Picco .28 tune issue - stalls after idling?

[TK5310]

Got a small but frustrating issue with my Picco .28, tune is spot on, great power, running real well, but if I let it idle for more than a few seconds when I take off again it loads up & stalls like it's pig rich on the low speed - but it isn't.

Does it every time without fail.

I'm thinking that perhaps I've got the idle gap set a little large, but it took me a while to get a handle on the tune for this thing so I don't want to go messing with it without being pretty sure of where I'm headed.

It runs about 220 deg, low speed is bang on - great response etc.

 

[Extreme RC Mods]

If it won't idle then the tune is not spot on, lean out the LSN until it will idle indefinitely. Below is a link to a tuning guide, it should help make tuning more easy for you.

http://www.extremercmods.com/content/tuning-popup.html

 

[TK5310]

It idles just fine! - will sit there idling indefinately.
But after idling for any more than about 15 seconds it'll load up & stall when you take off, restarts no probs but I have to clear it out with a few revs with the wheels off the ground, then it's fine again until I let it idle for a bit.

Got me stuffed, NFI why it's doing it.

I guess I'll just close up the idle gap & go from there.

 

[Extreme RC Mods]

The LSN is still too rich which is letting it load up even though it's not killing it, then when you give it throttle the extra raw fuel from the crank case comes up and snuffs the plug. Leaning it further will help stop it from loading up and then when you stab the throttle, it should launch properly. How much fuel has been run through the engine?

 

[TK5310]

That's what I thought, but leaning it off any more on the LS gets it making that 'bark' of a lean engine at idle & the temp does not drop while it's idling.
It's very responsive - does not behave like the LS is rich either, wheelstand or wheelspin on launch every time, or if I nail it while already moving it'll pull the fronts (& sometimes the rears as well LOL) & stand up on the wing.

Motor has about 1.5 gallons on it now & is loosening up nicely, still needs a little warmth from a butane torch to get it started, but not much.

I'm using fuel with 16% oil - could that be part of the problem? - at idle the air speed through the crankcase is low, oil could be building up despite it not being rich & snuffing the plug as soon as the throttle is cracked?

Or maybe it needs a hotter plug? - just using Hobao 'med warm' plugs, maybe switch to a warm/hot plug so it doesn't snuff as easily?

Plugs last a long time, I'm still on the 1st plug I put in it after initial break in.

 

[Extreme RC Mods]

It sounds as though the idle gap may be too large, at this point you could probably get it closer to .5mm and then set the LSN accordingly. I always recommend using a hot plug for break-in, it helps the engine start much more easy and it's not as sensitive to richer needle settings, I use McCoy MC#59's. 16% oil is a little high imo, I run fuel with 8%, 10% is a safe bet because the 8% is already more then enough.

I also recommend running 30% nitro content, it requires richer needle settings then 20% so you flow more oil by volume, it also starts and tunes more easily then 20% as well. You don't need to add a head shim to run 30% either.

 

[TK5310]

I'm running 25% fuel - all I can get locally.
Got an extra shim in.

Been thinking about mixing my own fuel - I deal with a lot of 1:1 racers, I can get methanol, nitromethane, suitable oils & corrosion preventatives pretty easy & a lot cheaper than buying premix.

May have to go ahead & mix up a batch with 30% nitro & 10-12% oil.

Can't do it for a few weeks tho.

Maybe for now I'll just reduce the idle gap (it's around 1mm now) & switch to a hotter plug, see how that goes.

I dunno that I'm comfortable removing the extra head shim - picco seems to think it's required for 20% & higher nitro content.

What's the downsides to leaving it in? - a bit less power? If that's the only downside I can live with it, she's not short on stomp as it is.

 

[Zdragon71]

The picco .26 max states to run 20-25% you do not need to shim the head any more but if you run 30% then you need to add a .01 mm shim to the existing shims. If your running 25% then you need to remove that extra shim you added.

 

[Extreme RC Mods]

There is no "need" to change the shims for 30% unless the fuel is really crappy. Manufacturers try to cover their butts from warranty so they list the worst case scenario conditions, and expect that even the least intelligent people can follow them. After modifying hundreds of engines and setting them all to my specs, which often includes raising the compression quite a bit from stock. I can safely say that adding a shim for 30% will cause the engine to run a little more hot with a slight loss in power. If that's the case then I would pull the shim and run 20-25% as it will likely run better.

 

[TK5310]

Ok, think I found the problem - carb had the smallest venturi available in it, it was covering up the edge of the idle gap so I couldn't see it, idle gap was actually about 1.5mm, way too big.
I machined the venturi out to the carb throat size - 9mm (it was 8mm) & set the idle gap to about .5mm.
Unchoking the carb should make even more power, especially in the top end.

Quick question about shims - there were 3 in there, but they were pretty thin, what total thickness of shims would you normally run (in thou or mm, whichever you know, I've got micrometers)?

 

[WinnersCircle]

Try a HOT plug. I think you will be amazed at how much difference it will make.

If you leave your glow ignitor attached to the plug and try to accelerate I'll bet the engine does not cut off. The hot plug acts sort of like the glow ignitor is still attached.

Lee

 

[TK5310]

That's how I'd clear it, start it up & give the throttle a few blips to clear it while the ignitor was still on.
Shouldn't need to do it now, would rather not go to a hot plug.