piper_chuck

My Feedback: (12)

I didn't take anything out of context, I just quoted what you wrote. You might have experience with aero engines, but you've admitted that you have never broken in the engine in question, or any marine engine. You've got a fixation on detonation without even understanding the characteristics of this engine. I suggest you pause and consider that perhaps not all engines are created equal and that you're not the only one with "more experience than you could shake a stick at".

Meanwhile, the OP has posted the same question at another site, where those of us who DO RUN MARINE engines hang out, and none of them has an issue with running high nitro in this engine.

Sport_Pilot

Don't get me wrong. I didn't say never, but the only rust I have seen on bearings is from bearings that have sat out in the rain. When I have felt roughness it cleared up after running fuel through it. My engineering experiance is that most bearing rust is very small specs on the balls and race, they quickly wear off and leave pits that cause spalling, the first sign is a whine. I suppose it is possible to have that much roughness from rust, I just have never seen this in 30 years of experiance.

Sport_Pilot

I suggest you read more carefully and be more civil!

piper_chuck

My Feedback: (12)

Timing, head clearance, shaft, lots of possibilities. What matters is the engine parts are different. I can't help that you assumed otherwise and were wrong. Any further discussion does nothing to get the OP's engine running right.

Sport_Pilot

Well I have seen several engines broken in that way without any problems. The only possible problem is with very tight ABC engines, if run too rich they could cool enough that they grab at the top and possibly throw a rod. Makes no dif as far as wearing in the piston to sleeve fit. In fact they best engines will have no noticible pinch left.

Sport_Pilot

Of which I have probably worked with every combination at one time. I assumed nothing. I asked the question. Could it be from detonation from overheating? I have had overheating problems runnning a K&B .40 that usually ran fine on high nitro (35%), caaused it to detonate. Ran fine after lowering the nitro. I opened up the cowling exit, ran fine on high nitro after that. Any high nitro engine can detonate when overheated. Using low nitro just eliminates that possibility and the possiblility of frying it from running lean till you are used to the engine.

piper_chuck

My Feedback: (12)

Different engines, different operating characteristics. What you experienced with a K&B .40 aero, while meaningful, does not necessarily apply to the K&B 3.5 outboard. Since the engine is designed for the percentages the person is using, the problem is most likely not the fuel. Changing to a lower percentage isn't going to help him get to the root cause(s). In fact, it will only introduce more complications, because after he identifies the real problem(s) and wants to go back to a higer percentage, he will end up having to retrim the boat for the increased power. I'm just going by what I've learned from reading the recommendations of the manufacturer, listening to people who have been racing them for what seems like forever, along with my own experience running these engines.

piper_chuck

My Feedback: (12)

K&B 3.5s are usually extremely tight. My latest aero has 30-40 runs ranging from 30 sec to 4 minutes, and it still occasionally stalls the starter due to the tight fit at the top. My latest outboard is also tight, but not quite as bad as the aero. Both of these engines were broken in this past summer. My first outboard, which I started running in the early 80s is still quite tight even after lots of running. I personally think the key to longetivity for these engines is to break them in somewhat (but not sloppy) rich, always run them a tad rich, and if there's any sign that they're lean, immediately throttle back (for the marine engines, this richens them considerably), and bringing them to shore.

jaka

Hi!
20-25 years ago we used the K&B 3,5cc R/C ABC engine in Quarter Midget pylonracing here Sweden. We used 80/20 castor fuel, and 7x5 wood or carbon props and got around 20000-23000rpm with magic mufflers silencers.

I raced several different models then, one being the Little Toni Formula 1 racer and also my own construction of the famous "Tsunami" racer build by Sandberg which he sadly crashed in -1991(?).
If I remember correctly the K&B 3,5cc engine was very tight and squealed when turned over cold.
It ran good though on just 80/20 fuel and the throttle response was very good even when using small 7x5 props. Carb used in those days was the black alumium K&B, airbleed carb which I think is mounted on your boat version too.
I think I used the K&B 1 plug then or the Rossi 2-4 plug.
I seldom used anything but the original shim and thus seem to indicate that the engine was set up to run on non or low percentages of nitro.

It is always good to try 0-5% nitro when an engine behaves funny on larger percentages of nitro and everything else is according to plans. Too much nitro in an engine set up for non or low nitro could play havoc to an engine and make it very sensitive on needle settings.
As for changing ballbearings...I would say this is a must according to my personal racing background. Especially so if the engine in question has been run on Castor oil fuel and sitting for as long two years.
But bad ballberaings is not the reson for the engine running bad.

Regards!
Jan K
Sweden

piper_chuck

My Feedback: (12)

Sounds about right, I'm getting around 19k with a 7x8 APC and a tuned pipe.
Yup, they sure do.
Considering the vintage, it was probably a K&B 1S plug, right? It's hard to say which carb he's got, there are several choices depending on which model engine he's got.
Wouldn't the presence of a shim be indicative of a lower compression ratio, which would be more suited to higher nitro fuels?
Sure, but if the engine IS designed for higher nitro, as the outboards are, it makes much more sense to look at other causes (needle set too lean, tank too low, prop too large, engine too low in the water, clogged cooling lines, etc) first. I know you're an advocate of low (no) nitro, low oil fuels, but that's not what these engines are made to run with.
Agree, if it wasn't put away properly, it's highly likely that the bearings and flex shaft will need to be changed.