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OS 75AX rebuild- question for Bax

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Old 08-17-2012, 07:10 AM
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ameyam
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Default OS 75AX rebuild- question for Bax

Hi,

I am restoring a 75ax that has given me a lot of trouble (it would overheat in normal running, lose compression and quit any time). Having eliminated all other possibilities, I am already almost committed to replacing the piston liner. Thing is one of the two screws that hold the carb down has slipped- or rather, the threading in the carb is damaged and the screw doesnt stop rotating. I showed this in a machine shop to have it filled back are redrilled. Its 3/16", the helicoil is difficult to get, so he is suggesting to redrill for 1/4" thread (one size larger) and put in corresponding screw. My question is, will this work? Point is, the piston liner is $109 and replacement carb is $70 (and not available till October), it better then to get a new engine

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Old 08-17-2012, 10:01 PM
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Default RE: OS 75AX rebuild- question for Bax

You can get parts for carbs the same as for any engine. Here's the 61A carb parts list:
http://www.gpdealera.com/cgi-bin/wga...pgm?I=OSMG2424

From your description it looks like you only need the Carb body which is p/n OSMG3409 (27482100)

The carb body is less than $40 at TH http://www3.towerhobbies.com/cgi-bin...?&I=LXSCL3&P=7 though it says "In Stock - Limited Quantities". I think that's still pretty pricey for that piece of kit, but it's a lot less than $109.

The myth about dropping by your local machine shop to have a small job done must have started somewhere in the 40's I think. Nowadays it's impossible for a machine shop to do anything like that any more. It usually costs $50 and up just for you to walk through the door.

I hope that this helps, Let us know how you make out.

Bob
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Old 08-17-2012, 11:15 PM
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Default RE: OS 75AX rebuild- question for Bax

Hi Bob,

you didnt read the post properly. The compression problems (probably resulting from misguidance in my first year of flying when I was sill learning) will need me to change the piston and liner. Thats $109.

In addition, threads for the carb retaining screw are damaged. I had fitted this in with some sealent around the screw but now want a proper solution. In the US, you have your garage tools to work on any thing that needs to be worked on, so getting something done from a workshop is expensive. But in India, I can still get it done fairly cheaply if the materials are available and its not too difficult a job.

If I get a replacement carb body, thats $40

Wen I looked up the 75ax carb yesterday, it listed as available late Sept. But they seem to have stock now. Its not that cheap really- on the $150 stuff (carb + piston-liner), I have to pay $10 shipping and then $50 customs duty. And even after all that, I may get it in a month, if at all. After all this, I still have to hope the engine will run properly with these. So, if it gets to $150, I should be considering getting a engine which is $209 anyway

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Old 08-18-2012, 04:17 AM
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Default RE: OS 75AX rebuild- question for Bax

Ameyam,

I stand quite corrected. It was very early in the morning when I answered your email.

Bob
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Old 08-19-2012, 09:34 AM
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Default RE: OS 75AX rebuild- question for Bax

There are two retaining clips that hold the piston pin in place inside the piston. Can anyone tell me how to get them off? can I reuse them when I get new piston liner or do I need to order those as well?

http://www3.towerhobbies.com/cgi-bin...?&I=LXCS91&P=Z

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Old 08-20-2012, 05:56 AM
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Default RE: OS 75AX rebuild- question for Bax

We use small long-nose pliers to remove the spring clips that retain the wrist pin. If you don't ruin them, you can reuse them. Put the piston/rod assembly inside a large plastic bag when you do this. That way, if the spring clips "jump" away, they'll be kept close by the bag.
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Old 08-20-2012, 07:29 AM
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Default RE: OS 75AX rebuild- question for Bax

thanks bax. that's what i wanted to know
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Old 08-20-2012, 07:48 AM
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Default RE: OS 75AX rebuild- question for Bax

bax,

small question- as I described,I nd to get a replacement carb as well. if i just get the carb body, how do I transfer the LSN to the new carb? So far, the thing only screws out to a point & stops

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Old 08-21-2012, 06:18 AM
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Default RE: OS 75AX rebuild- question for Bax

The low-speed adjustment doesn't come "out". You must disassemble the carb and then turn it so that it goes into the carb and falls out of its hole inside the carburetor body.
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Old 08-21-2012, 07:24 AM
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Default RE: OS 75AX rebuild- question for Bax

Ok, I already have the carb rotor out and the HSN out. So now I have only the spray bar and LSN in the carb. Once you get the LSN out the way you described, how do you put it back in?

