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Discussion Starter · #1 ·
Hello anyone, everyone. I have a pretty heavily modded 660 - 720 freshly built. Started right up, idled fine and ran superbly during half hour rides here and there during break in period. Finally grew balls enough to take it a few miles away to go riding trails for about 2 hours. Again, ran perfect the entire time. Upon my way home, (roughly a five mile stretch in 5th gear at a steady rpm range,) I noticed as soon as I slowed to a stop and returned to idle, it started smoking from the exhaust, then wouldn’t idle and died. It fired back up, however, I had to throttle it some and white/light gray smoke was puffing out. I cursed a little, pushed it into the garage and let it sit overnight. The following day I fired it back up. Started right up and absolutely no smoke present and idled perfectly. Upon letting sit and warm up, it started to smoke again. Gave it a few revs and still more smoke. Eventually it died and was kinda hard to start again. Any suggestions from experience on what this could mean? The smoke did not smell sweet of coolant, but like a stronger exhaust odor. Any help for what direction to go next is very much appreciated!
 

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Master of the Electron
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Gonna bet head gasket or cylinder bolts... What sandman suggested will point to the direction.
 
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Discussion Starter · #4 ·
Oil level is spot on and no contamination, coolant level was low, but not contaminated. Topped it off and checked for bubbles in the radiator while running/revving a few times, only turbulence present…the head is studded and have cometic gasket(s), have heard and read that was the way to go but yea, something is going on. Radiator and overflow tank lost coolant but wasn’t dry….
 

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Master of the Electron
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Does plug show any sign of oil?

I'm heading towards head gasaket, sorry. Leakdown test?
 

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Discussion Starter · #8 ·
In the meantime, even with studded head, top of the line cometic head gasket and advanced head torque spec., is this still a common issue? If the head gasket is the issue, other than a warped head, what would I do to fix this mofo? Lol. Yea m pretty sure when I sent the head out for valve work, I told them to make sure it’s true
 

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Your head might not be true even after the machine shop. Cuervo Racing has a video about this.

My guess is you have some antifreeze sneaking into your cylinder and burning up when your engine heats up. Once your engine cools, it seals up enough to not smoke. You could also check with a scope to see if you see anything leaking inside the cylinder. Keep us posted on what you find.

Here's the video I talked about:
 

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Master of the Electron
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Here in the forum it seems to show up a lot more often in engines that have already had the gasket replaced - relatively uncommon to hear abour stock head gaskets failing... but common to hear things like: 'I replaced my gasket and just xxxx later it is leaking again'.

My take from this (just educated supposition)? Some are due to poor/incorrect repair and assembly techniques, perhaps some due to low quality aftermarket gaskets and some due to over-stressing gaskets with heavily modded engines - higher cylinder pressures, etc.

Remember - the coolat is at around 12PSI above atmospheric and, during intake stroke, the cylinder is well below atmospheric pressure - so if the gasket isn't perfect, the coolant can easily get sucked/pushed into the cylinder.

This leak can be one way - sucking in coolant during the intake stroke while, during the compression stroke, cylinder pressure can actually push on the gasket, closing the pinhole.

So, It is common for coolant to be slowly lost through a small gasket leak with no other evidence to prove the path until it's apart again.
 
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Im with quadmaniac on this one. I tried one of those niche big bores, ran a couple rides then pop a head gasket. I pulled it off replaced gasket, same thing. Went back to stock and has been fine for 2 years now. The 2nd time the gasket went it was nearly identical symptoms as original poster
 

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Discussion Starter · #12 ·
Sorry i haven’t updated for a while, I just moved and the quad is temporarily in storage. So I’ll get the head checked for sure, but in addition I should go forth with an oem head gasket as well perhaps? Do they sell them for 102mm bore?
 

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Hello anyone, everyone. I have a pretty heavily modded 660 - 720 freshly built. Started right up, idled fine and ran superbly during half hour rides here and there during break in period. Finally grew balls enough to take it a few miles away to go riding trails for about 2 hours. Again, ran perfect the entire time. Upon my way home, (roughly a five mile stretch in 5th gear at a steady rpm range,) I noticed as soon as I slowed to a stop and returned to idle, it started smoking from the exhaust, then wouldn’t idle and died. It fired back up, however, I had to throttle it some and white/light gray smoke was puffing out. I cursed a little, pushed it into the garage and let it sit overnight. The following day I fired it back up. Started right up and absolutely no smoke present and idled perfectly. Upon letting sit and warm up, it started to smoke again. Gave it a few revs and still more smoke. Eventually it died and was kinda hard to start again. Any suggestions from experience on what this could mean? The smoke did not smell sweet of coolant, but like a stronger exhaust odor. Any help for what direction to go next is very much appreciated!
I know this doesn’t relate but My bike only does this when I don’t ride it for a couple days up to a week. I never figured it out but it doesn’t bother me much. It only happens on cold start once I heat it up and get it running no white smoke at all or anything and it runs like a dream.
 

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Fuel is black smoke.
White or gray smoke is oil.
Sounds like valve seals are leaking, because it stops as they heat up and expand, and oil slowly runs into the cylinder after the engine is off and cools down.
Then you have a small amount of oil in the cylinder on the next start up.
A fuel issue(black smoke) also will not just go away.
White smoke can also be coolant, but you would smell it, and see the resivoir level going down. It is also very rare for a coolant leak to be small and remain that way long term, but still possible.
 

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Master of the Electron
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Six, he said it's smoking AFTER warming up (1st post)- not valve stem seals, IMO.
 
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Master of the Electron
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Marco - in your case it's likely valve stem seals... but the original poster gets smoke while riding only AFTER it has heated up.
 
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