daz3nt

ive finaly flew my plane 6 time now but the engine have cut out twice and ended up smashing the thing in to the floor the first time but ive rebuilt it but the engine isnt 100% reliable can anyone give me some good pointers on tuning oh ye its a barrel carb on an sc .75 engine with ls and hs needle's

manks7477

What kind of engine do you have?
When does it cut out?
How is it set?
Does the engine lag when you go from Idle to Full Power? Does it cut out then?
Are you sure that this is an engine problem rather than a fuel problem?
What have you done to tune the engine?
Do you do a nose up test?

There are a ton of questions here. I was having similar problems that I figured out were not tuning issues, but rather fuel line issues (my line had a 90 degree turn in it that made fuel flow a problem). Please provide some more specific information.

daz3nt

Ive checkd the fuel lines ive set it to what i assume is factory settings then it dont start at all. the ls needle has to be out quite alot for it to run and it cuts out when i apply power. like the other day it was flying lovley i came in to land but was coming in a bit quick so i powerd up to try again and as i pulled up to turn the engine just lost all its power so i tried to idle it then go back to half power but it had lost its heat and fell to the floor. i am a beginer and dont know an aufull lot about glow engines i have always had eletrics.

j.duncker

My Feedback: (2)

Engine tuning is difficult to describe in words.

IMHO you MUST find a club and get their engine expert to check out your engine. It should not be necessary to join the club.

N.B. in most cases the engine expert is the guy who flies a lot and his engines start and run without continual adjustment. It is NOT the guy who is NOT FLYING AND FIDDLING WITH HIS ENGINE.

If you are anywhere near Luton PM me.

daz3nt

should this engine run a 13/6 prop because when i put it on this engine it hasnt got enough power to spin it it just dont rev

daz3nt

oh ye im in west wales

Montague

I use an APC 13x6 on my Super Tiger .75, and other guys around here use that prop with good results, I don't think that's your problem.

As others have said, it's rather hard to diagnose an engine over the web.

However, there are lots of threads on here about setting the mixture, using a pinch test to check the mixture and so on. If you dig around, you're sure to find one, there's lots of good info.

I'm guessing you have the mixture too rich on low end, or you are trying to idle too slowly.

If you get the engine running, and run it up to full power, then down to idle. Wait a second, then apply power, does it kill instantly, or does it burble and accelerate or die? Is there liquid fuel coming out of the exhaust at idle? Can it idle for just a second or two, but can't idle longer?

spiral_72

I've looked, but found nothing so far.

There's got to be a step-by-step instruction set on the internet on how to tune an engine properly from totally not running/completely out of adjustment, to running nice and reliable. It'd be nice if there were pointers on knowing when to change glowplug types/heat ranges and troubleshooting.

I'd love to build a page for everyone, but I'm learning myself. One guy swears by an OS#8 and another by an idle bar plug and another by an OS A3 and an OS A5..... what the? How do you know besides trial and error? At some point the engine manual is useless especially for a perfectly good older engine like I have. I've got advice, but it's just what has worked on my application. The same advice didn't work on a buddy's OS .46 that was doing EXACTLY the same thing mine used to do.

It sucks that you just try stuff till you find what works. It costs a lot of time and money.

opjose

As Montague said!

Additionally let the plane sit at idle for a while and note what happens to the engine RPM's.

If it tends to slow down over time, then the low end is too rich. A little of this can't be helped ( better that it IS a little rich ) but you don't want the engine cutting out.

Remember that the plane also leans out in flight, so keeping the plane flying is a bit of a balancing act.

It helps to perform all tuning with about 1/3 of a tank of fuel to simulate a lower fuel condition where the engine tends to run lean anyway.

Sik

Spiral, If you find that web site or anything else PLEASE let me know. I have an OS .46 FX that i know nothing about. HELP[&o]

Montague

spiral_72,

I feel your pain.

Really, engine tuning is an art, and it takes practice. I've been doing it for quite a while, and since I do a lot of flight instruction, I spend time tuning not just my own engines, but other guy's as well.

You're right, differnet engines will behave differently. There are tons and tons of factors. Glow plugs can be confusing as well, as you've noticed. The reason for all the plugs is that differnet engines will "like" different plugs.

Also, the fuel you use will affect your plug choice. Nitro is the obvious one, but oil percent can also affect engine running.

Your prop choice can also affect what plug is "best". And as we know, prop choices vary by person and airframe, even for the same engine. The exhaust system also has an effect on how your engine runs, though getting in to tuned exhausts is probably beyond the "beginners" forum.

Add in different ways of putting together an engine (ABC, Ringed), and engine design issues like the bore/stroke ratio, compression ratio, age of the engine and so on.

Oh, and don't forget the weather and where you are. There's a big difference between running and engine in Flordia in the summer and Denver in the early spring.

So, yeah, there's no "one answer". No one "best plug", "best prop", "best fuel".

There ARE techniques that generally work the majority of the time, and you can use to get your engine running well enough to fly and learn with.

-----

The reason a lot of guys like the OS 8 and the OS A3 is that they are reliable plugs with a medium heat range, and are a tad shorter than some, though technially still a "long plug". So they work well in most engines most of the time. I use the OS #8 in my Mosquito twin engine plane in a pair of OS .46AX engines. I'm not looking for peak power on those engines, but an engine out on take off will wipe out the plane, so reliability is everything.

