Hobbsy

My Feedback: (102)

I always do, the same physics apply to Diesel fuel as to glow, that's my take anyway.

1QwkSport2.5r

Figured some guys use it. Last time we ran my Fox on diesel we didn't use pressure. Would make sense as the midrange was fairly lean.

Thanks Dave.

gerryndennis

I use pressure, at this stage anyway.

There's two reasons for muffler pressure, throttling and fuel draw.

It's easier to get a carb to tune well from idle to full with muffler pressure. The increased pressure at high throttle helps increase the fuel flow, and vice versa. On a Diesel converted glow engine throttling is important to us so muffler pressure should help.

Glow engines often use large bore carbs to get the most power. These larger carbs don't draw fuel as well as a smaller bore would so the muffler pressure is necessary. Ideally we should go to a smaller carb ,when we convert to Diesel, to match the lower fuel flow and the lower peak rpm achieved on the much larger props it's now possible to use. I think I've already mentioned that you might have to restrict your carb to 80% or so open which is artificially restricting the carb bore. It may be with the smaller carb bore that fuel draw is sufficient that muffler pressure is not needed.

Another potential problem with muffler pressure is the oil that gets recirculated into the engine. This oil can be quite black depending on the engine and compression setting. Some say that this is not good for the engine. I assume the dark colour of the oil is due to soft carbon in the exhaust. I run JetA as my kerosene component which runs quite clean. The oil from my exhaust is normally no darker than a dark honey colour. I haven't noticed any undue wear or other problems with my engines so I'm happy to run muffler pressure at this stage. There's probably plenty of carbon and other junk recirculating through glow engines and no one seems to worry about that.

The only throttled and muffled engines sold as Diesels that I can think of at the moment are PAW, Irvine, and Enya.

PAW are more vintage style engines, the mufflers and carbs are a recent addition. They don't use muffler pressure but the carbs are quite small bore so it's probably not needed. Some say they throttle well others have changed to modern carbs to get them working.

Irvine Diesels are a more modern glow style Diesel, effectively a factory mod of their glow engine. The muffler pressure nipple is removed and the hole blocked. I suspect the carbs are smaller than the equivalent glow carb though.

Modern Enya Diesels use muffler pressure. I guess they think muffler pressure is OK.

I don't know what the general consensus is but I suspect if you need muffler pressure then you'll be fine to use it.

Dave H

1QwkSport2.5r

I'll try it both ways. The standard issue Fox carbs that I have are rather large and as such will probably need to restrict the throttle or switch to a smaller carb. I believe I might have a smaller OS carb laying around as well as a Perry that fits the Conley adapter. I should be in good shape. I got my fuel mixed up this morning - I used clear undyed Kerosene for home heating. I suspect it should run relatively clean.

Hobbsy

My Feedback: (102)

The Irvine .40 Diesel has a 6mm carb and the early Bumpy, glow, has a 7.5mm carb.

1QwkSport2.5r

So a .250-.270" carb choke would probably suit an 8cc diesel pretty well. Once I get past getting the engine running reasonably well and figure out the tuning a bit, I'll try a smaller carb. I figure I'm best off to start with a (mostly) box stock engine and get the learning curve straightened some first. One variable at a time.

Thanks for the info Dave.

gerryndennis

And FormerFarmer.

Dave H

1QwkSport2.5r

You're both Dave H, but maybe I'll abbreviate FDF and GND... LoL!

You both have been a big help. I think I'll take the Fox out back after dinner and give her a go. I have some hefty props to try out. 11x8, 12x7, 12x8 in APC, and a 12x8 and 13x5 MA.

I mixed my fuel in the garage except for adding the Amsoil ignition improver. I spilled a drop and I can still smell is 10 hours later. The ether smells better. Overall it cost me about $6 in ether, about $2 in castor oil, and about $.70 in kerosene to make a quart of fuel. Not bad if you ask me.

1QwkSport2.5r

So the diesel rookie got anxious and put the diesel head on and tried to give it a go. I made a short video of my first solo attempt to start the Fox .50 basket case engine on diesel fuel. I think it's gonna run! Here's a link to my video. Go easy on me, I'm new at this. http://youtu.be/9KWxr0uvOOs

Hobbsy

My Feedback: (102)

It's a start, no pun, it sounds promising.

1QwkSport2.5r

It was pretty late last night when I got to it. Yesterday was my new club's summer picnic so I was busy in the afternoon. Ill try to get the stand setup later and see what happens. I have to start somewhere.

