Upshur's opposed twin engine

[Brian Rupnow]

I had that incorrect valve timing figured out when I seen bubbles around the throttle cross shaft.--But if its leaking air out the carb under compression stroke, then it is also leaking air there during an intake stroke. I like this carb because It's so simple, but I also don't really trust it because it's so simple. I have made up a different intake manifold that will accept one of my tried and true Traxxas carbs. I just got word today from the surgeon that they are going to replace my other knee on the 14th of November.

 

[Sparky_SC]

I had that incorrect valve timing figured out when I seen bubbles around the throttle cross shaft.--But if its leaking air out the carb under compression stroke, then it is also leaking air there during an intake stroke. I like this carb because It's so simple, but I also don't really trust it because it's so simple. I have made up a different intake manifold that will accept one of my tried and true Traxxas carbs. I just got word today from the surgeon that they are going to replace my other knee on the 14th of November.

You seem very confused Brian.

If the carb is leaking under the compression stroke then the valves cannot possibly be sealing ! Also, on a proper intake stroke it cannot possibly leak, there is vacuum in the intake, the only thing that would happen is it would suck air into any leaks, NOT bubble out. Leaks in the intake or carb cause a loss of vacuum and/or the mixture to go lean.

I say a "proper intake stroke" because that assumes the valve timing is correct and the valves seal reasonably well.

A different carb won't change a thing.

 

[pileskis]

You seem very confused Brian.

If the carb is leaking under the compression stroke then the valves cannot possibly be sealing ! Also, on a proper intake stroke it cannot possibly leak, there is vacuum in the intake, the only thing that would happen is it would suck air into any leaks, NOT bubble out. Leaks in the intake or carb cause a loss of vacuum and/or the mixture to go lean.

I say a "proper intake stroke" because that assumes the valve timing is correct and the valves seal reasonably well.

A different carb won't change a thing.

Where’s the popcorn??

 

[Brian Rupnow]

Assuming the valve timing is correct is a big assumption. If the piston is coming up on compression and the valve timing is out, then one of the intake valves is being held open. The compressed air/gas will try to escape and consequently pressurize the carburetor. With my fat finger over the carb intake, the pressure is going to take any escape route it can find---thus bubbles around the throttle shaft.

 

[Sparky_SC]

Assuming the valve timing is correct is a big assumption. If the piston is coming up on compression and the valve timing is out, then one of the intake valves is being held open. The compressed air/gas will try to escape and consequently pressurize the carburetor. With my fat finger over the carb intake, the pressure is going to take any escape route it can find---thus bubbles around the throttle shaft.

As said before, you can verify if you are close just by looking at the rocker arms and the crank throw (piston motion) while turning the engine by hand. Verify each of the 4 cycles by eye. If something is wildly out you will see it. It certainly sounds like something IS wildly out.

In your example, the piston coming up on compression and a valve being held open, you easily could see that !!!!