On the other hand, in my combat planes where I'm running mousse cans for extra RPM, sometimes use extra nitro, use small props and wind the engines up to high RPM, I use K&B 7300 plugs because they work for me and do what I want in those engines. That K&B plug in a new OS .46AX will often cause it to run badly, I've seen that happen myself.

Montague

Ok, some helpful techniques that I use:

- Set the high speed needle first. On most engines, the low speed setting will be affected by the high end, and vice versa, but the effects of the low end on the high are less pronounced. (yes, the high and low speed mixtures DO affect each other to some extent. But it does vary a lot by carberator design. The non-airbleed OS carb designs are pretty good about the low speed not causing the high end mixture to change and vice versa. Some other carbs have more cross-effets (say what you want about OS engines, but their carbs are amoung some of the easiest to make work right, and many funky-running engines can be "fixed" by mounting an OS carb to it)).

- When I set the high speed, I run it up to full power, let it stabalize for a moment, then I pinch the fuel line with a solid pinch-and-release. This causes the fuel to drop off for a split second. Listen to the engine, it should pick up RPM and then settle back to the origional RPM. If no RPM change happened, either you didn't pinch enough, or the mixture is right on the edge of being too lean. Pinch again to verify, a little longer. If the RPM drops at all, you are too lean (or you held the pinch way way too long, but even then, if you clamp off the line, the RPM should pick up, the sag and die. If it didn't pick up to start with, you were too lean). How much pickup you should hear depends entirely on the engine/prop/etc, but you should hear some pickup. Unless you are marginal on power, or in a competition, backing off a little on the rich side is good for the engine anyway, so make sure you hear a good RPM change.

- Then set the low speed. You can do the same pinch test with the engine at idle. But I usually don't do it that way. Instead, I idle the engine down, hold a second, then pop the stick to full throttle. The engine should respond very quickly. If it lags and sputters, it's too rich. If it suddenly dies, it's probably too lean. But if it's really rich, it will often just die as well. So try again, bringing the engine up slower, and/or holding the idle longer. With some experience it becomes easy to tell if the engine is getting too much fuel or not enough fuel at idle. What I look for is a solid, smooth transition to full power with out a lot of hesitation or burbling. On a trainer, I'll usually take one or two burbles after a long idle, better a bit on the rich side than too lean. But on a student's plane that is about to start landing, I'll pay extra attention to the transition because I might need power in a hurry to avoid a problem.

A final note on mixtures, glow carbs are mostly pretty primative. You have a needle valve that controls fuel flow, and some kind of device to change the flow as the throttle changes. (the first is the needle valve, the second is the low speed needle). But the rate that the mixture changes though the mid-rage is a "guess" set in stone my the carb designer. It's one of the things that seperates good carbs from bad ones, and it's one of the things that makes an engine hard to tune if you run an overly large or small prop, or use a tuned pipe or do any number of other things besides run the engine as it was designed. Sometimes that means the wrong fuel or weather conditions as well.

More critically, a marginal fuel tank installation (too high or too low, or a pinch in the line), or a problem with the muffler pressure vent line or any number of other issues can often be compensated for on the high end with the needle, and at idle with the low speed, but not in the mid range. The engine either goes rich or lean in the transition because the fuel isn't being delivered quite like the designer expected.

I hope some of that helps.

Oh, and there are other ways to tune an engine. Some guys use tachs, some do a "lift test", some even try to use a thermometer to measure head temp.

manks7477

I have an OS .46 FX that is one of the best engines on the market period!

Great power, good on fuel, and once it is set up very reliable. Make sure you take a look at that low end, that can be a bit touchy, but you will love that motor!

spiral_72

I DID learn something very valuable Saturday though.... It was brutal hot outside with about 200% humidity. The engine tuning that has worked the last couple months with only a click or two from the main needle was leaning the engine out in the heat.

I had to richen it about 1/4 turn or more! I couldn't believe that much of a change. But now I know. High ambient temps means a less dense air charge and ya gotta crank up the fuel.

Montague

Want to be confused?

When air cools and drys, you get more air in through the carb. More air compared to the same amount of fuel means a leaner run. So if the air temp drops, you often have to richen the mixture. Which also means you often have to lean out an engine when it gets warmer and more humid.

But that's the opposite of what you just experienced.

When it is really humid and hot, engines also run hotter, which can cause an engine to over heat. To reduce the running temp of an engine, you run it a bit richer. So on a hot humid day, you might richen to reduce running temp.

So, when it gets hot and humid, you're engine performance will probably change. You may need to richen, you may need to lean out a little, depending on the running temp of the engine and what the weather has been doing.

Also, running temp is affected by such things as prop size and fuel, so two of the the same engines in the same airframe may need slightly differnet adjustments in response to a weather change if they have different fuel and/or props.

I've been flying on days where we had a 20+ degree temp drop as a cold front came through. After the t-storms had passed, all the engines needed re-setting.

Oh, and the OS engines often have a rather broad needle valve (a good thing), so it often takes more "clicks" to make the same mixture change on them than it does on some other brands of engines. Some engines are much more "peaky" and go from too rich to too lean in very few clicks. These engines are also more sensitive to weather changes.

Trollmaster

I'd try smashing it to the floor a few more times, see if that doesn't straighten it out.