Hi
Ive been running a new os 70 surpass in a cub I ran this engine ritch for a whole galon and it has been very reliable however sence i started a second galon out of the same case of fuel ive noticed if its just a touch lean i get a preignition type knocking sound at high rpm i back the needle out about 1/4 turn from peak as a rule with no troubles its been running a 14X6 prop and rarly gets ussed at full throttle i try and give it one or 2 blasts when dooing a llop just to clear it out and het it broken in right cant baby it forever. yesterday i decided to switch to the wooden prop as my take offs and landings have greatly improved and i no longer feer nosing it over. However when i did this shange over i also notices that the knocking noise was more previlent and happened a lot easyer as i tried to lean this thing out im guessing its just the way a 4 stroke runs or is it that the compretion is improving? when it was fresh out of the box i didnt notice it nearly as much prope size i feer may be to big it performs great on a 13X8 but it land at idle it wants to keep on going i broke the 13X8 and the 14X6 master air screw black plastick slowed it down for landings and i was thrilled also the 14X6 wood has caused it to come in a little hotter tempted to get a drag shute or crab it in for 1/2 mile before getting over the field. so prop size and engine noise durring lean opperation are my troubles tempted to try a 15X4 thank you all again for your time and safe flying

You're problem probably stems from the fact that you are propping that engine way too big!

a 14x6 is WAY too big for a 70. It's more like a 90 size prop.

Here are the recommendations for the 70 surpass: 11x8-9, 12x7-8, 12.5x6

Personally, I like to use a Graupner 12 x 7 on an OS 70

I thought so myself but the manual lists a 14X7 for scale plane and 13X8 the prop you mention is listed for a stunt plane i will buy a smaller prop today. its cheep enough for a go round as im buying a few for a OS 46 ax that i will put in a hangar 9 cap232-40 im gona get a list of them from 10X6 to 12.25X3.75 see what happens with that thing Rut Roh

I would try a 12 x 7 first. That's my "Go To" prop for that engine.

i will definetly give that one a go thanx a lot kinda thought the book was pushing it

It sounds like you know not to run the 4c lean, Maybe you need to
back off a little more than 1/4 after peaking it out?? Don't over
prop it.. I'm new to 4c engines, But I notice I get pinging when I
run too lean... Use Minn's prop suggestions.. But I would also try
a 12X4 0r 5....

Def sounds like an over prop issue I agree with all the above. If diameter in prop size is what you are looking for try a 13x4 my brother uses that on his Funtana with a Surpass 70 runs nice on 30% heli.

And in case the bluemagic has not learned this yet, the lower pitch will help slow down the speed of the plane when the engine is at idle.

ya the 4 pitch sounds like the ticket should realy slow down for a nice landing the cub needs to float a little more for good 3 pointers

The advice u got on going to the 12/7 prop is correct.The 70 surpass runs great on a 12/8 Master airscrew Glass prop. I've been running 4'rs for over 10 years now,and learned a very costly lesson that no one seamed to mention in the post's. Because a 4 stroke has a dead stroke between the compression stroke.you should NEVER run a wooden prop on them. The wooden props dont have the nessary weight and mass to carry the motor through the dead stroke.Yes i know,ur engines are running ok it seams,but without the load of the heavy prop,they are free wheeling and this causes the compression stroke to DETONATE when it comes up because it's not having to work to get the fuel load past the valves thus it pre-ignites causing back fires and basically a lean acting run. One other bit of advice BLUEMAGIC,since u have a gallon of fuel through the 70,it's probly a good time to check the valve settings on it. Set them with piston at TDC in the center of the compression stroke with motor cold. Set them between .004 and .01 thousants clearance between the valve spring and rocker arm.

good luck flying