OS FS Surpass II 91 Pump engine

[ahdofu]

Hello,
I was wondering whether some of the experienced folks can offer some help on this question. I've had the subject 4-stroke glow engine for a while. It normally turned a 14x6 prop at 9500 rpm on a 15/18 fuel. Last year, however, max rpm dropped to 8700. In checking the motor I noticed that the pump was leaking some so I replaced the pump. That did not change anything. I subsequently replaced the bearings and in doing so cleaned the engine block. Some buildup but nothing bad. Max rpm is now 8900. In replacing the bearings, I only rinsed the carb with fresh fuel and did not disassemble it. My question is whether it makes sense to disassemble & clean the carb or not and whether folks have seen a performance restoration by doing so. I am leery of doing this as there are tiny screws/springs on this carb but will do it if warranted. I suspect there is also a chance that the piston ring and/or the sleeve are worn and short of replacing those, I cannot tell whether they are.
Thanks,
Matt

 

[Konrad]

Is that with the same tach. If you can get the carb to run lean and rich then there is nothing to be found in rebuilding the carb as far as peak performance. ( I assume the carb barrel (throat) is opening fully)

How are the valves seating?

 

[ahdofu]

Hi Konrad,
I am not surprised to be hearing from you. You're one of the most prolific and helpful people that I've come across.

Yes same tach. Regarding the carb peak RPM does not seem to be affected much by the needle position. In the past I had the needle open one full turn plus a click or two. Now peak RPM is at 1.25 turns and it drops by perhaps 300 RPM when I open it two full turns. Regarding the valves, they're fully seated but I did not take them out. I suppose I can remove them and do a cleaning there too but like the carb, it is not something that I'd do, unless needed.

Matt

 

[Konrad]

Thank You. Many have another word for it.

Back to your engine. If both valves are not leaking by the bubble test (fuel on the back side of the valve as the piston is coming up on compression) then leave them alone.

My concern is the lack of much if any RPM change as the needle is pulled out (2 turns). This might indicate a restriction elsewhere in the fuel system.

You say peak RPM is now at 1.25 turns. Peak was one turn before the loss of power, isn't too much of a change (I place little importance on turn count, but if the turn count has changed that is an indication that something has changed). But that it won't go real rich is a concern.

If you found some lost RPM with new bearings then I'd have some concern with the rod journals (200rpm to me is often accountable with day to day atmospheric conditions). Where you able to measure their clearances?

Its been decades since I last ran any OS. I don't recall if there is a flow adjustment with the pump.

 

[ahdofu]

I just removed the rocker arms and added after run oil to the top and flipped the prop. No bubbles. That was a good suggestion.

I've ran the engine on a test stand so if there is a fuel restriction, that has to be in the carb or the regulator and not in the fuel tank or the filters. Short of disassembling the carb I don't know if there is something that is restricting the flow. There is nothing adjustable on the pump and nor on the pressure regulator.

Not sure what you mean by clearance measurements. Both bearings are of ball type. Examining the old ones, the rear one is a bit noisy but otherwise is OK. The front one is a bit sticky.

Matt

 

[Konrad]

Sorry not the valve steam (this is what I think you just tested if looking at the area where the rocker arms are located. But the back side of a poppet valve is thought to be the area opposite the combustion chamber. You are looking to see if the circumference of the valve head is sealing against the valve seat.

Connecting rod journals (one end towards the piston the other on the crankshaft pin).

Sorry the drawing has a a lot more info than you need, but the red should point you to the area to look for in the inlet and exhaust ports after flooding the areas with fuel.

All the best,
Konrad

 

[ahdofu]

Aah ... now I see what you're asking. With the rockers out (i.e both valves seated) I added fuel to both the intake and exhaust manifolds and now as I flip the prop I do see bubbles in the fuel.
Matt

 

[Konrad]

You got it. That's just a test.

If you can't get the engine to richen up when turning out the needles I think that's the issue. I'd look at the pump (bypass it on the test stand), clean the sintered clunk, make sure the low speed mixture isn't in so much as to block the high speed mixture orifice. Basically even a worn out engine should be able to run rich and lean. So solve that problem first.

