I have been working on several suggestions from RCU members on how to start my O.S. 91 pumped engine...It takes a lot of starting with an electric starter and full open throttle to just get the fuel from the carburator to the engine... The pump starts pump quickly but the fuel flow seems to stop at the regulator... My question this time is can I adjust the pressure regulator that is attached to the carburator..? There is a slot that a screw driver will fit and it does turn... Is this a way to adjust pressure from the pump? If so.. which way should I turn the slot to increase pressure? or should I not try to make any adjustments at the regulator? Thanks

If the fuel doesn't flow from the regulator you don't have much to loose

Before you do anything, see if fuel will pass through it when you choke the carb

If no flow by choking, something has the regulator plugged

The diaphragm could be old and cracked in the regulator

There is no adjustment in the regulator in the FS-91S II. The cover comes off and there is a diaphragm and spring. That's it.

How did you go with the suggestions? Did you try winding it over at full throttle with no glow heat for 10 seconds (opening out the high speed needle if required) as suggested? How about fuelling it through a T junction between the pump and the tank? Did you try that?

Are you sure the slot you have referred to is not the low speed mixture adjustment? There is no external way of adjusting the regulator pressure on the OS .91 pumper.

Here's the current regulator for the Surpass II

Thanks for the replies... yes I have tried everything as previously suggested... that did help a lot but I am just trying to improve the cold starting... on yes, I did not yet try the T connection on fueling.... but will do so... The pump is pumping from the start and squirting out fuel from the disconnected line to the regulator... No one mentioned that they are aware of a screwdriver slot on the side of the regulator....and that slot can be turned easily... Do I have a model that is different? I am not sure but I was thinking that my engine was a Surpass II that I purchased installed on the airplane...

Yes that is what my regulator looks like to me but mine is still attached so I am not seeing it completely... The slot on my regulator is just above the fuel in nipple and next to the main high end needle... so it sure looks like it is between the main needle and the fuel in side of the regulator... I will make a picture of it soon and post for your evaluation of what I am seeing... Thanks..

There was a previous version of the OS 91 Surpass. OS doesn't present any parts for it.

That regulator is a balanced "zero pressure" regulator. It will not let any fuel get past it unless there is a slight suction on the carburetor side. In other words, it is not a regulator of higher pressure to lower pressure but a "demand regulator". If there is no vacuum on the outlet side the regulator, there will be no fuel allowed to go to the carb.

That is your mixture control needle. If you don't have a manual you can get one here.

http://manuals.hobbico.com/osm/fs-70...i-p-manual.pdf

Thanks for the help.... I have a copy of the manual and the discussion on the pressure regulator is not clear to me but now another question...... You are saying that the screw/slot is a mixture control needle? if so, how do I need to adjust? Does it work like the low end needle on the engine because that would be confusing... It does turn but since the engine runs great and I am only trying to understand the starting procedures for the pump... would it be advisable for me to not try any adjustment on the regulator? I believe from what has been said here is that this engine and pump may be an older 91FS/pump? Thanks

It sure would be nice if this engine had a choke to work during the starting procedure....

This engine won't even choke due to the crankcase ventilation setup. It won't make any suction because air displaced in the crankcase is pushed into the intake. This means no fuel draw at starting. Follow the manual for starting. I would open the needle one turn and crank it over without the plug lit until the engine is wet. Turn it back in to the running setting and start with the starter. These engines will not hand start for the above reasons.

The mixture control needle is commonly called the idle mixture needle. There is a starting point explained in the manual if you have already changed it's setting significantly.

I want to be clear there is no adjustment on the regulator.

If this is the FS-91SII-P with the crankcase ventilation system, we are all on the same page.

If the pump and regulator are working properly, i.e., not gummed up from a previous run and allowed fuel to gel, you do not need a choke. If the pump/regulator has gelled fuel in it, the only way to go is to run it until it cleans up, or replace them. I'm for running it until it cleans up and returns to normal.

Put some prime into the cylinder by putting fuel into the muffler and then turning the engine over backwards by hand with the glow plug power disconnected, of course. Once you have fuel in the cylinder, the engine should start easily and the extra vacuum on the carb should pull the fuel through the pump/regulator assembly. It will probably run rough and require you to open up the main needle valve. Let it run until it gets warmed up thoroughly. After a while, the heat from running and the flow of fuel through the parts will wear away the accumulation and things should return to normal.

Also check your glow plug. You need an OS Type F plug in there in excellent condition.

Good luck and tell us how you fare.



Ed Cregger

Thanks to all for their input and help... I have tried the engine cold again today and it started and ran fine... I gave it about 5 seconds of wide open starting with the electric starter and then went to about 1/3 throttle and added the glow starter... after about 3 seconds of cranking it hit and ran fine... I had previously moved the idle mixture needle about 1/4 turn counter clock wise ... I only work on the starting after the engine is cold and not run for a couple of days... so today was a big step into the right starting pattern.... I can live with this... again thanks.. Richard

You're welcome.

Come back and visit even though all your problems are solved.

I don't like starting my engines with an electric starter. I have a brand new OS 91FS with pump that I plan to put in an Andrews Aeromaster bipe. What is the best way to start this engine without a starter (flipping)? I was planning on trying the tee in the fuel line as suggested earlier. Will this work?
Ralph White, Neoga, IL

Once you know the engine well, you can set up your glow four-stroke to start simply by twisting the spinner backwards against compression. It takes a little time to get the knack, but that is how many folks start their two and four-stroke engines these days.

Properly applied, an electric starter will not hurt your ball bearing engine. But it won't do your spinners any favors, that's for sure.


Ed Cregger

Yes, it should. I use this method myself with my OS 91 pumper. I don't own an electric starter.

Fill the tank with a reasonable amount of pressure with the tee junction in place and with the throttle set to WOT and you should get a bleed of fuel through the pump and regulator enough to prime the engine (especially if it's inverted and the fuel runs from the carb "down" the intake manifold and in through the port). Once primed, flip the engine over a few times at WFO, close the throttle to fast idle, apply the heat and backflip by flicking the spinner. Up she fires!

I've got an OS 91 Pump and use a Kavan starter. It has an honest 600 + flights and has been in several ships ; the latest a Kaos 60. I've replaced bearings twice and Hobby Services warranted a new pump several years ago. In cold off season Northeast temperatures , it would be murder starting it without electric... but I have started it by it by hand when the engine is relatively warm and primed. The instructions say use a starter so I have'nt been concerned about damaging it. I've been in this hobby awhile and have plenty of old scars from prop cuts when I flew Ukie combat , stunt, navy carrier and balloon bust back in the 60s and no longer desire getting wacked. The irony of this is that I have to hand start my DA 100 Carden Cap 232 ; something that always gives me reason to pause when I have it out.