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 !!!!

 

[werowance]

i hesitate to throw my hat into the ring but with respectful hopes of seeing this engine run: i personally have see the bubbles flow the wrong way twice. once was a 2 part issue which was backward starting and completly oposite direction on the cam timing. next was just pure on crappy carb build on my part. i do not like upshurs carb plans which apears to be the same one as the vertical single so built the chuck fellows version. and i had the luxury of another member which i will later pull up from my build log to give much due credit but he had redsigned chucks carb in 2 diff sizes and i love it. now mine was a single and i understand that Brian is dealing with more complex issues than i and i wish him the best. i wish icould just wave a wand or something and clear up the issues so i can see it run. best of luck Brian.

 

[minh-thanh]

Maybe: The timing overlap of the lobe is too much so the fuel suction ability of the engine is not much, you change the Cab to make the engine easier to suck fuel only solves the tip of the problem - For example, with a compression ratio of 6-1, the timing overlap is too much --> engine only has 3 or 4 -1 - of course the engine can run but that result does not give you a correct way to solve the problem, when you design and build another engine, the problem will repeat and again,..
The only possibility have bubbles at the intake port : incorrect lobe timing or the intake valve is not tight