Buying a used 4 stroke. What to look for - RC Groups
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Nov 04, 2017, 11:21 PM
Discussion

Buying a used 4 stroke. What to look for


Hi everyone,
I've found a plane for sale locally that I like and it comes with a saito 72. The seller was an experienced rc'er long ago and he built and ran the plane with the engine. He said that the plane has been sitting in the closet so center 2003.

Looking at the engine it looks fine and don't see anything that screens trouble .

Problem is I have no idea what to look for in terms of buying the engine (how to check for compression etc).

If anyone can give me tips on what to look for before buying I would be grateful.

A.J.
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Nov 04, 2017, 11:28 PM
AMA 46133
Quote:
Originally Posted by p51mustanglover
Hi everyone,
I've found a plane for sale locally that I like and it comes with a saito 72. The seller was an experienced rc'er long ago and he built and ran the plane with the engine. He said that the plane has been sitting in the closet so center 2003.

Looking at the engine it looks fine and don't see anything that screens trouble .

Problem is I have no idea what to look for in terms of buying the engine (how to check for compression etc).

If anyone can give me tips on what to look for before buying I would be grateful.

A.J.
Sorry you can't tell until you run it. Compression can be checked with a compression gauge.

https://www.rcgroups.com/forums/show...ircraft-engine

But you can't check for rusty bearings and rusty crank shaft until you open up to look at the inside.

You just have to take a chance. Even the seller with the best intention can sell you a bum engine because the seller doesn't even know sometime.
Nov 05, 2017, 12:11 AM
Hope to get out of life alive
kenh3497's Avatar
When ever I buy a used engine I figure the cost of a set of bearings in the price. With a four stroke (4C) often times the valves may be stuck open so the engine will have no compression. The best cure is to disassemble, clean re-oil and put it all back together. It's also possible the ring is stuck in the ring groove and that will also affect compression.

If the price is right, go ahead and buy the plane with the above in mind. I've been in the hobby since 1975 and have only worn out one engine. Maybe that's because I trade engines all the time. haha! Still to wear out an engine is not an easy ting to do. At the most you might need a new ring in addition to the bearings.

Ken
Nov 05, 2017, 12:15 AM
Registered User
Depends how cheap it is . If it were me the very least I would ask is for the backplate to be removed . Its no biggie and its not like tearing down the whole engine . If they refuse I would keep on walking . Others will disagree but its my $ and thats what I would do . Cheers
Nov 05, 2017, 12:24 AM
A man with a plan
Balsaworkbench's Avatar
What kind of a deal is he giving you? If it's cheap enough, you don't need to worry. But if he's trying to squeeze a premium price out of you, you are entitled to see the engine running.
Nov 05, 2017, 12:51 AM
Quote:
Originally Posted by kenh3497
When ever I buy a used engine I figure the cost of a set of bearings in the price. With a four stroke (4C) often times the valves may be stuck open so the engine will have no compression. The best cure is to disassemble, clean re-oil and put it all back together. It's also possible the ring is stuck in the ring groove and that will also affect compression.

If the price is right, go ahead and buy the plane with the above in mind. I've been in the hobby since 1975 and have only worn out one engine. Maybe that's because I trade engines all the time. haha! Still to wear out an engine is not an easy ting to do. At the most you might need a new ring in addition to the bearings.

Ken
Thank you all for answering. He's asking 150 for the plane and the engine on it. He says he's got nothing to start it up. I've got nothing either because I've flown nothing but electrics.
Nov 05, 2017, 01:11 AM
A man with a plan
Balsaworkbench's Avatar
So it's a crap shoot.
Nov 05, 2017, 01:11 AM
Registered User
David (dmrcflyr2) actually made a video about this particular question. Has a lot of good points.

How to: Tips for Buying A Used 4 Stroke Engine (6 min 1 sec)
Nov 05, 2017, 07:37 AM
Quote:
Originally Posted by alphatex
David (dmrcflyr2) actually made a video about this particular question. Has a lot of good points.

https://www.youtube.com/watch?v=zWLzhoW2wSM
This is an awesome video
Nov 05, 2017, 08:17 PM
Hope to get out of life alive
kenh3497's Avatar
Quote:
Originally Posted by p51mustanglover
Thank you all for answering. He's asking 150 for the plane and the engine on it. He says he's got nothing to start it up. I've got nothing either because I've flown nothing but electrics.
So you will be purchasing a 15+ year old airplane that has been sitting for at least 15+ years. Time is not on your side of on the side of the engine or airframe. About all you can do is turn then engine over to see if it has compression. IF, it turns over that is a bonus. It's possible it may be stuck. If it turns over but no compression probably stuck valves. If stuck it's a crap shoot. I mention this because I would used these points as price reducers. Also look over the airframe for warps from setting in the closet. I'm sure "stuff" got piled against it likely warping flight surfaces.

In this area there are quite a few RC auctions. Very often complete air frames, ready to fly less RX, will sell for $100.

Ken
Nov 05, 2017, 08:21 PM
Registered User
What plane is it?
Nov 05, 2017, 08:22 PM
Registered User
I TOBOR's Avatar
Offer him 50 or 55 bucks for the engine sans the plane.
Nov 05, 2017, 11:51 PM
| Saitopunk |
Bunnyshooter223's Avatar

FA-72A & Air Leak Issue


I have had recent trouble with my current running .72A that has the old style plastic case backplate.
It does not have the extra rib on the carb mount and so is loose near the intake at the head.

Basically, the intake gasket has some play due to vibration/inferior carb mount. I am talking air leak.
I had the signs in David McIntire's video on air leaks and later realized the o-ring worked loose.
I was chasing needle settings whereas my FA-50 was easy to tune...

The newer plastic back plate has a rib near its carb mount.

I was recommended by those wiser than me to buy the FA-82 metal back plate, gasket, and metal carb screws as a proper replacement--which i prefer anyways

I could probably add spacers at the head/intake pipe/o-ring as a temp fix. I do not know the long-term results of this

It's a dang nice engine otherwise and sounds great. I think it pulls over 7 lbs. static
Last edited by Bunnyshooter223; Nov 06, 2017 at 12:05 AM. Reason: dum editimg
Nov 06, 2017, 07:37 AM
Engine man
dmrcflyr2's Avatar
Quote:
Originally Posted by SeismicCWave
Compression can be checked with a compression gauge.
Here we go with the useless compression gauge thing again.... SMH I also have a video showing how useless this tool is too.

Model Airplane Compression Gauge by Hobbico (5 min 11 sec)
Last edited by dmrcflyr2; Nov 06, 2017 at 07:43 AM.
Nov 06, 2017, 07:42 AM
Registered User
I TOBOR's Avatar
Shooter, that O-ring on the intake pipe is supposed to be Viton not a regular O-ring. The Viton stands up to head temps better.


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