rancor411

My Feedback: (2)

I've got a brand new OS .91 FS II w/pump that I purchased from Tower about 5 years ago. It's been sitting on the shelf waiting for me to get back around to finishing up my GP Spacewalker. Well, I finally got it wrapped up and started the engine break-in per the instruction manual yesterday and ran into a snag. I cannot get the engine to run up to max RPM. The engine runs great, I just cannot get it to max out on RPM before the needle valve is completely closed.

So basically I've got two problems in my head. First, why does the engine even run with the needle valve completely closed? Second, why can't I get the engine to max RPM before it is closed? I'm expecting the engine to go to max RPM and then start to fall back off as the needle valve is closed some more but that just never happens. It keeps increasing in RPM all the way up to the closed point. I'm running a 14x6 prop and the engine tops out right between 10,400 and 10,500 according to my tach.

I completely disassembled the carb today and cleaned it out to make sure nothing was obstructing the needle/seat and couldn't find a problem. I then reset the idle mixture screw to the factory setting of two turns out according to the instructions. Fired it up again today and had the exact same experience as yesterday.

Any advice here is much appreciated. I've never ran a four stroke before now so I'm not sure what to expect. If it was a used engine then I would just assume the needle or seat were worn out but both are brand new out of the box and look to be in great shape when disassembled.

Thanks.

jaka

Hi!
First!
What do you mean "Max rpm"!

The rpm an engine turns is determined by prop! Not what a manufacturer says in a manual. The manual rpm recommendations is just that...a recommendation! That doesn't mean the engine will turn max rpm (What that is..) with every prop. Your .91 four stroke is is just normal if it turns a 14x6 APC at around 10000rpm.

To your second problem with a high speed needle that does not close the fuel coming to the carb. I have only one clue and that is a faulty needle valve seat! That's the only thing that can prevent the needle from not closing the fuel coming into the carb.

rancor411

My Feedback: (2)

By max RPM, I mean the maximum rpm that the engine will spin this prop. I know it is based on prop size is why I included that bit of info. I'm expecting the engine to spin up to max rpm for this prop, whatever that value is, and then I expect to see the rpms start to fall off as the engine is further leaned out. My problem is that all the way to the point where I reach a fully closed needle, the engine is still increasing in rpm. I never reach a point where the rpms start back down or stay steady. It would seem then that I may not be a max rpm for this prop.

Gray Beard

My Feedback: (-1)

When you read the instructions that come with an engine they give you working RPM range. Say 2000 to 10,000. That 10K is the max that the engine should be run at before you start floating the valves, not what the engine should be turning. Most people, myself included go through prop testing to see how well the different props fly the plane and then stick with that prop as long as the RPMs are within the working RPM range. I have several of the OS .91s without the pump and they usually run in the 8K to 9K range with the low pitched 15 inch APCs. That MAX is something to avoid, not try to gain. You want to over rev then go to a smaller prop and watch the RPMs come up.
I just fought this problem you have with the needle valve on one of my OS 1.20 pumpers. First thing to try is the needle valve assembly. If that doesn't work, like mine didn't, then the problem can be the pump itself or the regulator. In my case I replaced the regulator and discovered it was a hole in the pump diaphragm. A friend made up the new diaphragm for me, used my stock black plastic pump as a testing tool to see if he could fix the pump instead of having to buy a new one. It worked and he is posting up a how to thread here on RCU this week. OS makes things to be replaced instead of repaired, we feel that is just wrong!!!!!!!!!! Your diaphragm may have developed a crack from age and storage in both the regulator and pump. There can also be something as simple as rust in the needle valve seat.
NEVER TRY TO HIT THE MAX RPM ON THESE ENGINES!!!!!!!!!

rancor411

My Feedback: (2)

To be clear, the reason I'm trying to find max rpm is because the instruction manual tells you to do so. I'm not trying to find it so that I can abuse the engine. Part of the break-in process is to run the engine to max rpm for a few seconds at a time, then immediately going back to overly rich to cool the engine back down.

At least today is Monday so I should be able to get an answer from O.S. today.

Thanks

LesUyeda

As I recall, most engines tell you to adjust the needle valve for peak rpm, them open 1/4 to 1/2 a turn, so that when they are unloaded (as in flying), they will not run too lean. I have no clue as to what is causing you not to be able to find peak. I have a very old OS 91 (between the open rocker vintage and Surpass), and am running a 14 x 6 prop on it
and it tuned as one would expect it to.

Les

rancor411

My Feedback: (2)

Turned out to be a bad carb according to O.S.; apparently the steps I was following were correct as were my assumptions of how this should work. Hopefully will have it up and running soon!

saintcreed

My Feedback: (205)

rancor411
One thing you want to watch out for is getting a 4 stroke to lean....when it gets toooo lean it will backfired and throw the pop nut, jam nut, washer and prop...then just try to find all the parts that fly off except for the prop, which is large enough to see and find...just ask me how I know....one does not want to be in front or side of the engine when this happens...also you want to keep that engine around or just above 9000 rpm's...got to remember that the engine is going to unload in the air...meaning it's going to gain 700 + rpm's..and like Gray Beard stated, to high rpm's and then your valves start to float, not good...I've owned 3 of these engines and they all ran great...for my flying needs a APC 15 x 4w filled the bill....

rancor411

My Feedback: (2)

Thanks for all the advice. The new carb runs perfect and I am not having any problems now. Still haven't given the Spacewalker that maiden flight but maybe tomorrow. I'm working on my SSE build now.