No start after rebuild

[eprovenzano]

I just finished rebuilding the engine on my 99 400ex. I replaced the stock piston with wiesco, had the valve seals replaced, and had the cylinder honed. The bike wants to start, but does not. Its getting spark and fuel. The only thing I can guess is the timing is off.

Regarding the timing, I read the timing needs to be set on the compression stroke. How do I know which is the compression stroke? Also should the cam lobes be up or down?

[NacsMXer]

The compression stroke can be verified by lining up the T mark on the flywheel with the notch in the sidehole. Have you ever adjusted your valves before? It's the same process. You line up the T mark with the notch, and confirm that there is slack in the rocker arms. If the rocker arms are tight, you are on the exhaust stroke. Rotate the crank around again and line up the T mark to get to the compression stroke.

Lobes up or lobes down doesn't matter, as long as the piston is at top dead center. If the T mark is aligned with the notch the piston will be. If you put the cam in lobes down, the motor will be at TDC on the compression stroke when you put it back together. If you put the cam in lobes up, it will be at TDC on the exhaust stroke when it is put back together.

It is easiest to put the cam in lobes down, as this will make it easier to put the valve cover back on during reasembly. Then you need to verify that the 2 lines on the cam sprocket are level with the surface of the head when you bolt it to the cam. The 3rd line should be pointing straight up. Basically the lines on the cam sprocket should be at the 9 o'clock, 12 o'clock, and 3 o'clock positions when they are bolted to the cam.

So to recap, as long as the T mark is lined up, the cam lobes are down, and the lines on the sprocket are level with the head when the cam is bolted to it you should have the correct timing.

[eprovenzano]

I think I have found the problem. My son put the rings on and set the cylinder. He mentioned one of the new rings broke when he was putting them on the new Weisco piston, so he grabbed one off of the OEM piston that we took out. I don't belive their compatable.

So new rings have been ordered. I'll update as soon as they arrive.

[eprovenzano]

Update. New rings and it still will not start.

Since I had the valve seals redone, I'm thinking the valves are out of adjustment. I'm wondering is they are too tight and not closing. If they are too tight, the compression would be low, and would be blowing the intake air/fuel mix right back out into the intake, causing the backfire I'm hearing.

I'll adjust the valves tomorrow and update this thread

[2001trx400ex]

it has to be either the timing or the valves being out of spec, i had the problem with the timing i was one tooth off so check the lining on the fly wheel like the guy above said to do.

[eprovenzano]

Update: IT IS ALIVE.....

It was the valves... they were sooooo tight... They were so tight they never completely closed. During the rebuild, I had the valves reworked, new stem seals installed, and the cylinder honed. I didn't think about the valves, until a friend of mine asked if I checked the valves.

On the intake side, I has to back it off over a 1/4 of an inch before the adjuster was free. On the exhaust side, it to was way too tight, just not quite as bad as the intake side. I adjusted the valves to spec.

After I adjusted the valves, I hit the starter button; it turned over 5 times and fired.

I was starting to get really frustrated.... I've rebuilt my 2 stroke many, many times, but this was my 1st attempt at a stroker...

Now it runs fine, but seems to backfire a little, and doesn't want to idle as it should. I think I stil need to adjust the valves a little more.

[eprovenzano]

I readjusted the valves, and got it to idle, but it still wants to backfire little. When you open the throttle it revs fine, but may backfire a little, not always, just once in a while. I can hear the valves so they still need adjusted some more, but we are taking steps in the right direction.... I'm happy to hear it running again...