flybyjohn

My Feedback: (2)

Just about completed my 91 fx conversion head, just waiting on the contra piston seal. I decided to try a contra piston with a piece of high temp silicone tubing that was designed and use by someone who said they got 1 year of hard use out of it before it needed to be changed.

The contra fits pretty snug and takes a good push from your thumb to get it to seat in the contra cylinder. I bolted the head on the engine with a piece of heavy duty aluminum foil for a gasket. I wanted to see if it had any more compression than before. When I flipped the prop a few times the contra was sucked into the cylinder. The only way I could see this happening is if the threads of the adjusting bolt were letting in a lot of air. Is this supposed to be sealed with something?

I am just fiddling with this project right now and don't want to invest too much into diesel if I may not like it. That being said, I have about $15.00 into this head and enough material left over to make about 4 more. I looked into buying Davis diesel fuel but the shipping is as much as the quart. and I don't know how I will like diesels so don't want to invest in more than one quart to start with. I looked around on how to make some diesel fuel and can get jet A fuel from the airport for less than $4.00 gal. and found some 50% starting fluid at the local store for $2.40 for a 7.2 oz. can. I have a half quart of caster left from mixing glow fuel.

My questions: How much ether is needed for a good running 91 engine, what percentage oil should I run in my engine, and do I need the 4th additive for the fuel, called DII or cetane boost(amsoil). I can figure out the ounces as long as I know what% of each ingredient I want to end up with.

Well I will attach some pictures of the head and will accept any help you all can give me on how to go about starting this thing up.

Mr Cox

I don't see any seal on the contra piston, is it only a snug fit?
If you can move it with your thumb, I think it is too loose and you do need an O-ring seal (viton rather than nitrile).

If air and fuel goes past the contra that can push the contra down, the piston cannot really suck it down. So usually there is a small hole made above the contra to not allow for an overpressure to build up above the contra piston.

flybyjohn

My Feedback: (2)

Yes, I am planning to put in a seal. I had to order it online.

Mr Cox

Okay, just add a small ventilation hole then for the volume above the contra.

Recycled Flyer

By the looks of it your locking grub screw bears down directly onto the comp screw thread and if that's so its not the best idea for wear.

Better to have the locking screw bore start from the outer periphery of the cooling jacket so that you have enough bore length to include a short packing piece of brass rod between the two screw holes so that you don't damage the comp screw thread.

The only conversion head that I have has two Vitron O rings in it as extra security against comp loss and movement.

Mr Cox

On the subject of the comp screw. Instead of ending at a point you can drill it out slightly so that it ends with a small circle. The friction against the counter piston is then enough to keep it if from moving, thus no need for the grub screw...

flybyjohn

My Feedback: (2)

Recycle and Mr cox, There is a short piece of copper between the grubb screw and the contra screw. I found that it does need constant adjusting however because the contra screw is threading the copper. I figured that after the threads get fully engraved in the copper, the adjustment to the grub screw should be minimal. And as for end of the contra screw I will change the end to a flat circle. Thanks for the suggestions.

flybyjohn

My Feedback: (2)

I cut a bit more aluminum off the head to reduce wieght a bit more. It runs..... well kind of. I started with the muffler off for ease of priming and primmed a few drops down the carb and one on the top of the piston. Throttle was at 3/4 open. I started flipping and then every 5 flips I screwed the contra down 1/4 turn. After 5-7 minutes, and sweat dripping off my head, it popped a bit. It would pop and then run for a second then ran out of fuel. I then put some fuel in the fuel tank and did the same proceedure and it started and continued to run. From this point on I did not know what I was doing. I need to read more on how to tune a diesel.

While it was running I turned the compression screw in until it sounded smoother but then it stated to surge, so I leaned the mixture screw until the surging stopped, at this point it sounded like it was running good. I ran for about 1 minute and then started slowing down and died. The engine was extreamely hot and felt a bit froze up. I think I overheated it too much. After a few minutes the engine loosened back up and was able to be flipped again. I started it up again and played with unscrewing the comp screw and screwing it in to see what the sounds were like on both ends, but I wasn't able to get it as smooth sounding as the previous run. I hope I didn't mess up the engine with the overheating.
I did have a few sparks comming out of the engine exhaust as it was running.

Here are the specs.

Engine: OS .91 fx
Prop: 15 x 5 wood (Zinger?..cant remember the brand but a bit wider blade than a 14x6 wood Power Point)
Carb: super tigre .61 carb
Fuel:
- 1 can johnsen's 50% starting fluid 7.2oz
...contents from msds: 49% - 60% ether
....................................20% - 30% heptane
....................................5% - 15% propane
....................................2% - 10% carbon dioxide
....................................2%-10% Iso-Butane
....................................0.5% - 5% lubricating oil
- 5 oz lamp oil from Walmart (Flash Point: 142 Degrees Fahrenheit)
- 3.6 oz Sig Castor Oil

Now going out on a limb here, I am going to assume some things about the starting fluid to get started in the mix design. JD starting fluid is listed as 80% ether content, and the other 20 % is propelent and maybe some lubricating oil. The Johnsens list theres as 50% - 60% ether so I am going to assume that if it is 50% ether, then there will be 30% heptane to make a total of 80% and if there was 60% ether then there would be 20% heptane. For my fuel formula, I will assume 50% ether and 30% heptane. With this assumption, I will have 3.6 oz ether, and 2.16 oz heptane in the full can and the rest is propelent and oil.

