Ok ,

I have a new Magnum XL-70RFS 4-Stroke that seems to have a leaking exhaust valve. Its had this little leak since day one and seems to be my pain with this engine. I'm going to try to get it warrantied but it was in its box for a few weeks before I used it so the little card was not filled out in there timely manner. I've checked the valves several times to make sure they are not to tight , ran it on the bench , in the plane, and its only getting worse.

Can you lap the valves on these little things or do you need to get new ones ?? If you can lap them , what would I use thats fine enough not to hurt anything ?

They are also out of head gaskets for these so , once its apart it might not go back together for a while.

Andy

The head gaskets are generally reusable so don't worry about that.

People are using DuPont or Turtle wax rubbing compound, Bon Ami mixed with tooth paste.

I had one that way. I lapped the valves and achieved a perfect pattern on both the valve and seat, You could still hear it hiss through the muffler. I eventually started using it and it has never given any trouble. When I first started to lap the exhaust valve, I noticed the mating surface of the valve had grinding wheel chatter marks on it. I lapped these down but the valve still leaked.

I use Comet or Bon Ami cleanser and a little light oil as a lapping compound.

Ok , I'm going to try the warranty first , but if they won't warranty it I'm going to give it a go. I think this is a little more than a little his , its more like blow by ! [X(] Iif you crank it over casually by hand it can barrely feel compression , if you kick a little harder you can feel some compression. Idles good , but I'm thinking it should have a little more power. It has a 13x6 prop and does not have enough power to pull a 6 lb plane vertical.

Andy

Whatever you do, do not use the automotive valve grinding compound. Even the finest grade of Clover compound available at the NAPA auto parts store is too coarse and will eat everything alive. Clover makes some fine compound that you can get from McMaster-Carr but you and all your grand children would never use that much up the rest of their lives.

If its not really bad I was thinking about making a paste from a little oil and fine polishing rouge and trying that. I figure more than a tire cap full is probably to much.

Andy

That should work fine. At least try it. Look for a shinny pattern all around on both seats.

Thanks, I figure it can't get any worst at this time.

Andy

I have used Mothers aluminum polish with excellent results.

Motor ,

Thanks for the tip !!

AJ,
NO problem.

I always use regular automotive white polishing compound on the valves, but this is mainly
to clean the carbon off, and re-seal the valve surfaces. If I had some material to remove
I would go with regular automotive (orange) rubbing compount for a bit more abrasion.

FBD.

If you lap the valve seats, you can probably forget about warranty covering it! I'd try the warranty route first!

Ken,

The question was just incase the warranty did not cover it , and after the warranty does expire this is something that is good to know.

Andy

Check the fit of the valve to the valve guide. If this is sloppy, you will never get a good seal. If this is the issue, it should definitely be handled by warranty.

Andy, what you are describing here is not a problem, but the way these engines are. If you throttle is closed, there is almost not compression. At half throttle, you feel some.

If the engine idles good, fly it. There is nothing wrong with the valves. Loss of compression seriously troubles idle speeds. I have four Magnum four strokes from 52 through 91's and a couple O S four strokes. The O S exibits more compression, but doesn't run any differetly. It may result in a little more high end power, but doesn't effect the idle. Again as long as the engine idles well, your valves are OK.

On the top end, it sounds as if a good tuning is in order. If the engine has been messed with, IE the cam cover taken off, there may be a cam timing issue and/or valve lash issue.

A point on lapping these valves. Less is better. The best seat is with a lapped band of 0.003" or less. As the lap band increases, the seal becomes less effective. This is true of any valve. The valve seat in cut at one angle, and then the valve is cut with two angles, one at a lesser angle than the seat and one with a greater angle. This produced a sharp edge that shoud set about 1/3 the depth of the seat. This edge is what you lap to the seat. The less you have to cut lap off it the better the seal. The more PSI on the valve seal the tighter the seal. As the valve spring is the limit to the total amount of pressure you can apply to close it, any increase in PSI will be the result of a small SI (square inch). That

Hobby People are great in their customer service. Send the engine in and they will give it a good checkout and send you a detailed report of what they found and how the engine is running when it leaves them. When you get it back, don't start cranking the needle valves, but try it out with the settings they left. You may need to go a click or two one way or the other depending on the weather and fuel differences, but not a lot more than that. Speaking of fuel, what are you using. The Mag 70 likes a 15% nitro and 18% castor/synthetic mix.

Don

Don,

I should have wrote down the rpms I was getting with this engine. As far as tunning goes , I have retunned this engine from scratch three times and allway the same results. I purchased it new through Towerhobbies , so if somethings wrong I'd think it was from the factory. I've been told around 4 turns on the low speed should be close , not on this one. If this one is not at least at 5 turns in it will not idle below 3k . I've been running Byron 18% SC blend / 15%N to keep it good and oiled up. I have tried Omega , but runs about the same. I did send it in to see if there is a plroblem. If there is, good !! That would make my life simple , but if theres not I might not be a four stroke person. I'm not giving up , I'd really like to get this engine running good to fly my YAK with it.

