bear9236

My Feedback: (1)

I have an OS 70 surpass that is having problems.
I am running 15% nitro 20% 80/20 oil , OS F plug ,12-8 and 13-6
props. It will idle all day with no problem but when i try to set the high speed needle it gets to about 8000rpm and still running too rich it takes about two more clicks on the needle and the rpms jump to around 12000 and the engine shakes wildly. The transition is great it just won't tune on the top end.
I thought it might be the fuel tank causing it to go lean but i replaced the tank with one i tested and the same problem occured. Props are balanced well.
Any ideas would be greatly appreciated.

Thanks, Barry

Bax

My Feedback: (11)

Barry,

Slowly leaning the high-speed needle and having the engine suddenly jump in RPM and run very lean is a very strong indication that you are getting fuel foaming.

Fuel foaming is when the engine's vibrations agitate air into the fuel that's in the tank. You won't likely see bubbles in the fuel line because they'd be very small. The engine vibrates heavily because you are likely getting valve float, which means that the intake and exhaust valves can't move quite as fast as the engine's demanding, and they neither open or close fully.

You'll have to either add more foam rubber around the tank or remove some. Too little foam provides no vibration insulation, and too much foam does the same thing because the foam's too densely packed.

To confirm the situation, you can remove the engine from the model and put it on a solid test stand that won't allow much vibration, and where the fuel system is completely isolated from the engine's vibrations. The engine should perform normally.

bear9236

My Feedback: (1)

Bill,

Thanks very much for the response.
The engine is on a test stand. I will try to hold the fuel tank tomorrow and see if that helps.
If i can isolate the tank and make sure it is working properly i can rule that out. I'm so busy watching the tach that the tank might be foaming and i don't see it.

Thanks Again, Barry

GIFLYRC

My OS 91 got cranky and while it would idle perfectly I could not advance the throttle beyond 50 - 60%. It would get rough and die

Look in the venturie with the throttle wide open and look to see if at least 50 % of the cut out in the spray bar is visible

Adjust the mixture screw so that at least !/2 of the slot is exposed, set the high needle to 3 - 4 turns out for a starting position ands go from there

This may not be the problem, but it sounded so similiar to my experience that I had to put in my 2 cents

Roger Aka. GIFLYRC

bear9236

My Feedback: (1)

I tested the engine again this time holding the tank and it was better.
Then i pulled the tank pressure line off of the muffler and reset the needles and the engine ran great. The muffler was an empty chamber is this what the OS muffler looks like inside?
Could there be too much back pressure from the muffler or is the exhauste valve not fully closing causing extra pressure on the compression stroke?

Thanks for replys, Barry

GIFLYRC

It's hard to say what you should do next other than try and clear up what may be a misconception about valve operation. you asked

"Could there be too much back pressure from the muffler or is the exhauste valve not fully closing causing extra pressure on the compression stroke?"

#1- Unless the outlet of the muffler is crushed or pinched closed I t would not be causing too much back pressure and yes it is a small empty chamber.

# 2-valves not closing would result in "less" compression, not more!

I sugesst that you start with the basics and check the mixture screw adjustments as previously mentioned and then adjust the valves using the tools provided with your engine.

If you are either unfamiliar or uncomfortable performing these adjustments, enlist the aide of somone who has done it before
The adjustments are not difficult, however I hesitate to try and explane them for ffear of not being able to tell you accurately enough to avoid adding to the problem

Roger Aka GIFLYRC

bear9236

My Feedback: (1)

Roger,

The engine does not have a problem trying to run above half throttle, it has a problem tuning the last 1000 rpm's i should be getting.
When i said extra pressure on the compression stroke, i meant extra pressure from the piston coming up to TDC pushing extra air into the exhaust through the exhaust valve not completely closed. Like an air compressor works.

Barry

GIFLYRC

Ok ,I see that you suspect a low compression problem due to valve leakage and if that is the case I guess that you may need to lap the valves and valve seats

Not knowing what rpm you are trying to get there is no real way to determine what ( if any ) single thing is affecting the performance of your engine

There are so many variables such as

Fuel/nitro content/age

Prop/pitch/diameter/type

Glow plug/age/type

Ambient tempreture/humidity etc. etc.

How about it Bill Baxter, jump back in here any time
I'm no expert here, but am trying to help

Roger Aka GIFLYRC