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PAW .40 R/C needle adjustment

Old 06-30-2009, 09:08 AM
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mccarterj
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Default PAW .40 R/C needle adjustment

My PAW 40 burns way too much fuel from what I have read. My needle has been set to 2 full turns as the PAW paperwork suggest. I have turned it down to 7/8 turn now and it still runs fine. There appears to be a high pitch noise like metal on metal though at full throttle at times. How lean can / should I run this beast?

Old 06-30-2009, 09:53 AM
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Default RE: PAW .40 R/C needle adjustment

I don't know what you have read but the engine is tuned as usual a little rich (pinch test) and just enough compression for a smooth run a full speed. The fuel comsumption can possibly be lowered by using a larger prop, hence lower revs. but one cannot tune the engine for a lower consumption alone.
The noise you hear might be from too high compression or too little oil in the fuel?

The actual needle setting in turns will also depend on oil content. A lower oil content means a lower viscocity and higher flow for a given needle setting.
Just use proper fuel and do a pinch test, the fuel consumption will still beat glow by large margin.
Old 06-30-2009, 10:19 AM
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Default RE: PAW .40 R/C needle adjustment

Hi, do you have the Varijet carburator in your 40 R/C PAW engine?

See this link: http://www.rcuniverse.com/forum/fb.asp?m=5991650 and http://www.rcuniverse.com/forum/fb.asp?m=5992124

Old 06-30-2009, 12:57 PM
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Default RE: PAW .40 R/C needle adjustment

Hi first what fuel are you using?? 2nd I have an early PAW40 not a varijet carb I changed it over to an MVVS carb twin needle , and resolved the running issue great idle and transition to top speed

Above the 09 size engines I use conversions, TT,OS, and even an old K and B 20 with Davis heads. Some of the PAWs above 09 have had balance issues

You still may have a carb issue and still a little rich and compensating by overcompression thus 3 unknowns Carb, Fuel, compression, by the way I drilled and tapped the silencer on the PAW for a pressure tap and running tank pressure to the MVVS carb for even fuel delivery martin

Also Mr Coxs post is very worthy of your consideration
Old 07-01-2009, 08:53 AM
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Default RE: PAW .40 R/C needle adjustment

Thanks for the replies!
I just bought this engine new a few months ago, but it didn't have the varijet document in the box and I have looked in the carb a lot, as recent as two days ago and I don't have the varijet slit in the carb.
AMB, where can I get the MVVS carb?
Pinch test I will do soon, yesterday I few it for three 5 minutes flights now turned down to 3/4 turn open, one run was wide open the entire time, was really performing well, and it restarted easy, two flips, after the run so I don't think I am too lean yet.
When I shut down I do it via a fuel line pinch clip, at idle it doubles the engine speed when pinched and runs for about 20 seconds before shutdown, have to do pinch at full throttle though see how that effects it.
I use the Davis Diesel Power fuel sold by Tower. Davis wouldn't tell me the mix ratio when I inquired so I could tweak the fuel to correct proportions, only that my PAW engine needs extra oil due to my rod having no bushings. I checked the parts list for my engine and see bushings listed for rod, so I ignored that. I have the double bearing model.
Oh, and I am running a 12X9 three bladed prop. I keep raising the prop size and my top RPMS have stayed the same. Any larger and I will be mowing grass, I plan to make custom gear to get some more clearance.
Old 07-01-2009, 04:54 PM
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Default RE: PAW .40 R/C needle adjustment

The high pitched metal on metal sound is over compression, a lot of it judging by the way you're describing it. What prop are you running, should be about a 12x6 or 7.
Old 07-01-2009, 05:11 PM
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Default RE: PAW .40 R/C needle adjustment

checked the box the engine came in and I have the varijet carb, I can see the slit in the carb barrel too verifying that.
Compression doesn't seem to effect that sound, it only goes away if I open the needle up to about two full turns, I set compression to max rpm with a tach and no higher.
Old 07-01-2009, 07:47 PM
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Default RE: PAW .40 R/C needle adjustment

One of the hard things to learn about tuning diesels is the interconnectedness of the compression screw and the needle valve.

