didnt run race gas...ooops?

[400exbiggun]

Not me, my buddy has a ex that they used to race, this motor is a 425 im guessing with just a high comp piston. He took it out one day and didnt run race gas like they normally do, well about 45 min into riding he said he stopped riding becasue it developed a tap.

What exactly could cause all this problem, he said it still ran perfect so i doubt its really all that bad? I was orignally going to buy their spare 416 off of him, but now he is using that and im going to possibly snag this 425 depending on the damage.

[NacsMXer]

His motor is probably ok. But that "tap" you speak of is most likely spark knock from running too low of an octane gasoline. The octane rating represents the gasoline's resistance to spark knock aka detonation.

In case you don't know, spark knock happens when the combustion chamber gets so hot that the gasoline ignites before the piston reaches TDC and the spark plug fires. So you get a miss-timed explosion that is trying to force the piston back down as it is still traveling upwards in its stroke. This will cause many times the normal force to be exerted on the piston, rod, and bottom end (not designed to handle that stress).

If it is an isolated situation like you described, then you should be fine. Do this repeatedly over a long period of time and the piston, rod, and/or bottom end WILL grenade on you. This is why it is so important to use high octane fuel with high compression motors.

[400exbiggun]

ok, so my question to you I guess would be, think I should purchase this motor?

I recently got another motor off some retard on here who told me it was "low on compression" not thinking about anything I gladly picked up the motor, only to find the piston severly trashed, the cylinder walls need to be bored to at least a 416 because these are some pretty hefty scratches, the head has damage where the cam rides. Looks as if the motor was run very low on oil, or run super lean and overheated.
The bottom end moves 100% free, but there is some rod play which im not to fond of.

He should be calling me within a few hours with a price, no more thatn $300 I know which is a complete assembled motor with some bolt on parts like the hinson quick change clutch cover, barnett clutch, and possibly i think a TC cam.
If I aquire the motor would just getting a few gallons of power-mist and attempting to start the motor be alright? Just to hear the noises or should I pull the head off asap?

[NacsMXer]

Is this a constant audible tap it could be something mechanical. If it is a sometimes thing while riding, it is probably the detonation. I would crank it and listen to see where it is coming from as well.

300 is a pretty good deal for an entire motor. You could upgrade it with new parts and the overall cost would be pretty good.

[GPracer2500]

Originally posted by NacsMXer
[snip]...the gasoline ignites before the piston reaches TDC and the spark plug fires. So you get a miss-timed explosion that is trying to force the piston back down as it is still traveling upwards in its stroke...[snip]

Close--you've got the right idea but the details aren't quite right. Detonation ALWAYS occurs after the spark plug fires. Pre-ignition is when the fuel/air mixture is ignited prior to the spark plug firing. Pre-ignition is far more insidious, rare, and harmful.

What you've got to remember is that the spark plug does not fire at TDC. It fires many degrees of crank rotation BEFORE TDC. I'm not sure of the exact spark advance of a stock 400EX CDI, but it's around 30 degrees before TDC. Detonation occurs when a combination of things raises the pressure in the combustion chamber to the point that unburned fuel/air mixture spontaniously combusts in an irregular way. The forces that combine to cause this are the piston moving upward (compressing the space) and the spark plug igniting the mixture near the spark plug. When part of the mixture (near the plug) starts to burn (it burns from the ignition source and then travels outward under normal conditions, it does not explode), then the pressure from the expanding gasses causes the cylinder pressure to go through the roof. If too low octane fuel is present then this increase in pressure can cause the unburned air/fuel mixture that is at the edges of the cylinder to explode before the "flame front" has a chance to get there and burn that fuel/air mixture normally.

Again, it is not until after the spark plug begins the combustion sequence that cyclinder pressures get extreme enough to cause detonation. The reason compression ratio comes into play when discussing octane requirements is because the spark occurs well before TDC--the piston is still compressing while the fuel/air mixture begins to burn.

ETA: Mild detonation is not likely to destroy an engine in a hurry. If an engine is detonating all the time than, YES, something should be done to correct it. But, if it is just an occassional occurence (like only under the highest engine load conditions) and your willing to back off the gas when it happens, your engine will likely put up with some detonation throughout its life.

Pre-ignition, on the other hand, can destroy an engine in a heartbeat...

[JOEX]

Originally posted by GPracer2500
Close--you've got the right idea but the details aren't quite right. Detonation ALWAYS occurs after the spark plug fires. Pre-ignition is when the fuel/air mixture is ignited prior to the spark plug firing. Pre-ignition is far more insidious, rare, and harmful.

What you've got to remember is that the spark plug does not fire at TDC. It fires many degrees of crank rotation BEFORE TDC. I'm not sure of the exact spark advance of a stock 400EX CDI, but it's around 35 degrees before TDC. Detonation occurs when a combination of things raises the pressure in the combustion chamber to the point that unburned fuel/air mixture spontaniously combusts in an irregular way. The forces that combine to cause this are the piston moving upward and compressing the space and the spark plug igniting the mixture near the spark plug. When part of the mixture (near the plug) starts to burn (it burns from the ignition source and then travels outward under normal conditions, it does not explode), then the pressure from the expanding gasses causes the cylinder pressure to go through the roof. If too low octane fuel is present then this increase in pressure can cause the unburned air/fuel mixture that is at the edges of the cylinder to explode before the "flame front" has a chance to get there and burn that fuel/air mixture normally.

