Rebuild of 1979 YZ400

April 2011

Yes OK so the purchase and subsequent seizure of the engine during the tail end of the Catalina Grand Prix all happened very quickly, the re-build however is taking a bit of time. Why? Because I am a crappy mechanic and if it wasn’t for the help of my Dad (PJ) this bike  would still be in a thousand bits.

Lucky for me the British weather has been horrible over the winter which has meant that my Dad’s come over to escape from the freeze a couple of times and each time he does, the YZ and other projects get a serious propel forward.

The last time PJ came over he managed to somehow melt all the aluminium off the cylinder and flushed out the cases, and after installing some new top end bits, the YZ engine was rebuilt.

While it was stripped down I painted the frame and some other parts and the rather spongy and soggy suspension went to RaceTech for serious rebuild and to bring the 1979 suspension into 2011.

Now we are close to getting it all back together, only waiting on a used front brake plate I’ve bought off Ebay and some new throttle, brake and clutch cables. Then it’ll be a big boot to kick it, smiles and arm pump as I’m taking it straight to the race track…..

12th Dec’ 10

The Catalina Grand Prix might be over but the repairs to the YZ400 are certainly not. I braved it today and put the bike up on my bench and set about having a look and seeing what the damage is.

Two things are happening at this moment. Firstly, I’ve never really had to fix any major problem on a motorcycle (and I have damaged more than I can ever imagine) and secondly I’m going at this alone.

Now so far the finding, buying, shipping and racing the YZ400 was loads of fun, I had a blast; the whole experience up until now had been truly awesome, it couldn’t have been better. Now though, things could change, I’m getting stuck into a major strip down and I’m the first to admit, this is almost as daunting as the Pro race on Catalina Island.

Things started easy as it was just the basics, tank, fenders, exhaust, air-box and carburettor. The second I looked down the exhaust I knew it was trouble, it was almost white, even the little bit I did know about exhaust color, white means a lean mixture and lean mixtures always lead to engine damage.

As I looked up the exhaust port to see the piston, things just got worse, something serious had gone wrong and the end result wasn’t pretty, the piston was badly marked.

At this point I found myself at a somewhat dilemma. Of course I wanted to go further but with my limited engine experience, I’d rather step back and let someone else take over, but no one is here but little old me, so I stepped up and began taking off the cylinder head.

The 12mm bolts, all came free easy enough and the head was off and so was my enthusiasm for taking off the cylinder. So I tried and it was a lot more difficult. Because I used the small socket set and it’s designed for small not so tight bolts, I sheared off the small extension on the first tight bolt I came across.

Then the piston won’t come out of the cylinder, but I remember smacking one out of an RGV250 that Jamie Whitham had seized when I helped Team Grant out in the early 90’s so it was straight to the tool box for a hammer.

It worked, the butt of the hammer and a good few smacks was all it needed and in no time the cylinder was off and then detached the badly scored piston.

I’ve seen plenty of pistons in my time and quite a few with bad detonation but I don’t think I’ve ever seen one this bad. It looks like it was made of cheese and mice have been nibbling at it.

Because of my action of feeling the bike lock up, pulling in the clutch and then dumping it again 5 seconds later to see if it would re-start, and then when it did free up I continued until it would go no more, only did further damage. I’m finding bits of metal in the bottom of the engine too.

This is without a doubt going to be a major strip down, there is no choice.

The YZ400 was great fun until today but even as a surprise to me it still continues to be. I’m having loads of fun taking the machine apart and pretending to be a mechanic.

Problem found

I’m 99.99% sure I’ve found the problem with why the YZ400 seized up on the 4th and final lap of the Catalina Grand Prix Classic race.

The inlet manifold rubber is riddle with cracks, I knew this going into the meeting as it had perished badly in the dry Californian heat over the years and the cracks in it were as big and deep as a builder butt.

I hoped and prayed they didn’t go right the way through and the one’s I was worried about actually didn’t but there was a thin part of the rubber that did and it has a sizeable crack that goes right the way through. 

This means the engine was sucking in more air that it should have been doing and leaned the mixture out at a serious pace. By the look of the piston and the size of the crack, it’s a wonder the thing kept going as long as it did, let alone got me to the finish, even if I had to push it over the line.

It was poor maintenance that caused the seizure, and that I’m afraid was my fault for buying a bike and then racing it having done only the most basic of maintenance tasks, but I knew that going into it and was part of the fun, so as long as we can get this engine going again. I regret nothing.

I’m going to now take this baby apart and down to the bottom and build it up again having done all that is needed, the brilliant YZ400 deserves that at least. Then it’ll be back on track and racing again, as I’ve caught the dirt riding bug.

My Dad’s coming over again soon or a break from the bitter UK winter and I’ve now got a new project from him to work on, it’ll be brilliant to get my Dad’s professional hands on this fantastic motorcycle as his work is second to none.

I just know he’ll love it and want to make it one sweet looking original machine, so I’ll make sure it’s stripped down so he can see what it needs.

So far so good, as the YZ is being stripped down I’m starting to see that this bike has hardly been used but I’ve found a crack in the frame and the engine doesn’t look healthy so it’s going to be a while before it will be wheeling again and trying it’s best to throw me off. I can’t wait for that day though; I get excited chills just thinking about it, as this 1979 YZ400this machine is one hell of a ride.