Took me a long time to learn that as you open the needle you automatically increase the compression. More fuel = an increase in compression. So by opening the needle by 2 turns (seems a lot to me) you are really cramming fuel into the cylinder and therefore upping the compression.
Old 07-01-2009, 08:35 PM
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Default RE: PAW .40 R/C needle adjustment

McarterJ If you are getting proper transitions with the Varijet carb no worry about changing it out I used the MVVS carb because I had an extra
any twin needle carb of the right size would work You should cease stopping the engine by pinching off the fuel line you stated it runs for about 20 seconds you are running leaner and leaner till it stops and getting almost no oil to protect the engine stop it by closing the throttle ( cuts off the air) besides the fuel
no air no run martin
Old 07-02-2009, 07:30 AM
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Default RE: PAW .40 R/C needle adjustment


ORIGINAL: mccarterj

My PAW 40 burns way too much fuel from what I have read. My needle has been set to 2 full turns as the PAW paperwork suggest. I have turned it down to 7/8 turn now and it still runs fine. There appears to be a high pitch noise like metal on metal though at full throttle at times. How lean can / should I run this beast?

McCarter,

besides running too lean you're probably also running a poor quality fuel with insufficient/low quality (castor?) oil. Can you mix your own to PAW specifications? Apparently you are not alone in running PAW's and having problems with the fuel you are using.

Old 07-02-2009, 12:17 PM
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Default RE: PAW .40 R/C needle adjustment

Wow, so many comments. I think I didn't explain myself well, that I am not having any "problem" with the engine. It runs great! Good power, easy restarts etc., I just was using too much fuel so I started leaning it out, and leaning, and leaning, as I lean it out I also adjust the compression, but very little is needed. I really just want to know where anyone who has a PAW 40 with this Carb has their needle set.

REgarding my fuel, I have two different manufacturer fuels, one is the Diesel Power that tower sells and I can't remember what the other is. The engine pretty much doesn't care which kind I put in it, at least I can't get any different RPMs or power from either. Can both manufactured fuels be bad, I know one uses pure castor.
Old 07-02-2009, 07:52 PM
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Default RE: PAW .40 R/C needle adjustment

Whats the prop your running on it ?? If on the small end of recomended range for power/rpm's the fuel useage will be higher ie if similar to glow sized prop the fuel consumption will "allmost" be as bad as a glow. Low fuel consumption mostly comes from larger props utiliseing the diesels better torque too produce the desired thrust.
Stewart
Old 07-02-2009, 10:20 PM
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Default RE: PAW .40 R/C needle adjustment

This sounds like a basic question but did you give the engine a proper break-in? Iron/steel diesels are not plug & play.

If you are not very familiar with diesels, check out this site: http://www3.telus.net/dieselcombat/about_diesels.htm

Don't be concerned about how many turns on the needle, just tune it where it runs best. BTW, don't worry about adjusting the carb until it is completely broken in.

George
Old 07-04-2009, 10:05 AM
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Default RE: PAW .40 R/C needle adjustment

Thanks again for all the post.
Answers to some of the questions
my prop size is mid range according to PAW
I broke the engine in yes, but it did it near two full turns out on the needle which I think probably kept my engine temp two low for a proper break in. I now have a full gallon and half through it at what I believe was a really rich setting.
I completely understand richer increases compression due to more fuel in cylinder reducing airspace.
I have now adjusted the carb to shut engine off via the carb. Not a quick shutdown, it really wants to run and fights for life for about 10 seconds.

This morning I reread the doctor diesel book and decided to lean it out more and see what happens, I warmed it up, no change until I reached a certain point and suddenly rpms picked up and things got real happy, and at full throttle leaned it to max RPMs, readjusted compression a tad for smooth idle at new setting. The needle is now set at about 1/4 turn, but my throttle transitions are finally smooth, response is quick, have a few more rpms, and my fuel consumption is down to a trickle. I no longer hear that noise either, perhaps it is finally breaking in since it isn't so overwhelmed with fuel cooling the engine. I am very happy with this setting, it is just scarry being so low.
Old 07-05-2009, 08:36 AM
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Default RE: PAW .40 R/C needle adjustment

