Upshur's opposed twin engine

Brian Rupnow · Oct 23, 2024

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 · Oct 23, 2024

Brian Rupnow said:
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 · Oct 23, 2024

Sparky_SC said:
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 · Oct 23, 2024

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 · Oct 23, 2024

Brian Rupnow said:
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 · Oct 23, 2024

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 · Oct 23, 2024

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

minh-thanh · Oct 24, 2024

Hi Brian !
I intend to say more...
But in conclusion: Read what everyone has suggested
People are not trying to prove anything, just giving information and hope it will give you a different view on how to make a model engine and tune it...
Everyone has their own way but the end result is a running engine. The shortest or longest way,....depends on the decision.
Although there are differences in the way of doing things.... But your topics are always interesting to read - It is your topics that make the content of the forum, and that is much more valuable than some of the other content i have read on the forum over the years
Thanks for sharing !!

Jasonb · Oct 24, 2024

Still no mention of what compression you have?

You say there is compression with your hand over the top of the cylinder. You say the valves seal, you say your head gasket seals.

So you should have compression if there is nowhere for the gasses to go. Unless a valve is open and allowing them to blow out the carb

I'll ask again what is the compression like?

Brian Rupnow · Oct 24, 2024

Jason---I'm off tilting at other windmills today, but---the engine has very strong compression with the heads off and my hand held over the cylinders while rotating the crankshaft. The valves have been tested in the cylinder heads, and they don't leak. I haven't tried today to reset the valve timing, but when I do I expect the cylinders to have great compression. I do not have any way to test the cylinder compression when the heads are assembled on the engine.

dsage · Oct 24, 2024

Hold your finger over a spark plug hole. Not looking for an absolute value. Just that the valve timing is such that you have compression at all.
Oops. Assuming you can get a spark plug out.

minh-thanh · Oct 24, 2024

dsage said:
Hold your finger over a spark plug hole. Not looking for an absolute value. Just that the valve timing is such that you have compression at all.
Oops. Assuming you can get a spark plug out.

I use it regularly - But it has another problem: you have to have experience to know that the compression is good enough to make the engine run.

Jasonb · Oct 25, 2024

Only thing is he has Loctited the plugs in.

Brian You should be able to FEEL compression even if you can't see it or measure it. Turn the flywheel by hand and it should get harder as it approaches TDC then go very easy as it passes. I posted these earlier it shows how an engine will bounce due to compression. It does assume the whole engine turns over freely without tight spots giving false results

You were Feeling it when you had your hand over the end of the cylinder. It should feel more now as you are compressing 2 cylinders now if everything is sealed as you say, only thing stopping it is an open valve.

Being able to pull the plugs one at a time would allow you to check compression on each cylinder individually by feeling it. But you could hold the valve open on one cylinder at a time and see how the compression feels at the crank

gbritnell · Oct 25, 2024

Brian, my goodness. How many I.C. engines have you built, at least a dozen or so? All the elements are the same, valve seal, piston/ring seal, compression, fuel and spark. From what I have been reading you have all of those. Valve timing is just a matter of rotating the engine over until the exhaust closing and intake opening are at the same point. The piston should be at TDC. If not then the cam is out of time. Initial spark setting should be 8-10 degrees BTDC. Once all these things are verified the engine should run. As far as a carburetor goes, an opening with a metered jet in the middle of the bore will run an engine. Nothing fancy, just an opening to the atmosphere with fuel introduced. After all your engine accomplishments this should be old hat to you. You shouldn't have to reinvent the wheel with every engine.i get frustrated just reading about your frustrations.

Brian Rupnow · Oct 25, 2024

Be calm George. A friend of mine bought a rather ancient metal lathe and I gave him a 3 jaw chuck that doesn't fit it. Today I'm making him an adapter plate so he can fit the chuck to his lathe. The engine will still be here, setting on the end of my desk when the adapter plate is finished.---It's fun to machine something full size for a change.

minh-thanh · Oct 26, 2024

I said this once before .
You have to standardize the way you make an engine and how you tune it .
Every part of the engine has to be clean , the engine has to be clean after assembly !!!
The tolerances of the parts have to be smooth after assembly . You always make the parts when assembling to make an engine and you use the electric motor to force it to run for hours to wear the parts and become smooth . That is not the way I have seen it done on this forum and elsewhere - Only you use that method .
With model engines , we do it the way we want and like but we have to follow the principles as the companies that make full size engines or in repair shops : Clean , good enough tolerances , the parts smooth together .
Do not compare the tolerances of a model engine with a full size engine that is tuned for a race or F1 engine .
Make it clean and smooth that's what everyone has been doing, it makes things easy. It's a way to make your hobby a lot simpler.

GreenTwin · Oct 26, 2024

Brian does his own thing, for his own reasons.
I guess we all do to some extent.
.

pileskis · Oct 26, 2024

GreenTwin said:
Brian does his own thing, for his own reasons.
I guess we all do to some extent.
.

That’s true.
But eventually after asking for help, then ignoring everyone’s good advice, everyone is going to stop offering that good advice.
It is his turkey. He can burn it anyway he wants.

IC-man · Oct 26, 2024

I'll say it again but last time igot no comments.
Anyway the way I check for TDC and compression stroke on full size or model engines is by making a bubble on the spark plug hole ( either spit or washing up liquid) by rotating the engine slowly the rises at TDC and goes back down after TDC.
You can accurately detect TDC on compression stroke.
Try it chaps it works.

Wp356 · Oct 26, 2024

Pileskis,
I agree with your comment.

Upshur's opposed twin engine

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