Help with strange behavior on Picco P-0 .08 engine.
 

Hi all. I'm not a *complete* noob in regards to nitro, but I haven't completely worn-out an engine yet, so I'm still learning all the various kinds of misbehavior I can expect to deal with as a nitro engine ages.

So, I have a little buggy that runs on a Picco P-0 .08 cubic-inch engine. I've only run about two quarts of fuel through it so far, but because it's such a tiny engine in such a small car, it tops-out pretty quickly, and it's fairly common for me to end up running it at redline for maybe 15-20 seconds at a time. (I have no idea if that's unusually severe usage, but I thought it might be useful info.) I'm running it on 30% nitro (the manual recommends no less than 25%), with an O'Donnell hot glowplug (which isn't showing any obvious signs of overheating), and a stock 0.1mm cylinder-head shim.

The problem I'm having is, once the engine is hot, the idle drops a little after each high-speed run, so that after 4-5 runs the idle is so low the engine stalls. If I let the engine cool down for a couple minutes before starting it again, it has no problem idling until after I do a couple more high-speed runs.

In explaining this problem on other forums, I encountered some confusion, so let me clear it up in advance: I am *not* having trouble with the engine cooling off too much and choking itself while idling -- when the engine idles at all, the idle remains stable for as long as I feel like letting it idle. And in fact, I've got the low-end mixture tuned as lean as possible without the engine losing power during initial acceleration. The problem is that the idle speed drops after each high-speed run, until it can no longer maintain the minimum necessary idle speed without me opening the throttle a bit.

My best guess is that the top of the cylinder sleeve is overheating and over-expanding, and the engine is losing its pinch when it gets really hot after a few high-speed runs. But I'm not sure this is really the problem, and I'm not sure how to fix the problem. Should I use a thicker shim on the cylinder head? Should I use a colder plug? Do I need to send the cylinder sleeve away to be re-pinched? Or is something else wrong?

Any suggestions or troubleshooting tips would be appreciated.

Anyone? Any ideas at all?

You've probably answered your own question. Running full noise for 15-20 seconds will wear the engine quickly, specially if lean.

The smaller the engine the more delicate they are. Have you attempted to pull down the engine and inspect it? And have you check the clutch when hot.

Also 30% is quite a potent mix for a small engine.

I haven't completely disassembled the engine, but I have taken the backplate off, and there doesn't seem to be any play in the moving parts, and the sleeve is still polished-smooth with no scratches.

If it were just premature wear, wouldn't it have trouble idling all the time? It seems to idle just fine when it's not screaming-hot; in fact, when it first starts, it idles so high the clutch drags a little; it settles down after running for about 30 seconds, and idles smoothly, until I put it through a few speed runs, at which point the idle drops so low it stalls.

So if heat is a factor, what can I do to mitigate heat-related misbehavior? Would reducing the compression ratio with a thicker shim help at all, or am I pretty much looking at getting the sleeve re-pinched as my only viable option at this point?

A bit of research suggests that my current combination of 30% nitro, a hot glowplug, and the stock shimming may only work properly because it's been near freezing most of the time I've been running this engine, and that what I'm seeing may be the result of overheating when the weather is warm enough to go outside without a coat. Switching to lower nitro is an option, but since this is such a small engine it benefits greatly from the improved responsiveness of higher-nitro fuel, so instead I think I'm going to try adding another 0.1mm shim to see if that helps keep the engine from getting so hot. As I understand it, that will delay the ignition timing a bit. That would cause overheating if the timing were already significantly delayed, but since the timing in my engine is probably significantly advanced by the high compression, high nitro content, and hot glowplug, it should instead move the ignition timing to a more neutral position. Am I reading the tea leaves correctly?

At freezing point you already have a lot of O2 in the volume of air. So if there's plenty of 02 in the air then you should have more fuel in the mix. But then using 30% nitro adds even more O2.

I'd drop to 16% if possible, this would then allow you to richen the fuel mixture and also allow for more methanol for cooling.

Using a colder plug will retard the timing. Adding shims may decrease the compression as well but I'm not sure about how it affects timing.

PS: The smaller the engine, the tougher it is to tune.

That sounds like the problem right there. Fatten up that bottom.

Tiny engines have very little mass yet run just as hot as a big one so when you back off after a high speed run, the lack of fuel going through the carb and crankcase can no longer cool it. All that heat in the top of the motor soaks down into the crankcase and you get the dreaded vapor lock. You are going to have to run it a bit fat on the bottom and maybe even the top for high speed runs.

