is this pre-detonation or could i have a serious problem?

[Vern]

i was out gettin my m7 ready for a ride, you know toppin off the oil, checking everything out and such and i decided to clean the sheaves on my clutch. so i started it up to run a scotch bright on the sheaves while it idled(im sure not the brightest thing to do but im sure im not the only one to do it) and when i put pressure on the scotchbright pad against the sheaves when it would start to bog down i could here a nocking noise coming from the motor but its fine if i take the extra drag off the motor.

i do have high compression heads and my house is only at about 4500ft so i dont run it much down here. but i've never heard anything when riding at 6000ft+ where i normally ride and it runs fine. a week or two ago i had the y-pipe off to get a crack welded and checked out the pistons and such while it was off and everything looked fine. no sign of damage to the pistons or cylinders.

so could this just be a bit of pre-det. knock cuz of the low elevation or does it sound like there could be something major wrong?

thanks

brandon

 

[SLEDIOT]

Do you still have good compression? Does it knock onceits warmed right up? I hd piston slap in a quad 1 time that sounded like that also.........

 

[Vern]

haven't checked compression yet, like i said the only time i've heard it is if i fire it up for a few minutes at the house and put some sort of drag on it. never heard it when riding in the hills and always seems to run fine.

 

[bgreen776]

There is no such thing as pre-detonation. Pre-ignition or Detonation. One or the other.

How hard were you pushing?!

 

[pernikm]

I would be surprised if you heard detonation at idle. Cylinder presures are low and the pipe is not packing the cylinders.

 

[Vern]

There is no such thing as pre-detonation. Pre-ignition or Detonation. One or the other.

How hard were you pushing?!

whatever you say, i'm sure people got the point.

hard enough to get the clutch clean.

 

[MNM7141]

Without hearing it - it is difficult to diagnose. I would guess piston slap. Both my F and M7 will slap the piston when I first start them up - not everytime. Sounds kind of like a metallic cling and I have to be listening for it - it is pretty faint. Once warm it goes away. The larger the bore the more likely to experience piston slap. Check the compression, I bet you're fine. Paul

 

[mattymac]

no way! The thing that creates deto is heat, and lots of it!!! You cant generate enough heat at an idle to do that.


I have heard alot of noises come from clutches, waterpump/oil pump gear chatter at low RPM's. Typically not that out of the ordinary.

 

[Vern]

thanks for the info, i didn't think it was deto. but it was about the only thing i could think of other than something not very happy going on in the engine. it kinda sounds like a worn out clutch rattleing, but i just put a brand new clutch on about 2 days ago so its not that. its good to here people say this could be normal, cuz like i said everything looks fine and runs great and i'm supposed to head to island park next weekend for the expo and dont think i'd be able to afford to go if i had to tear it down. i'll have to throw a comp. gauge on tonight just to be safe, other than that im thinking im gonna just run it and keep an eye on things.

 

[Killerk]

One thing to keep in mind is you are right next to the engine without a helmet on, hood up.
I know that when I ride my sled to the outhouse at the cabin witout a helmet the engine sounds very noisy to me, almost scary noisy.
Same thing when loading/unloading the sleds in the trailer.
I get used to the sound of the engine with my helmet on, so with it off it sounds like it is knocking and is going to fall apart.
Especially my brothers cutler m7-m1000.

 

[Sledbrain2]

My 800 mc will do that once and a while idiling, I think its just a 2stroke stumbling, maybe running more compression makes it sound louder. I also run a high altitude head at 1200 ft .

 

[Vern]

One thing to keep in mind is you are right next to the engine without a helmet on, hood up.
I know that when I ride my sled to the outhouse at the cabin witout a helmet the engine sounds very noisy to me, almost scary noisy.
Same thing when loading/unloading the sleds in the trailer.
I get used to the sound of the engine with my helmet on, so with it off it sounds like it is knocking and is going to fall apart.
Especially my brothers cutler m7-m1000.

thats a good point, didn't even think about that.

 

[MTm7]

thats a good point, didn't even think about that.

mine makes a knocking noise as well... i only notice it when i dont have my helmet on.

 

[bgreen776]

You engine wasnt designed to be loaded at that low of an RPM. What your hearing is probably what most automotive guys would call "pinging", a condition you can hear if you put your truck in 5th gear at 25 mph and step on the gas. On a 4 stroke you could probably adjust the timing to get rid of the noise, not gonna happen on your 2 stroke. If your hearing one of the two conditions it is pre-ignition, caused by advanced timing. Since you engine wasnt designed to be loaded at that RPM I wouldnt worry about it other than to check your timing to make sure it matches factory specs.