I got the motor back together easy enough about 2 weeks ago but after messing around it for a few days, it failed to start. A week or so ago I took it with me to Texas World Speedway to watch the road races for the first time in a few years and figured I'd tap some of my very smart racing friends about ideas on why it won't start. They quizzed me on this and that and most of the items I'd already checked and re-checked, but nothing new came up. Lots of young guys who race motards kicked and kicked but other than the occassional "pop" nothing was working.

In my world when it "pops" it's usually a timing issue. Since these bikes don't have adjustable ignition timing it must be something else. I had drained the carb before storing it but I took the accelerator pump apart anyway, cleaned it and shot some carb cleaner through the jets and passages. I didn't check it before, but when I was done, the pump was shooting fuel just fine. Still no joy getting it started.

So, lets think about this. The plug is firing and at the correct time. The carb is putting fuel where it needs to go, so that leaves compression.

New piston, new rings, new cylinder and new gaskets don't leave a lot of room for options. I had checked the valve seating with some acetone and they were tight. When I removed the head I never even pulled the buckets off the valve springs so that should have been fine.

I'm using an '03 auto decomp cam so doing a compression check didn't make a lot of sense and I had checked cam timing numerous times. In a final attempt, I decided to check the valve clearances even though there was no reason they should have been off.

To my surprise, the left and center intake valves were so tight I couldn't get a .0015" feeler guage in there. The right intake was fine at .25mm and the exhaust valves were fine.

I removed the intake valve buckets and shims for inspection/measurement and they looked fine. On a whim, I reinstalled the shim and bucket and torqued the cam cap in place. Surprisingly, the valve clearance was now in spec at .15mm. Well, isn't that just peachy.

The only thing I can figure is that when moving the head around it might have gotten jarred or turned on it's side and those two shims got cocked in the valve retainer and didn't allow bucket to seat properly. Regardless, it was fine now.

I quickly put it all back together and it fired on the 3rd kick.

Excellent, now there is nothing left to do but finish up the details and go riding again. I'm having a problem removing the countershaft sprocket nut from the old output shaft that is still in the old crankcase. Even with my 1/2" drive impact wrench and 150 psi I can't get the nut to budge. I suppose I'll have to take it to the local shop and let them have a go with it. My impact wrench is almost as old as I am and was passed down from my dad who was an auto mechanic. Like me, it's probably gotten weak with age.

update......I sprayed some PB Blaster on the countershaft sprocket nut and let it soak for a while. Using my little nail gun compression/tank I hit with another couple of short 150 psi bursts with the 1/2" impact and it slowly gave up the fight.

Score another victory for the old guy.

 

did you JB Weld those cases back together Mike? :lol:

Hee, Hee.....yea, that's the ticket. I've got YZ426 cases all over the place here.

MIKE!!!!!...NOW someone will believe that and try it!.....GO TO YOUR ROOM!...(and work on the bike)...LMAO! :lol: :lol: :lol: :lol:

 

did you JB Weld those cases back together Mike? :lol:

Hee, Hee.....yea, that's the ticket. I've got YZ426 cases all over the place here.

 

did you JB Weld those cases back together Mike? :lol:

 

Unlike most of the HondaTwins you can pull the head and cylinders off these big bore single pretty easily. It's a good thing too since they call for piston/rings like every 20 hours or so, maybe more frequently. As you can guess, all but the hardcore racers probably don't pay much attention to those sorts of things.

Here is the lower end ready for the cylinder and head. I'd like to get this thing going by this weekend and maybe do some riding. It's not that hard but I've got a bunch of work stuff to do but I'll give it a try.

Here is the piston installed on the rod with the rings in place. The black marks on the piston are my notes for indexing the rings.

Here is the cylinder in place waiting for the front cam chain guide and cylinder head.

The cylinder is in place with the main head bolts and oil lines hooked up. Sharp eyed readers will note that I had to take the head bolts off because I forgot the washers on the bolts.

I love the color and condition of the new cylinder. I scratched the side of it to try and determine if it was just painted but nothing came off. I sure would like to know how they coat these things so I could replicate it on our twins.

I was going to install the cams tonight but it's late and the cams I have are for an '03 model while this motor is a '00. Because of this, the cam timing is different than what is called for in the manual so I've got to be a little sharper than my usual dull tack my brain operates at.

 

I'm starting to put the YZ426 back together and thought I'd show some pictures of what a modern day, after market, auto clutch looks like.

This is a Rekluse clutch and, for this big bore 4 stroke MX bike, it was the best mod I could have made for riding in the woods. There is no longer a clutch lever and if the bike falls over over comes to a stop it doesn't shut off or die. Obviously if it tips over it will quit after a short time due to fuel flow but all those times when you come up on traffic that might be stalled on an uphill section in mud or sand the motor just keeps running and you don't have to worry about clutching it. Consider it a big bore version of a Honda Trail 90.

While idling, the clutch plates are disengaged. As soon as the rpm is increased (this point is adjustable), the ball bearings are thrown out via centrifugal force and up the ramped slot they ride in. This action of the ball bearings pushes out on the center plate engaging the clutch. The motor doesn't bog but, rather, just takes off. Obviously, to much throttle too soon and it will act just like it had a clutch and flip you over if you want.

Once moving the clutch isn't used for up shifting or down shifting since there is no clutch lever anyway. However, if you do happen to kill the motor for some reason you better be sure the kick start works because there is no more bump starting down the hill.

Again, it's a very cool piece and well worth the cost.

.

 

I had always wondered what happened to this machine having saw it in your signature for a while now.

Let us know what comes of it. I am thinking about buying a similar bike to turn into a Supermoto race bike.

 

I have a collection of failed bits from the motors I've grenaded. My favorite engine failure was in my Porsche 914 that I put NOS on. I went off the road spinning into the woods...did what I believe was 4 full 360s before a tree caught me.

I've never so completly destroyed anything before or since

even my 140MPH impact with the armco on the NASCAR banks at PIR on the SV650 wasn't as destructive.

 

internal combustion is cool...

think you meant "internal explosion" :lol:

 

internal combustion is cool...

 

I can relate to that, but from a car angle.
I have a Vauxhall Zafira 150PS Turbo diesel. Company car and definately not raced (I still want a job after all).
At 35000 miles the gearbox threw a shaft right through the side of the casting it was out of warranty, but strangely Vauxhall repaired it, without a single word being said, within 3 days too :?

 

amazing how a high Hp motor can tear stuff up when it goes south isn't it? :lol: