I think I made a major mistake. I was reassembling my 350x's jug & head back onto the case. I follwed my oem manual ( I thought ) & lined up the marks, & put the cam in lobes down. Well, it turns out there a bunch of marks inside that timing cover & I was using the wrong one. I had tried to start the engine several times & realized something wasnt right. I now have found TDC & done it right, but it seems to be blowing air out the carb. Did I bend the valves or is it just that the honda dealer cut the seats so much I am way off on the valve lash & they are staying open?

Jim

 

You found TDC, but it is the wrong one. Remember, a four stroke crank rotates twice for every revolution of the cam. Instead of timing it so the piston is going down as the intake valves are opened, the piston is on the way up. This is commonly refered to as being 180 out (the cam is 180 degrees of rotation from where it should be). Take the bolts back out of the cam sprocket and roll the motor over till the holes line up with the oppsite cam holes. Hope this helps.

 

Are you saying that you have a 50% shot at getting this right the first time.. I just installed my cam last night, i haven't started it yet, but you are telling me it could be 180 deg. out. Damn !! I hope I hope I get lucky..
i have a question of my own here.. I was doing so on a 300ex.. There are actually two markings on the flywheel. Do I use the first one or the second one ?? One looks like a sidewards T and the other a upright mix of a T & F.
Hope you can help !!!

 

ALKIBURNINX IS RIGHT and did an excellent job of explaining that.(you da man) time the cam to the first T.BTW that blaster sounds fun,I had a 84 yz490 that thing was real fast!!!love that 4 speed.

 

Actually AlkyBurninX what you are noting would be 90 deg out.

The only problem that occurs when an engine is 180 out is ignition timing problems - all valve events are correct in conjunction with corresponding piston events. This is most frequently a problem in vehicles with a distributor like a multi-cylinder car engine (or any other non-wasted spark design).

What is really happening is the ignition fires as the intake is opening between the intake and exhaust stroke, instead of between the compression and power stroke. This usually results in fire being blown back thru the carb.

That said, I am fairly certain that the 350X engine uses a "wasted spark" ignition design. Meaning it fires every time the piston comes to TDC. This, in turn, means that there is no way for the engine to be "180 out".

With that cleared up.... It can be somewhat normal for some air to be blown back out of the carb at low RPMs (not much though). This is due to the camshaft design opening the intake valve slightly before the piston has reached TDC on the exhaust stroke.

What you are noticing may or may not be normal. What is your compression? That would be the first thing to test to determine if you bent a valve when your timing was set incorrectly.

Did you check the valve lash? It should be set to .005 on the Exhaust side and .004 on the intake side. It is certainly possible for the intake valves to be hanging open and blowing compression by. Check this first.

 

Exer: The F stands for fire (remember an engine fires before TDC), and the T means TDC (top dead center).

Sickmojave: I am kind of cheating. The bottom end will be a '82 yz465 with a 84 490 top end. The 465 had a 5 speed. I have talked about this engine a couple of times and you are the first to catch it.

86atc250r: You are right about it actually only being 90 degrees out. It is also still refered to as being 180 out. I have questioned this too. Maybe since the crank is really what is 180 out. You are also right about the wasted spark engines. For some unknown reason, I was thinking about degreeing in a motor and reading the wrong side of the wheel. (degreeing to BDC)

Jimster: I can't remember what the cam sprocket looks like on the 350x. If it is like the 250/300ex, and I think it is, there is actually two lines on it directly across from one another. They should line up horizontally with the head.

 

Alky..
Thanks for the info.. How can I tell if I am at the correct TDC (piston being at the top vs. the bottom). I cannot see the piston to be able to tell.. I guess I will figure it out !!!

 

Exer, I have timed a few motors since I last rebuilt my 250X, but here is what you should do. Turn your crank to the only T. It will be TDC. Then, put your cam sprocket in the timing chain and line up the two marks that are directly across from each other, with the head. Keep all slack out of the front length of timing chain. This keeps the cam from retarding when the timing chain tensioner is released. It helps to have someone hold the crank from turning by putting (I think a 17mm) socket on the flywheel bolt. When the flywheel is on T and the sprocket marks are lined up horizontally with the head, then you can install the cam. If everything is still lined up, it should be in time.

I know you have already done part of this, but if you have to retime, this is the best way. A few things to remember:
If you haven't rebuilt one of these engines before, use a good gasket sealer between the head and the rocker cover. I prefer Threebond (available at most ATV shops), but yamabond (yamaha's brand) is about the same stuff.
It seems every bolt in the rocker cover is a different length. Make sure you find the right bolt for the right hole to keep from stripping threads.
Take the bolt out of the back of the timing chain tensioner. Find a small flat blade screwdriver that will go into the hole. The tensioner is then wound counterclockwise (I think), when the shaft that goes into the engine is completely retracted, the assembly can be installed. It is spring loaded, so you will have to keep pressure on the screwdriver.
I hope this helps.