B.J. Cicada build

[werowance]

this weekend I had lots of garage play time so I decided to see if I could get anything to work on the little cicada. and I had some luck. not much but I did manage to get it to run for about 10 seconds multiple times.

so at the lowest compression possible on the compression screw it will fire.

I replaced my propeller which I believe is to small with a piece of wood transition strip like you use to separate carpet from tile on your house floors. made it about 7 or 8 inches long drilled in the center and used that. has lots more flywheel effect.

I remade the rod for the umpteenth time and fueled up the tank, primed the engine, and gave it a flick or 3 and it took off. sprayed fuel all over the side of my truck bed like an aerosol spray can.

now if I had to describe it, it didn't take of like a cox 049 that I used to play with as a kid but more like my little steam wobbler engine that just about has enough steam preasure to take off but not good steam preasure. kind of slow. but the fuel spray is just like I remember the 049's do on the string controlled planes I had as a kid.

I tried upping the compression and it will not fire at all on hire compression. and once it uses up the prime fuel it dies. (about 10 seconds or less)

when I hold my finger over the carb it will suck some fuel up then push it back in the tube with air. so it seems to me its not really sucking the fuel? but if I put the tank higher than the carb it will run through the engine and flood it.

maybe an inline check valve would help?

any suggestions on the carb? I believe it to be to spec as per the plans. I did a pretty decent job on grinding the needle valve and what not.

 

[Chris Murphy]

Ok-I may be a newby on this forum-and a tyro at making engines but I have 50 years experience operating these engines-and there are several points of information you could benefit from: first the fuel recommendations are fine-though in this particular situation you could do a lot worse than go the old 1/3-1/3-1/3 route for the three components-this mix has stood the test of time (for about 70 years!)-most of us consider it has a lot more oil than necessary (true!)-but that has the advantage of compensating (to a degree) for less than perfect fits in a homebuilt engine. Secondly-without seeing your mount, it is not clear what the tank level is in relation to the engine-you do NOT want gravity feed under any circumstances-that is a recipe for flooding and ongoing grief. Ideally you want the fuel level just below the carb level...reading between the lines your engine may be a bit lacking in suction....this is surprising as most sideports (ie piston ported intake) diesels have a lot of suction...sometimes embarrassingly so...which leads me to the next point-the needle setting. On a typical sideport diesel you generally only have the needle unscrewed about 1-2 turns from fully closed (this obviously is heavily influenced by the actual thread used on the needle)-if too far open you'll flood the engine and they then (especially sideports) get sulky and won't do much at all-not even fire...! Furthermore you run a good chance of hydraulicing them-ie getting so much liquid fuel in the cylinder that the engine cannot physically turn over-and if forced...something will bend or break...usually the rod as it is the weakest point....if you're really unlucky...the gudgeon pin and crankpin as well!

Now another problem/issue for you will be that you have no idea where the contra piston setting should be for starting-especially on a brand new homebuilt engine. An old trick used in this situation-and also on a commercial model diesel that has been 'fiddled' with and the setting completely lost, is to carefully screw the compression down until the piston and contra just touch-you have to proceed carefully of course-but you can feel the contact when it occurs-and of course at that point you can no longer turn the engine over-THEN back off the compression screw by ONE FULL TURN, and flick till the contra piston snaps back up to this position. That position should get you in the ballpark for starting. If your contra is too tight to return upwards to this new position -then you would have to remove the cylinder, and gently tap it up the bore again till it reaches the correct position.

Props-if anything I would regard an 8x4 as too small for this engine-and certainly so for initial starting from new-I would suggest a 9x4-the larger diameter will give you more flywheel effect-and keep the revs down to a reasonable level whilst you are running in. Bear in mind that these type of engines usually run at about 6-8000rpm-or about half the revs you get from a Cox reed valve. On a 9x4-assuming you've made it correctly to the published design-and the timings are as per the drawings I would expect a 9x4 to turn about 5-6000 rpm or so ....as the engine runs in (which could take anything up to about an hour of accumulated running) it would not be unusual for the revs on the same prop to gradually rise up to 1000rpm more than the first initial run.

One other method of learning to start these engines is simply to keep the needle shut, and squirt a couple of drops of raw fuel mix into the exhaust port, and flick the engine over-I cannot recommend using a drill as you have been doing-firstly because you lack any 'feel' on the engine-and secondly because a drill does not turn fast enough. Once you can successfully flick start the engine and get it running on a prime (it will only run for 1-2 seconds) consistently-then you can be sure you have found the correct compression setting.....and you can then proceed to gradually opening the needle. The difficulty with diesels is that with two controls-the compression and the needle-both of which interact to a degree-for a beginner to the type, there is potentially a lot to go wrong...the compression ultimately controls the ignition timing-which in turn is dictated by the load on the engine (ie the prop size)-the normal mode of operation is you generally have a needle setting adequate for starting (ie a rich mixture)-start the engine-(which may require you to reduce the compression by up to half a turn from the running setting)-adjust the compression for smooth running (ie increase it until the misfiring stops and the engine runs with a steady exhaust note) and finally screw in the needle to lean out the mixture for optimum running. If you go too far on the compression-the engine will start to labour-likewise too lean on the needle and the engine will starve and slow to a stop over several seconds-and overheat in the process.

Finally-someone has recommended you watch Brian Cox's Youtube videos on starting diesels-I fully endorse this suggestion-here's a couple to start you off:

He has plenty more-and you can learn a lot on model diesel starting and handling by watching them....listen to both his commentary-and also to the sound the engine makes as the compression is altered

 

[werowance]

ok, well that explains some of it. if I squirt some 5w-30 in the exhaust port and work the prop back and fourth (its hydraulic locked at this point from the oil) and work it until the lock is gone, then give it a good flick it will take off. the fuel is just a smidge below the engine so as not to gravity feed, maybe an inch lower than the carb. I was thinking about adding a lot more oil to the mix to see if that will do the trick, restricting the venturi a little bit like taking a piece of wire insulation that was stripped off that just fits the hole seems to give it the suction effect it needs.

was thinking that since the oil makes it fire up that maybe I need to cut a new piston thats tighter, thinking the oil was giving the seal it needed to boost compression.

oh and I have way since quit using the drill as I did break to many rods trying that.