400ex stock cam

[400exrider707]

Ok Im putting my EX back to stock and I was wondering if the decompression spring and plunger are necessary w/ the stock cam. I cant seem to find these little guys out in the garage anywhere and I would like to get it back together. Thanks

[K_Fulk]

If your going with out the decompression, I would take the decompression unit on the cam off also.

[400exrider707]

Whats the difference if its still on? Im having major problems getting this bike to even run now. Im extremely confused. Its getting spark, gas, air, and it will not run. It pops amlost every revolution showing that its igniting at some point, but it will not even hint at an idle or running. The only thing I've yet to check is compression because I dont have the right one that will go into the 400ex cylinder. I have no clues as to what the heck is going on now.

The thing ran fine a week ago. I took all my aftermarket stuff off. The only engine things I touched were the cam, and jetting. Now it will not even hint at running. So I figured timing. Heres the problem with that. How can you tell which TDC is the compression one without looking at the cam sprocket?? Its possible the cam sprocket is on upside down, so I cant really go off of the top mark. Any ideas?

[JOEX]

Put your thumb or finger over the spark plug hole when turning the motor over (by hand) to find the compression stroke.

When you feel the pressure build up, stop then put a piece of stiff wire in there gently against the the top of the piston then continue to turn the motor over until the piston tops out. You will then be at or near TDC on the compression stroke.

[lonestar13r]

Whats the difference if its still on? Im having major problems getting this bike to even run now. Im extremely confused. Its getting spark, gas, air, and it will not run. It pops amlost every revolution showing that its igniting at some point, but it will not even hint at an idle or running.

mine did this same thing once, i ended up taking my stator out and taking sandpaper and electro wash to it, put it back in and it ran fine since. it sat for only 3 day's and i went to start it , and it did all that. but it was confusing because it had air and was getting gas, half the time i checked my spark it had ,the other half it'd be dead so i finally came up with the stator deal.

[400exrider707]

Originally posted by JOEX
Put your thumb or finger over the spark plug hole when turning the motor over (by hand) to find the compression stroke.

When you feel the pressure build up, stop then put a piece of stiff wire in there gently against the the top of the piston then continue to turn the motor over until the piston tops out. You will then be at or near TDC on the compression stroke.

Ok that sounds simple enough to handle. Makes sense. Although if Im turning it over by hand I really dont need to put the wire in there, I can just check the mark on the flywheel correct? I've found that the T mark on the flywheel doesn't actually lign up perfectly with TDC of the piston. Its actually sitting down in the cylinder just a hair. Thanks joe.

[400exrider707]

Originally posted by lonestar13r
mine did this same thing once, i ended up taking my stator out and taking sandpaper and electro wash to it, put it back in and it ran fine since. it sat for only 3 day's and i went to start it , and it did all that. but it was confusing because it had air and was getting gas, half the time i checked my spark it had ,the other half it'd be dead so i finally came up with the stator deal.

I've thought about the stator as well, but from what I can tell its getting spark constantly, so I dont think thats the problem, but Im not ruling it out completely either. Thanks.

[aviator4]

Man, I don't know if I'd stick ANYTHING metal into the spark plug hole. you're asking for trouble if something goes wrong... you could score the top of the piston, or worse - the sleeve.

I'd recommended just using a clean drinking straw. It's long enough to keep from falling in. Worst case, use a small wooden dowel, but don't go putting a piece of metal rod or wire in there!!!

Have you made sure that the cam isn' 180 degrees out? at TDC, the lobes of the cam should be facing DOWN

[400exrider707]

Originally posted by aviator4
Man, I don't know if I'd stick ANYTHING metal into the spark plug hole. you're asking for trouble if something goes wrong... you could score the top of the piston, or worse - the sleeve.

I'd recommended just using a clean drinking straw. It's long enough to keep from falling in. Worst case, use a small wooden dowel, but don't go putting a piece of metal rod or wire in there!!!

