Top end rattle

Hey guys I know I only post here when I have a problem but I could use your collective help. I have a 74 850 in great condition. However, recently while riding along at 2800 rpm there is a noticeable rattle coming from the top end of the engine. All other RPMs there is little to no rattle. I checked the valve clearances and there good, the only thing I have done differently is use non ethanol gas and added a bit of seafoam to every fill up. Any ideas would be great.

Is it under load or free running? Ie. does it stay there when you open the throttle? Does it get worse when you open the throttle? Coming up with these answers could help diagnose the fault. If you get it when its on the centre stand, use a large screw driver and put the tip on various parts on the engine and your ear to the handle end. Tell us where you hear the noise loudest. Other that that , take it to a mechanic or someone who knows these engines.
Dereck

What octane gas did you switch to? Sounds like detonation. Seafoam may also be reducing the octane of the fuel. Quick way to hole a piston. Believe me the problems ethanol causes are insignificant to what detonation will do. What was the octane of the gas you were using before. Go back to it and see if it goes away.
IMHO the ethanol scare can be dealt with pretty easily. Put in stayup floats, vitron tipped float needles, modern petcocks and modern automotive fuel line. For around 100$ you can ethanol proof your bike and buy the best octane gas where its convenient. Put Stabil in if it's going to sit for more than a week or so. Drain the carbs if more than month. I've been running my 78 Bonneville on 93 octane 10% ethanol exclusively for over 15 years. It's all you can get in the Houston area without going to really out of the way places. I've ran my Norton and BSA for over five years the same way. Never any issues. The Bonneville also has a POR15 tank lining that's had 10% ethanol in it everyday for over 15 years and still looks like new.

Do you have those lock rings on the exhaust nuts?
Loose ones will rattle.

Maybe advisable to check the tension of the primary chain. If it is too loose you can hear a ratlle at this rev level while cruising (without load). But better you hear it as a too tight chain. I hear a light rattle on mine too. It is not the chain hitting the case, maybe the clutch basket.

Ralf

Towner said:

Maybe advisable to check the tension of the primary chain. If it is too loose you can hear a ratlle at this rev level while cruising (without load). But better you hear it as a too tight chain. I hear a light rattle on mine too. It is not the chain hitting the case, maybe the clutch basket.

Ralf

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I had a similar noise on my bike which has dry belt drive. At very similar rpm range to that quoted by OP. Yet there was no discernible snatch in the drive line just a noise and patch of vibration. After checking ignition and fuel systems i readjusted the primary as it was on the loose side of okay (but i felt it was better that way for sake of gearbox bushes and bearings) a close inspection of belt an primary cases showed no marks of any kind. The problem noise an rough patch was gone so it's worth checking if only to eliminate another possibility.

Decemberist said:

Hey guys I know I only post here when I have a problem but I could use your collective help. I have a 74 850 in great condition. However, recently while riding along at 2800 rpm there is a noticeable rattle coming from the top end of the engine. All other RPMs there is little to no rattle. I checked the valve clearances and there good, the only thing I have done differently is use non ethanol gas and added a bit of seafoam to every fill up. Any ideas would be great.

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Non-ethanol (WHAT OCTANE?) gas? Numb the timing up a few degrees. You will know if the fuel change was RELEVANT or COINCIDENCE. :idea:

Seafoam contains kerosene, which is very similar to diesel fuel. Diesel will not burn completely in a relatively low-compression engine. Check out youtube videos of Seafoamed engines pouring smoke out the exhaust.

Not saying this is the source of the problem, but when nothing else was changed......

Thanks for the tips. I was using 91non ethanol, I am hoping it was just a gas issue so Ian switching back to 89 regular gas. I have been riding quite a bit lately and the top end rattle is noticeable around 2700 rpm and it is more noticeable when letting off the throttle(no load) really hoping it's not a cam shaft.

concours said:

Decemberist said:

Hey guys I know I only post here when I have a problem but I could use your collective help. I have a 74 850 in great condition. However, recently while riding along at 2800 rpm there is a noticeable rattle coming from the top end of the engine. All other RPMs there is little to no rattle. I checked the valve clearances and there good, the only thing I have done differently is use non ethanol gas and added a bit of seafoam to every fill up. Any ideas would be great.

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I had that same experience in my 73 850. A 2500-2800 rpm rattle. It was really noticeable under trailing throttle. Found that my gas tank had migrated a red CH and was making contact with the frame. I put a little more carpet foam under the tank, checked the rubber mounting donuts and carefully remounted it. The problem was gone. My theory on this is that my Isolastics seem to fully kick in at 2800rpm and then things smooth right out. 2500-2800 would be where the engine is most clattery. (Not sure if clattery is a word or not but I like it)

Decemberist said:

Thanks for the tips. I was using 91non ethanol, I am hoping it was just a gas issue so Ian switching back to 89 regular gas. I have been riding quite a bit lately and the top end rattle is noticeable around 2700 rpm and it is more noticeable when letting off the throttle(no load) really hoping it's not a cam shaft.

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I mix my own brew using 91-93 unleaded. Add a little octane boost, a little lead substitute, and some ethanol mitigation. I would avoid Seafoam if I were you. If you want to use a deposit-cleaning additive, use Techron (techroline), which is what the top-tier fuel merchants use.

Decemberist said:

Thanks for the tips. I was using 91non ethanol, I am hoping it was just a gas issue so Ian switching back to 89 regular gas. I have been riding quite a bit lately and the top end rattle is noticeable around 2700 rpm and it is more noticeable when letting off the throttle(no load) really hoping it's not a cam shaft.

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Likely not the fuel, as you went up in octane. Save the 89 for your lawnmower, pump premium is a better choice. All kinds of stuff can rattle under certain conditions only. Worn/in need of adjustment iso's cone to mind...
Search for close proximity, it'll have left a mark. Head steady, primary, horn, etc...

My '74 started into the same annoying rattle late this summer. While performing some routine maintenance, I found the left-front mounting point of the battery tray to be cracked in two, right next to where it bolts to the iso mount. All better now.

Nathan

I had a bike that made a strange rattle once...

Eventually I isolated this to the fact it was knackered...

Fast Eddie said:

I had a bike that made a strange rattle once...

Eventually I isolated this to the fact it was knackered...

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That was a death rattle

/Steve.