Bought a used 400ex. Worked great for two weeks. One day was out riding for a bit then turned it off. 10 minutes later went to start it back up and while it was cranking over the noise coming out of the exhaust changed from a very definitive compression sound to a less definitive compression sound. But it still had compression, you could feel it putting your hand behind the exhaust.

Took it to the local Honda dealer, they said it was a broken connecting rod. They are apparently idiots. Wanted me to pay for a whole rebuild.

Thought it might be a stuck valve, took the caps off, and all valves move freely as they are supposed too. BUT, while spinning the crank (spark plug was removed), I noticed that at at one point in every rotation it got VERY HARD to turn, like two hands and a lot of force to get it to go, like it was a compression stroke, except the spark plug wasn't in.

Took the head off, everything looked fine. Turned the crank, turned easily all the way around. Springs aren't stuck, watched the piston move properly. Looked at the head, nothing looks bent or out of place to me. Everything moved freely, it does not make any grinding or any worn bearing noises. Put the head back on, turned the crank, same thing, gets to one point and it's very hard to turn. Spark plug was not in it.




I would appreciate any help, thanks!

 

did you check your timing marks to make sure your chain didn't jump time? that would explain the change in cranking noise, and its possible you are bumping a valve with the piston, making it hard to turn until you are past that point. your pictures didn't load for me, so I am shooting in the dark. with the information given.

 

No I did not check that.
Where can I get information on setting the proper timing?

 

There are 2 small covers on the side of your motor, that can be removed, some are slotted for a screw driver, some are taken out with an Allen wrench, one on the side is the bolt to rotate the engine, the one on the top is a view hole to find your timing marks. slowly turn the motor with a socket until you see the "T" mark line up with the line on the hole, that should be TDC ( top dead center) at TDC, your cam is in a neutral position with both intake and exhaust closed. At TDC, you should have a slight amount of slack on all 4 rocker arms, showing all tension is removed from the valves so they can be fully closed. If you are at TDC and the valves are not this way, then the timing chain has probably stretched beyond the auto tension aduster's ability to keep it tight, and you will need a new timing chain. Get the motor at TDC, check your valves and let us know what you find. Good luck !!

 

Excellent! Thanks for the help, I will let you know the results.

 

Checked the timing per your instructions. When the T mark was aligned, there was no play in any of the rockers at all.

Here's what I noticed. While turning the crank I observed 4 timing marks. The first two were not marked with any sort of label. The marks were probably about 5/8" to 3/4" apart from each other. As I continued to turn, I came across the second set of marks. The first of the two was labeled with an "F" the second a "T." Their distance apart was about the same as the first two, but the distance from the last mark of the first set and the first mark of the second set was probably an inch to an inch and a half.

As I was turning the crank, and I started to get to that "really hard to turn" sport, I noticed it was the exhaust valves opening, and that the exact spot where I had to grab it by two hands was at the "F" timing mark. It was hard to tell if the piston was at TDC at that timing mark or not.

So I agree that it probably jumped time. My questions now, is this an interference engine? Would the exhaust valves be bent?

Where can I get instructions on replacing the timing chain and properly setting up the timing?

Thanks!

 

2006 400 ex won't start has good fuel and good spark. I have drained the bowl and I it will fire every now and then but won't start up. Any help?

 

Fit a new plug. If that doesn't work, clean the carb out. If that doesn't work, do a compression test 90psi should be enough to get it to start.

 

YES A HONDA 400EX IS A INTERFERENCE ENGINE IF YOU HAVE A HOT CAM!! If it is a stock engine I would believe it would be as well if it jumps timing.