All the best,
Konrad

 

[ahdofu]

The pump feeds the pressure regulator and the regulator is an integral part of the carb. If I were to bypass the pump, I'd still have to use the pressure regulator and in that case the engine may not run. I have attached a pic of the carb. The black thingy in the pic is the pressure regulator. There is a diaphragm inside which is supported by two springs. The diaphragm looks OK as well as the springs but I have no way of knowing whether the unit is OK or not.

 

[Konrad]

Low pressure fuel (no pump) should force the regulator full open. Yep, there are a lot of sub-systems that could cause performance issue besides a worn out engine.

 

[ahdofu]

I will give that a try. What about the lack of pressure in the fuel tank? There is no nipple on the exhaust that would pressurize the tank.

 

[Konrad]

The pump and tank pressure is to deal with adverse attitude in the air. In steady state of the test bench one should be able to make just about any engine run as long as fuel head pressure is kept constant steady (fuel tank level with the spray bar).

 

[ahdofu]

I will give this a try once it stops raining and will report back

 

[ahdofu]

Had a break in the weather and ran the engine on a stand with the pump connected. At 1+ turns peak RPM is 9000. At two turns, full throttle RPM drops down to 6800. I cannot explain why I was getting less of a variation before. In between I had removed the head to check on the valves, removed and re-installed the rockers, reset the valve lashes and rinsed the carb again.
Matt

 

[Konrad]

So is she now running OK? Or are you still looking for the 500rpm (9500 vs 9000)? How many hours are on the engine?

Not sure why you had to remove the head to run the test, but that doesn't matter.

 

[ahdofu]

Although the motor is running OK, I am still looking for the 500 RPM. I have not kept records on the hrs used. I bought the motor new in 2004. I don't visit the RC field that often so if I were to assume 5 visits/year x 4 flights/visit x 15min/flight x 16 years, I get 80 hrs.

I had removed the head before I asked the question. At the time I was pondering whether I should be removing the valves or not and I ended up not doing so after our correspondence.

 

[Konrad]

How fresh is the fuel. If stored for a year in a plastic* container water can enter and more to the point nitromethane can pass
*Plastic actually is semipermeable.

Are you using the same prop? Props vary between units, balance and stuff. Also different brands offer different loads to the engine for the same diameter and pitch.

 

[Konrad]

Ok, just to level set expectation. Few model engine actually have that many hours on them. So good job! Also with ground time I think the run time is actually a lot more.

I think we can eliminate the fuel delivery system as the cause of the power loss. Valves appear to be seating so that's not the issue. That leaves load (prop), ignition timing (Glow Plug?) energy (fuel) and then engine condition (RIngs and rod bearings).

If you only go flying 5 times a year I suspect the fuel is old and lost a lot of its nitromethane.

Glow plugs do deteriorate, but you aren't likely to notice a sudden power loss. (I assume you are using the OS "F" plug or Thunderbolt four cycle plug).

And then a change of props can have a huge effect. (Not so much with molded props like the APC but the great wooden one did have some variability).

And last but also least likely is engine condition. Worn rod journals will in effect lower the compression ratio, along with causing piston stability issue. That then cause the ring to have sealing issues.

From here, behind my key board, I think your loss of power is most likely in the fuel.

All the best,
Konrad

 

[ahdofu]

I use Powermaster 15/18 fuel which comes in a sealed can. I opened this can this past Fri. However this fuel had been sitting on a shelf for two years so it could be that the ingredients have deteriorated some. Unfortunately due to the pandemic, glow fuel is still not available locally. Otherwise I would have tried some. New shipments have been ordered by my local hobby stores but it may take a while before they're here.

I use an OS F type plug and every season I switch to a new one.

I am still using the same MA 14x6 prop but will give it another try with new 14x6 props from MA & APC.

Thanks for all the comments and suggestions. My regards Konrad.

Matt

 

[Konrad]

If the prop is the same let's not add another variable. I like the metal can of Powermaster but 2 years might be an issue. (don't throw it out until you have proven it is a problem).

Is this an exercise in tach racing, or have you noticed a drop in the performance of your aircraft?