Calling all diesel experts

I have a PAW .19 that occasionally "misses" or "burps". I don't think it's the "under compressed burp" and it doesn't seem to be related to the airplane attitude or maneuver like the infamous Fox "burp". It seems to be completely randome. It hasn't quit yet but it sure is un-nerving. It's side mounted on an ARF Flight Streak. The the fuel tank is a non-uniflow atomsphere vented "Perfect" brand wedge shape tank and I have a fuel filter in the line. Prop is a 9-5 "Zinger" and the fuel is from Eric Clutton. Weather conditions are typical Houston in the summer. Any suggestions? Fuel flow issue?

Contact Eric Clutton - AKA Dr Diesel. I'm convinced that he knows more about them than anyone else. Plus, he is a PAW dealer and has a publication that tells it all.
913 Cedar Lane, Tullahoma, TN, 37388. 931 455 2256.

Bob Z

One additional comment regarding Dr. Diesel - Eric sells a very good diesel fuel, "Olde English Brew", which is $30/gal delivered to SE Texas. If you aren't certain your fuel is fresh (and of the correct composition) you might try his fuel.

Another "stinky engine" guy,

Just try tweeking the compression up a bit at a time and see what happens, just don't over-compress.

Ron,

You say completely random so not even specifically related to i.e. inside or ouside manouvres? More during beginning of flight or the same all the way through? Tried leaning it a bit, 1/4 turns each time?

>Ron,
>
>You say completely random so not even specifically related
>to i.e. inside or ouside manouvres? More during beginning of
>flight or the same all the way through? Tried leaning it a
>bit, 1/4 turns each time?
>
>Rgds, Bram Anker

I'm just learning to do the beginers maneuvers so far. i.e. no outside squares yet. just wing over, inside loops and attempted inside squares, out side loops (more like a "potato")... I can fly level inverted (sort of). and yes there doesn't seem to be any corelation to maneuver or inverted attitude, or overhead position in the wing over. My needle setting procedure (possibly all wrong...) is after the engine has warmed up, I set for max rpm then go richer until I hear the rpm drop off just a little. I have been VERY careful to not over compress. So, that's a good point: it may be getting richer after launch... It may also be unloading a bit after launch so it may be under compressed a bit. I did mess with this a bit on the last 2 flights of the day: but just a bit: 1/4 turn leaner on the needle on one flight and 1/8 turn more compression on the last flight. I Don't know which way to go on the settings. Does it tend to unload a bit in flight and become under compressed? Or, do diesels get hotter in flight and get over compressed. Do diesels get leaner or richer after launch? Would a "uniflow" tank work better?

Interestingly enough, on the bench, where I can use a tach, it seems to be insensitive to needle setting... unlike my old Fox .35 where one "click" would make a big difference. I don't know if it works that way on the airplane.

Ron,

I would surely try giving it a little extra compression, just a little more than peak compression on the ground. Don't look at numbers, 1/8 turn may already be too much. I don't know how much extra turns you give the needle after peaking, generally 1/4-1/2 a turn will be enough and if you say it is insensitive to the needle setting rpm wise but still richen it untill you hear the rpm drop of you may be running it way too rich.

Yes, generally a diesel will cool a little in flight due to unloading and the effect can be quite substancial. Try the effects of whipping; does it cool down? Or holding back, sometimes you can actually overheat it holding back long enough.

A uniflow will also work on a diesel and should give you a steadier fuel flow throughout the flight. Weather it gets richer or leaner in flight tells you more about the fuelsytem setup then about the engine. There is no diesel specific behavior in this respect I think.

I agree with Bram. I have two PAW 19s in my stable one standard the other twin ball race. Both work really well. This is what works well for me.
1. Use good proven fuel. I used a cheaper variety for some time and got variable and poor results. Not applicable in the States I'm sure, but I use here in the UK the Model Technics D2000 (D1000 running in) which is superb. Different engine with good fuel!
2. When started go leaner then back off compression a little. You should be able to walk your engine up this way. I end up with NV about 1.5 turns open. Don't overcompress (of course) but as Bram says you may be either/and slightly rich or undercompressed.
3. I run both mine on 36-40 in. profile planes the heaviest of which is 25oz. I use APC 10 x 4 or Bolly 9.5 x 5 and launch is about 10,000 rpm. On 60 ft lines or 55ft in windy conditions I get long steady consistant runs.
4. I haved tried uniflow several times but uniflow and I don't get on! Just stndard open vented tanks are fine.
Good luck
Adrian

