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        Discussion norvel .074 break in

#1 uspsjuan Sep 30, 2012 10:55 PM

norvel .074 break in
I have run 3 tanks of 25% nitro through this engine and although it seems to be running fine, i just can't seem to get it to get above 12500 RPM. manuel says i should get close to 17000 running a 7x3. i am running a 6x3. Any advice would be helpful. thanks

#2 surfer_kris Oct 01, 2012 12:28 AM

It sounds like something is seriously wrong. Check for head leaks or a bent conrod etc.

#3 uspsjuan Oct 01, 2012 11:33 AM

new engine. was thinking about removing one of the head shims, or seeing if i can find some 30% nitro fuel.?

#4 GliderJim Oct 01, 2012 12:41 PM

You certainly don't need more nitro - something else is wrong. Did you start it with an electric starter? You could've bent the rod that way. Remove the back and take a look.

#5 surfer_kris Oct 01, 2012 01:20 PM

The engine will do +17000rpm on a 7x3, with three head shims and 10% nitro, if anything the nitro level might be too high...
Make sure that you use at least 20% oil, all castor.

#6 uspsjuan Oct 01, 2012 04:25 PM

using Hobbico 1/2 A fuel, 25% nitro. a major pain to take the engine out of the airframe to open up the back to check for a bent con rod. I will try another tank or two of fuel to see if things improve. will keep you posted

#7 Bare Oct 01, 2012 06:09 PM

Erm I'm thinking that you would need a.. quart.. of Fuel run through it,
Before believing that it's Broken in.

#8 GliderJim Oct 01, 2012 09:50 PM

Anyone ever figure out what percentage oil is in the Hobbico 1/2a fuel?

#9 uspsjuan Oct 02, 2012 07:40 AM

The bottle does not say what oil the oil content is. I just know that the wing is an oily messn after i run a tank of fuel. l

#10 surfer_kris Oct 02, 2012 08:51 AM

Tower says 18% of a special Klotz oil...

You can always add a little bit of castor to be on the safe side, the amount of oil on the wing doesn't tell anything...

#11 OkiThumper Oct 03, 2012 07:44 AM

It may be something as simple as needing additional run time.:popcorn:

#12 Dave Campbell Oct 03, 2012 10:46 AM

These engines are notorious for being both very tight at TDC and wearing like iron during break in. This is bad because the first runs ideally need the cylinder warmed with a hot air gun before cranking to prevent crank damage. Even after 2 seasons of flying, mine will still lock solid at TDC because the pinch is still that tight..... I am turning an APC 6.3-4 at about 16,500 on 30%, which is about 1k short of what the guys on RCU say I should get when the engine is worn in. Apparently, the .074's just decide to 'wake up' one day and make good power, but I haven't seen it yet. I have like, 9, of these engines and am hoping that they don't all go this long to being run in.... All that said, I am very happy with the power and performance of the one I am flying.

12k tells me something is wrong with your motor. I'd suspect a bent rod, an air leak, or a tight fit on the crankshaft fit.

#13 OkiThumper Oct 03, 2012 11:48 AM

As you say, it is possible for hidden damage or correctable defect along the way. The tightness issue sort of reminds me of the old days, where one would run their piston / sleeve lapped motors, running rich (then called "4-cycle") accumulating at least 1/2 hour run time prior to flying. Otherwise, one could not maintain full speed by leaning out the needle.

If no damage or leak, perhaps the solution is to fly, running slightly rich, accumulate some time on it then recheck RPM?

Also, another possibility is if the OP lives at an elevation greater than 2,000 feet (610 m), there is a corresponding loss in horsepower, due to the thinner atmosphere. At for example, 4,300 feet (1 310 m), the air pressure is about 12.5 psia (0.85 atm) versus 14.7 (1 atm) at sea level. At 6,500 feet (1 981 m), it is 11.5 psia (0.78 atm). Change in elevation air pressure in atmospheres (atm) is proportional to the power produced. Figuratively speaking, figure on about a 15% power loss at 4,300 feet, a 22% power loss at 6,500 feet, and etc.

Therefore, the RPM at elevation will be naturally less than at sea level. If this is the case, then a 3,000 to 4,000 drop in RPM may be considered normal. This is why flyers at elevation will normally replace an engine with one the next size up. (Example, .061 for a .049, a .30 for a .25, a .46 or .50 for a .40, etc.)

The other option that sometimes works is to substitute the largest diameter prop permissible by the engine manufacturer. Propeller performance is much more noticeable at elevation than at sea level. :popcorn:

#14 gkamysz Oct 03, 2012 02:33 PM

Elevation has nothing to do with engine RPM on a given prop. As elevation increases the air density decreases both the engine and prop work with the reduced density. Engine power output does drop, but so does the required torque to drive the prop. The result is virtually no change in RPM with air density.

I suspect a bent rod. The engine will have to come out sooner or later. 12kRPM on a 6x3 is less than 50% the power it should make. This indicates a problem be fixed not additional break-in time.


#15 surfer_kris Oct 03, 2012 03:26 PM


Originally Posted by Dave Campbell (Post 22902310)
I am turning an APC 6.3-4 at about 16,500 on 30%, which is about 1k short of what the guys on RCU say I should get when the engine is worn in.

You might have to much nitro in the fuel, mine does +17000rpm on that prop and 10% nitro. Check the number of shims and optimize for top top revs, staying on the side of slightly too low compression.

Another thing to look for is the throttle barrel, it is important that it lines up with the carburator bore at full throttle. It can be off sideways, as it rotates on it's threads, or it might not open fully if you have the throttle stop screw still installed (just remove it, it is not needed).

A cox babe bee will do more that 12000rpm on a 6x3, there is certainly something seriously wrong...
Take off the engine from the plane and run it in on bench before you put it in a plane.

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