Andy

Andy, first the Magnum service, Hobby People (another branch of Great Planes like Tower) is second to none. They even helped me out with an engine I bought on Ebay.

The 70's carb is different than any of the other four strokes I have in that the idle jet screws out to a stop, not in to one. When I first got mine running, I was very disapointed with it. I could get it to run, but it was ragged and the idle was awful. Little power. Finally I started from scratch on the tune up. The manual tells you the inital setting of the Idle, I think it is screwed out to the point of flush whith the housing and then in a turr or two. I kept trying to screw it in to a stop and at one point I had it flopping around in the spray bar as I had screwed it in so far that the threads disengaged.

I sat both jets to the manual and then opened the high speed another 1/2 turn. I fired up the engine and let it warm up some at this setting. I then leaned out the engine for maximum RPM, turning the high speed needle a couple clicks and the waiting for the speed to stablize before the next adjustment. Once I found the max RPM, I went rich to lower the top end by 800 RPM (after the engine is broken I change it to 400 rpm ). Now throttle down to the slowest speed that the engine will still run at, don't worry what this speed is yet. At this throttle setting, lean out the idle for the maximum RPM, again waiting for the engine to stablize between each 1/8 turn max adjustment. The pressure of the screw driver will change the setting, so remove it from the needle when making the final adjustments. Once you have found the mix settings on the low end, then go back and set the high end again. Now repeat the idle settings again. This time your stable idle will be in the mid to low 2000 range. Fine tune it at the lowest setting so it will run for a least 30 seconds at an idle without stalling. You should be around 2200 for a newer engine. Once you have it at this point, go back and do both high and low one more time. Now you will work on transistion.

With the engine fully warmed up, go full throttle for 10 seconds or so to clear up any loading and then go to a full idle. Let it set there for 20 seconds or so the jam the throttle full open. The engine should jump to life with no heasition. If it stutters and stammers and slowly picks up speed, the idle is still to rich. Close the idle jet 1/16 turn at a time and then check the transistion until there is no hesitation. If the engine dies as soon as you jam it, the idle is to lean so back the idle jet out 1/16 turn at a time checking the progress until it jumps to life.

My 70 has had less than a half gallon through it and it is running better and better each day. I am cranking a 12-6 prop and it is under proped wit it. I can easilly turn 11,500 on the top and 2100 on idle. My plane is a 60" span 3D scratch built from plans in RC Modle World's magazine for a Ruperts Dad. It weighs in at 5.5 lbs and will go unlimited verticle. In fact after about a 20 ft roll on take off, you can do a 90 degree turn up and go out of sight.

I'm glad you sent in the engine without mucking with it. I think you will find it a very good engine. Just don't expect much in the way of compression feel when you get it back. They don't show anything like a two stroke's compression feel.

Don

The valve can leak some and still run flawless. I had a Saito once that hissed through the exhaust valve and still ran perfect. I think with added pressure, heat, vibration, speed etc. of combustion that it seals a little better. It the leak is enough though it must be fixed. One thing I would check before lapping is to make sure the valve stem is not sticking or warped. It still is better to have them sealing totally.

Once the valve leaks get bad enough that you can't start the engine by hand it's time for a rebuild.

I would seem any leak at all could lead to the valve or seat being flame cut and progressively becoming worse.

....yep, any leak is no good. Take the part to the solvent tank....fill the port with
solvent....look at the combustion side of the head. If any solvent leaks past the
valve seats....it needs to be re-seated.

FBD.

Many of our engines are not bubble tight. The good Japanese engines are produced to tolerances which might get them bubble tight. The Chinese engines, well if you've visited a Chinese factory and dealt with Chinese suppliers you know the story, especially in the hobby industry. I know first hand.

Leaking valves end up burning the valves in gasoline engines. This doesn't happen in our model engines, they simply don't burn hot enough. Methanol, running rich to prevent detonation, results in a relatively cool combustion chamber and no burnt valves.

One thing to question is why is the valve leaking to start with. There has to be a cause. The Saito I mentioned earlier that would run with a valve leaking was run hard and over reved and probally run lean by the looks of the engine. The valve face had a ring or grove worn in it. I did polish it up some by putting it in a drill and very lightly using some very fine sandpaper. Must be careful here as if you take off metal on the valves sealing face it goes into the valve pocket more. The valve seat was ok but if it had been bad in newer Saitos it would have to have been replaced becaise the seat is is chromed aluminum. Larger galoline engines warp valves but I doubt that the small valves in glow engines warp easily.