How hot should my engine run and why is low end needle not working?

[Carl Richardson]

Hi guys,

I've read so many articles on setting up my Magnup 40 GP engine that I'ma bit confused.

My engine seems to be getting extremely hot to the point where it will instantly boil water if you put a drop on it.

I have hte high end needle set to maximum revs and then I back it off a quarter turn. there is plenty of white smoke from the muffler and fuel hitting my hand so I can;t see why it gets so hot.

At the same time I'm struggling wth the low end needle. Even if I screw it all teh way in at idle the engine continues to run. how does it do this at idle?

When I screw it out it makes little differnce to teh transition to full throttle.

Any ideas guys?

[Peter Miller]

Water boils at 100C. Your engine will be running at about 180 to 190C.

You do seem to have a problem with your idle screw.

[fly boy3]

I had a similar problem with a hot OS many years ago. Found I was using wrong fuel with not enough oil in it. Cheers

[John Muir]

The Magnum GP40 is a very old engine. It doesn't have an idle needle, it has an air bleed screw. To set up the air bleed screw, unscrew it a long way out, stick the tip of a pin into the tiny hole in front of the carb and screw the, er, screw back in until it just 'grabs' the pin. It should be reasonably close at that. Remember that to lean it out at idle you need to unscrew it and screw it in to richen it up, the opposite of a twin needle carb.

The main needle on the Magnum is prone to wobbling about and leaking badly. A bit of fuel tubing over the threaded bit can help, but it will still need to set up quite rich or it will probably conk out. It will produce, on a good day, about the same power as a modern ball raced 25 and is extremely light for a 40, so watch out for your CoG moving aft if it's replacing an Irvine 40 (or something). Don't overprop it. 10x6 tops, maybe a 10x5 or 10x4. Even might struggle with an APC 10x6 and would be better with the lower loading of Master Airscrew. Anything bigger and it will overheat badly. TBH it was cheap and a bit rubbish when it came out, but could pull an old trainer about adequately so long as it wasn't too heavy. I had one in a Chart Mascot, which was designed for a 25-40 engine and it worked. The conrod eventually broke.

P.S. Don't forget to take the pin out.

[Carl Richardson]

Thanks for all teh quick replies guys.

It certainly is an old engine. I bought it when I was 15 for my trainer and have only just started using it and I'm 33 now.

When it was running earlier in the garden at full throttle I did notice that fuel was leaving somewhere as I coult feel it hitting my hands as it was blown back. It wasn't from the muffler either as my habds were up wind of the muffler. Could this be an issue? It looked to be coming from the drive shaft where it exits teh front but its so hard to tell when you have a 100mph wind throwing it back so fast.

I'm going to strip the engine down tonight and clean and tighten it all back up.

I'l try the air bleed screw as you said by pinching a needle in the hole.

I'm not sure what you mean about tubing over threaded bit? do you mean on the high speed needle? how can I get tubing on without stopping the needle from going back on? I did notice the clips either side of thbe needle that make it clip were very loose on the needle sides so I took the needle off and pinched them together to tighten it up. Its much better now.

As said it is light and as said I had put it in place on an Irvine 40 which has a leaky carb. I had to move the battery pack to under the fuel tank to keep the COG forward. The irvine did come with a spare carb when I bought it off ebay so perhaps I should look at reinstating that engine.

As for props I was runnign a 10 x 6 Master airscrew but it snapped so I have ordered some 10 x 6 APC replacements. However as a temporary measure I have cut an 11 x 7 down to 9" as advised online. Could this prop chance cause the overheating at all?

I also have an OS40 AX in a Seagull PC9 that I got a good deal on but I'm nowhere near good enoguh to fly it yet. I could always take that engine but I'd just really like to get some use from this Magnum that I've had for so long.

