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Author Topic: Magnum 36 Problems  (Read 1374 times)

Offline Robin_Holden

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Magnum 36 Problems
« on: July 26, 2022, 03:26:43 AM »
Morning folks from North Yorkshire , England.

Took out a Magnum 36 the other day to install into one of my profiles.
It’s a converted ex R/C motor , a nylon Venturi , blocked boost port ,  with a single head shim.

Always kept boxed , clean and well oiled , whilst in dry storage.

When I tried to turn it over it ‘jammed’ , didn’t want to turn over.
Took off the cylinder head . Turned over perfectly.
No bearing issues.

Put the head back on , and bingo , didn’t want to turn over.

Any suggestions fellas ? Any contributions much appreciated.

Robin.
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Offline Motorman

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Re: Magnum 36 Problems
« Reply #1 on: July 26, 2022, 07:02:13 AM »
Did it jam like a bind or like it was blocked? The first thing would be to check head clearance.


Motorman 8)
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Offline Tim Wescott

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Re: Magnum 36 Problems
« Reply #2 on: July 26, 2022, 09:01:16 AM »
Was the plug installed?

How tight were the head screws?
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Offline Robin_Holden

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Re: Magnum 36 Problems
« Reply #3 on: July 26, 2022, 02:21:11 PM »
Quote from: Motorman on July 26, 2022, 07:02:13 AM
Did it jam like a bind or like it was blocked? The first thing would be to check head clearance.


Motorman 8)

It was blocked.
I’ll check the clearance as you suggest.
Much obliged ,
Robin.
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Offline Robin_Holden

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Re: Magnum 36 Problems
« Reply #4 on: July 26, 2022, 02:22:21 PM »
Quote from: Tim Wescott on July 26, 2022, 09:01:16 AM
Was the plug installed?

How tight were the head screws?

They were as normal , not over tightened.
The plug was installed.

Robin.
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Offline Mike Scholtes

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Re: Magnum 36 Problems
« Reply #5 on: July 26, 2022, 04:04:27 PM »
I have a Magnum 36 (square head) converted from RC with a delrin venturi but without blocked boost port. Its a great runner and very powerful for its displacement. I have never encountered the problem you describe.
Will it turn over with the plug removed?
How is the boost port blocked? Any chance it is interfering with piston movement?
Are you sure the head is installed front-to-rear correctly, ie not reversed?
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Online Brett Buck

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Re: Magnum 36 Problems
« Reply #6 on: July 26, 2022, 06:21:17 PM »
Quote from: Motorman on July 26, 2022, 07:02:13 AM
Did it jam like a bind or like it was blocked? The first thing would be to check head clearance.

   Agreed, and it may be that this issue is caused by another common modification - "dropping the liner" (usually to try to reduce the exhaust duration) without adding the necessary additional head shims could give it insufficient  or negative head clearance. I am not sure about the Magnum, but many engines have liner shims to set the liner height, if those were omitted (on purpose or accidentally) you would likely have a clearance issue.

    I can check my Magnum engines to see if there is anything tricky about it in this regard. Maybe tomorrow night, I have just about had it after 3 straight days of driving and a 2-hour traffic jam at the NV-CA border today, courtesy of CALTRANS.

    I also note, yet again, that *the engine runs fine as it comes* for stunt, and modifications are unnecessary, or in this case, detrimental.

      Brett
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Offline Tim Wescott

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Re: Magnum 36 Problems
« Reply #7 on: July 26, 2022, 06:49:00 PM »
Quote from: Brett Buck on July 26, 2022, 06:21:17 PM
I also note, yet again, that *the engine runs fine as it comes* for stunt, and modifications are unnecessary, or in this case, detrimental.

My Magnum 36 is bone stock except it has a venturi instead of a carb, and it runs great.  I have a few motors that have been "improved" and they run -- not so great.

Which is why seeing

Quote from: Robin_Holden on July 26, 2022, 03:26:43 AM
... blocked boost port ...

makes me worry that you may never find joy with this engine.
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Tim Wescott

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Re: Magnum 36 Problems
« Reply #8 on: July 26, 2022, 06:50:40 PM »
Try just holding the head on in the correct position and turning the prop.  You should be able to hold the engine with the loose head in one hand, and turn the prop with the other.  If you can feel the head move when the piston hits TDC, then for whatever reason, you have negative piston-head clearance and you need to fix it.
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AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Online Brett Buck

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Re: Magnum 36 Problems
« Reply #9 on: July 26, 2022, 07:39:32 PM »
Quote from: Robin_Holden on July 26, 2022, 03:26:43 AM

When I tried to turn it over it ‘jammed’ , didn’t want to turn over.
Took off the cylinder head . Turned over perfectly.

    On further consideration, I am suprised to hear that, I would have expected the piston to bind up in the liner near top dead center, and push the liner up, not just pass over TDC with no effort. This is intentional, they have a tight fit and a lot of taper and usually squeak over TDC even with a glow plug in. If it turns over freely without pushing the liner up, I wonder what the fit is like.

    Actually, are you sure that the "jammed" engine is not just binding up at TDC due to taper and being cold? Or is it actually hitting the head?

    Brett
« Last Edit: July 27, 2022, 10:39:44 AM by Brett Buck »
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Offline Motorman

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Re: Magnum 36 Problems
« Reply #10 on: July 27, 2022, 08:31:35 AM »
+1 on what Brett said. Magnum 36 came through with various piston fits. You might have gotten a real tight one.
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Offline Steve Helmick

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Re: Magnum 36 Problems
« Reply #11 on: July 27, 2022, 10:23:43 AM »
I've seen one Magnum XLS .36 that had two head gaskets from the factory. With one removed, the piston hit the head. Didn't make it a bad engine at all...maybe better than most. I suspect the counterbore for the cylinder flange was too deep, but it wasn't mine and it ran just fine with two head gaskets.

The first of multiple Magnums I've had (all are PRC produced) said to break-in with at least 20% castor oil. More recently, the instructions said  to use fuel with at least 20% oil, with no mention of castor. I think that's because you probably can't get any fuel with 20% castor oil.

Regarding cylinder "pinch", I haven't seen a Magnum with anything similar to an ABC K&B or Rossi .15, but way more pinch than any of the ABN OS engines.  D>K Steve
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