1QwkSport2.5r

Okay.. First real solo run. Later runs not caught on video I opened the throttle to about 80% but struggled to keep it running, but I don't believe I went overcompressed but maybe once and that was only slight. The needle setting as shown in the blurry end of this video is pretty lean so I suspect I got too lean. Seems the good starting point is about 1.5 turns open from lightly seated and about 3/4 turn back from fully seated compression screw for starting. Once started and smoothed out (warmed up), backing the compression off retarded it. I'll have to spend more time with it to get a better feel for it and learn the sounds it makes to determine what action is necessary. I read the pointers on the last page several times but need to re-read a few more times and get more time on the engine to get the ear for it.

Heres the video: http://youtu.be/YYQvhEVCmK8

Ill just add - the oil color coming out the exhaust port was the color of the fuel pretty much. Pale yellow.

gerryndennis

Hey Quick,

Great stuff,

It's a little hard to diagnose over you tube, but you're getting there I think.

You could hear the engine start to warm up and smooth out (advance) at the 1 minute mark.

You bought the compression up toward the correct setting but it might have been slightly under compressed the whole time. You can increase until it's dead smooth.

It looked like you were leaning and compressing at about the same time, hard to tell from watching. At the very end I think you were under compressed (retarded) then started to die lean (retarded), then backed off the Comp (further retarding) which let it die (nice and safe though), well done. Get the compression up a bit more to just stop the burping before leaning (don't go as lean as you were there though).

The engine dieing like that is because it was lean (retarded), perfectly safe. It just winds down sounding 'soft'. If it goes 'hard' and you lose RPM with it still running then it's over advanced, that's when you need to back off the compression (or close the throttle) to recover and prevent damage. If you leave it like that it will heat up (further advance) and eventually stop hot and perhaps seized.

I reckon it's time to fit the muffler and get the carb settings sorted.

Well done mate,

Dave H

BTW I'm definitely not FDF and he's not me. Not sure where that came from?

1QwkSport2.5r

I figured it was dying lean. I'd rather be lean and undercompressed anyway, so it's always better to be safe especially when engine parts are scarce. I'm still uncomfortable with the engine, but mainly because I don't know all of the tell-tales just yet. I know what retarded sounds like so I'm halfway there. I'm concerned I may need to drop the liner maybe .005" because on a subsequent run I wasn't leaned all the way and was nearly bottomed out with the contra. So I may need to pull one liner shim out which would put the liner .005" lower than it was from the factory. The head spacing is still in the .035-.040" range with the Davis head which sits .165" into the liner and the piston to top of liner at TDC is .206".

FDF is a different Dave H. LoL.

gerryndennis

Oh OK, I get it. Sounds like too many Dave H's

I'm currently running my Davis head with no shims at all (mine seals fine without shims), as long as the piston doesn't hit the head you'll be OK. I think I mentioned I replaced the compression screw with a longer one as well (careful not to screw it in too far though). Now that I've removed the shims the original screw may well have been OK.

The shims do make quite a difference. I think I took the shim out then backed of the compression about a turn.

Would it be easier to machine the .005 of the sealing section of the head rather than the case?

I watched some of your air boat videos, they're pretty impressive.

Dave H (one of them)

1QwkSport2.5r

I do not have any shims between the diesel head and liner. It seals fine as-is.
No, because we already milled the top of the crankcase. 020" to fit the MDS .40 P/L when we thought it was hopeless to get replacement Fox parts. I cut brass shim stock to put under the Fox liner to have the Fox liner installed in the same position it was in before we machined the case. If I didn't use any shims, the port timing would be much different. Lowering the liner would open the exhaust port later and close it sooner which would reduce its overall timing which I would think would be good for a diesel... It would also increase the compression ratio some allowing a less aggressive (my terminology) contra setting. I don't think I want a longer screw until I really know and trust what I'm doing...
Thanks! I'm still on the hunt for speed. Once I get my Jett .56 on that outrigger, I wouldn't be surprised to bit 65mph+. Looking forward to doing some repair work on a couple boats this winter in that quest for speed and good steering. I might even build a boat for the diesel if I get motivated enough...

1QwkSport2.5r

I'll add this because I forgot to mention it before. I was not simultaneously leaning the needle and increasing compression. I increased compression until it smoothed out and leaned the needle after. I decreased compression after thinking I was overcompressed and it died. Being I don't know the noises (I call them tell tales), I didn't know if I was too lean or overcompressed. Turns out I was just lean and probably at peak lean mixture. It did throttle from idle to 80% throttle really well though.

edit: adding rpm figures on glow for comparison purposes.
Fuel is 5% nitro 20% castor
plug is Fox idle bar with gold post
carburetor is Fox EZ-Adjust .330" choke
APC 12x7 - 9,480rpm peak, 2,280rpm idle
APC 12x8 - 9,270rpm peak, 2,100rpm idle
MA K-series 13x5 - 9,780rpm peak, 1,950rpm idle. Throttle transition was nice on all props.