Have you made sure that the cam isn' 180 degrees out? at TDC, the lobes of the cam should be facing DOWN

Ok yes I understand how it works but here is my problem. With the cam and cam sprocket off, how on earth can you tell which TDC of the piston is the compression stroke? JOE I dont think your idea will work either now that I think of it. Its going to be building compression on both upstrokes with the cam off, because without the cam its not opening the exhaust valves. So far I have had the cam in both ways, so I've tried both TDC's with the lobes down and still have the same problem. I hold the starter button down and it will NOT start at all, on either TDC. It turns over and is getting gas and spark. I have a new plug, the carb is clean and the air filter is clean. I put on a timing light and its lighting up right on the F mark so that ignition is timed properly and getting good spark. It cranks over and fires ONCE only. It fires once almost every revolution but will not even hint at starting. Im getting ready to roll this thing off a cliff!!! HELP!

[JOEX]

Originally posted by 400exrider707
Ok yes I understand how it works but here is my problem. With the cam and cam sprocket off, how on earth can you tell which TDC of the piston is the compression stroke? JOE I dont think your idea will work either now that I think of it. Its going to be building compression on both upstrokes with the cam off, because without the cam its not opening the exhaust valves. So far I have had the cam in both ways, so I've tried both TDC's with the lobes down and still have the same problem. I hold the starter button down and it will NOT start at all, on either TDC. It turns over and is getting gas and spark. I have a new plug, the carb is clean and the air filter is clean. I put on a timing light and its lighting up right on the F mark so that ignition is timed properly and getting good spark. It cranks over and fires ONCE only. It fires once almost every revolution but will not even hint at starting. Im getting ready to roll this thing off a cliff!!! HELP!
My method was intended for when the motor is together.

How are you checking the timing without it running?

[400exrider707]

Originally posted by JOEX
My method was intended for when the motor is together.

How are you checking the timing without it running?

How would you check valve timing with it running? You cant you need the valve cover off to see the marks on the cam sprocket. If you meant actual iginition timing Im just holding the starter button down and using the timing light that way. It lights on the F mark like it should for spark, if thats what you meant!?

[JOEX]

Originally posted by 400exrider707
How would you check valve timing with it running? You cant you need the valve cover off to see the marks on the cam sprocket. If you meant actual iginition timing Im just holding the starter button down and using the timing light that way. It lights on the F mark like it should for spark, if thats what you meant!?
I meant ignition timing with the light. It should be done when the motor is ldling. I'm not sure how accurate it will be using the starter.

[400exrider707]

Originally posted by JOEX
I meant ignition timing with the light. It should be done when the motor is ldling. I'm not sure how accurate it will be using the starter.

Well I cant get it to idle or even start so I dont know what the heck to do.

Is there no way to know which TDC its at with the cam and cam sprocket off??? Right now I will either have it right on or 180 off, but either way it doesn't seem to start. Im lost!!

[JOEX]

Originally posted by 400exrider707
Well I cant get it to idle or even start so I dont know what the heck to do.

Is there no way to know which TDC its at with the cam and cam sprocket off??? Right now I will either have it right on or 180 off, but either way it doesn't seem to start. Im lost!!
I don't think it matters as long as the T mark is lined up on the flywheel.

I'm thinking you have an issue(s) somewhere else......
Ignition system not strong enough
Too much fuel
Not enough fuel
Sheared key

Troubleshooting sucks:ermm:

[400exrider707]

Originally posted by JOEX
I don't think it matters as long as the T mark is lined up on the flywheel.

I'm thinking you have an issue(s) somewhere else......
Ignition system not strong enough
Too much fuel
Not enough fuel
Sheared key

Troubleshooting sucks:ermm:

It seems like most of the troubleshooting is done and thats whats killin me. Im thinking issues somewhere else as well, but the more I think of that cam it bothers me. The first time I rebuilt it thats all I did was put the piston at TDC and put the cam in with the lobes down, but doesn't the ignition only spark at one TDC?? So that would need to be the one I have the cam set for.

Also Ive put it back to stock. The cam was the only motor thing I've touched and I put stock main back in w/ a 40 pilot because I have a UNI with a vented lid. So I think the gas situation should be correct. Its getting air as well and the spark seems good. Any other ideas? A sheared key doesn't seem right but I guess its possible. This engine was running right before I switched everything back to stock and has not run right since.

[JOEX]

Originally posted by 400exrider707
It seems like most of the troubleshooting is done and thats whats killin me. Im thinking issues somewhere else as well, but the more I think of that cam it bothers me. The first time I rebuilt it thats all I did was put the piston at TDC and put the cam in with the lobes down, but doesn't the ignition only spark at one TDC?? So that would need to be the one I have the cam set for.

