Timing/Decking/Backfiring Help

[bh2285]

Ok, If you all would help me get this problem fixed that I cant seem to figure out I would be so happy...But anyway If you want to get the most detail search for my other posts. I have a noise that is only when I let off the throttle. Like for example when I rev it up and hold it everything sounds peachy but as soon as I let off the throttle I get this noise that sounds like a backfire to me. At first I told members here my problem and they told me to change the chain and guides... well I did that and I put in a CRF450 chain and and new front guide. My guide was destroyed had chucks missing and was just flat out beat so I thought for sure this new chain and guide would solve my problem. Well guess what all it did was make this noise louder.

So...My head and jug have each been decked .025 for a total of .050. When I had the old timing chain in there I could not get the marks to line up(it was in between a tooth) so I advanced it. Im guessing that this was because of my decking and chain strech. Now that I have the new chain in and new guide the mark line up pretty damn dead on. But my noise got loader with makes me belive that it is the timing and that the noise was never the chain is was my quad backing firing...But the backfiring was not coming out the tail pipe it was coming from the header/chain area which is what made me belive that it was my chain as people here stated. So my question is could this noise be that my exhaust valves are closing at the wrong time there for causing some kind of backfire that cant get out of the cyclinder? Would this all be caused by me not advanceing the timing. Or should the timing be advanced because of my decking and if so how much. Im so lost to what this noise could be?? Also why would it get louder if I have the timing marks dead on when before I had it advance probley a degree and it was not near as load as it is now. please ask questions Ill try to make this more clear if you dont understand what im saying I really need help and I have no idea what I should do next??


BTW: When I first got this motor it had a XR400 cam in it and it didnt make any noise it ran and sounded great...Then I got a stage 2 hotcam and installed it not thinking about how it was decked and tried to line the marks up but they would not line up so I put the as close as I could and ever since then ive had this nose..AHHH!

You can see my setup in my SIG ive got the stage 2 hotcam in there now and it is all adjusted to the spec card that came with it.


THANKS YOU GUYS FOR ANY HELP!

[bh2285]

No One?

[firefighterjosh]

I could make up something You got me:(

[bh2285]

Man O Man, Ive guessed already I dont like it.

[firefighterjosh]

Maybe the Vavles need adjusting????:ermm: :confused:

[bh2285]

I wish it was that easy!

[firefighterjosh]

I had a 1984 impala that had a similar problem. The cam in it wore causing the exhaulst vavles not to open all the way. Since I am not there and can't hear it I can only guess what I would do. I would prabably tear it down put new vavles, seats, rods and everything into the head. Sounds like maybe the old cam wore spots and thats how it broke in. Now the new cam isn't worn to those spots and can't function like the old one? Do you get what I am saying???

Example:
If you have old sprockets and put a new chain on it will wear your new chain out faster due to the sprokets are already broke in from the old chain.

[bh2285]

ahh..You could be right but I really belive this has to do with the decking and timing. Because I have 2 base gaskets to make the valves clear. I know this throws off the timing..Im just going to advance it and see if it will stop...I just need to know where to stop advancing like how much is too much?? 5 degrees?

I searched and it seems this is a big problem with decking. Theres a couple other posts on this.

Plus I even put the old cam back in to see if that was the problem it was not. I know I messed up the timing because I didnt remember that it was decked when I went to put the HOTcam in(didnt look at how it was just tore it apart...VERY DUMB) and just lined it up as good as I could thats when it all started.

[bh2285]

I just want to know how to advance the timing? Can I assume 1 tooth on the spocket is 1 degree? How much should I advance? God Im so freakin lost someone please help me im reading all the post on advancein the timing and im more lost then ever now !

[firefighterjosh]

I dont know how to do timing on 400s. call Baldwin racing???

[JOEX]

There is an adjustable cam sprocket available, I think GT Thunder sells it. The stock sprocket is not adjustable other than moving it a tooth forward or back, I don't know how much each tooth is but i'm sure it's alot more that 1 degree.

[bh2285]

So there no way to advance the timing? I guess Ill risk it and just move it up 1 tooth at a time and see what happens.

[boricua]

its a pretty big risk. you run a chance of hitting the piston with the valves. or it not running at all.
look into getting an adjustable cam gear. the adjustments on them should be much less than a tooth, it only needs small increaments....
you say it sounds like its backfiring.....do you know for sure its not? if there is a leak at the head to header mounts, it can backfire from there.
now that i think of it.....if your chain was stretched, its possible that your valve(s) have hit the piston and are bent causing them to stick, just a little or close a bit slower than normal,
FYI, intake valve opens @ 5* BTDC
intake valve closes @ 40* ABDC
exhaust valve opens @ 40*BBDC
exhaust valve closes @ 5* ATDC
this was from my clymer manual......
you can use this to determine if the valves are opening and closing at the right times, turn engine over manually (obviously)

[bh2285]

Originally posted by boricua
its a pretty big risk. you run a chance of hitting the piston with the valves. or it not running at all.
look into getting an adjustable cam gear. the adjustments on them should be much less than a tooth, it only needs small increaments....
you say it sounds like its backfiring.....do you know for sure its not? if there is a leak at the head to header mounts, it can backfire from there.
now that i think of it.....if your chain was stretched, its possible that your valve(s) have hit the piston and are bent causing them to stick, just a little or close a bit slower than normal,
FYI, intake valve opens @ 5* BTDC
intake valve closes @ 40* ABDC
exhaust valve opens @ 40*BBDC
exhaust valve closes @ 5* ATDC
this was from my clymer manual......
you can use this to determine if the valves are opening and closing at the right times, turn engine over manually (obviously)


Hum, I dont know if it for sure backfiring but I do know I checked all my valves when I had the head off I put solvent in the head and I had only one exhaust valve that was leaking a little bit..But it was not that bad only dripping every couple secs. I dont know if this could cause my issue I didnt think it could. But I know the head is not cracked or anything like that. I do have 2 base gaskets to make the valves clear but they never hit and when they did hit because of one gasket they got replaced.

All that valve stuff you typed how do i check that i dont understand all those terms..

[bh2285]

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