So my fuel mix I want is: 25% ether, 25% castor, and 50% kero.
My total volume for my mix will be an assumed 14.4 oz.

This makes my volumes needed as : 3.6 oz ether, 3.6oz caster and 5 oz kero + the 2.16 oz heptane = 14.36 oz

Questions, 1. Will this fuel mix work for this size engine?
2. Does anyone have any sugestions on how I should tune this engine?
3. Should I use a smaller carb, like one from a 46 size engine?

Thanks for all the help.

flybyjohn

My Feedback: (2)

Well I looked up some tuning instruction on the DDD site and tried to tune the engine. I couldn't get a very good tune and towards the end of the tank I started getting what sounded like a bad knock I couldn't get rid of it by compression adjustment or fuel mixture. I disassembled the engine when done and rod connections looked ok. My rear bearing looks a little sloppy. Just replaced it a little while ago. This engine doesn't seem very efficient. I hope to be able to get it to work, but I am really getting frustrated with it now.

AMB

well you were diligent careful planning but I feel the issue is your source of ingredients your % of mix is ok but think if you used john deere starter fluid 80% ether when sprayed out and propellent gone maybe 95% ether plus lube spray into a bottle for collection
also Amsoil cetane booster should be used 1.5 to 2% added (ethyl hexanitrate) it should run without overheating and much easier to adjust regards martin

Oh is the lamp oil de-odorized kero?? mostly

Mr Cox

To me it looked better with the cooling fins in place. Try to get a commercial fuel, that way you can get familiar with the engine before you play around with your own fuel mix.

The engine shouldn't really get very warm, it sounds like you have use a little too high compression ratio. It is better for the engine to run with a compression setting where it misses a little, don't try to squeeze out the highest power. As the engine heats up it will need a lower compression setting so once it starts to overheat this can be a vicious circle...

gerryndennis

G'day Flyby,

Congratulations on progress so far.

I'm wondering if you are trying to tune at full throttle?

As Davis says sometimes the glow carbs are too big. You may get full power at around 80% throttle opening and any further opening makes the tune go out with no increase in RPM. The answer as you suggest, can be a smaller carb or simply restrict the opening to around 80% or no further open than maximum RPM.

You might find it easier to tune at a partial throttle setting because of this (I realise the low speed needle will come into play but you can worry about that later when you adjust the idle).

Tuning Diesels is different to glows as Davis says, if your glow started to die lean you would richen the mixture. If your diesel starts to die it may be over compressed, richening the mixture at this point (a natural reaction) will make it worse (more fuel in the mix takes up more room increasing compression). So the correct thing to do would have been to reduce compression in this case.

Diesels need the mixture and needle to be balanced with each other. If the compression is adjusted to suit, the engine will run smoothly rich (good for running in) or lean (more power).

The symptoms of lean/under compressed (ie ignition retarded) is burping, missing, loping or the engine going brrrrm brrrm brrrm (sorry, hard to describe but you will know it when you hear it) note that the symptom is for both lean and under compressed. adjusting either will smooth the engine out but you would normally bring compression up in this case. When I say 'lean' here I don't mean too lean in the glow sense, I mean too lean for the compression setting. The engine may still be very rich.

The symptoms of rich/over compressed (ie ignition too advanced) are the engine hardening up in sound, slowing down, over heating, and knocking. Again leaning may fix the problem but once the engine goes hard it's best to reduce compression straight away. Again when I say 'rich' I mean too rich for the compression setting, the engine may still be quite lean.

Start the engine the way you did. No fuel in the tank (or needle closed) and prime only. Adjust the compression till you get a short run. Now you have a good initial compression setting. Open the needle till it continues running and warm it for 30 seconds or so before adjusting.

To tune the engine you want to start lean/under compressed ie burping (don't make me do the noise again). This is a safe setting. So after starting the engine back off compression till you get the engine missing, then bring compression up again until it just smoothes out.

Next lean the mixture until it misses again, then bring up the compression to smooth it out. Keep doing this till you have the needle setting you want (normally as lean as possible for max power/economy or richer if running in). If you lean too far the engine will start to die, if you are 100% sure that you are dying lean richen back up, but then immediately back off compression for the burping and then increase again to smooth out (Davis calls this the 'safety miss'). Remember one of the symptoms of over compression is also the engine slowing down or dying, richening in this case won't help.

If at this lean running setting the engine starts to overheat you will get the rich/over compressed symptoms (timing advanced), you may have to run richer or you may have a cooling problem.

Once you've done some running you can look for the other symptoms Davis mentions ie very dark or black oil means over compressed, clear oil means under compressed, lots of carbon means too rich etc.

I have found that my engines throttle better with a slightly richer tune.

Don't worry if this all sounds complicated, it's easier to do than write.

I don't claim to be an expert on this but I have had good results with two converted glow engines (one Davis and one Mecoa head).

Good luck

Dave H

Recycled Flyer

Just wondering if running it with a muffler may help to get the state of tune back?

Oh, and the idea behind hollowing out the comp screws contact end is so that the parting off point (left from the lathe work) on the inside of the contra piston does not offer up an almost frictionless spike around which the screw can unwind from the constant hammering it receives.

It is virtually impossible to eliminate the parting off dimple so the workaround is to concave out the comp screw.

If you had sparks coming out of the exhaust then it sounds like you may have a glowing carbon build up that may be advancing the engine independently of the amount of compression - if that is true you are never going to tune it.