OS Max FS-70 Surpass help needed
04-18-2004, 02:11 AM
#1
Senior Member
Thread Starter
My Feedback: (8)
Join Date: Sep 2003
Location: Washington,
IL
Posts: 449
Likes: 0
Received 0 Likes
on
0 Posts
OS Max FS-70 Surpass help needed
I have an FS 70 four stroke that fires up and idles perfectly and runs great until up to about 2/3 throttle. At that point it stumbles off and on. At times it will run strong for a short period then stumble again.
I have installed a new glow plug and adjusted the valves per the instruction manual but the same problem continues. I tried running the engine off a different fuel tank to rule out any fuel delivery problem - the engine still ran the same way on the second tank setup so I am reasonably certain that the problem has something to do with the engine.
I suspect a possible air leak someplace but I have taken off the carb and the gasket and o-ring at the manifold joints look to be in good shape. Could the o-ring be possibly allowing air by at only higher RPMs? No matter how I adjust the needle valve, the same unreliable top end operation exists.
I started disassembling the engine at one point wanting to inspect the piston/ring/liner to make sure all looks OK but I couldn't get the head off for some reason. I got the screws out without problem but the pushrod tubes seemed to be holding the head on. Is there a trick to getting the tubes to release? I didn't want to mess anything up so I quit and reassembled the engine. I didn't find any rust in the engine so it appears to be in pretty good shape. The poor top end has me baffled.
I have considered ordering new gaskets and o-rings for the engine but the one at the carb looks to be in good shape so I'm not sure if that is a good approach or not.
I bought the engine already installed on a plane that had been built by another flyer who had passed away so I don't know much of the engines history before I bought the plane. I am at a bit of a loss on where to go from here. Any suggestions on what I should try next?
Thanks a bunch!
Bryan
I have installed a new glow plug and adjusted the valves per the instruction manual but the same problem continues. I tried running the engine off a different fuel tank to rule out any fuel delivery problem - the engine still ran the same way on the second tank setup so I am reasonably certain that the problem has something to do with the engine.
I suspect a possible air leak someplace but I have taken off the carb and the gasket and o-ring at the manifold joints look to be in good shape. Could the o-ring be possibly allowing air by at only higher RPMs? No matter how I adjust the needle valve, the same unreliable top end operation exists.
I started disassembling the engine at one point wanting to inspect the piston/ring/liner to make sure all looks OK but I couldn't get the head off for some reason. I got the screws out without problem but the pushrod tubes seemed to be holding the head on. Is there a trick to getting the tubes to release? I didn't want to mess anything up so I quit and reassembled the engine. I didn't find any rust in the engine so it appears to be in pretty good shape. The poor top end has me baffled.
I have considered ordering new gaskets and o-rings for the engine but the one at the carb looks to be in good shape so I'm not sure if that is a good approach or not.
I bought the engine already installed on a plane that had been built by another flyer who had passed away so I don't know much of the engines history before I bought the plane. I am at a bit of a loss on where to go from here. Any suggestions on what I should try next?
Thanks a bunch!
Bryan
04-19-2004, 11:57 AM
#2
My Feedback: (11)
RE: OS Max FS-70 Surpass help needed
One of the head bolts is accessed only when you remove the rocker arm assembly. The head just comes right off, then.
Your engine sounds like the idle mixture's much too lean. This will cause problems because it can interfere with high-speed fuel flow. Richen the idle mixture a LOT and then try running the engine. You'll likely have to start the engine at part throttle and keep the battery on the plug.
Get the engine to full throttle first, get the top-end set, and then retard the throttle to work on the idle.
People talk about air leaks all the time, but we've found them to be, in fact, very rare. Only very old engines would have theo-rings so deteriorated that they leak.
Your engine sounds like the idle mixture's much too lean. This will cause problems because it can interfere with high-speed fuel flow. Richen the idle mixture a LOT and then try running the engine. You'll likely have to start the engine at part throttle and keep the battery on the plug.
Get the engine to full throttle first, get the top-end set, and then retard the throttle to work on the idle.
People talk about air leaks all the time, but we've found them to be, in fact, very rare. Only very old engines would have theo-rings so deteriorated that they leak.
04-19-2004, 12:35 PM
#3
Senior Member
Thread Starter
My Feedback: (8)
Join Date: Sep 2003
Location: Washington,
IL
Posts: 449
Likes: 0
Received 0 Likes
on
0 Posts
RE: OS Max FS-70 Surpass help needed
Thanks Bill,
If I am understanding your suggestion correctly, the low end needle is probably not set right which is throwing off the setting on the high speed needle. Is this correct? I realize you will be just speculating without seeing the engine run, but is it maybe the fact that the low end needle is maybe too rich which causes the high speed needle to tend to be too lean? I have noticed the engine has a lot of fuel coming out of the crankcase breather nipple - even at idle. Maybe a sign that the low end is too rich... The throttle response does seem pretty good, but I have more experience with two-strokes so maybe my inexperience is playing a part here. Usually I look for the engine to load up on acceleration if it is set too rich on the low end.
When disassembling the engine I expected to find the o-rings to be in poor condition with the way the engine has been acting but found they look quite qood which definitely didn't support my air leak theory at all. I generally try not to touch the low end needle on my engines until I am comfortable that they are broken in but in this case I don't know the history of the engine so maybe the previous owner got things fouled up. When it runs well it really pulls nice and strong so if I can just get it to be reliable I can tell it will be a really nice engine.
Thanks for the info on the extra bolt. I hadn't removed the rocker arm bolt because it was really tight and I didn't want to mess up my only allen wrench trying to force it. All of the bolts were pretty tight but this one was more so than the others so I gave up on it.
I'll give your advice a try. Hopefully I can find the magic settings where everything works in harmony without too much trouble.
Bryan
If I am understanding your suggestion correctly, the low end needle is probably not set right which is throwing off the setting on the high speed needle. Is this correct? I realize you will be just speculating without seeing the engine run, but is it maybe the fact that the low end needle is maybe too rich which causes the high speed needle to tend to be too lean? I have noticed the engine has a lot of fuel coming out of the crankcase breather nipple - even at idle. Maybe a sign that the low end is too rich... The throttle response does seem pretty good, but I have more experience with two-strokes so maybe my inexperience is playing a part here. Usually I look for the engine to load up on acceleration if it is set too rich on the low end.
When disassembling the engine I expected to find the o-rings to be in poor condition with the way the engine has been acting but found they look quite qood which definitely didn't support my air leak theory at all. I generally try not to touch the low end needle on my engines until I am comfortable that they are broken in but in this case I don't know the history of the engine so maybe the previous owner got things fouled up. When it runs well it really pulls nice and strong so if I can just get it to be reliable I can tell it will be a really nice engine.
Thanks for the info on the extra bolt. I hadn't removed the rocker arm bolt because it was really tight and I didn't want to mess up my only allen wrench trying to force it. All of the bolts were pretty tight but this one was more so than the others so I gave up on it.
I'll give your advice a try. Hopefully I can find the magic settings where everything works in harmony without too much trouble.
Bryan