[John Muir]

The tubing would go over the threaded part of the main needle where it enters the carb body. Just a small piece. When you screw the needle in it compresses the tubing to give a sort of makeshift seal. The oil coming from the front of the engine is perfecty normal. The engine doesn't have ball bearings supporting the crankshaft. There is instead a brass bushing at the front which is lubricated by oil in the fuel which is channelled along a groove in the crankshaft to the front bearing and gets expelled just behind the prop driver.

The prop could be an issue in that 9 inches might be too small and over revving the engine. I wouldn't like to guess at what the pitch would be now with the tips missing. Might not be very well balanced either. Personally, I'd never try to modify a prop in any way other than to balance it, and even then I'd be careful.

I'd suggest you use another engine. The Magnum is only barely adequate for the job of flying a Ripmax Trainer and will never be as easy to start and set up as an OS or Irvine.

OTOH, you do like a challenge don't you?

[John Privett]

The Magnum GP40 that I had (still have somewhere) is/was a clone of the OS FP40. One review in a French mag (actually, the only review I ever found of it!) reckoned that it was a clone to the extent that virtually all parts could be interchanged between it and the OS.

Mine however didn't inherit the OS reliability and ease of use. It was very tight until it had run-in, and was the engine that forced me to buy an electric starter. It was also very fussy about what plug it would run on, an Enya No.3 or one unidentified plug found in the bottom of my flightbox seemed to be ok - anything else and it would either idle but not run at full throttle, or it would run at full-throttle but not at idle. I think it only ever found its way into one of my planes for some reason!

[Frank Skilbeck]

If the engine doesn't slowdown at high revs or nip up then it's not running too lean, your cut down 11 x 7 to a 9 x 7 is probably not a good prop for your trainer, the reduced dia will reduced the static thrust while the 7" pitch will try and make it fly faster if it can overcome the airframe drag.

On all the airbleed carbs I've had the airbleed screw has had negligible effect on the running.

What fuel are you running?

[Carl Richardson]

Thanks for the suggestions guys.

Mr Muir - I love a challenge but the other night my trainerr deadsticked on me again but this time over the sea into an offshore wind. I just managed to get it down on the dry stuff. I know its not a good idea flying over the sea but on the large beach I was flying on the tide was still relatively high and its otherwise a safe place to fly.

I won;t try again until I get one of my new 10x6 props tomorrow. That and until I can run a full tank of fuel with some throttle variations to simulate a flight as best possible.

I'm running 10% nitro fuel. Not sure on casstor content but it was recommended by local model shop.

The engine would run at full throttle for 2 - 3 minutes and then just cut out without change of throttle.

I've stripped the engine down tonight and rebuilt it. Everything appeared fine except I did notice one thing. At the bottom of the con rod where it fits over the crank there is a small lubrication hole in the conrod. This faces to one side and I'm not sure which way it was facing when I took the engine apart. I have assumed that it needs to be fitted in such away that the hole moves forward in a scoopig motion in order for it to push oil through it. If initially this hole was facing the wrong way then perhaps this could of contributed to teh overheating factor.

It may be that my engine doesn;t overheat at all. Its just that I read on another site somewhere that you should be able to touch the head of the engine after its been running. There is no way you can do this on mine without getting a nasty burn. Perhaps for a split second but not hold it.

I'll give it another run tomorrow and check as many suggesitons as possible. I want to try and run this engine if I can. As for power it seemed to pull the plane along just fine last week. I was doing rolls, loops and steep climbs without too much problem.

I'll keep you updated. Thanks again guys.

[John Privett]

Posted by Carl Richardson on 29/07/2013 23:07:56:

It may be that my engine doesn;t overheat at all. Its just that I read on another site somewhere that you should be able to touch the head of the engine after its been running. There is no way you can do this on mine without getting a nasty burn. Perhaps for a split second but not hold it.

Well I'm certainly not in the habit of putting my finger on the cylinder head of any of my engines, but I think the result would not be pleasant!

The only similar "test" I've heard of is for heli engines, where the recommendation I was given was that the engine should be running rich enough that the backplate should be cool enough to hold your finger on for a few seconds. I'd imagine that would be considerably cooler than the head.