I have never flown a PAW so am not a PAW expert. I would definately go to a uniflow tank and that may well solve your problem. An occasional burp is OK. If the burp is during maneuvers, that suggests a hair lean. If there is any slowdown in maneuvers that suggests overcompressed. You may be running a little undercompressed and making up for it by running a little rich. If your exhaust is running clear to light honey-color, I would suggest compressing 1/8 turn at the most, and then seeing if you could lean the needle a little more. Suppose you have a diesel with compression and needle valve set just right. If you rich the needle the engine will start to overcompress and slow down. If you lean the needle, the engine will undercompress, start misfiring and eventually dies. Hope this helps.

The GeeBee looks neat - more details?

>The GeeBee looks neat - more details?
>
>Tony

Hi Tony

I made most of it up as I went along! I made a GeeBee500 from Pat Johnston's plan (via Stuka Stunt) and liked it so much that I made a rough copy of the fuselage about 60% I think. I made it of 5/16 solid balsa with 1/16 extra skinning (to make it 7/16) but the front 4 ins were 1/16 ply butted up to the 1/16 balsa (is that clear???). The wings are a real mish-mash. Do you know the Midi-Slow by Dave Clarkson? Aeromodeller plan but not sure of the date. It has a flat wing profile (36in wing) It's a really good, simple to build plan and goes like smoke with any 2.5 or better with 19 size engine. Well, I have built several and on one I experimented with a conventionally shaped wing and it did OK too. I used this wing for the GeeBee with 3 in tips!!!
The good thing about this wing is its simple construction. 9 ribs only out of 1/8th. No spars either and a built up LE. You can build the whole thing in a couple of days or so. The GeeBee took a week start to finish and flies really nicely. I may make another!
By the way I fly near Canterbury with other Old Codgers. I run a Saito 4 stroke in a flapped stunter (based on a Maikis design) some Midi-Slows, the GeeBee and I've just finished a Grumman Hellcat to try my hand at Basic Carrier Deck. I've got pulses up the lines rather than a third line system. Tried it last weekend and very pleased. Can send some photos if you are interested.
Good to talk to you
Adrian

I remember the Mini-slow? came out with an Aeromodeller 'Control Line Special' in the late 60's.

I might have a go - I have the GeeBee plans somewhere....

I've not flown this season - my flyin buddy doesn't call????

Look at my sites from UK-type nostalgia....

What diameter spray bar are you using? There are thick and thin and have had a problem of getting good engine run on the thin spray bar. I couldn't work it out, but more knowledgeable flyers lent me the thick spray bar and it cured the problem. I didn't know there were different PAW spray bars btw. The thick one is the same dia. as the thread for the needle, the thin one is noticably wasted.

Under compression normally gives a consistantly poor run with a steady miss.

I'm not a diesel expert, although I use several of them, including the PAW. I have been to a contest in Birmingham England where Tony Eifflanender could reliably start his PAW in a combat plane in one flip! And the thing ran like a Nelson .15 glo! Quite amazing!

I just went and checked... My spraybar is the same diameter as the threads... the larger one you discribed. What about the location of the hole in the spray bar? There was a thread about the best location for a single hole not straight parallel to the venturi...

Thanks to everybody who jumped in here and gave me some ideas to try. I'm really impressed with this engine. Power wise I think it's in the same ballpark as the OS 20/25LA that has been well described here on the forum. When I get this little "hic up" straightened out I'm going to try it on my "Primary Force". The problem I'm having must be unique to my specific plane/fuel system/engine setup or operation since no one else here on the forum seems to have experienced it. It really is easy to start: increase the compression a half turn, choke until it bumps, a flip or two and it's running, back off the compression a half turn while it warms up. No batteries, no glow plugs, no starters. etc., etc. The folks who use them report a good steady constant speed. There is a bit of a learning curve for us non-experienced users but that would be true of any engine setup. The big advantage to me is reduced noise level (a long story). A tounge muffler on a two stroke glow engine is a bit too loud... I can muffel it down a bunch without killing the power. Just a long tube after the muffler does an amazing job.

