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Discussion Starter · #1 ·
Hello there, first time posting here and hoping for help. At this point I'm pretty desperate to fix my bike but haven't found any answers online or from my mechanic. Bike is a stock 78 cb550K and it's blowing white smoke out of number 2 exhaust. This happens after riding it for 5-7 mins when the bike is up to temperature. It only happens about 7-10 seconds after coming to a stop. So idling at a red light, for example, the bike is spewing white smoke. Can't tell if it has a fuel or oil smell, but it has a smell. Mechanics checked compression and all cylinders are reading well. So it can't it be valve stem seals or piston rings, or can it? They cleaned the carbs in august last year and it was stored correctly over winter. I installed a Dyna S ignition recently but the smoke was blowing before that so I think it's irrelevant. Internet says white smoke is oil but my mechanic thinks its fuel. His recommendation was a stuck float needle but I've checked those and they seem fine, albeit on a cold bike. The bike starts well on choke then I can get it to a nice idle fairly quickly. The throttle can be a bit choppy under acceleration when I haven't ridden in a while but that usually clears up about 5 mins into the ride. If anyone has any info, please point me into a direction. I can't take it to the mechanics cause I've got no money since the pandemic and so I'm hoping to fix it myself or I'll have to reluctantly sell. Thank you!
 

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White smoke is generally indicates a coolant leak (not your issue), usually a blown head gasket would be the likely culprit. Blue or grey would indicate oil burn.
 

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Agreed, Black smoke is oil burning, blue is raw fuel and white is typically a sign of coolant, but this is an air cooled bike. You must have something cooking inside there that burns white and that is a short list.
Aluminum burns white.
 

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Discussion Starter · #4 ·
Thanks! I will start a top end rebuild. Maybe do piston rings and valve stem seals while I’m in there. Hopefully I didn’t damage any internals.
 

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Do it again now and you will know if something is getting worse. It's easier then an engine tear down.
 

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Discussion Starter · #8 ·
I guess I could have mentioned I did it about 2 weeks ago. I don’t really have the money to go get another one done. Is there a possibility it’s not compression related?
 

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First off, you can rent compression testers from a decent rent-all store for all you need it and if you don't get real numberers you have nothing. Record them in your owner or service manual.
Think of your engine as an air compressor that just happens to run on exploding gasoline, because all of the things like pistons rings and valves are to make and contain compression. Being in spec does not make it great, being the same as when it was new would be great, anything else is a sign of wear tear or damage.
 

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Discussion Starter · #10 ·
First off, you can rent compression testers from a decent rent-all store for all you need it and if you don't get real numberers you have nothing. Record them in your owner or service manual.
Think of your engine as an air compressor that just happens to run on exploding gasoline, because all of the things like pistons rings and valves are to make and contain compression. Being in spec does not make it great, being the same as when it was new would be great, anything else is a sign of wear tear or damage.
I appreciate the advice. Thank you. I will look into renting one. I got real numbers when my mechanic did it two weeks ago. I just can’t find the paper it’s written down on. I vaguely remember them all being within the 140 range. When looking in the manual it says 170 but when reading up most people say the values being close to the same is the most important. Mine where all within 10-15% of each other.
I may just sell the bike as is.
 

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My first thought was worn valve guides but if there is suppose to be a seal around the valve stem that may be all it is.
...and a bad valve guide seal won’t affect compression readings. It’ll just let a little oil drip into the the exhaust.
 
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Leak down test usually is more helpful than a simple compression test. And a good tool and skill to have around.
But worn.valve stem seals might be the most obvious and simple cause and my guess...
For intermittent white/whitish blue smoke. Particularly at idle when vacuum levels are higher and might draw oil past the seals.
 

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Discussion Starter · #14 ·
Leak down test usually is more helpful than a simple compression test. And a good tool and skill to have around.
But worn.valve stem seals might be the most obvious and simple cause and my guess...
For intermittent white/whitish blue smoke. Particularly at idle when vacuum levels are higher and might draw oil past the seals.
Thanks for the help. I think this is the general consensus and I’ll try and swap them on my own.
 
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