Burnt valve?

How do you know whether you have one? Had a limited peak through the plug hole and it looks clean. Apart from the performance problems , is the only to take the head off or buy an endoscopy camera?

First step is a compression test, then a leak down test. If the leak down shows a problem and the valve clearance is correct then it's a bad vale/seat and further tear down is needed.

-Just my 2 cents.

Long time ago when I burned an exhaust valve in a CB250 it was backfiring loudly in the exhaust pipe
Didn't have a compression tester. Took off the head and ground in new valve. Old one was definitely and obviously burned
What makes you think you might have a burned valve?

This is a carryover from another thread. So the compression is even on all cylinders and the valve clearances have been set, the one thing that comes to mind are the valve springs and or valve guides. It's not firing on one cylinder so a leak down test should confirm it's a bad seat or valve. It will NOT confirm a bad spring issue being it is not running.
Steve

Yes they've been compression tested and they're about 125psi. Something is a miss for 2 reasons. Firstly if I cover 1, 3 and 4 carbs one at a time the motor falters and dies. When I cover 2 carb, nothing happens, it's like there is no suction. In addition, I have noticed oil leaking from number 2 exhaust header. Not coming from the cover gasket, it's tactically coming from the port itself, well that's what it looks like. I guess the next step is a leak test?

P.S, I think the clearance is out on number 2

You have compression on number2, so are you 100% sure you have fuel and spark? Check that at the very least and do a leak down test.

Adjust the valves.
Steve

Tyler wrote: You have compression on number2, so are you 100% sure you have fuel and spark? Check that at the very least and do a leak down test.

Excellent spark and number 2 carb seems? to be pushing air/fuel in. A leak down test is the next step I think

Hey JoHNy,

If all these are met (this is my understanding of your descriptions)

1. Cylinder compression of no. 2 is the same as the others at about 125 psi with a standard compression test
2. Blocking the intake of cylinder no. 2 with your hand has no effect on the engine running
3. You can't feel any suction while you are blocking the intake / carb inlet of cylinder no. 2
4. The ignition checks out in that you have spark at no. 2 AND cylinder no. 3 is working properly
5. There is a noticeable oil leak at no. 2, definitely coming out from the exhaust port
6. You have carefully adjusted the no. 2 valve clearances and re-checked them at least once more time afterward

Then my guesses are thus (all and any, regardless of how likely or unlikely):

1. Your no. 2 carb is set wrong - the butterfly or slide (throttle) valve is closed and not opening at all for some weird reason
2. Your fuel level in no. 2 bowl is set too low, or the float valve is partially obstructed in some way so it runs low when the engine is running
3. Your head gasket is leaking under high pressure, with the engine running only (I had a big block Chevy 427 that did this once - the block deck surface was poorly machined)
4. Your no. 2 spark plug is defective - won't fire in high ambient pressure (I had a CBX do that with a brand new set of NGK plugs once - hard to notice with the big six, but after one painted head pipe didn't change color in the rain, I new something wasn't right....)
5. Your no. 2 cylinder is pumping excessive oil, too much to even allow it to fire - either bad ring seal or a broken valve stem seal - easy to spot as the cylinder and plug will be very "wet" with oil
6. Your cylinder head is cracked through to no. 2's combustion chamber - look around the exhaust valve guide first, then around both valve seats in the combustion chamber near the spark plug hole.
7. You have a massive intake vacuum leak between the no. 2 carb's throttle valve and combustion chamber - possible the carb manifold to head junctures, but could also be due to someone "hitting air" while attempting to port the intake runners (in liquid cooled car engine porting we used to call it "antifreeze mining", "secondary water pump", or "unintentional glycol injection systems")
8. No. 2's exhaust head pipe is blocked - with god only knows what....

Did you bench sync the carbs and make sure the slides/butterflies move the same?
Steve

JoHNY wrote:

Tyler wrote: You have compression on number2, so are you 100% sure you have fuel and spark? Check that at the very least and do a leak down test.

Excellent spark and number 2 carb seems? to be pushing air/fuel in. A leak down test is the next step I think

Leak down test is really the way to go about checking cylinder sealing....I use 90 - 100 psi.

Most think the result of % leakage is the key to the leak down test, but myself - I put the emphasis on listening to where the hissing sound is coming from, and how loud of a hiss it is! Get a mechanic's stethoscope - they are cheap and work well for this. Pull the exhaust off, the carbs, and open the crankcase vent cover, too. Listen everywhere for escaping air. On a used engine, a very small amount of hiss form the exhaust valve / port is pretty common. That is thin carbon deposits stuck on the valve seats and faces and is to be expected on carbon heavy old used engines with tired / weak valve springs. The intake side might hiss ever so slightly, but should seal if you quickly snap the valve open - closed with a screwdriver on the follower. A spray of soap and water can help detect cracks on the head or leakage at the head gasket (look for bubbles). I also check at TDC AND BDC of the compression stroke. Be sure to rigidly and strongly secure the crankshaft from rotating in either direction while checking at / near TDC!!!!!!!! I've seen 1/2" drive breaker bars, complete with sockets and extensions fly off the ends of crank shafts10 yards or more...

My exhaust valves were seating on carbon due to bad seals. The bike ran great even though it would burn 1 Qt in 100 miles. When I put in new valves and springs I lost all my clearance. I should have "tipped" the valves so I'm at the bottom of the shim range. It's possible that's happening here.
Steve