Again, it is not until after the spark plug begins the combustion sequence that cyclinder pressures get extreme enough to cause detonation. The reason compression ratio comes into play when discussing octane requirements is because the spark occurs well before TDC--the piston is still compressing while the fuel/air mixture begins to burn.
WOW! Now if I can retain all that:p

Thanks for all of your informative posts:)

It's people like you and a few others, including NacsMXer that really help out this site:)

[2004exrider]

Uh oh watch out wilkin someone else is movin in on your posts:eek: :D

Jimmy

[NacsMXer]

Originally posted by 2004exrider
Uh oh watch out wilkin someone else is movin in on your posts:eek: :D

Jimmy

Haha, yeah. I ain't no Wilkin or an expert for that matter, but I try to help out if I am able to do so. Thank you for your kind words JOEX ;) What's better is people who ARE experts like GPracer2500, who step in and clarify things that need to be clarified :)

[krt400ex]

Originally posted by 400exbiggun
ok, so my question to you I guess would be, think I should purchase this motor?

I recently got another motor off some retard on here who told me it was "low on compression" not thinking about anything I gladly picked up the motor, only to find the piston severly trashed, the cylinder walls need to be bored to at least a 416 because these are some pretty hefty scratches, the head has damage where the cam rides. Looks as if the motor was run very low on oil, or run super lean and overheated.
The bottom end moves 100% free, but there is some rod play which im not to fond of.

He should be calling me within a few hours with a price, no more thatn $300 I know which is a complete assembled motor with some bolt on parts like the hinson quick change clutch cover, barnett clutch, and possibly i think a TC cam.
If I aquire the motor would just getting a few gallons of power-mist and attempting to start the motor be alright? Just to hear the noises or should I pull the head off asap?

I would pull it apart right away and re-build it. I don't know what a crankshaft for a 400EX, but my RM250 crankshaft costs $465. That's a pretty expensive part. Hot Rods sells a 460EX kit for $731, and you can have L.A. Sleeve install it for around $200. So you're looking at at least $1,350. For only $300 I would buy the engine and do it. The Hot Rods kit runs on pump gas so you won't have to worry about burnig your engine apart. Also if the damage where the cam rides is substancial you're looking at more cost. Remember if you go to the honda dealer, they're going to charge like there is no tomorrow. Hot Rods kit is actually less expensive than OEM Honda. Here's some of your options. Let us know how every thing works out.

Originally posted by GPracer2500
Close--you've got the right idea but the details aren't quite right. Detonation ALWAYS occurs after the spark plug fires. Pre-ignition is when the fuel/air mixture is ignited prior to the spark plug firing. Pre-ignition is far more insidious, rare, and harmful.

What you've got to remember is that the spark plug does not fire at TDC. It fires many degrees of crank rotation BEFORE TDC. I'm not sure of the exact spark advance of a stock 400EX CDI, but it's around 30 degrees before TDC. Detonation occurs when a combination of things raises the pressure in the combustion chamber to the point that unburned fuel/air mixture spontaniously combusts in an irregular way. The forces that combine to cause this are the piston moving upward (compressing the space) and the spark plug igniting the mixture near the spark plug. When part of the mixture (near the plug) starts to burn (it burns from the ignition source and then travels outward under normal conditions, it does not explode), then the pressure from the expanding gasses causes the cylinder pressure to go through the roof. If too low octane fuel is present then this increase in pressure can cause the unburned air/fuel mixture that is at the edges of the cylinder to explode before the "flame front" has a chance to get there and burn that fuel/air mixture normally.

Again, it is not until after the spark plug begins the combustion sequence that cyclinder pressures get extreme enough to cause detonation. The reason compression ratio comes into play when discussing octane requirements is because the spark occurs well before TDC--the piston is still compressing while the fuel/air mixture begins to burn.

ETA: Mild detonation is not likely to destroy an engine in a hurry. If an engine is detonating all the time than, YES, something should be done to correct it. But, if it is just an occassional occurence (like only under the highest engine load conditions) and your willing to back off the gas when it happens, your engine will likely put up with some detonation throughout its life.

Pre-ignition, on the other hand, can destroy an engine in a heartbeat...



That is exactly what I was going to say... I guess he just beat me to it.. :rolleyes: ... well said GPracer2500, I work at a ATV sales and repair shop and didnt know a single thing about what you just said but its all good.. someday I will be that smart... :rolleyes: I doubt it..

[bh2285]

So...I have a question??? If this is happening would this happen right when you let off the throttle and then go awy while you are on the gas and so on? Because I have all the stuff that is in my SIg done to my quad and I am running 93 gas with booster and I get this noise thats like a tap right when I let off the gas it only lasts a sec then when I rev it and hold it it sounds and runs fine but when i let off the throttle i get that noise for a sec agian and agian...Now I rode one whole day with it making this noise and it never shut down on me or anything it did seem to be running hot.

[krt400ex]

Originally posted by bh2285
So...I have a question??? If this is happening would this happen right when you let off the throttle and then go awy while you are on the gas and so on? Because I have all the stuff that is in my SIg done to my quad and I am running 93 gas with booster and I get this noise thats like a tap right when I let off the gas it only lasts a sec then when I rev it and hold it it sounds and runs fine but when i let off the throttle i get that noise for a sec agian and agian...Now I rode one whole day with it making this noise and it never shut down on me or anything it did seem to be running hot.

Ya got me. Never heard of anything like that before.

[bh2285]

Yeah..I think it could be the problem my only other thought would be sloppy cam chain as read in other post. Any help anyone?

[bh2285]

Could this be my cause maybe too?

[Chino886]

Originally posted by bh2285
Could this be my cause maybe too?

Detonation is a "ping"-ing sound, not tapping like yours

I am guessing yours is more prominent on the right side of the bike.

[bh2285]

Yes, its on the right side of the motor for sure...

[Chino886]

Originally posted by bh2285
Yes, its on the right side of the motor for sure...

That is probably your cam chain slapping around on the walls or the chain guide, I would not ride it too much at all because you are probably leaving marks.........