Setting up and running model 4 cycle engines.

[Brian Rupnow]

If your ignition points are driven by the camshaft, check to see that your timing is not 180 degrees “out of phase”. A spark occurring when the piston is at top dead center on the exhaust stroke isn’t going to do you much good.
If your ignition points are driven off the crankshaft, then you will get a spark every time the piston is at top dead center, so you can’t be 180 degrees out of phase.
Sooo---You’ve got spark, you’ve got fuel, the spark is coming at the right time, but still no joy. 90% of engines that will not start when they have spark and fuel are refusing to start because either the valves aren’t sealing or because the piston ring isn’t sealing. You need compression to get things happening. Oh Yeah—One last thing to check—when the piston is up at top dead center on compression stroke, you should have .005” to .010” of clearance between the exhaust valve lifter (be it rocker arm or direct tappet) and the stem of the exhaust valve. Otherwise, the exhaust valve will be held open enough to let all the compression escape.
Now What?—Remember that compression spring which holds the intake valve closed. That should be a VERY light spring. Just strong enough to pull the valve closed against any friction between the valve stem and the valve cage. When you are turning the engine over rapidly with your variable speed drill, you should see some movement as vacuum pulls that valve open. It doesn’t move very much, but it does move. If it isn’t moving, you might need to cut half a coil off that spring to lighten it up even more.

[Brian Rupnow]

If still no joy, now we are getting right into heartache territory. You should be able to feel pretty strong compression when turning that engine over by hand.—But with a new, stiff, engine, sometimes it is hard to tell if you’ve got much compression or not. Do what I do---Put a 6” v-pulley on the crankshaft, put a 2” pulley on that spare ¼ hp motor you have in the shed, coat everything with lots and lots of oil, take the sparkplug out, and let the electric motor drive the engine for half an hour to loosen up the bearings.
If after this you still aren’t sure about compression, pull off the cylinder head. Put your thumb over the top of the cylinder, and turn the engine over by hand. You should feel a really strong compression. If you don’t, then your rings aren’t sealing. Sorry---Go make new rings. Keep making new rings and installing them until you do feel really strong compression when turning the engine over with your thumb over the end of the cylinder.
Put the engine back together, hook everything up, do all the tests and see if your engine runs. If it does, you’re golden. If it doesn’t, then sorry, you are suffering from the model engine builders curse. Your valves aren’t sealing.  #1—Relap the valves---maybe if you are really, really lucky, that will fix it. Of course you will have to put the entire engine back together again and try it to see if relapping the valves worked. (This is difficult, because now that we have cut that “handle” portion off of the valves, you have to remove the valve springs and the valve keepers, and whatever else is in the way, and remove the head from the engine, and grip the end of the valve with your finger chuck in order to spin the damned thing.)---And don’t even think about using automotive valve grinding paste---it is way to coarse.—Remember—600 grit carborundum paste.

[Brian Rupnow]

Or, #2--You might have to make new valve cages and redo the seats. You might have to make new valves. This is about the time that you begin to wish you had taken up hot air ballooning or mountain climbing instead of getting involved with model engines.
Once you do finally get those valves to seal and get the engine reassembled, the engine will simply have to run. There just isn’t anything left to prevent it.---Brian Rupnow

[b.lindsey]

Thanks for taking the time to share all your experiences Brian. I feel sure it will help many, me included!!

Bill

[bruedney]

I concur with Bill

Thanks for sharing a great resource.

Now we just need to file it away for future use

 

Bruce

[Zephyrin]

I like this developed study, wow, lots of experience in it, I have recognized there most of the setbacks I've meet, thanks to share !

[Ian S C]

Got a motor of my own design that wont go, it gets fuel, spark every rev, valves seem to seal. looks like I need to look at the piston rings, maybe leave the top cast iron ring, and use an O ring below it.  It does blow smoke from the exhaust, and has been known to back fire through the fuel mixer/ carby.  Ignition is via points and coil, fuel automotive petrol.  Bore 2", stroke 3", the basic design comes from volume one(1898) of "Model Engineer", without the hot tube ignition.
Ian S C

[Mosey]

 


Brian,
Thank you for your generous provision of this material. You have helped me in the past with some of it, but it is great to have it all in one place. I printed it out and will keep it at my workshop desk for reference.
This is a goldmine of good information which will help many people especially rookies like me.
Mosey

[gerritv]

Thank you Brian. While I haven't progressed very far with my Jan Ridders glass cylinder engine (too many diversions with the 'new' house) your advice will be of benefit when I do get to fire it up.

Gerrit

[Art K]

Brian,
Thanks for your comprehensive study on how to set up a 4 stroke. Back in the day when I built my Upshur vertical single, and I finally got it running but really crappy. I called my dear departed Dad. Described how it was running & without skipping a beat he says cams advanced to much, retard it, he was right it was 3 teeth off. Its run like a charm ever since. I have also learned to read the cam timing diagram since then.
Art

[Zephyrin]

Kill Switch/..... /there is nothing worse than having an engine start and immediately rev up to a point where it literally explodes before you can pull a wire off of the battery or pull off the spark-plug wire. Don’t ask me how I know. Just trust me on this one!!!

it is just where I am with my new 4st engine, how come a IC engine doesn't start at a quiet pace, just like a steam engine ?

[RayW]

Hi Brian,

When I built my Wyvern, I had great problems initially getting a good seal on the valves, but having only run it a couple of times the compression has built up to the point where if I spin the flywheels against compression, they will kick back really strongly.
The Otto that I am building now has no compression at the moment, and I suspect that I need to do more work on the valves and seats to get a better seal.
Interestingly, when I used to help run a big 12 1/2horsepower Hornsby engine at a local windmill, that is exactly how you started it, by turning the massive flywheel rapidly against compression at which point it would (hopefully) bounce back in the right direction and fire. Ocasionally, though, it would bounce back again and start in the wrong direction! Very exciting (and smokey).

[Brian Rupnow]

One more thing, while I think of it. New valves seldom EVER seat perfectly, no matter how well they are lapped. Of course, the secret to having a running engine is to get them to seal "enough" that the engine will start and run.  Now the thing is, with brass valve cages and steel valves, once the engine begins to fire, the force created by combustion will force the valves into the seats to a point where eventually they will seal, perfectly. If you have an engine that "wants to start", and everything is adjusted properly to the point where the engine will fire but not "light off on it's own" or starts but only runs very briefly, this is what I do. Hook up the battery and confirm that you have spark occurring at the right time. Fill the gas tank and make sure that you have the needle valve open about two turns, and if you have a throttle, set it about 1/4 of the way open. Bolt or clamp the engine down to your workbench and drive it with an electric motor. Make sure that everything is lubricated well, and let it run.--The main proviso here is that the motor must be firing when it is being driven. After about 10 or 15 minutes of this, as long as the engine is firing, the valves will begin to seal better and better, until they seal well enough for the engine to run on it's own. Of course you can dick around with the needle valve while the engine is being driven until you reach the point at where the engine is firing most consistently. Likewise with the ignition timing if you have adjustable ignition timing. This trick has saved me from insanity two or three times when I had done everything correctly and the engines still refused to start.---And--ALWAYS have a fire extinguisher close to where you are trying to start the engine.