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O.S. 70 surpass looses rpms at full throttle.

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Old 12-30-2012, 07:46 PM
  #1
harlanb
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Default O.S. 70 surpass looses rpms at full throttle.



Hi all.
I have a O.S. 70 surpass 4 stroke that I bought on ebay (probably not the best place to buy a reliable engine).
The engine seems to run and idle well. However, the engine rpm will peek at about 90% throttle.
When I push the throttle the extra 10% to full throttle, the engine rpms are reduced by 300 to 400 rpm.
I am using a new glow plug, and have adjusted the valves twice. i have also tuned the low and high speed settings as best as I can.
Can any one help me understand why I am loosing rpms at full throttle?



Thanks

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Old 12-30-2012, 09:06 PM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

G'day Mate,
It's probably lean, richen 1/4 turn, if it's on a plane, lift the nose vertically, & make sure the RPM's don't drop.
Then fly it, if it looses RPM in the air, throttle back & land & richen some more, until it runs without loosing RPM's.

Cheers
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Old 12-30-2012, 09:39 PM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

alan0899
thanks for the tip. I have tried to richen and lean the fuel mixture. the engine acts the same regardless if it is running lean or rich.
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Old 12-31-2012, 04:26 AM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

IMO:

To make 'like new' one OS 4 stroke is quite easy.

-Buy the 2 main bal bearing and replace them .
-Buy one piston ring and replace it .
-clean all the engine before remount it, and not lose to clean the valve conduct and valve polishing .

You'll find again all the lose power .
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Old 12-31-2012, 05:03 AM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

I've notice it too, probably valve float so I don't push it.
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Old 12-31-2012, 05:58 AM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

My guess is that the LowSpeed needle is too lean, that renders the HighSpeed needle ineffective above a certain point. Richen the LS needle about one half turn then try to peak the HS needle.
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Old 12-31-2012, 06:36 AM
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Default RE: O.S. 70 surpass looses rpms at full throttle.


Quote:
ORIGINAL: Hobbsy

My guess is that the LowSpeed needle is too lean, that renders the HighSpeed needle ineffective above a certain point. Richen the LS needle about one half turn then try to peak the HS needle.
Bingo!

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Old 12-31-2012, 01:37 PM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

Thanks for the tip rosnik,
I am not sure what you mean by this statement, can you clarify?
"-clean all the engine before remount it, and not lose to clean the valve conduct and valve polishing ."

thanks
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Old 12-31-2012, 01:39 PM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

Thanks for the tip Hobbsy,
I did adjust the low speed idle for the best idle speed, but I will try to richen the idle screw and see if that helps.

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Old 12-31-2012, 02:15 PM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

Always be willing to sacrifice a few idle rpm in favor of the best transition. Take car, Dave
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Old 12-31-2012, 04:22 PM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

The low speed needle is for engine running like the high speed needle. The low speed is more important to engine performance than the high speed needle, imo. Too rich causes problems all over the place, and too lean causes backfires. Tune where you get the best transition *just* rich enough where it doesn't wheeze or backfire as you advance the throttle.
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Old 12-31-2012, 04:44 PM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

Hobbsy
Ibow to your supreme knowledge.
Idid exactly as you said and the engine ran great. No rpm drop off at full throttle.
Iam starting to figure out that for each low speed setting, there is high speed setting that works the best for the low speed setting.
If your low speed setting is not correct, you can still adjust the high speed setting to get a seemingly optimum setting evan if the low speed setting is off.
You have to go back and forth to get the right combination.
Thank you again for the info. I hope to be knowlegable enough to help people on these forums.

Thanks again.
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Old 12-31-2012, 04:46 PM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

Thanks for the info btw. I am starting to figure this out.
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Old 12-31-2012, 07:54 PM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

An incorrect low speed will screw up attempts to set the high speed. Hobbsy always advises to peak the high speed, leave it there, and set the low speed correctly. Go back and set the high speed a few hundred rich. This really is the best method to tune a 4 stroke,
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Old 01-01-2013, 06:16 AM
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Default RE: O.S. 70 surpass looses rpms at full throttle.


Quote:
ORIGINAL: blw

An incorrect low speed will screw up attempts to set the high speed. Hobbsy always advises to peak the high speed, leave it there, and set the low speed correctly. Go back and set the high speed a few hundred rich. This really is the best method to tune a 4 stroke,
I follow his tuning method on my Enya 4-stroke as well. I use a tach as I don't quite have the 'ear' for setting the needle on a 4-stroke just yet.. Still new at the 4C scene.
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Old 01-01-2013, 06:52 AM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

1Q, my very first RC engine was an Enya .46 MKII which I still have. I know that many on here advise to not get a fourstroke first. The only mistake I made, (according to some), was using Fox fuel with 20% castor for its first two seasons. This did not have the devastating effect that some think. It only got a brown muffler. I used the same fuel in a Saito .80 and 1.50, no ill came to them either.
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Old 01-01-2013, 09:05 AM
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Default RE: O.S. 70 surpass looses rpms at full throttle.

I like gadgets and have a newer TNC tach. I use it most of the time out of curiosity for the numbers. You can tell more by listening though.
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Old 01-01-2013, 09:36 AM
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Default RE: O.S. 70 surpass looses rpms at full throttle.


Quote:
ORIGINAL: Hobbsy

1Q, my very first RC engine was an Enya .46 MKII which I still have. I know that many on here advise to not get a fourstroke first. The only mistake I made, (according to some), was using Fox fuel with 20% castor for its first two seasons. This did not have the devastating effect that some think. It only got a brown muffler. I used the same fuel in a Saito .80 and 1.50, no ill came to them either.
I've been in RC for over 15 years, but most of that is with cars only. I wanted to have a 4C just to have one and it so happened I fount the Enya. The instructions say to use castor oil or a high quality synthetic, and being of the "old school" belief, I use all castor in my aero engines. I save the synthetic oil for the car engines that rev to 40k, the thick castor hold the rpm back some.

With my Enya 60-4C, I find if I start rich and lean it until it runs smooth, it's still 1000rpm rich of peak. By smooth I mean clean four stroke, no missing or popping. The 600-700rpm I gain using the tach doesn't sound much different to me. Again, this is my first season with a 4-cycle, and being an old Enya, it's got it's own learning curve. I still find it comical seeing castor "goo" spraying from the exhaust at fast rpm. I don't know why.
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