the frame the frame make sure there's no bare spots that's grounding it self out.

Disconnect the kill switch see if this grounding out.

The trans was used but in good condition.

Maybe not that good. The wear that causes this is at the locking lugs/slots on the sides of the 3rd and 5th gears on the main shaft. The other two forks are not involved. The lugs are supposed to have a very slight undercut so that the driving loads hold them together. When this wears away to the point that it's gotten slightly reversed, they push away from each other until the disengage by flexing the fork, then the fork snaps them back into engagement and it starts over.

If it only does this in one gear, and the problem can be controlled just by increasing or decreasing the load on the drive train with the throttle, it's the trans. An engine performance issue like spark plugs or jetting would affect operation in all gears.

GrayRacer this my old trans it worked fine it didn't jump out of gear.

I put a WR trans in my YZ and it was just sitting on a self in my garage before I gave to Dennish87.

This why I think it not the trans I could be a shifter fork.

I using the same shifter forks that came out my YZ for the WR trans in my 426.

But he might not pushing the bike as hard in the lower gears and revving out in 5th farther this why I asking it could be the jetting or is the hot start hanging open just enough can cause the motor to stumble ask me how I found this out LOL.

Most of my riding is wide open in top gear 5th so I know when I was using the YZ trans it worked fine.

The WR trans works better for the type of riding I do now still 5th gear I am in most of the time more top end speeds but at normal cruise speeds I not revving as high anymore.

Edited by marv02

Dennish87 you said you still using the old Chain and sprockets swap out to you're new chain and sprockets and see if this could be it the old one could be so worn it skipping teeth.

Try the other stuff before you tear engine apart again.

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