Also Ive put it back to stock. The cam was the only motor thing I've touched and I put stock main back in w/ a 40 pilot because I have a UNI with a vented lid. So I think the gas situation should be correct. Its getting air as well and the spark seems good. Any other ideas? A sheared key doesn't seem right but I guess its possible. This engine was running right before I switched everything back to stock and has not run right since.
It sparks on both up strokes.

Bad gas, water in the gas
Air leak
The spark may look good but not be strong enough under compression.
Not enough compression

It could be many things:ermm:

[400exrider707]

Originally posted by JOEX
It sparks on both up strokes.

Bad gas, water in the gas
Air leak
The spark may look good but not be strong enough under compression.
Not enough compression

It could be many things:ermm:

Ok well if it sparks on both upstrokes how could a cam really be 180 degress off?

Gas is fresh,

No air leaks at all its tight! I dont have the lid on the air box right now but there is a UNI bolted on, dont know if that makes a difference.

I dont see how the spark would all of a sudden not be strong. It was running perfectly right before I took the hot cam out.

Compression is one thing I DO need to check still.

Also would it make a big difference if i didn't put the decompression plunger and spring back in? i couldn't find it to put it back in but didn't think it would make a difference cause the hotcam didn't use it.

BTW thanks for all the help so far JOEX.

[aviator4]

It can be 180 out because the cam turns 1 rotation for every 2 rotations of the engine. The valves don't open 180 from each other, the patter is more "staggered" than that. So, if it's 180 out, You have a valve open when it should be closed, and one partially open with it should be closed.

"180 out" just means that cam is 180 degrees backwards, not hat the valves are opening 180 degrees from where they should. When the cam is in backwards, NONE of the cam geometry works as it should and the engine will "pop" or backfire becasue it's sparking when one of the valves is still partially open.

The decompression plunger won't make a difference as far as wheher it runs or not. Sound like the next thing to check would definatley be compression.

Also, it's possible that you flooded the bike at some point and fouled the plug.. it doesn't take much. You might want to try a NEW plug.. Take the old plug out and turn the bike over a couple of time to blow/dry out any gas that might be in there, then try a new plug, make sure the fuel is on (this one has caught me more times than I will admit) and the choke is on.... then give 'er a go.

[400exrider707]

Originally posted by aviator4
It can be 180 out because the cam turns 1 rotation for every 2 rotations of the engine. The valves don't open 180 from each other, the patter is more "staggered" than that. So, if it's 180 out, You have a valve open when it should be closed, and one partially open with it should be closed.

"180 out" just means that cam is 180 degrees backwards, not hat the valves are opening 180 degrees from where they should. When the cam is in backwards, NONE of the cam geometry works as it should and the engine will "pop" or backfire becasue it's sparking when one of the valves is still partially open.

The decompression plunger won't make a difference as far as wheher it runs or not. Sound like the next thing to check would definatley be compression.

Also, it's possible that you flooded the bike at some point and fouled the plug.. it doesn't take much. You might want to try a NEW plug.. Take the old plug out and turn the bike over a couple of time to blow/dry out any gas that might be in there, then try a new plug, make sure the fuel is on (this one has caught me more times than I will admit) and the choke is on.... then give 'er a go.


Ok yeah I had a brain fart with the 180 out thing. I understand I just was being dumb there, anyways

Compression is next on my list to check.

Brand new spark plug jsut put in and I did crank it a few times to clean it out same results. Fuel is definitely on and I have tried it with the choke in nearly every position.

[K_Fulk]

:o Sounds like mine did when i had my cam one tooth out of time.

[400exrider707]

Originally posted by K_Fulk
:o Sounds like mine did when i had my cam one tooth out of time.

Its definitely not out by one tooth, its either right on or 180 off. I never took the cam sprocket off of the cam chain I zip-tied it down to it so it cant be one tooth off.

Also does it actually spark on compression and exhaust like JOEX said?? that doesnt make sense to me.

[aviator4]

Yes. To make life easier on Honda, the stator causes the plug to spark on every revolution, which means on both compression strokes. This is called a "wasted spark ignition" system and is very common on both ATV's and Bikes. The only advantage is that is makes the ignition system simpler. The wasted spark causes little energy loss and no harmful effect on the exhaust stroke.

