Death rattle?

[FatBiker]

I have this old bike..............

I just acquired an old '82 RM 250. Seems to run ok but I've noticed a disconcerting "rattle" in the engine. I only notice it at idle and it seems to be the same whether cold or hot. Sounds like it could be down low like something in the bottom end but it's been so long since I've been around 2 strokes that I can't remember what sounds mean what. On an automobile I can tell you in less than 2 seconds what any tick/knock/thump means but on this little "whing-ding" 2 stroke I'm lost! :ohmy: Doesn't sound like piston slap, not metallic enough to me, but I'm really hoping it is that as opposed to the bottom end. Is there some easy way to tell the difference without having to crack the case and measure stuff?

 

[Ol'89r]

Is there some easy way to tell the difference without having to crack the case and measure stuff?

Just remove the cylinder and piston then, grab the rod with your hands and try to move it straight up and down. If you feel any clearance or hear any clicking, you may have a worn or damaged big end bearing. You will have some side to side movement, (that's normal) but, you should have no up and down movement.

If it is only at idle, it's most likely piston slap.

Good luck.

 

[ellandoh]

sure its not spark knock??

 

[FatBiker]

Thanks Ol'89r, I'll do that check. ellandoh, how can I check that? What would cause spark knock? Timing? Or could there be some mechanical issue? It has fresh high octane fuel with good mix oil in it so I think I can rule out junk fuel. Perhaps a jetting issue? The bike appears stock, OEM pipe and no obvious external upgrades. Anyway, thanks for your prompt replies guys!.

 

[ellandoh]

well im in no way telling you not to check to be sure but , i do know that the two strokes bikes especially my buddies yz125 do make a little bit of a sound that if youre not used to would make me think it was going to pop. and it is only at idle also. but, if i thought i had spark knock at idle i would first try opening the air screw. i dont know but if my bottom end was out of spec and "rattling at idle" i would guess it would be "RATTLING" when i gassed it

 

[FatBiker]

well im in no way telling you not to check to be sure but , i do know that the two strokes bikes especially my buddies yz125 do make a little bit of a sound that if youre not used to would make me think it was going to pop. and it is only at idle also. but, if i thought i had spark knock at idle i would first try opening the air screw. i dont know but if my bottom end was out of spec and "rattling at idle" i would guess it would be "RATTLING" when i gassed it

That makes sense to me, although I am a bit hard of hearing (too many years in construction) and when it revs the "other" sounds may be drowning it out. That being said, I think you may be right, if it rattles at idle it will prolly RATTLE when revved. :nod:

 

[FatBiker]

Well I bought a set of used "good" cases off of fleabay to replace the cracked one (will replace both) on my bike so I guess this will be an opportune time to check everything out eh? I'll let you know.

 

[helio lucas]

just to add two things:
most motocross bikes have primary gear with straight theets. with the normal use it starts to get more and more wear at the theets and making more noise. "rattle" i guess (not very high frequency sound).
not sure about your bike but bikes with power valves, like the yz125 ellandoh mention, do have a lot of rattle, especially if the power valves are worn and clean.

 

[FatBiker]

Theets? I assume you mean teeth right? The bike makes the sound while sitting still and out of gear. The noise goes away when it's in gear and rolling. Anyway I think that it may be the primary gear rattling on it's shaft while idling. I think that's what you meant. Is it?

 

[whenfoxforks-ruled]

You got it! The clutch basket and idler gear are on the list also. The pipe is tight, no leaks, big dents, and the silencer is intact with fresh packing? There are some set ups that make an odd noise while idling, not really what they are intended to do, but, its an odd sound. Somewhere between a ring and ting? I suspect its the pressure and sound waves moving oddly? Sadly, all the other answers could be also. Old air cooled European mx bikes made these sounds, regularly. The 82 was the 2nd year water cooler, and Suzuki was on the correct path, or best at the time. And I believe all the major engine parts are still available. My 78 RM does not show thrust washers that carry the big end of the rod, the od is just big enough to keep the bearing centered, that makes for some racket around idle. I am not for sure when they started making the washers with a large enough od to carry the rod? Add that to the possibilities. Vintage Bob

 

[helio lucas]

Theets? I assume you mean teeth right? The bike makes the sound while sitting still and out of gear. The noise goes away when it's in gear and rolling. Anyway I think that it may be the primary gear rattling on it's shaft while idling. I think that's what you meant. Is it?

i am sorry for my poor writing. theet is what i mean.
the primary gearing is one gear on the crankshaft and other on the clutch. they are before the clutch and gearbox.
most racing bikes have straight theet on that gear to prevent frictional losses like street bikes with obliquous theet to cut down the noise from the gap between the theet.
a pic worth more than a thousand words:

 

[helio lucas]

its an odd sound. Somewhere between a ring and ting?

bob, if i am understanding correctly those sounds come from ressonace on the pipe walls or cooling fins on the top end. if a pipe is made of very thin sheet, say 0,5mm thick mild steel, the sound of the bike is pretty awful. nothing like the ring ding of a original, well tuned bike ;)

 

[whenfoxforks-ruled]

In Helio's pic, The outer gear on the clutch basket has a hypoid cut. The smaller idler gear is a beveled straight cut gear. It has been a while! One of them makes an odd sound, and can clank back and forth? Vintage Bob

 

[FatBiker]

Well, I just learned something! It hurt my head a little though! Lol! So is it safe to assume that those theet are driven directly off of the crank? Or is the smaller gear on the outside the one? This is obviously due to the choppiness of the single cylinder low idle characteristics and the flywheel like load of the clutch basket? :think:

 

[whenfoxforks-ruled]

The big gear on the basket is driven by the small gear on the crank. There is no such word as theet in the English language, but, teeth is? Helio's English is a ton better than when he joined, he is from Portugal if you do not know. I could never translate it back to his language! And he is correct as usual! Vintage Bob

 

[_JOE_]

Helio's english is better than most Americans........

 

[FatBiker]

Helio's english is better than most Americans........

Yeah, I have to agree. At least I got what he was talking about. There's some people around which I work that couldn't have got me any closer with their limited command of the English language.

 

[helio lucas]

. There is no such word as theet in the English language, but, teeth is? Helio's English is a ton better than when he joined, he is from Portugal if you do not know.

i got the teeth now :bang:
thank you, and learning better english was one of the reasons i have stayed here almost as long as you bob.
i still have to think in portuguese and traslate to english after. sometimes not very successfully, but google help me ;)

 

[whenfoxforks-ruled]

I am learning also! Vintage Bob

 

[FatBiker]

Well, we'll know soon enough. My replacement cases showed up today and I just ordered the gaskets about an hour ago. I'm gonna have to learn on the run I guess! :yikes: