silver.kiwi

I'm having a problem with my XL .91RFS that defy's my abilities to correct. I'm hoping one of you out there has the answer.

Over the past 10 years I've owned several of these engines and they all served me well. This one, however, is giving me quite a headache. It's mounted in a GP Super Skybolt that I've had a couple of years but was put aside for about 3 months. When I returned it to service I could not get the engine to produce good power. The main needle seemed to want to be screwed in extremely lean and there was only a one or two click turn between too lean or too rich. Too lean and the rpm would naturally sag as the engine heated up and too rich would result in a loss of rpm and power. There was no sweet spot.

I have now replaced the carb. and every fuel/pressure line. I've cleaned the tank and replaced the stopper, clunk and associated lines. Then I pressure checked everything to 10 psi and found no leaks. I have inspected the bearings and adjusted the valves and installed a new OS F type glow plug. Fuel is Omega 15%.

With the new carb. main needle open approximately 1.5 turns and the low needle at the factory setting, the engine only turns 8,700 rpm with a 13x6 master airscrew prop. Occasionally the rpm might briefly rise to 9200 rpm. That should be close to normal. What baffles me is that without touching anything the engine will start to run rich and loose 800 to 1,000 rpm with much smoke from the exhaust and rough performance. The idle is fine and so is the transition from idle to full throttle. So why is this thing suddenly running rich for no apparent reason after tuning the the needle to what would be a normal setting and then it acts as if you deliberately opened the needle to a very rich setting. Any suggestions to help me get this pig running normal again would be most welcome.

LOCKTUMBLER

If its the latter type with the crankcase breather from the cam box to the inlet manifold I'd give that a good check out, externally and internally, for damaged pipe/loose nipples/leaks/blockages. Also, IIRC (could be wrong) there is a timed passage way from the crankcase to the came box via the crankshaft - intermittant blockage??

EscapeFlyer

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I know people won't like this, but I have never gotten a magnum to run well on Omega fuels. Power Master and Byron fuels worked great. It made a huge difference.

Brian

silver.kiwi

Thanks, I'll take a look. I did give the associated tube a quick looksee but I will open up the cam box and do a more detailed inspection of that area.

silver.kiwi

I don't discount what you are saying and anything is worth a try, but truthfully I haven't had any problems with Omega in any of my engines before.?????????

EscapeFlyer

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It is crazy. I don't know why this is the case.

Another thing to consider.. Magnum uses course threads on their hi speed needle and a single ratchet spring. There is the strong possibility of false needling. It wouldn't be quite as noticable with fine thread needles because there wouldn't be as much play.

I would check the o ring seal and try to put even pressure on both sides of that needle if possible. The spring in conjunction with vibration could be pushing the needle against the wall and creating strange runs. I would assume this would be even more likely if the engine was well used.

What glow plug do you use?

Brian

silver.kiwi

The entire carb. assemble is brand new (needle valve, O rings etc.) and the performance is the same as with the original carb. Glow plug is OS F type..

WMB

My Feedback: (1)

My .15 had the same problem. If the hi speed needle was pushed a little to the side, it would go out of tune. I removed the spring that keeps the needle from turning and put a piece of fuel line over the hi needle assy. That kept the tune in place for the day. It was still difficult to find the sweet spot due to the coarse threads.

TomCrump

The needle threads on the Magnum .91 four stroke are identical to those on a 91 Surpass, and other OS 4 strokes.

I think the adjustment problems lay elsewhere.

the pope

I seem to remember ( but I could be wrong ) something about dodgy valve springs ( maybe even a different brand ! ) , Have you checked to see if one maybe broken etc . Have you turned the engine over slowly while watching the rockers to see that the valve train is working ok ? Have you checked the exhaust nipple for blockages ? Been there done that ! Cheers the pope

silver.kiwi

Thanks, but I have done all that you suggested when I set the valves. I thought that I might have a sticking intake valve or maybe valve float due a weak springs but all seemed normal. Before I put on the new carb., the first indication of a problem was an extremely lean setting (3/4 turn open) of the main needle in order to get anything close to full power. I did the usual fix of placing fuel tubing over the needle to stop any potential air leaks but that didn't help at all. I then assumed a bad spray bar and orderd a new carb. No luck there either.

I guess the next thing to do is tear the engine down so I can inspect forward of the rear crankshaft bearing, although for the life of me I can't see what that would have to due with the engine suddenly running rich and never reaching full rpm when leaned. To my thinking, that all points to a fuel metering problem.

blw

My Feedback: (3)

Magnums run fine on Coolpower and Omega down here. The most common nitro content is either 10% or 15% from the LHS.

Some Magums do have loose needle valves because I remember tuning and flying them for other people. It seemed the ratchet wasn't worth much. I like tuning Magnums. They're easy natured and seem of run well. I have a friend who knows next to nothing about engines and his Magnums never give him any trouble.

silver.kiwi

It's been my experience with Magnum and other brands,that the needle valve may loosen with age and some people feel this will cause an air leak. It has also been my experience that air leaks around the needle are the result of the "O" ring hardening and air leaking around it, not loose threads and this is the cause of the engine running lean.

This is not my current problem as my engine suddenly begins to run rich and loose power.

1QwkSport2.5r

Nearly all engines using muffler pressure will leak fuel OUT of the needle valve, not air leaking in. A bad O-ring will compound this problem. Usually if an engine will only run with the needle nearly closed, that usually signifies the engine doesn't need muffler pressure as it's probably "pumping" too much fuel to the needle valve. If the problem didn't exist previously and suddenly now it does not run/adjust properly, something changed. In a 4-stroke, valve lash can have a detrimental effect on performance (or lack thereof) if too tight or too loose. Being that this engine is performing differently than previous engines of the same model, perhaps something has been updated or changed like cam timing or compression ratio. I think the problem lies within the fuel system most likely.

blw

My Feedback: (3)

The o rings often get overlooked.

1QwkSport2.5r

Provided the carburetor on a particular engine has them. Worth looking into though if the NV leaks a lot.

blw

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Under the carb too. And intake manifolds if a 4 stroke.

wcmorrison

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Worn out one Magnum .91 four cycle and working on another one now. I also have Maggie .52 four cycle, just no plane for it at the moment. I get great service out of the Maggies. I also have Saito's, OS and YS four strokes but seem to continue to use my Maggies.

blw

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I've always like them.

RDJeff

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I got a .52 four stroke a while back, still trying to figure out what plane to put it on. Today I picked up a .91 and 1.20 NIB, $100 for both! I'd better get busy building it would seem.

hobbynutt

You really lucked out ! The 52/4s works great on a trainer. Better low speed performance, when doing practice approaches. Not a big fuel drinker !

flyboy2610

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This club has gone nearly 2 years without a post.
Can't have that.
I currently own a Magnum XL61 (not the A), just got an XL70 RFS (will be going into a rebuilt, re-covered taildragger conversion Hobbico Superstar 60 (http://www.rcgroups.com/forums/showthread.php?t=2682779)), and will soon have an XL52 RFS for my Top Flite Elder 40, when it eventually gets built.
I do have a question: what are the differences between the XL61 and XL61A? I know the XL61A RNV is a rear needle valve engine, but that don't enter into it. I'm just asking about the regular carb XL61A.
I can't find a manual online for the XL61, and seem to have 'misplaced' my original copy. I did download one from Global Hobbies website for the XL61A.

wcmorrison

My Feedback: (2)

Flyboy2610, did you look for it in the list of manuals shown at the top of the engine thread? It may be there. The carb on the A has a screw in block, bolt or cap where the HSN needle valve would be located. So essentially, it is the same carb except the needle valve itself is relocated to the back of the engine. I am told one can remove that cap and install the needle valve ala standard carb found on the non-A model. I have just acquired an XLS 61 A and it runs fine. It is on a Great Planes Big Stik 60 and while it runs great it seems to me to be a bit underpowered. I usually install a Maggie 91 four stroke rather than a 60 two stroke. With the Maggie four stroke I get off in about 20 feet with the XLS Maggie two cycle I take a couple of hundred feet to get off. I will try a bigger pitch prop and see what happens. But have been disappointed in the combination of the two, GP Stik and Maggie XL.

flyboy2610

My Feedback: (1)

Hi, wcmorrison, thank you for taking time to respond. The website where I found the manual for the XL61A is here:
http://globalservices.globalhobby.co...als/Magnum.asp

I believe what you are referring to is known as the XL61ARNV. There are pics of both that engine and the XL61A in the manual list. Open up the manual for the XL61A, and on the left side of the bottom of page 1 it shows the two variants side by side. I can't see any external differences between the XL61 and the XL61A, so I was just curious what would be the difference. Maybe they just called it "A" to designate the model with two carb variants?
This engine seems to have enough power to haul the Hobbico Superstar 60 around OK.

the Wasp

My Feedback: (1)

hey guys I don't think I have replied here, I have joined the Magnum Club,,
a couple flying friends had the Magnum 91 4St and I have always wanted one cause these guys had such good luck with them, I mean their engines just ran and ran for years..
so I built my son a ARF 76" span Edge and I bought and mounted a Magnum 180 4St in it, the engine runs great and has great power and is easy on fuel for a 30cc engine, I broke it in on the bench with a gallon and a half on 10%, then once in the plane I switched to 15% Wild Cat, it turns an APC 17X8 the same as people have reported their Satio's 180 do on 15% (8,800 Peaked). (the Manum manual reports peak power is at 9,000), I tried a Xoar 17X8 Sport, boy it really pulls because the RPMs dropped to 7,800, it ran and pulled well in the air but I could feel on the throttle the engine was a bit over loaded, so now on Monday (IF it's a nice day) I am trying a Xoar 17X6 Sport to try to get the RPMs back up a bit,,

my only 2 complaint is the loose Needle Valve, but I have not had a problem with it yet, and the fact that it idles a bit too high with a full tank of fuel, once the fuel level has lowered to say 1/3 or 1/4th she will idle down incredibly low and smooth (with a full tank of fuel it idles too high so I have to hold the Tail or she will Taxi off unwantingly)

anyhoo, as for today the engine has only 2 & 1/2 gallons burned threw it, my son gave up flying for girls, and the plane sat for 4 years so late this year I bought it from him, I started it at home and it stated hard, once started I ran 10oz threw it, then 3 weeks later at the field she came to life on the 2nd back flip and she ran great..

my only requests is I get comments about my "idling" problem, I have the Low Needle set, and if I move the Low needle ether way (rich/lean) my Idle gets worse.. I wanted to try a Regulator like the Cline Regulator, but I get mixed feedback, some say a Regulator will work for my problem and others say it wont work for me, maybe I just need to put more time on the engine ???


a little Magnum info here,
as I was surfing Magnum's 4St parts and I found the Magnum 70 uses the Magnum 91 Camshaft, I find that interesting..

Question, did Magnum stop making the 75 and 91 2St ?? I have not seen these 2 engines on Magnum's Site for a long time now

thanks guys
sorry, long post
Jim