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Discussion Starter #1
Brought my bike in for the 24K service and it seemed to be running OK when I dropped it off. Got a call from the service guy who said there was a problem with the exhaust valve in the horizontal cylinder and that it was letting something like 80% of the combustion gasses through. He couldn't believe I hadn't noticed it. The net is I needed a new exhaust valve.

So my question is, what might have caused this? Sadly the bike has been outside under a cover this winter while our garage is being enlarged, but other than the last few weeks when the weather has been horrible, I've been riding it regularly (with no apparent change in power).

Is this a common problem? I haven't seen any posts on it, so I kind of doubt it, but wondering if the tech gurus can shed any light.
 

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Not necessarily. In fact, I'm not sure how that could cause this... ???

It's probably casued by very loose closing clearance and or very tight opening clearance. Both of the 904cc 2V injected bikes we've purchased new had an exhaust valve that was always open. That's how they were adjusted from the factory. It was really like the assembler had swapped the shims for the valves on the horizontal cylinder. If you take the numbers and swap the shims, you'd get something that is almost ideal. But as it was, the exhaust valve opener had like -.002" (always held open) and the closer had something like .012" clearance. If I hadn't have adjusted the valves so aggressively on BOTH the MH900e AND the M900ie, they would surely have suffered a burned valve.

This is why I declare that valves need good initial adjustment at 1000 miles, but something that I believe dealers would call "an internet problem" and "never happens" etc etc etc.

Sorry to hear it happened to ya.
Chris

PS - FWIW, Guy MArtin has developed some kick ass OEM replacement valves that actually will improve performance over the OEM parts and cost like 1/2 as much. Thing is, you'd at least need to replace both exhaust valves...
 

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On this subject, when I was tuning my carbs last night I had the airbox off and after a short ride around the block I had the bike idling for a while in my garage. I shut it off and lifted the tank I noticed light smoke coming out of the carbs. I have never seen this before except on my old chev truck, but I have never run a bike with no air filters before and checked it immediately after riding. Could this be the same problem?
 

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Nope, this is normal. Especially with FCRs. Due to power pulses through the engine in both directions, you would see a mist of fuel vapor standing above the carbs while running. The occasional light back fire happens and this kinda pops. Also, could be steaming from the heat of the intake tract.
 

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Whew! all this talk about valve replacement got me shakin.
Sorry to hijack this thread.
 

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Discussion Starter #7
Not necessarily. In fact, I'm not sure how that could cause this... ???

It's probably casued by very loose closing clearance and or very tight opening clearance. Both of the 904cc 2V injected bikes we've purchased new had an exhaust valve that was always open. That's how they were adjusted from the factory. It was really like the assembler had swapped the shims for the valves on the horizontal cylinder. If you take the numbers and swap the shims, you'd get something that is almost ideal. But as it was, the exhaust valve opener had like -.002" (always held open) and the closer had something like .012" clearance. If I hadn't have adjusted the valves so aggressively on BOTH the MH900e AND the M900ie, they would surely have suffered a burned valve.

This is why I declare that valves need good initial adjustment at 1000 miles, but something that I believe dealers would call "an internet problem" and "never happens" etc etc etc.

Sorry to hear it happened to ya.
Chris

PS - FWIW, Guy MArtin has developed some kick ass OEM replacement valves that actually will improve performance over the OEM parts and cost like 1/2 as much. Thing is, you'd at least need to replace both exhaust valves...
So Chris, given that my bike is at 24K, I'm guessing this initial setup problem is not really applicable to my situation. Maybe a bad valve adjustment on the last service?

BTW, when a valve is "burned" what exactly happens?
 

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Michael,

i read your post yesterday. one thing bothers me. 80% would give you a dead miss at idle and stay extremely rough and sluggish up to maybe 4,000 rpm. i doubt you wouln't notice this, especially on a V twin. mabe a second opinion is in order?
 

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Discussion Starter #9
That seemed weird to me too, but I don't think these guys are shysters. It was in getting the 12K service anyway, so I said to fix the thing. Now if I could only get my speedo/tach back so I can get it home...
 

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Heres a couple examples of burnt valves, albeit the one is a pretty extreme case.

 

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When valves burn, the part that interfaces with the valve seat is typically eroded by hot gasses. This part is only suppose to be exposed to post combustion gasses, but if you expose it to gasses that are still burning, it will erode and fail to seal.

M
 

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When valves burn, the part that interfaces with the valve seat is typically eroded by hot gasses. This part is only suppose to be exposed to post combustion gasses, but if you expose it to gasses that are still burning, it will erode and fail to seal.

M
Thats acutally not *completely* true. Valves can withstand gasses that are still burning. The reason valves burn when they don't close all the way is that they can no longer transfer heat into the valve seat, and into the head. This is when they can't take the still burning gasses, because they just get hotter and hotter, and eventually deform or crack or other assorted bad things.

Kevin
 
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