magnum 70 4 stroke, no compression
 

I have a Mag. 70 4 stroke that was serviced by Hobby People. It has no compression what so ever. Checked the valves and they are set at .005. What ever they did to this engine they wanted to charge me 143.00 Any suggestions.

RE: magnum 70 4 stroke, no compression
 

Most 4 stroks feeel like no compression and if valves have some clearence just go and fire it up and probley OK - mine feel like worn out also and run great and used a lot

RE: magnum 70 4 stroke, no compression
 

Put the tach on it and it was turning a 12/5 at about 9200. Seems like a lot of vibration at slower speeds

RE: magnum 70 4 stroke, no compression
 

magnums dont have a whole lot of compression, not like os and saitos

RE: magnum 70 4 stroke, no compression
 

Boy,,, I have 2 91's that has tons of compression. They really do run great. Start and fly, no fooling around with them

RE: magnum 70 4 stroke, no compression
 

Yep. Those rascally valves opening sure do mess up the compression. :D D'oh! You still should have some tightness from the sleeve/piston, but even that is slight in ringed or broken-in 4 strokes.

RE: magnum 70 4 stroke, no compression
 

In my experience Magnum 4 strokes have weak valve springs and poor valve seats. It might get better with some running and if it runs OK for you then I would ignore it. However if you can not get it to idle under 2500 rpm you may have an issue that needs addressed.
Ihave shimmed the factory valve springs and found better idle on an otherwise horrible engine but ultimately the valve seats are a weak spot.

RE: magnum 70 4 stroke, no compression
 

Gene, often the ring rotates and lands in a spot where all of a suden the engine will feel like it has no compression. I'm not sure where this magic spot is?? Maybe in line with the exhaust valve?? This is an ask Bax question.I use pretty much all four strokes, YS and OS and I have seen/felt this a number of times on my own engines. As soon as I fire up the engine and it runs for a while it goes back to normal. I have had engine gurus tell me about this little quirk but I still don't have a full understanding. Run it and check it again.

RE: magnum 70 4 stroke, no compression
 

SPRINGS- just replace with OS springs when the ones get weak

RE: magnum 70 4 stroke, no compression
 

Gene

I'v run about 3 tanks full land it still has no compression. If you remember this is the one that was sent back to hobby people because I had lost a valve spring. They were suppose to do a lot of things to it and wanted to charge me a 143.00. It was a real mess with getting the engine back. I guess I should have let them keep it.

RE: magnum 70 4 stroke, no compression
 

If it lost a valve spring it probably has a bent valve.

RE: magnum 70 4 stroke, no compression
 

I recall the problem, I just thought you have been running it with good results. If after three runs then I would guess it's in the top end. All mine require is to be fired upand all is well again. They could have bent a valve when they were putting it back together?? You know I'm not a Magnum person so I know nothing much about what will fit and won't fit. If OS springs fit then by all means I would replace them. Try removing the rocker cover and hold down the lifters with your thumb a bit then bring it up on the compression stroke and see if you get compression. I lost complete compression on one of my OS engines, none at all!! Seems that dumb ass that is married to my wife forgot to tighten down one of the rockers. At least he didn't bend the push rod when it slid under the rocker!!!:eek:
Gene

RE: magnum 70 4 stroke, no compression
 

A Magnum XL .70 RFS should be able to turn a 13x6 APC or MA K-Series prop around 10,000 rpms.

My first course of action if I owned a .70 RFS that wasn't up to par on power would be to send it in for service. Since you've tried this already and you haven't been happy with the result, I'd recommend that you shelve your engine or you sell it for a "spare parts" engine on an auction site. You will still get a fair price on E-Bay for it if you describe the engine as weak on power output. Folks will assume you don't know how to tune it and they will take the chance that they will be able to do better. You can cash it out with a clean conscience as long as you describe the power output problem.

If you don't want to roll the dice on another Magnum XL .70 RFS, you can try the Thunder Tiger F-75s for a similar price:

https://www.thundertiger4u.com/store...idProduct=1144

You will probably learn a lot if you take the time to order different replacement parts and continue to tear down and rebuild your .70 RFS. Michigan flying seasons can't be all that long, however, and dumping a bad engine instead of chasing down the problem could cost you a lot of flying time and extra expense.

Good luck and good flying!

