LS3d

I have a DA 50 which i bought new and now has apprx. 6 gallons throught.After breaking in the engine i adjusted the highspeed needle so i could get 7300 rpm on 22x8 xoar wooden prop.The engine has run fine since then.However this weekend i checked the max rpm with the tachometer only to find out that max rpm was down to 6700 rpm.I then tried to adjust the highspeed needle but to my surprise the max engine rpms remained unchanged eventhought i started at 2 1/2 and went down to 1 turn.The only thing that was changing was the temperature of the engine.The high speed needle seems to have no effect on the max rpm of the engine.
Is this normal? Or i am missing something.
I am running the engine on fully synthetic mobile 1 at 50:1.The engines idle and transition to full power are very good and the enginee is very reliable without any drama or dead sticks.
Any help would be appreciated

Tired Old Man

My Feedback: (1)

That kind of rpm loss is often indicative of a stuck ring. Running too lean or too hot causes the ring to stick in the groove at the exhaust side of the piston. Depending on how hot it became the ring could be stuck for a very short distance or up to halfway around the piston. The type of oil being run will not prevent a stuck ring if the engine gets too hot.

It's worth a top end tear down to take a look. If it isn't too bad the ring can be removed, clean out the ring groove and the ring, and reinstall. Be sure you replace the ring with the same side to the top of the piston as when it was removed.

All the above applies if the rpm loss has been determined by using the same size and maker of prop as was originally used. If you've recently changed props to a deifferent size and maker the issue may well just be the prop. If you are using the original size and type of prop the condition you described is not normal.

LS3d

Yes. I am still using the same prop as before. Wouldn't i have any indication if i was running the engine too lean?

altavillan

My Feedback: (1)

You are seeing symptoms of a clogged high speed jet. Try running the highspeed needle all they way in to dislodge crud around the tip. Then reset the needles and run it. Might check the in-carb filter too. Pull the fuel pump side to access the filter.

It's easy and necessary to check the ring occasionaly. A dirty or stuck ring will scorch your engine and ruin it.

outssider

My Feedback: (23)

high speed needle =my da does the same thing, has from the beginning and everything is clear and clean. i have read other threads about da50 and it seems to be common.

rpm drop= the only prop i've had that would turn up that high was a mens 22/8 (7400), I don't think it loads the engine as much as the other brands. mine is now turning a xoar 23/8 at 6500 and never runs hot. was it a very cold day when you got your original reading of 7300 rpm ? air temperature difference can play a part in the engine rpm. also cold engine (beginning of flight) verses hot engine (end of flight)

your reading of 7300 with 22/8 xoar sounds high...

LS3d

I have already opened up and cleaned the carb but there was no change.I used to get the reading of 7300 rpm at the beginning of flight.
How can i check if the ring is stuck or dirty?How can i decide if the ring only needs a good cleaning or replacing?I would appreciate it if someone could guide me through the steps
from opening up the engine to cleaning or replacing the ring as i have never done it before.

Tired Old Man

My Feedback: (1)

If nothing else has changed, and you know the carb is working as it should, a substantial drop in rpm IS a sign that the engine had run too hot or too lean. When an engine loses compreession it looses rpm, up to the point it will no longer run.

An inspection is simple. Remove the spark plug. Remove the 4 screws or nuts holding the cylinder on the case. Rotate the piston to the bottom of the stroke. Gently pull the cylinder up and off the piston. Look at the ring. Does it expand out past the edge of the piston or is one side stuck flush in the groove?

LS3d

I have just removed the cylinder head and checked the ring.It is loose (i can twist it in the piston groove without any difficulty).The color of the ring is shiny silver.I have also noticed that there is a lot of carbon deposited on the piston head.All the piston head is covered with it.Is there any way that i can clean it?
How am i supposed to put the cylinder head back.When i removed it the ring opened up slightly and is bigger than the piston ?

Tired Old Man

My Feedback: (1)

Without seeing a high resolution picture it's not possible to determine if the top of the cylinder needs to be cleaned or not. Excessive amounts of carbon can casue pre-ignition, reducing usable rpm. The only other possible cause of an rpm drop if everything else is good is a change in the ignition timing. That only happens if the hall sensor has been moved.

Replacing the cylinder is actually pretty easy. Carefully slide the cylinder onto the piston and compress the ring with your fingers while gently sliding the cylinder over it. Don't force it. If it does not go on easily then the ring has not been compressed enough for the cylinder to pass. It will go easy or not at all. Start over if it does not go.

Back to the carbon build up. Too much oil or the wrong kind of oil casues excessive carbon.

JoeAirPort

My Feedback: (41)

Several gallons of Lawnboy ashless will put all that carbon on the piston. So will DA's excessively rich needle settings. Air temperature will not drop the R's that much. Something's wrong with that engine. I had 2 different carb's, same brand and model, one would run 300 more RPM's on the same DA. So some DA50's will turn more R's than others with just a carb difference. Yours has dropped too much. Must be something major.