What else is there to be removed and reinstalled in the new carb body?

Basically, is there a write-up or manual or video to do this?

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Old 08-23-2012, 06:08 AM
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Default RE: OS 75AX rebuild- question for Bax

When you remove the idle mixture adjustment, it goes into the carburetor. To replace it, you just carefully put it back in. You will need a very thin screwdriver to engage the slot and then turn it counter-clockwise to get it in.

Your engine will have an exploded parts diagram in the included manual. If you don't have a manual, it can be downloaded from here:

http://www.osengines.com/parts/index.html

There is no writeup or manual for working on O.S. engines. There are books on general engine topics, though. Here's a popular one:

HIGZ2042 BOK012 ALL ABOUT ENGINES US$23.95

This is available through any hobby retailer who sells Harry Higley products.
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Old 08-23-2012, 10:48 AM
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Default RE: OS 75AX rebuild- question for Bax

Actually, the part has already gone out of stock at TH. The problem is in the tappings for the carb retaining screws- one side hole threads have slipped. The new carb wont be available till Mid September. So, while I wait till the carb becomes available, I am going to have the one side that has been damaged re-tapped for a slightly larger screw- 1/4 or 2mm, whichever tap I manage to obtain. Since the function is just to hold in the carb in the casting, this arrangement may serve the purpose. Else, I have to order a new carb anyway

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Old 08-23-2012, 06:05 PM
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Default RE: OS 75AX rebuild- question for Bax

Bax,

if you (Hobby Services) have some 75ax carbs and piston-liner in stock, is there some way you could send me one along with the TH order I am about to place?

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Old 08-27-2012, 06:16 AM
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Default RE: OS 75AX rebuild- question for Bax

We don't stock the parts here, but draw from the Great Planes central warehouse. If the part is on backorder at Tower, it's because it's on backorder with OS at Great Planes. We would not have the part here, either.
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Old 10-14-2012, 09:32 AM
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Default RE: OS 75AX rebuild- question for Bax

Bax,

I redrilled the carb for 4mm screws and it turned out OK. After thorough cleanup of the engine, I reassembled with the old piston-liner and ran the engine at idle only today. Since I had moved the LSN last time, I had to reset it. Downloading the latest manual from the Tower Site, I note that the factory setting is now for the LSN to be flush with the carb body. I distinctly remember that earlier it used to be 1.5 turns out from fully closed at closed carb. There is a vast difference between the two settings and further, the carb body where the LSN sits itself is recessed, so knowing what exactly is the flushed position is difficult. I see that the manual has been re-written to include the 95ax.

Anyway, I did try to set the LSN so it was flush with the carb body. A pinch test revealed that the engine gained rpm as soon as the tubing was pinched so I guess I am close to the ideal setting. Thats about as much as I can do at home, remaining setup, final tinkering is at field.

Just to cross-check my LSN setting on the 75ax, if I hold the carb barrel closed and turn in the LSN as far in as it can go, how many turns out from that position is the factory setting?

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Old 10-15-2012, 06:23 AM
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Default RE: OS 75AX rebuild- question for Bax

The idle mixture adjustment is called the "mixture control valve" in the O.S. instructions. For the .75 AX, you close the throttle until the venturi is just closed. Then turn the mixture control valve clockwise, gently, until it stops. Then turn it 1.5 full revolutions in the counter-clockwise direction. That's the initial starting position. You then adjust it as needed once you have the engine running and are working to set the idle RPM and mixture.
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Old 10-15-2012, 06:35 AM
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Default RE: OS 75AX rebuild- question for Bax

Thats the point Bax, thats how I used to set it earlier. But the new manual says it differently. This new position is several turns out even beyond 1.5 turns. Then, there is also the issue, that the carb body is not flat as in the pic but recessed in a bit. So exactly what am I supposed to do?

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