One other thought occured to me: It's on an ARF Flight Streak with a (standard issue) very soft nose. I don't know if diesel fuel foams or if there migh be a vibration problem unique to my particular plane? It certainly didn't do this on the test stand.

I've got a uniflow tank on order today from Brodak, I'll recheck the prop balance, try some other props, and try the suggested compression and mixture adjustments.
Wouldn't you know? Crappy weather is forecast for this weekend

Hi Ron
Just to come back once more. There are different spraybar assys incl. those for the throttled engines where the spraybar holes are in a different longtitudinal place due to the different width of the throttle assy. Also some had a single hole whereas most of mine have two holes in the spraybar at 90 degrees. The recommended alignment is rotate until you can't quite just see either!
I may be totally wrong but I think your problem is settings rather than spraybar or tank. The common thread of all the above contributors is a tad leaner and a tad more compression. My gut feeling is just that. Although I run glows I am really enthusiatic about diesels apart from the high fuel costs! Once you have mastered the twin controls they are a delight. Get the right amount of prime in the cylinder head and they're away first flick. I use a baby PAW on a Bristol FF model and launch with a big prop and the engine 'burbling' for a realistic flight. Magic! PAW are great. I have a lot of time for them and if you get stuck a call to Paul or Tony will sort it.
Hope the weather improves so you can sort your problem. Regards Adrian

I may have found the problem... I was using the OEM supplied tank and when I went to pull the fuel filter off the line the pickup tube came out of the tank. It must not have been soldered in properly (or at all).

Off the top of my head I don't see how this could have any effect. But now I have a nice shiney new Brodak uniflow tank on it. We'll see.

As a side note I'd rather have ARF manufacturers leave out parts rather than include junk parts.

Ron
By all means invest in the Dr. Diesel book
John

>Ron
> By all means invest in the Dr. Diesel book
> John
Got one on the way...
Thanks for the tip

I used to use P.A.W. diesels as practise engines back in the days before glow engines were used to save my Oliver Tigers from unescary wear, I had a similar problem as you are talking about and it was due to not enough D.I.I. back then it was legal to buy amyl nitrate which was the diesel ignition improver of choice, then a P.A.W. needed if my memory serves me correctly 3% of course this may be different with more modern P.A.W. engines but when amyl nitrate wasnt available an extra 1% commercially sold diesel ignition additive was nescary if you can get some you may want to try adding some to your fuel if you havnt managed to get the engine running correctly yet. Increasing the compression wont do much more than make the engine labour more and slow it down.
Doesn't Eifflander (PAW) sell fuel, or at least give a recommended fuel recipe/mix with the engines anymore in the instruction booklet supplied with the engine??.

.......... DON'T LET THEM GRIND YOU DOWN KEEP FLYING ............

Nothing much has changed at PAW in 40 years, except more sizes added to the lineup. Buying diesel fuel is not real easy here, but far easier than buying ether. The same D.I.I. available in England is not available here. I always thought an occasional burp was good...it means you are not overcompressed, which is bad news. Steve

...got some better runs on the PAW .19/ARF Flight Streak this weekend. Increased the compression a bit and a bit richer mixture. Cut off at the end of the run is still ragged. Lots of burps before it shuts down. The first flight was funny: launched way under compressed which got worse in the air. as the power dropped so did the flight speed. finally landed with the engine still running and one of the guys ran out and grabbed the model as it "taxied" by...

I got to thinking about the ragged cutoff. I put a uniflow tank on but I left the plug off the over flow vent. don't know if this could cause the burpin' and stumblin' at the end of the run or not. Something to try next time... also want to try some different size props...

The learning process goes on

Ain't this sport great?

I lost the overflow plug (uniflow tank) in flight with a ST G20-15 diesel yesterday. For whatever reason, it went way overcompressed and ragged. It is not unusual for diesels not to give a clean cut off; maybe a half lap of burping and missing.

Hi Jim. I think one of the reasons a diesel is a little slower to shut down is due to the lower fuel consumption. Those last few drops which get sucked quickly through a glow take a bit longer in a diesel.