OS 91HZ slow to clean out

[Drivelayer]

Good Afternoon All,


So I have an OS91HZ, it has a regulated carb from a OS105 on it, and it runs well but I have to force it to 'clear out'.


What is mean is when I first lift off and even if I fly around it seems somewhat weak and sluggish. However if I do something that forces the rpms up they will stay up and the engine performs well the rest of the flight. What I often do is a hard climb out for few hundred feet in IU1 then drop the heli with negative collective, forcing the engine RPM's up.


I have not seen any other engines do this. This one has always acted this way. I have tried it on Gov and throttle curves with the same results.


I am running 15% cool power, 8:1 gearing and after I cleared it out I am turning right at 2000 RPM at the head.
I have a pipe, looks like a hatori, but I bought it used and i'm not sure if it is a knock off or not


High needle is about 1 1/8 turn out, mid is about 3/4 out and low is at 11 o'clock. if I lean the low then at the end of the flight I hit TH and it takes several seconds for the engine to wind down.


I do not have a great way to do video as I often fly alone but if really necessary I will post one.


Thanks in advance for any suggestions.

[ticedoff8]

I though it was a carb issue. But, the carb part number is the same:
OS 105HZ-R parts list:
OSMG2494 29088040 O.S. Carburetor 61G 91HZ-R Speed 3D 3C

So, I would look for the typical "too rich in the mid-range" issues.
Maybe the diaphragm is leaking?
Maybe there is too much pressure in the tank?

[Drivelayer]

hmm, too rich at mid is definitely possible, I am about 3/4 out from closed.


I pulled the regulator apart and the diaphragm looks good.


how would I check tank pressure, by the end of the flight I do get a lot of pressure. in fact I've had to put a clamp (made out of a piece of scrap fuel line about 3/16th in long) on the carb nipple because it has blown the line off before

[Drivelayer]

I had several flights today. leaning the mid range needle. the transition from low to high is smoother but still the same, I almost have to force it to smooth out and once I do it makes good power and sounds great.

A better way to describe it might be forcing it to come on the pipe... kinda weird.

[banditpowdercoat]

Regulated carbs I am finding act different than the regular carbs. Mine I have found the midrange needs to be almost closed. I have actually closed the mid completely on mine didn't know it. Mine at 1300ft elevation is a little over 1/2 on mid, and 1 3/4 on high and I am rich on the mid. Huge smoke in mid, but still heats up with my bad collective in tic tocs so I keep it fat in midrange. I havent found lean yet on my engine either as I cant hear the difference and running with Telemetry temps, seems hot when in fact I bet I am still rich.

Have you got you high needle set yet? Must do high before mid. And I've heard stories that some top pilots run their mid just a couple clicks from closed

[Drivelayer]

yeah I think my high it pretty close. I don't have telemetry but when I land right after tic tocks and feel the base plate it is hot but I can hold my finger on it for several seconds

[ticedoff8]

Basically, the 3-needle carbs only use the hover needle to make slight influences in the mixture in the mod-range.
There is no set point where the hover needle takes over.

If you treat it more like a 2-needle carb, it might help

If you mid-range is too rich, and it seems like the high speed of okay, I'd set the hover needle back to the factory recommended point and start to lean out the idle.

That is how I turn my 3-needle carbs

[Drivelayer]

so do I understand the hover needle is the mid range needle, leftmost needle

[ticedoff8]

http://manuals.hobbico.com/osm/91hz-r3d-manual.pdf
Page 23

I believe the answer is yes - the "left most".

IMHO - set the "medium speed needle" to the factory recommended settings 1st.
Then adjust the high speed for a good "slightly rich" setting for flying and the idle needle for a good reliable idle

If it loads up and tried to die during the transition from idle to high speed, I would start by leaning out the idle until I knew that I could not get any leaner on the idle (leave the high speed as it was - don't burn up the motor).

Then, start messing with the mid range needle

[Drivelayer]

thanks, I appreciate the info.. one day I will get the ear for it

[Drivelayer]

So help me understand the manual please.


P27 reads, in reference to initial settings:
2. High speed needle-valve:
"Approx. two half turns open from fully closed position"


To confirm I understand that as one full turn open from closed.


I think my high is currently at 3/4 turn open from fully closed and it is running pretty well (I think)

[ticedoff8]

Of course, the point of the "factory settings" is to keep the engine from running lean at the initial startup / 1st flight.

It will run at "2 1/2 turns" (two and a half turns open) from fully closed.
It will also run a 2 "1/2 turns" (two, half turns or 1 full turn open) from fully closed.
And both will be rich enough to get you through the tuning phase without ruining the motor.

But, as you found, it seems to run pretty well at 2 "1/33 turns" open (hehehe) - or in American English, three-quarters of one turn open.

I haven't checked my YS 120SR-X lately, but 3/4 turn open on the high speed seems a little small.

Depending on the outside air temp, you may want to enrichen it when the OAT is colder and lean it when the OAT gets hotter.

[Drivelayer]

so just to close the loop on this topic after much fiddling it does appear that closing the mid range needle was the answer. as I leaned it out more and more the engine became much more responsive not only in spool up but also recovery after a load.

Thanks for your help, I certainly wouldn't have leaned the mid without your input

[banditpowdercoat]

What settings you end up with?

[Drivelayer]

not exactly sure as the heli is home and I am out of town but I believe the low is running a little rich, probably 11 o'clock, mid is between 5/8 and 3/4 turn and high is right about 1 turn out

plenty of power, great smoke, decent spool up and I can hold finger on back plate for about 8 seconds before I have to remove it

[banditpowdercoat]

Sweet. I know I am still super rich still. Just wanting to know some others settings so I know I am ballpark. I once leaned the mid to fully closed and she still didn't have any issues. But my high was so rich.

[Drivelayer]

I have the regulated carb, I've read that you can actually run the mid nearly closed as long as your not lean on the top or bottom needles

[banditpowdercoat]

Same. And that was why it got closed all te way off. I thought I was good on main. But was at 1.75 turns. So still pretty rich I guess

[Drivelayer]

i have to say once I get it dialed in well the power the 91 put out was crazy

[banditpowdercoat]

Nice. As it got pretty good power now. Hehehee