Building a twin cylinder inline i.c. engine.

[Sparky_SC]

Yes, my compression gauge does have a leak down valve.

So, try it on a known good engine that runs, see what it reads. That will confirm/condemn the test setup.

 

[Brian Rupnow]

Here is an interesting video. I can't get any pressure to register on my compression tester, yet an airline ( with a check valve) screwed into the sparkplug hole will quickly blow up a balloon, and continue to blow it up as long as the engine is spun over.

 

[tractor162003]

Will the compression tester read on the good cylinder?

 

[Sparky_SC]

Here is an interesting video. I can't get any pressure to register on my compression tester, yet an airline ( with a check valve) screwed into the sparkplug hole will quickly blow up a balloon, and continue to blow it up as long as the engine is spun over.

A normal human can blow a balloon up much faster than in that video AND a human can only generate about 2PSI !!! In short, totally worthless test. 2PSI ain't going to get a engine to run !

 

[Ghosty]

Brian, do it the old fashioned way, just hold finger over spark plug hole and spin engine over and then check the other cylinder, and compare.

Cheers
Andrew

 

[Brian Rupnow]

The automotive compression tester that I used has a dial that registers from 0 psi up to 300 psi. I will look for a dial that reads from 0 to 40 psi, and if I find one will change the dials. Where am I with the engine as of right now?--The cylinder nearest to the flywheel is firing and the exhaust pipe is getting hot. The cylinder farthest from the flywheel is not firing at all. The engine is not running on it's own. I have switched the sparkplug leads around on the sparkplugs, but it made no difference. (The sparkplug leads both fire every 360 degrees). Tomorrow I will search for a 0 to 30 psi dial and will try to find out what's up with the non firing cylinder.

 

[Ghosty]

Brian, did you swap the spark plugs between the cylinders. I may be a bad spark plug. Just asking
Cheers
Andrew

 

[Brian Rupnow]

No, didn't do that. May try that tomorrow.---Brian

 

[Brian Rupnow]

So, today the testing goes on. First step was to remove the sparkplug from the non firing cylinder and test it. Yes, it does spark when laying out on the starting table. Next step was to hook my air compressor with 60 psi pressure to a line plugged directly into the sparkplug hole. As I turn the flywheel, air comes out the exhaust pipe when the exhaust valve is open, or air comes out the carburetor when the intake valve is open, and when both valves are closed no air is escaping anywhere. That shows that the valves and the piston ring are not leaking air pressure. (Same test was administered to the cylinder which is firing.) Next step was to use my other airline with a check valve and balloon and plumb it directly into the sparkplug hole. As with the other cylinder, the balloon gradually gets blown up as the engine is turned over by an external power source. The displaced volume in each cylinder is less than 1 cubic inch, so the balloon does not blow up super fast. The intake valve on the cylinder being tested does not begin to open as soon as it should---instead of beginning to open at 15 degrees before top dead center it begins to open right at top dead center. I can adjust that particular cam if I have to. One of the first things I will do today is to get rid of the 12" long flexible rubber hose that reaches from the sparkplug adapter to the readout dial on my compression tester, so the gauge can be directly coupled to the sparkplug hole. If I still get no reading, I will look for a zero to 40 psi dial for the compression tester, which currently has a dial registering from zero to 300 psi.

 

[Steamchick]

I suspect the 300psi gauge may not move the needle at below 20psi... I have seen many that are slow to start!
Do you have a tyre pressure gauge - not accurate, but should be within +/-20% of the max reading. Usually they work as low as 10 or 5 psi...
K2

 

[Jasonb]

Brian, as Ghosty says swap the plugs between cylinders, and see which cylinder fires. It may be sparking in open air but not when actually in the cylinder if it is a bit faulty or there is a problem with the coil giving a weak spark to that cylinder.

 

[Brian Rupnow]

Swapping the plugs didn't change anything. I have just ordered a 0 to 60 psi gauge from amazon to replace the 0 to 300 psi gauge on my compression tester and it will be here tomorrow. I don't want to do anymore trouble shooting until I get a gauge that will give me a reading on both cylinders.

 

[petertha]

One good thing about having your air hooked up for leak-off testing, is you can turn the pressure down low & audibly hear your valve timing hissing & compare both cylinders apples to apples on the protractor wheel. You mentioned non-fire cylinder inlet is appears to be late 15-deg, how about exhaust? Maybe the cam has slipped or was not set correct to begin with?

 

[TonyM]

Have you checked the fuel to both cylinders? You now have twin carbs.

 

[Brian Rupnow]

One gas tank feeds both carburetors thru a splitter in the fuel line. Cam may have slipped a bit. I will check that today.

 

[werowance]

im just wondering, your ignition setup is wasted spark right? so it fires both plugs at the same time meaning one plug fires on exhaust and the other one is firring on compression strokes and then after 4 more rotations its hitting the other cylinder on compression. so that said your points cam would i guess need 2 lobes ? or would it need 4 lobes? vs 1 lobe like the webster for example? im not really certain my self so my question is as much for my own learning as a possible cause of your problem as well.

edit - i guess 2 lobes or the proper gear ratio driving just the points cam only could be done. but if points cam is riding on the valve camshaft then i think you would have to use 2 lobes

 

[Brian Rupnow]

No, the ignition fires every 360 degrees. The cam only has one lobe. Both plug wires fire at the same time. The cylinders fire alternately. So--cylinder one fires, then after 360 degrees cylinder two fires. the engine has a firing pulse every 360 degrees. The cam rides on the end of the crankshaft.

 

[werowance]

ah, ok, i also looked back to where you were building the crank and i see both cylinders are hitting tdc at the same time, they are not 180 or opposed to one another which is what i was thinking. so both cylinders hit tdc simultanously and both should fire on compression at the same time so that makes sense to me now. sorry but it gave me a reason to research wasted spark even more and watch some youtube videos anyway. so at least i learned more. good luck to you. i know you will get it soon.

 

[Brian Rupnow]

Yes, they both get a spark at the same time, but only one has a charge of gasoline vapour to make it fire. They are set up so that one cylinder is coming up on compression while the other is coming up on exhaust. The cylinder coming up on exhaust gets a spark too, but it doesn't do anything because there is nothing in that cylinder to ignite. That's why it is called a "wasted spark" system.

 

[Brian Rupnow]

I need help. The attached drawing is from Malcolm Strides drawings for the cam set up on the Bobcat engine. where does that 102.5 dimension between the cams come from.---Brian