This is my 1st OS engine, I have broken it in the way the manual says...I have got my starter problem solved thanks to you guys!! My next qeustions is this...how do you know if your engine is running too hot? I just ran it in the yard, first it would crank up.....but when you gave it throttle is stalled out...assume it was running to rich...leaned it out just a couple of clicks and it ran fine.., ran it approx 4-5 minutes....it just seemed to be awful hot...just wondering if thats normal? I have been running super tigre .27 engines and run them hard and they dont seem to be as hot as the OS 75ax. Btw this is on a airboat...figured you plane guys knew these engines inside and out.

You should be able to hold your finger on the bottom of the crank for more than a few seconds. If your engine suddenly stalled when you accelerated it was probably lean. You might have the low end rich and the high end lean. Good luck.

The best way to tell is to use an optical thermometer.

The cylinder head measured at various locations, should not go above 340 or so degrees and typically a bit less.

If it gets above this you might damage or seize the engine.

With normal cooling after a flight your engine will be very WARM to the touch... warm enough to make you feel uncomfortable holding your hand against the cylinder head, but not so hot that it feels like you burned your hand.

When properly tuned and ventilated the AX is a pretty cool running engine, but you'll tend to load it up more on the ground than it will get in the air....

So it may run hotter on the ground... but it should NOT stop, smoke or give any other indications of overheating.

BTW: tighten the head cap screws after a good run. They come loose with the heat.

Thanks for the info...believe me I am paying attention. I just built an airboat and put some pictures up in the airboat section under "Post your pics". But I think I opened a whole new can of worms going to a bigger motor...lots of stuff to learn...ie balancing prop, learning how to adjust carb accordingly..whats too rich..whats too lean, etc.Should be a runner though!! Like I said earlier I have been running these Supre Tiger .27 and they are alot less adjusting and dont seem to get as hot. The OS is a superb engine from what I understand.

Vital and Critical Question Here:

Is this boat configured as a pusher or tractor?

If the boat is a pusher And you run the engine static four or five minutes you are going to over heat it regardless.

Break the engine in on a test stand using a conventional tractor prop.

John

John, its a pusher, do you think as it runs accross the water it will pick up sufficient air to cool it? , I was running it on the ground in the middle of the day when it was getting hot.

Yep, I was thinking the same thing John mentioned.

If you are running it pusher style, your not getting any cooling.

When you have it at full throttle, do the pinch test. While at full throttle, pinch the fuel line, just before it goes into the carb, for just a second or two. If the engine immediately quits, you are probably to lean. If the engine revs a little higher and then goes back to its original speed, then you are not to lean. You want the engine to run about 300 rpm below the highest rpm you can get. A tach is helpful if you don't have an ear for tuning. Some guys do, some guys don't.

On a new engine, you should run it a little rich for three or four tanks.

As for hot. If you can grab the cylinder for a second or two and it feels hot, but not to the point you feel it's going to burn you, you are probably OK.

Tom

Yup thats the reason it is getting hot. Yes it will cool when you run the boat but you do not just sit there and let it overheat you put it in the water and go.

Truck on down to the hobby shop and get the recommended size break in tractor prop and if you don,t have a test stand just install this prop on the boat and let it blow over the engine, just tie the boat down.

Do your break in process from square one as indicated in the manual for the engine. Only after doing this do you reinstall the pusher prop and maybe a click or two richer on the main needle and let her rip in the water.

The AX series are superb engines and very user friendly.

John

marshrabbitt,

assuming you are using a puller prop and that should be a 14x8 since its a new engine. Close the HSN fully and then open 3 turns. Start the engine and let it run at full throttle for a few seconds. Then very carefully (watch out for the prop) slowly close the HSN. You should hear the engine exhaust pitch rise and then drop. When it drops you are too lean, so open it back up till you hear the pitch rise again. If it goes to the max and then drops, now close the needle to get the max again. Once you achieve the max engine pitch open the needle 1/4th turn. That would be your setting.

We open the needle back from max rpm because the engine will unload in the air (run a bit faster as the load will reduce once in motion) and for some safety to keep the engine rich. We assume you have not touched the LSN screw yet. The factory set for that is rich, leave it there. You will need to repeat the same once in your airplane, do it with a helper holding the airplane vertically up as this is the worst position for the engine from the fuel draw point of view (most chances of going lean in this position). While not everyone will agree to this, preferable to use synthetic-castor mix in these larger engines and use minimum nitro (2-5%) for more relaible operation.

If I were you I wouldnt run in the backyard (though I did do it in my appartment yesterday for a few seconds) lest the neighbours complain.

Ameyam

An engine running too lean and hot has a distinct odor. Also, when it stops running, if you spit on the head ~15 second later and it still sizzles and boils off the spit, it was running way too hot.

Ameyam the original poster is talking about a boat, and one with a pusher configuration.

Sorry, my bad. Didnt see the last line

Ameyam

Ran the boat in the water today and the engine stayed cooler...alot cooler!! I need to figure out why it keeps cutting off...the engine, when I rev it up a few times and go to take off if shuts down...if I just give it full throttle it takes off and as long as i stay over half throttle it runs fine...its when I start to feather the throttle from a lower rpmit shuts down. Carb needs some sort of adjustment.

Yes there is no doubt that the midrange adjustment is to rich. The mid range adjustment on all the AX's except the .46 are on the left side next to the rearward angled needle valve and is that tiny screw on the end of that long cone. To lean it you screw it in slightly. Do read up on it in your manual.

John

Thanks John...appreciate all the help. I read the manual and I am the type of person that if someone shows me hands-on and explains what and why ...it sinks in. That is the low speed needle you are talking about right?

I fly several pusher planes.

As noted the engine will get hot even in flight, because it lacks the cooling action of the prop's airflow, even when using a smaller prop that does not load the engine down as much as a standard prop.

I find that I have to richen the high end a good bit, enough to force a marked reduction in available power, to keep the engine in it's operating range and prevent my engines from seizing.

I learned this the hard way with one pusher...

After reading one of Downunder's posts about head temps I purchased a laser temp meter... with that I could find the optimal tradeoff between richness and temperature rise.

I adjust the HS needle to keep the engine head operating well below 360 degrees ( at the hottest point I can measure ) on the ground at full throttle.

That worked great and resolved my pusher engine overheat problems.