I have a new Magnum 70 4s running a 12/6 master airscrew. The problem is that is does not seem to run as fast as it should. I have approx 5-6 tanks of fuel through it. I don't have a tach but it just doesn't peak out. Runs very rich in the air. any suggestions

GOIRISH. First off GO IRISH. I hope this is a better season than last year. Now as far as the Magnum Mine was doing the same until I put a tach on it. I had the high end way to rich. Get a tach it will help you out a ton. I went from 12k to 15 k . JMHO.

12K to 15K is WAY too many RPMs for that engine. you should keep it around 10K. That is a good engine, I have one on a 40 sized P51 that runs great.

Not sure what the RPM's are as I don't have a tach. It just seems like it needs to step up another notch. Pulls the plane OK just leaves this slobber all over the plane. I am using sig 15%. Maybe I will try the wildcat 15%

I am using a 12/6 should I step down to a 11/8? I think it needs to run more lean than what it is. However only a couple 2-3 clicks and it seems to be too lean. A little adjustment seems to affect the running a lot.

You may have an air leak somewhere that is giving you fits on adjusting your carb. Is it leaking fuel from around the intake anywhere? There should be an o-ring where the carb seats into the the intake tube, and (if I remember correctly) another o ring where the intake manifold goes into the cylinder head. make sure those are there and tight. Sometimes with an air leak, one has to adjust the carb real rich to get it to run right, and then, two clicks leaner, all of a sudden it's too lean and dying.
15 percent is exactly what I run in mine. I think it has a 12X6 prop as well, that sounds right.
If I remember right, we clocked my P51 at around 90 mph or so with that engine, so it is plenty capable.

I've been using a Magnum 70 fs for a couple years not. The carb on this engine is different that the other Magnum or OS four strokes I have. The idle jet screws out to a stop vs in to a stop. Getting the idle jet out of whack really screws up the top end. When things don't seem to be adjusting right, start from the begining.

First, buy a tach. You can't adjust a four stroke without one, at least until you have flown the engine for a few gallons and are fimiliar with it's settings. That knowledge doesn't transfer to another engine either. You really need a tach to set up the engine and to keep it set.

All of the four strokes I have, have a very narrow peak. IE lean the high for peak RPM and just one click or two leaner will result in the stalled engine. Adjustments on the highspeed needle should be made 1 click at a time and a count of five at a minimum between clicks. The low speed should be adjusted in icrements of 1/16th turn, and remove the screw driver after each turn. Again wait for a count of five at a minimum before the next adjustment.

To tune this engine, start with the instuctions in the manual for the starting needle settings. Don't adjust the idle from this point until you have made the inital high speed adjustment.

From the manual inital settings, using a tach, start the engine and let it warm up for a miute or so at mid throttle. Go to full throttle and check the RPM/s Start going lean, 1 click at a time and check the RPM's . When you have reached your maximum, go back to the rich side for a 500 RPM drop. ( after a couple gallons of fuel through this engine, this rich drop can be reduced to 300 RPM)

After you have found your high speed spot, the set the throttle for the slowest speed you can and still keep the engine running. Without changing this throttle setting, start leaning out the low speed 1/16 turn at a time for maximum RPM.

Go back and adjust the high speed again.

Go back to the idle and again set for the slowes RPM that you can get and keep running and again adjust the idle for max rpm again. At this point, the engine should idle between 2000 and 2200 and be able to hold this idle for a minute or longer without loading up.

Now, do the final tweek for transistion. Run the engine up to full RPM for a cound of 10 or so and then cut it back to full idle. Let it idle for a count of 10 or so and punch full throttle. Don't go up slowly, but as fast as you can move the stick. The engine should jump to life now without heasiation. If the engine sputters before gaining RPM the idle is to rich and you need to adjust 1 16th turn lean until it jumps with heasitation. If the engine dies as soon as you punch it, the idle is to lean. Go rich by 1/16th turn until it jumps.

Once you have the engine at this point, you will seldom need to adjust the needles. A change in temp and/or humidity may require one click. A change in fuel may require one or two clicks. Generaly speaking, you shouldn't need to make adjustments.

I'm not sure about the fuel you are using. I use the Omega 15%. It has a blend of castor and synthetic oils and a total oil of 17 or 18%. Your fuel should be the same mix. The magnums recomend a castor only or castor/synthectic oil mix around the 17% to 18% range.