Now ow to get some numbers on diesel...

Hobbsy

My Feedback: (102)

Since you used a 12" prop on glow the Diesel numbers are going to disappoint you, the .50 normally turns a 10x6 at about 14,400. By using an APC you should get better numbers than I did on the .50. The Graupner 12x7 I use has wide blades.

1QwkSport2.5r

Well, I ran those props to have a comparison to the diesel power. From what everyone says, the diesel should turn the bigger prop better than it will on glow. Moreover, several fellas have said the .50 makes a great diesel conversion - if it doesn't turn those bigger props that much better as a diesel than glow, why bother converting it?

Also, your last Fox .50 glow you ran on page 173 of the club Fox thread showed a 13,089rpm figure on a Robbe 10x6 and 10% fuel. You said "it's as strong as it should be". If the .50 usually spins a 10x6 at 14,400rpm (an MAS prop?) then it's not as weak as some guys make them out to be because that rpm figure rivals what my TT Pro .46 can do which is a coveted engine by many. I got a 13,680rpm reading on 5% nitro and APC 10x6. By some accounts, we have weak .50s if comparing to that 14,400 figure. Maybe I should put that Bowman ring in after all and chuck the Fox ring?

I remember reading the .50 diesel should turn a 12x7 at around 9,900rpm. If that's the case with mine, it's not that much stronger than it is on 5% glow fuel so I question why so many say these engines make such great conversions... Until I get some peak rpm numbers from mine on diesel, I cannot say if it's as strong "as it should be", but it seems to not be as worth it to run on diesel as it seemed to be based on what I've read. I'm not beating anyone up, just making observations.

1QwkSport2.5r

Bumping this up in hopes someone will have some insight as to my post in reply #144. Thoughts anybody?

Glow power - ~9500rpm on 12x7, Diesel ~9900rpm same prop. What is the real advantage besides 400rpm? Diesel Fuel cost is double if not triple that of the test glow fuel...

fiery

My Feedback: (2)

http://news.davisdieseldevelopment.com/

1QwkSport2.5r

Some things of note in there, but it still doesn't answer my question. What makes the Fox .50 such a great diesel conversion when power is similar between both fuels on a big prop? I understand the principle of diesel, and I'm all for it... I'm just unimpressed with the difference in power between the diesel and glow setups. If the diesel runs 1,000rpm faster than it did on glow on the same big prop, that's one thing... Expecting a 400rpm difference or so - doesn't really make it worth the money spent on fuel and converting the engine?

To be clear - I'm not nitpicking. I'm just trying to understand what is so great about the engine..? I will say that I'm glad it still runs and I haven't wrecked it yet...

1QwkSport2.5r

Another question for the conversion guys... Not sure it would work without wrecking the engine or not, but do the Fox .40 compact case engines fare okay under diesel operation? I have a head that will fit the .40 I have albeit needing a few shims.

steve111

I guess that if the improvement was really that cut-and-dried, everyone would be doing it - which clearly they aren't.

To to be fair, though, your 400 rpm improvement actually represents about a 9% gain in bhp and a 13% gain in static thrust, so it's not altogether insignificant. The extra cost of fuel isn't quite as bad as it seems, either, because a diesel will use considerably less of the stuff. I suppose it really just comes down to personal preference and the desire to try something different. I've got plenty of good purpose-built diesels, and have never felt the need to convert any of my glow engines - but good on you for trying it out.

1QwkSport2.5r

I haven't gotten an actual tach measurement on mine yet. Just comparing my top glow numbers with another fellas peak diesel numbers. I like different, and I already have the stuff to run the thing so I'll stick with it to learn it well. Maybe I'll fly a diesel, maybe I won't. I'll wait and see. This Fox .50 was a real basket case when I got it and it too replacing several main parts in the engine to get it running again.. Might as well see if it'll keep running or blow up. I ran it more tonight to get more of a feel of the tells and such, and get the other carb dialed in. Swapped to a MkX twin needle full spraybar carb and gave it a go. Got the overcompressed sound down, heard that a couple times. Got my main needle set (at about 85% throttle) and started working the low speed needle for good transition. Ran out of fuel - a 2oz tank doesn't last long in an 8cc diesel. So my video doesn't show me adjusting the idle mix and transition. It does transition without quitting but it revs a little slow which I suspect is just a little rich or a little lean. It doesn't miss or burp, just revs slow. Slower than it did the very first time I ran it as a diesel with my buddy last summer. So it needs a bigger fuel tank and a little more tuning, but otherwise I think I got the idea pretty good. It's all about listening to the engine and the sound it makes. My oil color from the exhaust was close to a light maple syrup color which tells me I'm right in the ballpark. http://youtu.be/pH1phEe7ags