[400exrider707]

Originally posted by aviator4
Yes. To make life easier on Honda, the stator causes the plug to spark on every revolution, which means on both compression strokes. This is called a "wasted spark ignition" system and is very common on both ATV's and Bikes. The only advantage is that is makes the ignition system simpler. The wasted spark causes little energy loss and no harmful effect on the exhaust stroke.

Ok thats good, cause Im pretty srue that totally eliminates my valve train timing. I now need to look into compression. Although I dont see how I would have lost compression. It ran then I took it apart and now it doesn't.

[jhr400EX]

I ran into the same problem with my 400. If you have the stock cam and take the decompression mechanism out, it will not run at all. As soon as I put mine back in, it fired right up and ran fine.

[400exrider707]

Originally posted by jhr400EX
I ran into the same problem with my 400. If you have the stock cam and take the decompression mechanism out, it will not run at all. As soon as I put mine back in, it fired right up and ran fine.

Well if thats all that it is then I guess I should order that little *** spring and ball. Although how would it run without it with the hotcam in? Is it possible that it just runs easier with the hotcam cause its opening the valves longer and farther?

[400exrider707]

Anyone??

[K_Fulk]

Is the decompresion thing still on the cam behind the gear mount? If it is you need to take that off if your trying to remove it competly..

[400exrider707]

Originally posted by K_Fulk
Is the decompresion thing still on the cam behind the gear mount? If it is you need to take that off if your trying to remove it competly..

It is on the cam. I didn't know that would make a difference!? What does it matter if its still on? I couldnt find that plunger and spring to put back in. Will this really make a difference?

[400exrider707]

Will it really make that much of a difference. I might just go ahead and buy the plunger and spring.

[400exrider707]

Originally posted by 400exrider707
Will it really make that much of a difference. I might just go ahead and buy the plunger and spring.

[K_Fulk]

Originally posted by 400exrider707
Will it really make that much of a difference. I might just go ahead and buy the plunger and spring.

Sorry my computer has been down.

I don't have a technical answer for you but thats just what ive been told.

[bradrenea]

I am not positive, but I think without the spring and plunger the docompression mechanism will stay engaged, causing the valves to stay open a small amount, which in turn would not let it build any compression. Either buy the spring and plunger or take the decompression ring off the cam.

[400exrider707]

Originally posted by bradrenea
I am not positive, but I think without the spring and plunger the docompression mechanism will stay engaged, causing the valves to stay open a small amount, which in turn would not let it build any compression. Either buy the spring and plunger or take the decompression ring off the cam.

Ok now that makes sense. Thanks Ill try something out.

[400exrider707]

Originally posted by bradrenea
I am not positive, but I think without the spring and plunger the docompression mechanism will stay engaged, causing the valves to stay open a small amount, which in turn would not let it build any compression. Either buy the spring and plunger or take the decompression ring off the cam.

Can anyone verify this? I need to have this thing running this weekend!

[K_Fulk]

Thats what I was told also. Just couldn't remember until brad reminded me.

[aviator4]

I don't know for sure if thats right or not, I would invest the couple buck in the spring and plunger and put 'em back in and try it... worst case is that you're no worse off now than before.

No one has asked this yet, but why did you take out the stage one cam? If you still have it, why can't you just put it back in? Have you made ABSOLUTLEY sure that you didn't jump a tooth on the bottom cam chain sprocket? There are two of them, you can only see one with the rocker arm cover off.

[400exrider707]

I took out the hotcam to put it back to stock to sell the quad, I have already sold the cam. I know for a fact it didn't jump tooth, I have CRF chain so its not stretched at all, and I never took the sprocket off of the chain, I only unbolted the cam from the sprocket, the sprocket was zip-tied to the chain so its either dead on or 180 off.

[bradrenea]

I think what he is saying is that it could have slipped a tooth on the lower (crank driven) sprocket.

[400exrider707]

Originally posted by bradrenea
I think what he is saying is that it could have slipped a tooth on the lower (crank driven) sprocket.

No I know it hasn't. It was running perfectly fine right before I switched the cam, and the sprocket never came off of the chain so it didn't jump any teeth. Im going to just order the spring and plunger.

[400exrider707]

Ok heres the deal I put the plunger and spring back in and voila! Started right up. So for future reference if you dont have the decompression plunger and spring in, using the stock cam with the decompression mechanism in place, it wont start!