[Martin Harris - Moderator]

One thing worth doing (assuming you haven't already) is to pack the bay around the fuel tank with pieces of foam rubber. This can reduce foaming in the tank which causes bubbling - as can an out of balance propeller (or spinner) so it would be worth investing in a balancer - even the fairly cheap ones are well worth having.

If you haven't checked yet, I'd make sure that the air bleed hole at the front of the carb hasn't been blocked by congealed caster oil as it's been stored for a long time.

Edited By Martin Harris on 30/07/2013 00:44:24

[John Payne]

Hi guys. I endorse the "don't touch" approach with cylinder heads just after running, way too hot for healthy fingers! Another thing to check, if it is running with a silencer pressure feed, is the pressure take off on the silencer. It could be gummed up with old castor and not pressurising the tank at all and causing a vacuum instead, which in turn stops the engine. Seen this a couple of times before. Cheers, John.

[Steve Hargreaves - Moderator]

All good advise from the guys there Carl....I don't think its over heating...an aircolled engine will be bloomin' hot after a run....Peters guess of 180-190C sounds about right to me....way too hot for fingers!!!

Another possibility is a small hole in the internal tube that links the clunk to the tank outlet.....such a hole is covered by fuel when the tank is full but as the fuel level drops the hole is uncovered air is drawn in & the engine stops.....the further down the tube...the longer the motor will run before stopping!!!

Another couple of thoughts.....a 10x6 sounds about right for that engine.....not all conrods are symmetrical & should be marked before removal, lets hope you got yours on the right way round... What was the piston like when you stripped it? Dull aluminium or a bit browny?? If the latter then this might be burnt on castor oil which will certainly slow the engine down when it heats up as the castor residue goes all sticky.....polish it off with wadding polish....

It may be that this is an old engine & has simply "worn out". Fading away as the engine gets hot is a classic sign of old age &/or wear in the piston/liner or crank bearing....a new 40 will only cost around £50 & would probably be a wise investment as far as reliability goes...

Hope that helps...

[John Muir]

Hi Mr Richardson (seeing as we're getting all formal),

I've been thinking about my Magnum GP 40 and I remember it being very sensitive to fuel (I think it liked a bit of nitro, so your 10% should be good) and plugs, as John said earlier. I bought an Enya no 3 specially for it and that helped. I always had to keep mine running rich or it would simply stop and it gave off loads of smoke in the mid range. I had to leave the throttle about a third open and let it idle a bit fast as well or it would cut when the throttle was opened back up. However, this type of engine was around for a while and I'm almost certain that somebody at my club used one in a Super Sixty for a while quite recently, and it had a twin needle carb. His went ok for quite a while I think but eventually the propshaft broke and the prop assembly disappeared into Loch Earn. So if the carb was changed during the engine's lifetime, yours could have either. Quality control may have been a bit dodgy on them but may have improved as time went on. They were made by the Taiwanese company that became Thunder Tiger and their later engines are very good.

Anyway, it will fly your trainer, just not with the authority or reliability that a modern OS40 would. Mine would turn a 10x6 at about 10500rpm. I have an SC36 that will do a shade under 12000rpm on the same prop. Deadsticks are an absolute pain when you're trying to get stick time. I'd still put the Irvine back if it's been fixed or fit your OS. You may be lucky and have a good Magnum, but why take the chance?

Mr Muir.

[Peter Miller]

Actually up to 190F was not a guess. It was information from Clarence Lee, the Americna engine expert who work for K&B. I quoted his figures in my Engine bay coliumn back in the 80s. At the time I was playing with some Celsidots. Little stickers that one could stick to thinbgs (Cylinder heads) and they would cvhange colour if they exceeded the predetermined temperature.

On sagging as the engine gets hot. Could be varnishing of the cylinder liner, a brown deposit on the cylinder walls. IT cana be removed with fine steel wool.

[Glyn R]

You can still get OS made Irvine 40's for around £40. Brilliant motors, get rid of your problems and save yourself a lot of time.