Hi, I just rebuilt a 400ex, pretty much just the top end. I got the engine all back together, it's turning over, getting air, getting fuel, making a spark, you know... all the stuff it takes to make an engine go... it's just not going. It won't start. If I put the choke on, and hold the throttle, it turns over and eventually gets a super loud backfire out the muffler. I dunno if that will help anyone, but maybe it will. I'm just looking for suggestions on what to do next. Thanks for any and all help!

 

Sounds like you have the cam in backwards or you have a major timing offset. It's backfiring for a reason and this is usually the case. Double check your timing marks and make sure it's at TDC. I would take the rocker cover off and eyeball the cam to make sure it's in right when you line the marks up.

 

By having the cam in backwards I mean 180 degrees off.

 

well, i had piston at TDC, the "T" mark lined up on the crankshaft, the 2 marks on the cam sprocket lined up with the other mark pointing upward, put the cam lobes facing downward, and I read online that it doesn't matter at that point whether you put the lobes facing upward or downward because the 400ex fires a spark on every upstroke.

Let me put a little more info in here. It is a 2000 400ex with the 440 kit. The guy that gave it to me had it sitting up for a good while, at least a year I would say. He had the valve cover off when I got it. The cam and rockers were in very bad shape so I replaced them (bought a cam and sprocket and valve cover with rockers already installed). I put everything back together as stated online and in my manual. I'm getting flow through my carburetor whether it be from the bottom "drain plug" (whatever it's called), or when I take the carb off and press the throttle... gas sprays out. Now, the carb was dirty and so was the fuel line coming from the tank. I cleaned out the jets (they were clogged). Gas is spraying so I don't think it's a carb issue. I'm getting a spark.

Could it be the CDI? I'm reading that sometimes the engine will turn over like it's going to start, but it won't and sometimes that's from the CDI.

 

I just went buy a compression tester to help rule out low compression from either the rings being bad (which they shouldn't be because the bike was picked up running), valve leaking, or a valve being closed or open when it's not supposed to be. To me, those are the most likely problems.

 

Is it safe to assume that as long as I'm getting a spark, that the ignition system is working properly? Is it possible for the plug to be firing at the wrong time or something like that?

 

Ok the other thing I can now think of if your valve adjustments are not correct since you stated that you installed all new rockers. IF you have over tightened the valve rockers it will cause one or both to be just slightly open when it's supposed to be closed. Check the valve clearances on the intake and make sure you even have clearances between the rockers and intake valves at TDC

 

You could squirt a little fuel in the carb, just to be sure you didn't miss something obstructing the idle/low speed passages...

Otherwise, I think I'd do the compression test you already have planned, and maybe double check the valves. Is the replacement cam new, or did you happen to measure the lobes before install?

Lastly, Im not that familiar with the EX, but can a person check spark timing with a timing light? Even if it won't run, you could pull the plug so the starter will spin it over faster and see if it's in the ballpark while cranking.

 

the replacement cam wasn't new, but it looked new. it was in very good condition

 

He's getting spark because it backfires. No can't check timing like a car that I know of. Run the valves and check your cylinder pressure. My money is on the valves so far. I have a 450r that had a right intake valve wear the seat, causing it to hang open by 4 thousandths, enough that it wouldn't start, backfire like crazy when cranking and when it did run would have a hanging idle just like a vacuum leak. It would only start with full choke and lots of gas.