Well, that's an explanation I hadn't thought of. Are you saying the fuel can boil inside the carburetor inlet, and block the flow of liquid fuel into the engine? Also, can vapor-lock last long enough to explain why the idle would be lower for a sustained period of time (~30s) after a high-speed run?

I already swapped in an extra shim last night. I was wrong about the stock shimming, it was 0.3mm instead of 0.1mm, so adding another 0.1mm didn't change the compression as much as I thought it would. So I added another 0.1mm to the stack and put it back together. It still idled afterwards (but it was 2am so I didn't rev it), and I was a little less worried that adding a shim would be a terrible mistake.

I also cleaned the carburetor with CRC Carb Cleaner, because I was wondering if some crud had gotten stuck in the spraybar again. I didn't see any visible buildup, unlike last time I cleaned it. (I had accidentally reinstalled the screen in my fuel filter backwards, and it dumped a load of tiny fibers into the carburetor...doh!) However, when I ran it this evening with the clean carburetor, it was back to being pig-rich just like after the first rebuild, and I had to lean it out just to get it to rev-up without stalling. I suppose I should ask: Is it normal to have to clean the carburetor every 20 runs or so, even if I'm using a fuel filter?

I'll keep an eye on the low-end needle when the weather gets warmer. Unfortunately I bought this vehicle going into winter, so it's been near freezing the entire time I've been running this engine, and there has been EXACTLY ONE position for the low-speed needle that didn't cause it to lean-out or choke.

I know, I just use 30% for the better torque response (the engine is seriously tiny -- the pullstarter is larger than the crankcase), and supposedly using high-nitro fuel makes the engine a bit more tolerant of temperature changes, and living in the Mid-Atlantic US where it can be 40 one day and 90 the next, temperature-tolerance is a huge advantage.

Are you sure about that? I read in a few places that higher nitro content allowed a richer mixture (and thus, more cooling from the fuel) because of the extra oxygen in the fuel, like you mentioned earlier.

I knew that using a colder plug would retard the timing, but the hot plug has been running for months and the coil is still shiny, so I'm inclined to think the plug itself isn't overheating -- even if the rest of the engine *is* overheating.

As I understand it, adding shims delays ignition timing, because it takes time for the compressed fuel/air mixture to absorb the heat from the glowplug, and if the compression ratio is lower, the compressed fuel/air mixture isn't as hot to start with, so it takes longer to absorb enough heat from the glowplug to reach ignition temperature.

Oh yeah, I know. This little 1.3cc engine is my first nitro engine, and I can tell I jumped into the deep end of the pool, but I really wanted a *small* nitro RC to start with. When I get a larger one this summer, it's going to feel like I'm driving a Cadillac by comparison.

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EDIT: I just realized I replied "I know" to everything you said. I don't mean to sound unappreciative, if nothing else you've helped confirm that what I know is actually correct.

Just thought I'd follow-up on this. The problem was sort of a partial-vapor-lock scenario. Adding shims to the cylinder head helped the engine run cooler, and that in turn reduced the temperature of the carburetor body, which keeps the fuel from flashing to vapor and choking out the intake air as soon as the fuel gets sucked out of the spraybar.

Im not entirely sure about dropping the nitro content as nitro is packed with o2 and it's the methanol which cools you down. My hypothesis is the less o2 you have the less methanol, and that's not what you want.

But then take into account the small combustion chamber and it becomes a juggling act to work it all out. Sounds like increaing the comnbustion chamber helped increase the fuel to air mix. Perhaps also look intio the clutch to help richen the LSN mixture at idle and counteract the bogging down with the clutch.

You do need a richer bottom mix to help cool the engine off power. Seems like you are getting closer to a happy medium.

well done.

h

P-0 engine, is small and HSN & LSN don't treat it like your .12, .15 or .21 engines. 30% maybe to high. That high nitro you may have to add head shims. 20% is fine. Larger the engine, adjustment call for 1/4 or 1/8 turns. P-0 1/8 or smaller. A hair turn on the LSN is much. Get idle speed to speed you are comfortable. Start turning your LSN CW,Idle goes up yu are leaning it out, Stop there. Give it throttle, Idle speed up? Turn LSN CCW al little. Idle speed should drop to normal 1/2 sec - 1 sec. Use a temp gauge and tell me the temp. your running. All needle adjustment are touchy with this small engine.

That's pretty basic tuning advice Geezatec, and I've already got the hang of it at this point. The problem I had was the compression ratio was too high. Lowering the compression ratio by adding a cylinder head shim fixed the problem. The engine runs great now.

Higher % you add shims, lower % take out shims. It doesn't only raises or lower compression, it changes the timing of engines. So with the glow plug.

Yes, I understand that. I'm using a hot glowplug to compensate for the delayed ignition timing.