Yes those DA50 high needles are virtually useless. Just crank them to two turns out and forget about them. All tuning is done with the low needle. Trust me on this one.

Truckracer

My Feedback: (19)

Don't forget the ring is pinned against rotation on most gassers. Be sure the ring gap is positioned over the pin in the ring land on the piston before trying to compress it or you will break the ring for sure.

Truckracer

My Feedback: (19)

Late DA-50s have carbs that have high speed needles that adjust in a normal fashion .... not at all like the ones that were so common for most years. I called DA and asked them about this and they denied that they had changed anything but new DAs now adjust normally! Maybe Walbro just changed something .... I don't know. A trip back to the DA hospital might get an engine back with this carb mod ... mine did! A different carb and I didn't have a carb problem when I sent the engine back!

LS3d

What is the best way to clean the carbon from the piston head without causing any damage to it?

Flypaper 2

If the high needle is set right, the plug should be a tan colour. If it's light tan or white it's lean. Dark tan or black is to rich.

Truckracer

My Feedback: (19)

How much carbon buildup do you have? Can you estimate the thickness? Is it a hard carbon or relatively soft ..... solid in appearance or broken and powdery. Is the piston top completely covered with carbon or are there areas where aluminum shows through. A picture would help here. What one person would think is excessive carbon, another would say is normal and just leave it alone. I worry more about carbon in exhaust ports and mufflers than carbon on internal parts. Good oils help to prevent carbon buildup in these areas.

Tired Old Man

My Feedback: (1)

Good point about carbon in the exhaust ports. Another with the ring stop pin. If the ring won't compress then it is installed wrong over the pin.

AndresAM

True, My DA 50 high end needle is useless. The engine runs fine though.


I wonder what modifications has DA made to their lates 50's.(there has to be an improvement with time) The numbers posted in the forums for newer DA's seem to be higher compared to the early models. My DA 50 is around 14xx and it seems weak compare to others here. I am getting 6800 RPM's with a NX 22x8 and I have seen several posts with numbers of 7300 for the same prop [X(]. I know you can't believe everything that is posted in the forums but there seems to be several DA 50's getting better numbers than mine???

Andres

branded

My Feedback: (9)

Other causes of lost RPM........
Bad, or low ignition battery, or bad ignition module....A battery that's low, or won't supply full power at high RPM will cause the RPM to drop.

arobatx

LS3D has asked a couple of times about carbon removal, and I don't believe he's gotten any responses. I've heard of some quality service depts for some of our more popular engines return engines from service 'decarboned'. I too am curious about this process.

Chad

Truckracer

My Feedback: (19)

In post 15 above, I asked some specific questions about the carbon buildup in LS3D's engine. There are different kinds of carbon buildups (some are more of a varnish than a carbon) and the type can determine the removal technique. No response from LS3D yet. When I get the time, I'll write a few words about carbon removal but I would bet the Bugman and Pat Roy would be experts on the matter.

arobatx

Well, I 'nudged' Pat, Pe and Elson via PM to see if they would share with us any tips and tricks regarding carbon removal....I hope this isn't taking this thread off topic, I am just running with some things I read from others above....

Chad

Truckracer

My Feedback: (19)

This could be a good topic for its own thread ... especially with winter coming on. In one of Pat's comments, he requested a picture of the piston to determine if carbon removal was even necessary. What one person would consider excessive carbon, another might consider no problem at all! If we could stay on subject and not get too personal, carbon removal and simple engine PM could be a very interesting topic. I fear that some of the methods for carbon removal could be considered controversial though (even though they work well) and a few feathers might get ruffled. But on the other hand, you can pretty well read the health of a gasser just by looking at the piston top. You can tell even more by looking at the piston skirt and ring land area. Oh heck .... go for it!

pe reivers

Decarboning? It's just a plain old cleaning process. Be it mechanical or chemical, excess carbon is removed, so the engine can function as designed again.
Here is where modern ISO LEGD (Jaso FD) oils come to the resque. They have so many detergents, that the engine stays clean and decarboning becomes a thing of the past. I just wonder what all those detergents do to lubricity of the oil. Racing oils still depend on engine maintanance, so have less detergents to keep abreast of carbon formation. More of the oil is used to lubricate. Added detergents are just enough to last a race and training sessions I suppose?

arobatx

Well....LS3D happened to ask, and also happens to be the initiator of the thread....so what the heck, I'd love to hear what folks have to say about carbon removal since we're talking about it here.

Chad

rc bugman

My Feedback: (30)

Ok Chad,

I will reveal my secrets of carbon removal. I take a putty knife or a flat chisel xacto blade and gently scrap the piston. When most of the carbon is removed by scraping, then I have a very soft wire wheel on a bench grinder to remove the rest. If the carbon is soft and black, the task is easy. If the carbon is hard and tan (typical of amsoil), the removal is a chore and I usually sign the task with DNA in the form of blood. Hard black carbon is also a task and I usually DNA sign my work.

Hope this helps

Elson