Engine Knocking, Predetonation, Help!
 

I've got this terrible sound coming from my OS 70 Surpass. This just started happening recently as it's beginning to get colder here in Minnesota. This engine ran beautifully this summer. Currently, it idles great, small puffs of smoke, nice and slow. When the throttle is advanced (it has good response by the way) is starts to make this terrible sound that I'm almost sure is predetonation. When this begins, it starts to knock, the RPMs sag, and the engine becomes louder. I've tried running it 1/2 turn leaner than usual, 1/2 turn richer than usual and this does not help. For the past month or so, the problem used to go away when the engine warmed up. Today, even when the engine was warm, the same thing happened. It also sags when in a vertical position in the air, but NOT on the ground. It's leaving a huge smoke trail, and the wing is coated in what's now a brown oil. It used to be the nice color of the WC 18% Total Synth. that I'm running. I feel like it's not too lean, but the sag in the air would seem to indicate otherwise. Does anybody know what's wrong? Thank you!!
Reid

Engine Knocking, Predetonation, Help!
 

How old is the engine? Sounds like time for a rebuild: new piston, ring, connecting rod, sleeve and bearings.

Cold weather will ofter bring out the worst in a worn engine, as the cooling and intake air are so cold the engine will run cooler, and parts inside never expand as much as they do in the summer. If internal clearances are borderline on a hot engine in the summer they will almost certainly be too loose in the winter.

A less daunting alternative may be first to check the valve train, make sure things are adjusted correctly and see if anything is sticking or broken.

Engine Knocking, Predetonation, Help!
 

I checked to see if any of the valves were stuck. They're not, they both move up and down without sticking or clicking at opposite times. How do I check this to make sure it's working properly? Should I check valve clearances or what? I'm going to take it to my friend at the field tomorrow and see if he can help too. Any more input would be much appreciated!
Thanks!
Reid

Adjusting valves
 

I'm not sure if you needed this or not? If you don't, maybe someone else does?

Adjusting the Saito, Magnum, OS, ASP, or Y-S valves can be accomplished by anyone with the right directions. The valve clearance is the same for all of them. Here is the method I have been using with Saito and the other 4 stroke engines.

Go to the auto parts store and get a blade type feeler gauge with .001, .002, .003 and .004 (one through 4 thousandth's INCH) blades. .002 is the minimum valve clearance and .004 is the maximum recommended by the manufacturers. I like to set to the minimum clearance so that the valve is open the maximum amount of time per cycle (more gas in, more exhaust out!).

The engine must be absolutely cold (not run that day - room temperature is fine).

Remove the valve cover(s). You may have to tap it (them) with the handle of a screwdriver to loosen them - pull it (them) off carefully, the gaskets (if present) can tear.

Bring the piston to the top dead center (TDC) of the compression stroke. It’s very easy to feel. If you are not sure, rotate the prop counter-clockwise until you feel compression, then remove the glow plug and look to see when the piston is at the top of the stroke. If you can't see, insert a straw or small plastic tube (only) vertically into the glow plug hole and slowly rotate the prop counter-clockwise until you feel the piston is at the top of its stroke.

Hold the prop at the exact position of TDC. It helps if you have 4 hands to do this job, but can be done with only 2. You should now be able to wiggle both rocker arms up and down a bit, as they should have some play.

Do one rocker arm at a time. Complete one, then do the other.

Using the tools, unscrew (counter-clockwise) the nut on top of the rocker arm about 1/2 turn. Leave the wrench on the nut.

Insert your Allen wrench or screwdriver in the top of the adjustment screw on top of the rocker arm - this is the shaft, which the nut is tightened onto. If you have an older engine, it will have a small straight screwdriver slot instead of an Allen wrench fitting. It helps if the wrenches are just about even with each other when loosened so you can hold them with one hand.

Open your feeler gauge and spread out blades .001, .002 and .003. Make sure no other blades are sticking to them! Fold all the others back into the handle.

Looking directly at the junction between the rocker shaft and valve stem, try to insert the .002 blade between the rocker arm and the valve stem shaft. I use the side of the blade, but some folks prefer the end of the blade. If you cannot slide it in, turn the Allen wrench or screwdriver counterclockwise a 1/4 turn, and press down on the push rod end of the shaft (the one with the nut on it) with just light finger pressure until you can slide the blade into the gap. Don't open the gap too much, or the push rod can slide out of the socket on the bottom of the rocker arm. If that happens, just re-center it and continue.

Hold the nut in place with the wrench, and turn the Allen wrench or screwdriver slowly (small amounts!) clockwise while sliding the .002 blade back and forth between the rocker arm and valve stem. When you have a very slight drag (Very Slight!) on the .002 blade, hold the Allen wrench or screwdriver in that exact position and tighten the nut down. Doesn't take a lot of pressure. Use a fingertip only at the end of the wrench to tighten the nut snugly. Remove the Allen wrench or screwdriver and nut the drive wrench. Check again with the .002 blade for a very slight drag.

Now, rotate the crankshaft two complete turns so that you are exactly at top dead center on the compression stroke again. Recheck your gap. The .001 should slide easily in and out without any pressure. The .003 should not fit, but may if you press hard, because it will cause the valve stem to open the valve slightly. The .002 should fit exactly with a very slight drag. If not correct, and repeat the procedure. A little practice here and you will have it down! Don't use much pressure fitting the blades, they are thin enough to slice your fingers!

Do the other rocker arm.

While the covers are off, use a small screwdriver to check that the shafts on which the rocker arms rock are tight - again, not much pressure here.

Oil the assembly before putting the covers back on.

Turn the engine upside down and put a drop of oil on:
the contact point between each rocker arm/valve stem,
the contact point between each rocker arm/push rod,

Tilt the engine sideways - put a drop of oil on each valve stem shaft through the spring - don't touch the spring, just let it drip through so the valve stem seal on the head will be lubed during the next step.

Turn the engine upright: Put a drop of oil on each side of the rocker arm where it rides on the rocker shaft. Finally, put 4-5 drops down each push rod tube. Put the covers back on. You're done. Do about once a year or 10 hours flying time on the engine.

Do the lubing before starting the engine anytime it has not been run for more than a month. Squirt several drops into the crankcase drain nipple and move the engine around to get the oil into the bearing at the same time. I use a 50/50 mix of Marvel Mystery oil and automatic transmission fluid for my lube oil.

I hope this has not been too basic - absolutely no insult is intended, just trying to help.

Engine Knocking, Predetonation, Help!
 

Could be the denser air charge can cause overcompression? If so try a colder plug. The K&B 4C plug is pretty cold. You didn't say what percent nitro you are using, you could lower the percentage of nitro. I don't know if you are hearing detonation or not, it doesn't sound the same as a car it doesn't knock it's more of a rattle.

Solved!
 

Thanks W8ye! Your instructions for how to change the valve clearances were great. I did just like you wrote and found them to be off by .002" (still within OS's tolerances). I don't think that's what solved my problem, but I do feel like it's running much stronger now than it was before. I think what ended up doing it was a carb flush along with 3/4 of a turn out on the needle. It smokes like crazy, runs strong, and will transition well as soon I sort out the low end. Thanks to everyone who provided help!

Reid