Overfloating carb and smoke from intake manifold

[jyrki]

Hi,
I changed membranes of carbs, cleaned carb at same time, checked float levels. But carb no 1 or 2 is overfloating (from bigger hose) when machine gets warm.. I tried to syncronize carbs, but vacuum is much higher in second cylinder, and after removal of vacuum measurement hose, I noticed smoke coming out from intake   Valves are also adjusted and new piston rings installed...
Any idea? to stop overfloating... I'm for my first time synchronising carbs, so if I'm totally out of proper range with second carb?
See attached picture of smoke coming from intake..
br Jyrki

[FJmonkey]

The overflow of the carbs is most likely crap stuck in the float needle or a stuck needle. Smoke from the top of the engine is likely oil that dripped during valve cover removal or the valve cover gasket (bolt grommets as well) leaking. If the smoke clears up after a few runs then don't worry about it. If it continues then look for signs of oil where it can leak from the valve cover.

[jyrki]

Hi, smoke  is coming from underpressure measurement connection, after removal of measurement hose, so from intake manifold/connection. I will clean again the carb...

[balky1]

Burnt intake valve?

[jyrki]

Hopefully not... I noticed that sealing rings between carbs are not in the petfect condition, so I chabged those O rings. Next step to try..

[jyrki]

What makes intake valves burning?

[FJmonkey]

Tight valves. When the gap is under spec the exhaust valves can't close completely. This lets combustion gas past as well and cooks them.

[great white]

Typically, it's an exhaust valve that "burns". Exhaust valves see the extremes of temperature that intake valves typically do not.

As the engine runs, the exhaust gasses erode both the valve seat and the valve itself. This causes the valve to recess into the seat area, which reduces the clearance at the cam lobe. Left long enough without the clearances being checked and the valve can get into a situation where it does not close fully on the valve seat. This only accelerates the valve wear and can result in a burned valve as a localized spot will usually fail first. This causes a localized area where exhaust gasses will "jet", which causes even faster errosion in that one localized area. If its left long enough, you end up with this type of damage:



Intake valves are closed when combustion occurs so the narrow sealing area between valve and seat don't see the same combustion heat as exhaust valves. Combine this with the fact that when the intake opens it only passes clean (and relatively cool) air and fuel and the intake valve is thermally more stable. Intake valves typically don't have thier clearance wear down as fast as exhaust valves, but if it started "tight" and was run for extended mileage without a valve check, you can get into a valve "burn" situ on intake as well. Poor valve sealing on the intake side is far more likely to be insuffeceint valve clearances than a burned valve. You will notice the engine coughing back through the carbs long before you end up with a burnt intake valve. Be cautious when thinking it's a burnt valve if your bike is "carb coughing", lean and rich mixtures can cause the same behaviour (ie: carb coughing). Check the valve clearance and then go into the carbs next if it's coughing back.

Burnt valves occur due to insuffecient sealing at the valve and seat interface. The most common cause of this sealing issue is insuffecient clearance at the cam lobe/lifter....

[FJmonkey]

Yea, what he said... Check your valves even if they sound alright. A tight valve is silent and can cause a failure. A loose valve will not, ie. a Tappy valve is a happy valve...

Consider this a Public FJ Service Announcement (PFJSA). Seems like a Trump text when put like that. 

[ribbert]

Intake valves are closed when combustion occurs so the narrow sealing area between valve and seat don't see the same combustion heat as exhaust valves.

Aren't exhaust valves also closed during combustion?

Noel

[FJmonkey]

Intake valves are closed when combustion occurs so the narrow sealing area between valve and seat don't see the same combustion heat as exhaust valves.

Aren't exhaust valves also closed during combustion?

Noel

Except when they are out of adjustment and allow hot gasses past them.

[ribbert]

Intake valves are closed when combustion occurs so the narrow sealing area between valve and seat don't see the same combustion heat as exhaust valves.

Aren't exhaust valves also closed during combustion?

Noel

Except when they are out of adjustment and allow hot gasses past them.

That's true Mark, and a tight inlet valve would be subject to the same thing but the context above was of a theoretical cycle.

Noel

[FJmonkey]

That's true Mark, and a tight inlet valve would be subject to the same thing but the context above was of a theoretical cycle.

Noel

Fair enough, just a gentle poke. Something for us to remember when I get to visit the land down under. Its been too long since my wife has seen family.

[jyrki]

Hi, many many thanks for your replies and hints! My first time checking valve clearances of yamaha after machinerebuild...I will open the engine and check if there is damages on valves. So on I am on learning curve with Yamaha ☺ 

[great white]

Intake valves are closed when combustion occurs so the narrow sealing area between valve and seat don't see the same combustion heat as exhaust valves.

Aren't exhaust valves also closed during combustion?

Noel

Well, I know you know what I meant because of your experience. But for the benefit of those that don't have as much mechanical knowledge: yes, they are both closed during ignition.

I was keeping it simple as the discussion only required a simplistic answer. Without posting a novel on the combustion cycle and engine construction, it's easy to pick holes in a short internet forum post. People typically don't want to read several paragraphs on an internet forum, they just want a simple answer.



Both valves are closed during ignition but only the exhaust valve sees the higher combustion temps as it deals with evacuating the spent charge. The "Blow" part of Suck/Squeeze/Bang/Blow.



The intake valve does not see the same temperature swing for several reasons from boundary layer effect to the cooling properties of the intake charge. Not to mention that its small profiles (ie:edges) are not exposed to the high temp exhaust gasses so its relatively protected from the rapid heating effects by the virtue of it's effective increased mass by being in contact with the valve seat/head while the "hot" part of the cycle is in progress.

The only cooling (essentially) the exhaust valve gets is the contact area with the valve seat where it can conduct it's heat to to the head, although boundary layer also helps protect it (like it also does for the intake valve) during the charge ignition.