Good you got your PAW .30 running OK. Here is a photo of mine running on the test stand. Flew it in a sport model years back. A reliable engine and likes big props. Jack
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Old 07-05-2009, 08:40 AM
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Default RE: PAW .40 R/C needle adjustment

Typo, .30 should be .40 Jack
Old 07-05-2009, 10:21 AM
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Default RE: PAW .40 R/C needle adjustment


ORIGINAL: mccarterj
...I no longer hear that noise either, perhaps it is finally breaking in since it isn't so overwhelmed with fuel cooling the engine. I am very happy with this setting, it is just scarry being so low.
You might consider a few heat cycles; that is, running it at peak for a minute or so (so it is fully up to operating temp), then shut down by pinching the fuel line. Let it cool to ambient temperature, then start it and do the same thing again. Each heat/cool period is a "heat cycle". Half a dozen of those should ensure that it is fully broken-in, and remove all doubt.

George
Old 07-06-2009, 06:08 AM
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Default RE: PAW .40 R/C needle adjustment


ORIGINAL: mccarterj
The needle is now set at about 1/4 turn,
It might not be the needle that seats at your 0 turns. It might instead be the screw that bottoms out, I have had that problem. The construction is very simple with brass holding the needle stem, sometimes this is off and the needle can not be closed completely.
So don't worry too much about the setting actual turns, it is the performance that counts...
Old 07-06-2009, 05:59 PM
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Default RE: PAW .40 R/C needle adjustment

Mr. Cox is right about the brass bottoming out. I had a feeling things still were not right and bottomed out the needle and it ran the same. Took the carb off and examined carefully. The hole in the round bar that feed fuel was facing wrong, it is supposed to face accross the air flow. Corrected this and reassembled. Totally different beast, but I couldnt find a happy spot at all any more. I suspected fuel problem, the Dr Diesel gallon I was using was almost empty and I had fueled and unfueled many many times out of that can the ether must be going flat. Decided to eliminate the fuel question and made a batch of homebrew 50,35,15. Very happy now, I don't know Dr Diesel recipe, I will try a fresh can of it I have, if it doesn't behave with close to these settings I am not going to use it. I can't find a supplier that will tell me their percentages here in the US so I guess making my own is the best way to go. I can keep the batches smaller that way too.
Old 07-06-2009, 07:04 PM
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Default RE: PAW .40 R/C needle adjustment


ORIGINAL: mccarterj

Mr. Cox is right about the brass bottoming out. I had a feeling things still were not right and bottomed out the needle and it ran the same. Took the carb off and examined carefully. The hole in the round bar that feed fuel was facing wrong, it is supposed to face accross the air flow. Corrected this and reassembled. Totally different beast, but I couldnt find a happy spot at all any more. I suspected fuel problem, the Dr Diesel gallon I was using was almost empty and I had fueled and unfueled many many times out of that can the ether must be going flat. Decided to eliminate the fuel question and made a batch of homebrew 50,35,15. Very happy now, I don't know Dr Diesel recipe, I will try a fresh can of it I have, if it doesn't behave with close to these settings I am not going to use it. I can't find a supplier that will tell me their percentages here in the US so I guess making my own is the best way to go. I can keep the batches smaller that way too.
For a TBR conventionally ported iron/steel diesel like the PAW you probably couldn't beat the "Oliver Tiger brew" fuel formula of 20% castor, 32% ether and 48% kero. On top of that (which comes to 100%) add 1.5-2.5% cetane booster as DII (Amsoil) or IPN. It makes a sweet running fuel which has been well proven over half a century.

Some things to watch are the qualitiy of the castor and the kerosene. Some castor has platelets that precipate out of mixed fuel after a while and causes all sorts of running problems. Low quality kero also causes running problems. All kero isn't good kero for diesels.
Old 07-06-2009, 07:23 PM
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Default RE: PAW .40 R/C needle adjustment

I have three PAWs, a .19, .40 as above and a .60, the very best fuel I've run in them is Davis Plane Fuel.
Old 07-06-2009, 08:22 PM
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Default RE: PAW .40 R/C needle adjustment

Ah, I see the stuff I got from Tower is The Davis ABC fuel, have to see if I can find the plane fuel someplace.
Thanks for that tip.

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