cappaj1

My Feedback: (1)

I just purchased a O.S. FS-120 III Surpass 4-Stroke w/Pump as it was suggested due to tank location.

When is a pump necessary, or is it just a case of 'better, but not required'?

What are the disadvantages of having a pump? I have to prime the pump before starting which makes it more difficult to start - that's one - are there others?

Is there any test prior to mounting an engine to see if I indeed actually need a pump?

I guess what I'm getting at is are pumps often used when people think they need them where they could get by without one?

psuguru

I didn't think that priming was a disadvantage, because you have to do it anyway, usually by choking the engine.
The OS carbs are pressure regulating types, giving better throttle reponse. Perry pumps "regulate" at the pump although I don't think it's quite as good a system.
Other than that there's more expense, I can't think of any disadvantages at all.

cappaj1

My Feedback: (1)

Thanks, I guess I'm happy I have the pumped version of the engine, but I already have one and also a non pumped version of the same engine and the non pumped one starts alot easier, ie. less priming, so just thought if it was just as good, why bother. But it seems it can't hurt, but only help.

jerrysu29

The pump is actually to add additional pressure in the fuel system. Some of the 4 stroke engines do not have a pressure tap in the exhaust to run a pressure line to the tank and require the use of a pump such as the OS Gemni 160 twin. Some Scale planes have to have the fuel tank installed further away or lower than recommended, and require the extra pressure to supply sufficient fuel pressure whether it be 2 or 4 cycle, even with a pressure line in the system (from muffler to the tank) I am not the best at explaining things but I hope this helps.
jerrysu29 Call Sign Wild Man

combatpigg

My Feedback: (3)

I had the pumped 1.20.......MKII [IIRC] and it ran perfectly, once you got it running. The pump occasionally needed to have fuel drawn through it with a syringe to "wake it up". It could be that it would get gummed up. It never did fail to run and I figure that OS decided it was necessary to pump it so it would sell to the pattern crowd and to the sport guys who wanted to see 10 pounds worth of plane have unlimited vertical. These engines would purr right on through the worst aerobatics you could throw at them. They sold like hotcakes around here when they first came out....and $400 was a lot of money back then.
IIRC, some guys tried to run it with medium fuel tubing instead of large and had problems.

Jetdesign

My Feedback: (8)

Most engines get their fuel from vacuum pressure formed in the cylinder during a stroke. This is effective when a fuel source is near by, however the suction is not great enough to draw fuel through fuel tubing from a distance. The further the fuel tank is from the engine, the more force is required to push fuel through the lines.

Fuel tanks in RC planes are usually placed behind the firewall for this reason. In some applications it is not practical or not desired. Twin engine planes sometimes share a central fuel tank, sometimes there is no room behind the firewall due to airframe design, or sometimes people prefer a plane with a constant center of gravity - one that does not change as fuel is used.

Pumps allow the tank to be placed a greater distance from the engine, or maybe even below the engine. A pumped engine can still be used if the tank is near by, so it is just more versatile than a non-pumped engine.

Another advantage is that pumped engines don't really lean out in the air. They can be tuned to max RPM, and maybe just a few RPM rich to be on the safe side. There is no need to tune down 300-500 RPM with a pumped engine like you would on a non-pumped engine.

I would say the only disadvantage, if you want to call it that, is having more parts. All engines need to be primed, it just may take a few extra flips on a pumped engine to get fuel throughout the whole system.

cappaj1

My Feedback: (1)

Thanks, that explains a lot to me. By the way, I have both a pumped and non pumped 120 before the pumped 120 I have on order, and both use medium fuel lines and I haven't had any problems. But I don't go vertical for long periods either

cappaj1

My Feedback: (1)

Thanks, Gar, very good explanation and it makes it much clearer to me. One thing though, I have both pumped and non pumped 120's and I can tell you from experience, it's more than a few extra flips to get the extra priming, for mine anyway. Maybe I'm doing something wrong though.

bruce88123

I have K&B .61's with and w/o Perry pumps and do get a bit more power from the pumped engines. They use carbs with slightly larger bores too so this also helps. Remember these engines are just air pumps so if you can get more air in/out you should get more power. With more air you need more fuel to keep the mixture right so you may need a pump to keep up. The K&B pumpers run great in all positions better than the non-pumped. I'm working on a pusher right now and the tank will be almost 2 feet away. I've bench run this arrangement with the pumper and it runs well in all attitudes.

Gray Beard

My Feedback: (-1)

The $400.00 dollar pumper is the new price, when the III model came out it wasn't that much more then the unpumped model, the huge jump in price was just over the last couple of years, OS was never cheap though. Your new pumper operates on a pressure pulse and draws the fuel from the tank to the pump to the regulator to the carb and off you go. Your tank must be vented or it acts like when you stick a finger over a straw and suck. Your engine shouldn't go rich or lean in the air, the fuel flow is regulated. The only disadvantage to them is when something quits working. There is a diaphragm in the regulator that will go out with age, they live a very long time though so not to worry. The pump itself can have problems and it is a part that can't be rebuilt, it needs a replacement, again, not to worry they last a very long time. My first 1.20 had a little choke attached to the carb, I still have a couple of them in my shop. It was just a wire rod attached to a flapper that you could twist and the little flapper went over the carb, one or two flips and the engine was choked, just like sticking your finger over the carb. Some people like to stick a finger over the muffler and give the engine a few flips to prime. I just hit mine with the starter and it takes a few roll overs and fires right up. I have to store some planes on there noses and the nasty castor will flow into the pump and gum it up once in a while. If you don't store them on the nose and you run your engine dry at the end of the day you will never have this problem. If you do you just need to clean it, not very hard to do, I use my heat gun and a pressure bulb full of fuel when needed.

bigtim

My Feedback: (13)

here is a couple of photos of my older pumped OS 120 spII it has a metal pump mounted on the back plate pre surpass III plastic pump version.

if you stick the starter on it for a few revolutions it will self prime pretty quick, OS recomends using a electric starter on there newer 4 strokes so priming isn't really a issue at all.

as for advantages the reason I decided to use it in this particular plane was to allow me to mount the tank on the CofG to balance it, and allow me to move the battery and reciever in the normal tank position behind the firewall.

this engine doesn't change in RPM no matter what angle its in upside down or vertical also the tank position like many have posted is never a issue.

pic #3 shows my vent tube mounted into the cowl flush with the outside, I just place a catch cup under it to pick up the overflow of fuel when filling up a big difference is the need to vent the tank with a pumper for fuel flow,for non pumped engines the fuel system needs to be closed for the muffler pressure to be affective pushing the fuel to the carb.

Gray Beard

My Feedback: (-1)

I have one of those older models too. It runs but not very well. I think someone opened up the pump at some point and it is just off some how. I have been using it as a parts engine for others, just screws and what ever I can use, it's a shame because when they run well they are very good engines. Nice to see one still in use.

psuguru

Nice plane.
I think I must have the Sullivan Wimpatron starter, because it won't turn my OS FS91-II-P over compression.

cappaj1

My Feedback: (1)

I have a cheapy Hobbico [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXL396&P=ML]starter[/link] with a couple 7.2V NiMH car [link=http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXTJH4&P=ML]packs[/link] in series and it turns over my 120 four strokes without even flinching

Jetdesign

My Feedback: (8)

I have the semi cheap-O starter (hobbico hi torque) with two 7.2v car packs also, works pretty well, although I recently had the opportunity to run it off a 4 cell Lipo, and BOY what a difference!

Gray Beard

My Feedback: (-1)

I have the cheapest high torque starter sold at Hobby People and it really zings my engines pretty well. I have one of those auto jump starters that I use for power. That thing is even powering my flight box at the same time. Worth the 40 bucks I paid for it!! I have even used it to start one of the guys cars out at teh field.