Clanking / clanging noises at high rpm

weefek

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So, I get my driver's license back in about 1 month. That means I still have a month of relying on my bike for trips into town. After that I can have some real fun with this bike! To start off I'm going to throw my rebuilt new engine on (fresh bearings, blahblah etc see my other thread about it) with proper squish gap and everything and focus on rebuilding the engine I'm currently running. Already have a new jug and piston and bearings and what not in case any of that needs to be replaced. I think eventually maybe V5 phantom85 but who knows what the future holds. Regardless.

Lately (the last... 50 miles or so, I'm at around 500 miles total on my bike/engine) at high rpm (full out) 33mph or so on a 44t rear gear so around 7200~ish rpm (by the maths, I don't have a tach on it) there is a weird clanging noise. I'm not sure how to describe it. I've had a decent amount of experience with automotive 4 stroke engines (ford FE engines, chev 350's, modern VW 4 strokes, subaru weird ass engines, buick V6s, etc) and it doesn't sound like rod knock or anything i've ever heard before. It literally only happens above 7000rpm or so.

I have a feeling it has something to do with the clutch bevel gear / crank bevel gear. When I go over large bumps or gaps in the road the same noise occurs. Everything is tight obviously before anyone asks (motor mounts, head bolts, etc).

The best way to describe it sounds like marbles in the engine. It's definitely not predetonation, I'm running 94 at the moment (because it was the only gas i could get at the time that has no ethanol in it). I've taken the head off a few times and it doesn't look like there is a hot spot (which can also cause pre-det regardless of octane).

Any ideas or has anyone else has this issue and figured it out? I've got my spare engine ready to throw on but I'm worried that this one will die and/or explode on me during a trip into town (around 5 miles one way).
 

weefek

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I don't run a chain guard so it's definitely not that. My experience with cars / engines have given me the ability to know where sounds are coming from... it's coming from the engine. Whether it's top end , bottom end, clutch side, etc I don't know but I know it's coming from the engine. Not just that experience but also my experience in automotive production... I've been working on press lines and weld cells for a long time now, they bring a unique skill set... I know some older guys that can tell what's wrong with a press based on the sounds they make...

I'd imagine such a sound from a chain guard would be more 'tinny' as it's fairly thin metal.
 

Chainlube

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I don't run a chain guard so it's definitely not that. My experience with cars / engines have given me the ability to know where sounds are coming from... it's coming from the engine. Whether it's top end , bottom end, clutch side, etc I don't know but I know it's coming from the engine. Not just that experience but also my experience in automotive production... I've been working on press lines and weld cells for a long time now, they bring a unique skill set... I know some older guys that can tell what's wrong with a press based on the sounds they make...

I'd imagine such a sound from a chain guard would be more 'tinny' as it's fairly thin metal.
Well, if you have the provisions to get the rear wheel off the ground and run it, you might be able to diagnose it that way. A short length of air line tubing (aquarium) or stethoscope to listen to suspect areas can narrow it down. My first guess would be the chain clinking off chain stay because that has happened to me.
 

weefek

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Well, if you have the provisions to get the rear wheel off the ground and run it, you might be able to diagnose it that way. A short length of air line tubing (aquarium) or stethoscope to listen to suspect areas can narrow it down. My first guess would be the chain clinking off chain stay because that has happened to me.
Good idea. I haven't done this. I will do this tomorrow. It could maybe very well be chain related , but i'm pretty confident it's coming from the engine top or bottom end. I admit I could very well be wrong. I have a mechanics stethoscope i'll try that. I don't have any gosh darn fish.

The chain stay is pretty far rear-wards, this noise is coming from the engine area 100%.
 

TheWizzerd

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Could be detonation. It can sound clangy and clanky on these little engines. The cooling fins can ring and ting and sound clangy when it happens, and the detonation makes the clanky sound. Combined with the vibration these get close to top RPM, it sounds pretty awful.

May see if your timing slipped if you have done any mods to allow your magnet to slip. If you have changed fuels, or got a batch from a different station lately, this could potentially be the issue causing detonation.

Not saying thats what it is...but its a possibility. I had an engine sound exactly as you describe and it was the fuel. It was given to me by one of my paramotor buddies so I dont know what it was. I suspected as much since it was the only thing that changed, and after that tank was dry, I filled it with what I normally use, and it has never happened again.

Another potential thing to check is a broken ring..... but they normally sound bad throughout more of the RPM range, but I have had some that were fairly quiet until the engine was loaded and not particularly noticeable at idle, or revving a bit unloaded. Depending where and how it broke.
 

coubob

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I'm new to this, but maybe the gear the clutch pads are in.
 
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