As far as the oil on the side of your plane, while it will be a lot less than from a 2 stroke, you will still have the oil. You can route the exhause for a minimum of oil, but I've never seen a plane that didn't have some.

A four stroke swings a bigger prop than a two stroke. I run 12x6 zinger wood or 12 1/4 x 3 3/4 Wide APC composites on my Mag and OS 52 four strokes. They will both spin either of these props near 10,000 RPM. You 70 should spin a 13x6 or 14x4W near the same RPM. I wouldn't think of using a 11" unless it was around 9 pitch. Even the 14x4 is a bit under propped in my mind, but I love the way it pulls.

Make sure you are using and O.S type F plug.

Make sure your tank and lines are good. Any sign of air in the line will cause engine problems. ANY. Even the little fine ones. There should be no evidence of air, even the little bubble that stay at the carb nipple. It is getting fed from air further back. I've found it best when chasing down this problem to pull the tank and replace all of the tubing from the clunk forward with fresh tubing. Do it once and do it right and you won't have trouble for many gallons. On the fuel tank issue, if you have an ARF supplied tank, replace it with a Dubro or Sulivan tank. I've seen to many of the ARF tanks split at the seam.

Get a tach and adjust your engine and enjoy it. It is one of the best in the fleet.

Don

Don
I really appreciate your help very comprehensive. I will pull the tank and look at that. I have other magnum 4 strokes 2x 52 and 2 x 91. All run great. I know this one will also when I find out what is causing this. There is no difference from about 1/2 throttle to WOT Engine stays the same. I wasn't sure if the valves needed to be adjusted or not

I'll check mine after while and see exactly what prop I have on it. I can't remember right off hand, I haven't flown it since last summer. I'll go out to the trailer in a minute and check on it and get back to this thread.

Thanks 2slow2matter

Hi,
I have a tach, and will bring it to the field wen. I think I have my plane ready, may fly it wensday.
see ya ther Irish.

Thanks Sir Stick. I went out to the field this morning and just run the engine to help break it in. Just me and my coffee. Are you bringing the one that you had at the field last Wed.

Ya, thats the one. we have to look it over but I think its ready.

Hay I got a question , the other plane we are putting together for Tom the one just like mine. We put the rear stabilizer on today and it cocked and out of line with the Fuse. Its quit bad, I will bring it along wen. and see what you guys think of it. Tom want to cut it loose and reglue, and I think he is right.But I want to see what you guys think first.

Irish, when I first got my70, I ran about three tanks through it for break in. Then when I was ready for the first flight, I couldn't get the engine to run well at all. At one point I was seriously thinking of doing a little hammer adjustment to it. Following the suggestions from the guys at the field, only made things worse. I ran across a couple post suggesting the method I described and what a difference. That carb seems to have a differnt mind for inital adjustment than either the 52 or 91 and I have both. Let us know how it works out.

Don

Yea you want that to be straight. Otherwise-you will find that it will be very hard to trim out without sometype of adverse yaw.

Will do Don. Thanks again

I've read where some of the Magnum 70 XL RFS got out of the barn with the wrong valve springs. Story goes they run fine initially but the engine begins to lose power, runs good but just loses power. Go to their website and do a search. The few posts I've seen indicate that new springs eliminate the issue immediately.

Clay

OK, I checked and indeed I have a 13 X 6 prop on mine. That is definitely the ticket as it swings it with authority, so fix your carb problems, and go up to a 13 X 6 prop and you will love this engine.

Or, a 12 X 8, but I would more prefer the 13 X 6 if it were me....

I agree that 13x6 is the correct load for a 70 four stroke. 12x8 will work fine too. With a 13x6 prop and 15% nitro fuel you can expect about 10k rpm. If you get about 10k your engine is running correctly.

My friend Fred is breaking in his Magnum XL .70 RFS right now, and one issue he ran into was that the spring tension on the high speed needle wasn't tight at all. Basically, when Fred would try to dial in the high end needle, the engine would rev up toward peak rpm and sound great, but as soon as Fred let go the high speed needle would start turning on it's own.

The vibration from the engine would cause the needle to start spinning on its own back toward counter-clockwise. The engine would richen up and the rpms would drop until the vibration wasn't enough to turn the needle anymore.

Your .70 RFS may seem way down on power due to this same problem.