Building a twin cylinder inline i.c. engine.

Home Model Engine Machinist Forum

Help Support Home Model Engine Machinist Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
The pushrods on the rear cylinder (furthermost from the flywheel) look to be bent near the rockers when zooming in on the picture. Is this just a illusion from the camera view?
At this point you have two single cylinder engines mounted on a common base plate. You have separate carburetors, separate ignitions, separate cams.
That's the problem I don't understand.
The engine has 2 cylinders, 3, 4...just a 1 cylinder engine assembled together, just a little more complicated about the crankshaft, camshaft, distribution but he always gets into trouble.
Hi Brian!
You take your 1 cylinder engine (of course it runs.). Compare it with this engine - About compression, ignition, valve timing...-, that gives you a way to solve the problem!
The pushrods have a slight bend near the top to clear the heads of the manifold bolts. This does not impair the operation of the pushrods. The engine is not firing at all now. The ignition timing is correct, and a sparkplug laid out on the engine deck fires quite happily as the crankshaft is being rotated. The valve timing appears to be correct. The fuel is Naptha gas, same as I use in all of my engines. I have a few more diagnostic tricks to do today to see what is wrong.
Naptha gas is a clear liquid that used to be burned in gas lanterns and other in house gas appliances. The big plus with Naptha is that it doesn't smell bad when burned. Ordinary pump gasoline stinks to high heaven when burned indoors.---Brian
I've just finished preliminary tests on both cylinders. They will both blow up a balloon if a fitting with a check valve is screwed into the sparkplug hole when the engine is being rotated. All this really shows is that the valves are not leaking--not much anyways, or the balloon wouldn't blow up. Then, when plumbed into my air compressor, both cylinders will hold 50 pounds of air pressure without letting any air escape thru the exhaust or intake, which means that my valves and my rings are sealing well. Of course you have to manually rotate the engine by hand until you reach a point in the cycle where the engine is up on compression on that cylinder and both valves are closed to do the test.
Naptha gas is a clear liquid that used to be burned in gas lanterns and other in house gas appliances. The big plus with Naptha is that it doesn't smell bad when burned. Ordinary pump gasoline stinks to high heaven when burned indoors.---Brian
Is this what we used to call white gas? That is what I used to put in my Coleman stove and lantern.
Regarding your engine testing, it seems like everything on the individual cylinders is okay mechanically and electrically. Seems like the only other possible variable is the fuel-air mixture in the cylinder. Unless the timing is off on the one cylinder when the spark plug fires.
Right now, I'm kind of lost in the forest. The fuel/carburation side of this engine seems to be okay. I have played with the ignition timing a bit and got back to where the engine is firing some of the time while being rotated. The easiest thing to try is a different fuel, so I will try some automotive gasoline in the tank to see if that makes any difference. I could take out one piston and con rod and try to get it running as a single cylinder engine. That requires a considerable amount of work and I'm not sure I want to go there yet.
Is there any chance your test fixture is spinning the engine the wrong direction?
Hi Brian - A little off the actual topic, but what is Naptha gas and where do you get it? What are the advantages of using it?
in the USA we commonly call it coleman fuel found in the hunting/camping supply area in stores such as walmart. hope that helps. and yes i had the same question a few years ago myself. use it for the coleman lanterns and camping cook stoves alot
only thing i can think of is that i notice you have an idler gear between crank and cam shafts. all the engines i have built which are very few are geared directly to each other without the idler. meaning they spin in oposite directions because of this.
now i know you know this already but just thought it might be worth asking, are the cams setup with keeping in mind the rotation of the cam? i could just see me rotating it in the wrong direction because im just not used to having that idler in place is all. i also know that having an idler is not a problem. just hoping maybe its as simple of a solution as you set them up not thinking about the rotation is all
I quit Coleman fuel due to the price. Also I think the little engines hit a little harder with pump gas-no ethanol. AV gas is nice but not so easy to get.

Brian, have you tried turning the engine over by hand, no spark plugs, correct direction, and simply watching the valve opening and closing in relation to piston location and direction of travel and see if it looks correct?? Also see if the spark occurs when it should. I've timed my engines this way with no degree wheels, DTIs etc. Sometimes simple is your friend.
Vietti--I don't mind the price of Coleman fuel, as I don't use a lot of it. The valve sequence is good, at the point where the piston should fire it is at top dead center and both of the valves are closed. The cylinders fire alternately. Spark timing is correct.
Turn it to TDC on one cylinder then manually press one of the valves momentarily on that cylinder to release any pressure. If you lightly put your finger over that carb, do you feel suction as the piston is on the intake stroke? You should also notice fuel being drawn in at the same time. Don’t press very hard, it may help to have your finger a bit wet and just press hard enough to seal the intake as the suction may be subtle. Check both cylinders obviously. I use that method to prime my engines by repeating that process for a few “intake” cycles.

Do a similar test on the exhaust stroke, you should be able to feel a bit of pressure on the exhaust pipe as the piston is rising on the exhaust stroke. For this test, drop the piston to the bottom of the power stroke then momentarily press a valve open to equalize the pressure first.
Last edited:
I wonder if your ignition coil is no good. I know it sparks when you take the plug out, but a short inside the secondary winding of one side could result in a very weak spark that fails under the compression in the cylinder. Might be worth checking both secondary windings with a multimeter and making sure they have the same resistance?

The only other thing I can think of trying is to give the engine a squirt of starter fluid. If it runs you can be sure compression and spark are good enough, and can focus on your air fuel mixture as the remaining issue.
Hi Brian, Another (stupid) suggestion - which you may have already tried. Reverse the whole wiring (HT and LT) on the ignition coil, if it is a "double-coil" device. I once knew a motorcycle that had one half fail under HT (engine compression sparking) and that was identified by swapping both low tension and high tension wiring. The fault changed cylinders.
But the rule goes... "it is ignition" - until you find it is Carburation, and "it is carburation" until you find it is ignition"...
Stupid question.
Your two cylinder heads are mirror image of each other with intake port inboard and exhaust ports outboard.
Does the cams and plumbing match this setup?


Latest posts