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New Project Bike: '99 YZ400f

Joined
Jul 15, 2018
Messages
166
Location
US
First Name
Trevor
I've been looking for a project or bike to part out over winter break to fuel my need for fun and funds.

Today I looked at a 99 yz400f. It was listed as not running. Seller said it ran last year but hasn't since, and he doesn't know why - it has good compression, a rebuilt carb and no leaks. Asking $800

Well, I checked it out and after seeing all the issues that weren't disclosed I told the seller I probably wasn't interested anymore. I offered $300 on my way out and he took it!
Ten minutes later, my apprentice @randypower and I had unbolted the frozen front brake caliper and loaded the new bike up

The issues I've found so far:
-Badly leaking left fork seal
-Doesnt run... Has spark and fuel
-front brake caliper stuck
-no kickstand
-exhaust is some random slip on pieced together
-front tire is super old and shredded
-trail computer is dead
-fuel valve doesn't actually do anything but turn
-front brake lever played twister too much

The good:
-big ol gas tank for woods riding
-grips are nicely worn in and might not tear immediately
-included a neat trail computer
-bars are straight and beefy
-has twisting footpegs... Not sure if this is a plus
-excel gold rims
-rear tire is almost new
-aftermarket seat and cover
-great compression
-new spark plug


I cleaned it up a bit and here she is! I've done project threads on other forums and since I've been more active on twt, I figure it's the perfect place to write this up.


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Joined
Jul 15, 2018
Messages
166
Location
US
First Name
Trevor
Got it running today, but it thinks its a two stroke - that can't be good:

Looks like white smoke to me, and it hasn't gone away after running a bit. I'm guessing the head gasket could be blown out.



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_RG_

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Oct 9, 2009
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Bryan-sort of-Texas
This seemed like an anomaly at the time....guy rode off into a collapsed mine up in the mountains. His four stroke was laying front wheel down (drastically) for quite a while before we managed to get him and the bike hoisted up. The engine cranked but smoked like I'd never seen. It stayed that way for at least 30 minutes or so. It was so bad we had to put him in back so we could breath. I can't explain it mechanically. Makes me wonder if riding that project around for a while might be a good thing before tearing into it.
 
Joined
Nov 27, 2007
Messages
1,179
Location
West Texas
This seemed like an anomaly at the time....guy rode off into a collapsed mine up in the mountains. His four stroke was laying front wheel down (drastically) for quite a while before we managed to get him and the bike hoisted up. The engine cranked but smoked like I'd never seen. It stayed that way for at least 30 minutes or so. It was so bad we had to put him in back so we could breath. I can't explain it mechanically. Makes me wonder if riding that project around for a while might be a good thing before tearing into it.
Crankcase breather hose attaches to the air boot/ air box.. When the front tire was that low in the mineshaft, it caused some of the crankcase oil to go through the breather hose and then into the air box. A few ounces of oil directly sucked into a four stroke carb will smoke like a locomotive for 20-30 minutes. If the crankcase was even a bit overfilled at all, it makes everything even worse and you end up with quite a bit of oil in the air box. The ensuing special effects are what you then experienced.

I too would run the bike a while and see if it clears up. Projects are fun, but those early Yamaha four strokes are basically prehistoric compared to even the YZFs that are 8-10 years newer (and the newer ones are dirt cheap on the used market). That looks like oil smoke to me but it should be easy to tell after you run it a while. Let us know what you find.
 
Last edited:

budzrex

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Yep, somehow my XC-F ended up inverted on a ride day and after I extricated myself from my odd landing position and righted the bike
it smoked something crazy for a while. I knew it was from the oil that got sucked into the carb and had the vent line from the valve cover to the
carb full of oil. But since I got a chance to smoke out my buds that led me down the path that led to the inversion I did not feel bad at all
 
Joined
Jul 15, 2018
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US
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Trevor
Interesting stuff! I've got school to focus on for the next couple weeks, but the project will be my focus over Christmas break.

I checked the airbox for oil and it seemed empty, but the oil level could be high. I'll investigate further once I have time and keep the thread updated.

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Well, I ended up tearing it apart. Exhaust had a lot of oil in it and so did the head where I could peak through the exhaust ports.

Took the top end off. No play in the crank bearing, piston was covered in buildup but it looks ok. Valves were a bit out of spec on both intake and exhaust so I'll order some shims to fix that.

