J_L_G

I've acquired three diesels now. I have had a plain bearing PAW .29 for a couple of years. It's in an ARF Flite Streak and has been flown a couple of times.

I have a brand new PAW .40 TBR CL engine with a beautiful dark purple cylinder top and head. I've run it about 5 times on my test stand for break-in. I plan to run it (heat cycles) another 5 or so times to break it in further.

I have a used Irvine .40 RC engine. As I've posted before, I was concerned that the head was not adjustable. Since then, I gacquired a brand new head for the Irvine.

I have a couple of questions. On the PAWTBR can I convert it to RC by getting a carbeurator? I know Eric Clutton carries carbs. What is a good carb for this engine. Anyone tired the Conley (Perry) carbs on diesels?

The Irvine .40 RC engine apparently has another problem. With the head removed and the piston nearly at TDC, one can hold the piston and yet rock the propeller in an arc about 10 or 15 degrees. There's a kind of bump I can feel in the middle of this arc. I'm assuming the fitting of the crank to the wrist pin or perhaps to the chank shaft is worn.

I know what great responses I get from you all and I thank you for your help. John

AMB

Use any RC carb that will fitThe rock back and forth at TDC is normal its just the geometry of the swing of the rod, I just checked my Irvine 36 and 40 they both act the same 10 degrees of swing or so and no piston movement martin

J_L_G

AMB:

This is a big help. I appreciate your checking your Irvines and getting back to me with this information.

John

Lou Crane

John,

Your PAW 40TBR CL should fit the PAW Carb for the 40TBR RC perfectly. Eric Clutton should have, or be able to get one for you, with any additional instructions for setting up the VERY simple PAW carb. I've heard rare comments on loose fits on the rotating drum in the carb, but going through Eric C should assure you get one that fits, works and is right. It is a very simple carb, almost to being crude compared to some of today's items, but it is effective. Part of that is that diesels don't have fussy needs in terms of carburetors. Once you pick through the process and succeed, you'll wonder why all the fuss with the other engines...

The Irvines, as AMB says, don't sound like they have excess rod play on the wrist pin and crankpin. That's probably where the slack comes from. Analytically, think of it this way. The rod, with no bearing slack, only moves from 100% of rod length to the foreshortened length of 99.14% of rod length (presuming a total 'rock' of 15°, half of which is each side of Top Dead Center piston position *TDC). Cosine of 7.5° is .9914; the piston would only move that last .0086 of rod length from TDC to +/- 7.5° if NO slack.

That leaves bearing clearance, if equal at wrist- and crank-pins, at 0.43% - or as decimal, 0.0043 - times rod length between pin centers.

This may also have something to do with the 'bump' through TDC... Running pressure and load conditions push the piston down (well, duhh...) and may collapse the bearing clearances all the way down to the oil film minimum. IOW, turning it through by hand may not provide that pressure, and you may be feeling a head strike, or the pinch at the top of the stroke. If it is a serious head strike, the engine would probably have shown signs of damage by this time.

As to glow engine carbs on diesels, they don't always (ever?) swap properly for the same displacement. Diesels are more limited in their fuel/air ratios than glows. Methanol burns well at settings so rich a diesel would more likey flame out, or, worst case, hydraulic-lock and break things. A lot less volume in a diesel's combustion chamber than in a glow's; liquid fuel is incompressible.

Good results have been achieved by using a glow carb for a much smaller engine. That could be an expensive cut-and-try game, plus the awkwardness of fitting some carb not designed for the engine. Straight-converted glow engines often show max RPM at around 2/3 (OEM throttle) open, with no significant gain, possibly even a loss, from there to WOT.

Perry HAS made a few carbs for small batch orders to adapt specific glow engines to a more likely successful throttled diesel use. Also, the difference in fuel constituents may affect the seals and plumbing in a glow fuel carb. That's why we have separate fuel line materials for gasoline(spark) and methanol (glow) engine use. Diesel fuels usually can use gasoline-fuel tubing...

gkamysz

My Feedback: (19)

You may find that a glow two needle carb will tune just fine at WOT and idle, have a rich mid range. If you happen to get a carb that was designed for high nitromethane fuel, this will be very obvious. I find that air bleed carbs are just fine. An OS .20 air bleed carb would probably be just about right. I don't know if it will fit though. I ran an OS .10 FP carb on a PAW .35 I had. it was much better than the original PAW carb. I understand that the new PAW Vari-jet carbs are much better.

The key is carb throat area. Just match what the existing venturi or carb uses.

AMB

On the carb issue. the larger glow carb could be sleeved, but using a smaller carb is easier, also with larger (glow stock)carbs on diesel WOT is around 75% sort of acheives the same end point, my glow conversions run about this , wide open they will bog martin
the one exception I has was the MVVS 40 which required a smaller bore carb

Lou Crane

AMB mentioned a possible solution to the excessive choke area in a carb for equal displacement diesels...

ENYA, in at least their (OEM) D25 BB TV has what looks like a glow carb, with a sleeve to reduce choke area to be more appropriate.

It works well. I imagine it could be useful to convert other carbs. Should do a little math to have a solid idea of the change in area. Choke area at WOT can be estimated as the area of the full choke diameter, less the area of the spraybar or stub. The fuel outlet diameter doesn't change, but its length inside the choke changes with the amount the sleeve obstructs. Perhaps start with the sleeve reducing WOT to 60% of the glow area?

It doesn't take much change in ID to make a change like that...

Hobbsy

My Feedback: (102)

It is really not necessary to use a smaller carb since our model carbs are, variable venturi carbs. Just limit it to 70 to 80% opening. Occasionally when I set up an engine with the carb at full open then reduce the carb opening the engine actually sped up and ran smoother and easier.

J_L_G

P.A.W. .40 TB control line. I would sing the praises of this engine. I've been running it on milder days here and it is one of the easiest starting engines of any I've owned and I have about 20 different, mostly glow engines. A two drop prime down the snoot and 2-3 hand flips and we're off. I've been running it on Davis Diesel ABC fuel with an 11x6 propeller for break-in purposes. It runs at 9300-9600 RPM with no fade. If I push it to 10,000 I get a little fading so I don't do that except in little bursts. It has run for 30 minutes on a full 12 oz tank. Nice engine by P.A.W.

On the issue of getting a carb for it, I've gotten the concept that larger carbs do nothing for diesels. I'll get in touch with Eric Clutton and see if he has a Vari-jet carb for this engine. I appreciate all the knowledgeable comments. John