RE: magnum 70 4 stroke, no compression
 

The ring could have broken when they were assembling it?? It's just something simple but without being able to look the engine over it's hard to say. There isn't a lot that will remove the compression like you are describing.
Gene

RE: magnum 70 4 stroke, no compression
 

REPETE,REPETE-Just fly the darn thing as said before I have the 70 Magnum and it feels like no compression when hann flipped dry or wet and I use a starter and it takes up my 3 lb skydiver on a Sr Kadet -And no whimpee bad motor will do that

Yoy can go crazy and spend a lot on the motor or you can just fly and forget all the ,try this ,try thatstuff and nothing gets done , -GO have fun and quit messin around with it OK

RE: magnum 70 4 stroke, no compression
 

OK, OK, I will do that. Thanks for the advice.

RE: magnum 70 4 stroke, no compression
 

I have a Magnum .91, .70, and .52 all bought used and they all have good compression. The .91 runs better than the .70. IMHO at $143, it was not worth fixing. When the .70 are on sale they run for about $150, sometimes less.

RE: magnum 70 4 stroke, no compression
 

I did not end up paying the 143.00. told them they could keep the engine. We settled for about 35.00

RE: magnum 70 4 stroke, no compression
 

Gorish, how many hours on the engine before the HP service? What type of fuel?
Depending on the type of fuel and number of flight hours you could have heavy carbon buildup on the exhaust valve and port. Believe it or not the carbon can and will actually accummulate on the valve and port seats (not just around them) and rob compression. A good cleaning can restore some of the compression. Just a thought.
Cheers.

RE: magnum 70 4 stroke, no compression
 

No more that 2 or 3 tank fulls. 15% full synthetic. Was never mounted in a plane. Only run on the bench. When I purchased it from Hobbypeople new. It would only turn about 7000 13/6. Talked with hobbypeople and they said that some of the 70's got out with weak springs. They would send me new springs. I told them I would not know how to change them. They then said send it to us and we will replace them. Did that. Got it back and it would turn about 9200. Put it in a baggie with some oil and it stayed that for about a year. When I took it out it did not have any compression. Took the valve cover off and one of the valves with stuck. I pushed down on it to free it up and the valve keepers came off. I tried to put it back on but ended up losing the keepers. called Hobbypeople and told them what had happen. The gent on the phone told me to send it back in to them and they would put the keeper on. It was a little over a year old. Was told that it would be covered under warranty. Well, when they got it fixed they told me it would be 143.00 because it was out of warranty. They said that Mag 4 strokes only had a one year warranty. However, the box that it came in clearly stated two year warranty. Said they had no record of that engine. Even though they replaced the weak springs. So that is my story and I'm sticking to it. One of the people on RCU has offered to take a look at it and see if he can do something with it. BTW, the new magnums with the blue valve covers to state a one year warranty. Here is a picture of the box showing the 2 year warranty

RE: magnum 70 4 stroke, no compression
 

Thanks for the clarification. For some reason I thought the engine had hours on it Prior to HP tinkering with it.
$143 for a couple keepers and 10 minutes worth of work? What else did HP tech support supposedly do?

RE: magnum 70 4 stroke, no compression
 

I think they said that they put new valves in it, valve springs, a ring and not sure what else. Ask them who gave them permission to do all that work. They said that when the tech gets in they do everything. Here is a picture of the engine.

RE: magnum 70 4 stroke, no compression
 

goirish, did they ever tell you what they allegedly did that could possibly cost $143.00? I'm not sure about Magnum engines, but Saito and OS engine valve keepers can be had for under $20.00 for the full set and labor to put them on is just minutes. Did they tell you what else they did to bring the cost to $143.00?

RE: magnum 70 4 stroke, no compression
 

Understand that I did not pay the 143.00. Told them to keep the engine. They called me back a couple of days later and said that it would be (I'm guessing about 37.00 for the parts) That is what I paid + shipping. I have several magnums and love every one of them. Start and flyno fooling around with them. This deal has left a bad taste in my mouth. I think I am looking at Thunder Tiger 4 strokes.

RE: magnum 70 4 stroke, no compression
 

I don't know why this bothers me, but it does. Does anyone know what their labor rate is? Goirish says that the parts were approxomately $37.00, which leaves $106.00 for labor. Let's suppose they charge 53.00/hr for labor, that is two hours. There ain't much you can't do to one of these engines in two hours. Must be something they did that goirish doesn't know about or they were attempting to get deep in his knickers. That is what I am concerned about. This hobby is expensive enough without a major player being greedy.