Cleaned the piston, I might hone the cylinder a little, and once I clean the old cylinder gasket off I'll put it together again.

I'm thinking it might have been overfilled with oil because the piston doesn't show signs of blow by. I guess the valve seals could be bad but I find that less likely.

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OldTLSDoug

Living the Dream.
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What is making the piston wet? Is that coolant?
 
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Trevor
What is making the piston wet? Is that coolant?
Probably. That was either from my hands or from coolant dripping out of the head when I pulled it apart.

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Joined
Jul 15, 2018
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I'm currently undecided on what to do.
I don't own a valve spring compressor, but I'd like to replace the valve seals. I can just put it together as is, but if I need to do the valve seals afterwards I'm not sure I could without having to remove the head again.

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Joined
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Hello, I don’t have a spring compressor but found a way. Never been into YZ. Hope the pic can give you some ideas.
Oooh good idea! My neighbor might be able to lend me a tool but if not I'll probably rig something like that up, thanks!

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Joined
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Autozone has some for around $20 I think, it's a good tool to have in the box. I have one somewhere at my dads place
 
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Borrowed my neighbors valve spring compressor tool, got new valve seals on the head tonight, cleaned up the valves, and put the head back together.

I was feeling good so I put the head on the bike and as I installed the cams I realized that the exhaust cam does not line up well with the timing chain.
The intake looks and feels perfect, but the exhaust cam teeth seem different and don't match the timing chain right. Ugh.
The timing chain sits on top a bit rather than falling in to the teeth if that makes sense.

I've read of people swapping cams from other bikes with auto decompression, so maybe this is the result of an incorrect cam swap.

Took pics of it along the way and a current bike pic as it sits now
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Joined
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Borrowed my neighbors valve spring compressor tool, got new valve seals on the head tonight, cleaned up the valves, and put the head back together.

I was feeling good so I put the head on the bike and as I installed the cams I realized that the exhaust cam does not line up well with the timing chain.
The intake looks and feels perfect, but the exhaust cam teeth seem different and don't match the timing chain right. Ugh.
The timing chain sits on top a bit rather than falling in to the teeth if that makes sense.

I've read of people swapping cams from other bikes with auto decompression, so maybe this is the result of an incorrect cam swap.

Took pics of it along the way and a current bike pic as it sits now View attachment 250844View attachment 250845View attachment 250846View attachment 250847View attachment 250848View attachment 250849View attachment 250850View attachment 250851View attachment 250852View attachment 250853

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Is that a new cam chain? If not, I would go ahead and replace it due to the age of the bike

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Is that a new cam chain? If not, I would go ahead and replace it due to the age of the bike

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Not a new cam chain, but the idea of this bike is to fix it and sell it for the least money possible. I'm not sure I want to invest in a cam chain right now.

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Joined
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Not a new cam chain, but the idea of this bike is to fix it and sell it for the least money possible. I'm not sure I want to invest in a cam chain right now.

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Potential buyers take note!!!!! Maybe could have worded that a little differently. Good luck.
 
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Well, there's some weird tricks to timing the 450f exhaust cam when it's on a 400, but the first step is always to time the intake as usual.
I can't get that to work.

When I have the engine at TDC, the dots on the cams don't lay flat with the head, they are crooked. If I move it one tooth to fix it, I get the same crooked dots just in the other direction...

I'm thinking that it could be timing chain stretch or maybe it's normal when people use these improper exhaust cams that move the chain a bit. Right now if I assume the chain is not the issue I have two possible positions, advance or retard the timing. I'm not sure which is better.

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Alright, looks like even people using the auto decompression 450 cam get better timing than I'm seeing. I'm gonna continue putting this together for now to measure the valve clearance. Then I'll get shims and a timing chain on order.
Yippee

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Here's the cam at TDC with the best timing I can get out of it currently:
(just look at intake on the right, exhaust is not correct)
Not terrible, but worth fixing
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Jul 7, 2004
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I've ridden a WR 426 many times and the YZ250 in their steel tube frame formats out in the California dunes and I really prefer how they ride and deliver power compared to the racier aluminum framed versions. I just think they're super easy to ride, even for us who suck at it.
 
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Mar 6, 2009
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Oak Ridge North, Tx
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Cool project! - If you decide to keep the bike and ever need a strong bottom end for the YZ400F I have one sitting under my work bench that you can have anytime.... I think I even have the exhaust header and a few other bits.
 
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