The muffler pressure is very very low at low throttle settings though, so the engine needs to draw fuel on its own, essentially, at idle.

As the piston travels up and the crank inlet opens, a low pressure is created inside the crankcase. The pressure outside the crankcase is higher (and inside the fuel tank) which then the fuel is pushed into the carb. At high throttle settings the muffler pressure provides an extra boost pushing fuel into the engine albeit not much.

Not much fuel is required at idle, so low pressure is all that is required. It works exponentially.

The rest is basically what I said.

Okay. Here is my question with a picc attached, on the side of the muffler is a hole tapped for smoke, its just an open hole. there was jb weld over it but i had to take it off. now my question is, is this causing my issues, not supplying enough back pressure? if so it makes sense. let me know

That is a problem because anything that reduces muffler pressure , reduces the available pressure to push fuel from the tank . By all means , seal that hole back up by whatever method works best for ya .

Course , that may not be the ONLY problem , but it's always best to start with the obvious first , and we all know OS did not sell the engine with a hole drilled there !

nah the hole was drilled for smoke like 10 years ago. and i fired it up it ran like a top. baby screamed. even held the plane up and shook it and it didnt die, but when i went back to idle it died sometimes.
I think this is my only issue honestly

sounds like the idle mix or midrange needle valve is too far open allowing fuel to run back into the tank. Or there could be an air leak in the idle mix system ( bad o rings) this carb has at least three orings on idle needle/ spray bar mechanism. The orings could be worn or cracked from drying out.

Jerry

An engine thats been sitting around will normaly have some issues with the old caster oil left over from the last run, simply clean out the carb and flush engine with new fuel reassemble carb with new o rings replace glow plug, plug that hole, start engine with the needle valve out 3 turns and tune from there, the fuel is the best thing to clean out old caster , after all thats what put it there in the first place..........

the more i run it the better it gets, so once i plug that hole ima just maiden this thing.

I still don't see any real problem, it just sounds like it is set too lean, have you even touched the needles?

If the needles were not needed, they would simply have omitted them from the beginning...

yes ive touched the needles.

Anyone wanna play a guess that engine game? also got mine running perfect

In lieu of acting not to smart is the 61 ringed or abc? If ringed could be a stuck ring in the ring groove. Just a thought.

Just an FYI, if you did not know, the 2 clunk system is known as Uniflow venting. Common with control line models. You want one clunk attached to your carb line, and the other clunk connected toy your muffler pressure line. Not your fuel dot. This helps maintain a consistent needle setting throughout the tank level. Typical problem is to go lean as fuel level drops. This eliminates this issue.

A clunk on the exhaust pressure line? Would that not blow a lot of bubbles in the tank, a condition to avoid at all cost? The reason I put a clunk on the fill line (fuel dot) is so that I can de-fuel the tank through the fill line at the end of a flying session.

Look up "Uniflow" venting. A sI am sure nobody will take my word for it, and I really do not feel like trying to explain it. It works. Bottom line.

The tank will still have pressure whether the pressure line has a clunk or not, so why would this set be any different than a pressure line with no clunk? I can't see how it will stop the engine going lean as the level drops? With a clunk it will aerate/foam the fuel?

Just trying to understand how it works.

https://www.google.com/search?q=unif...g&ved=0CB0QsAQ

If you run your drain line (fuel dot) to the bottom of the tank, it will do it's job. It does not need the complexity of a clunk added, to properly drain the tank.

It is imperative to have the vent line, or entire tank for that matter, sealed. No leaks.

Most R/Cers do not know about this type of venting, but it is superior to the standard method. It is standard in CL stunt flying.

Anybody ever fly a CL plane? The engines go lean at the end of the tank, if a standard vent is used. Any successful CL model is either using a Uniflow tank, or a pressure system.

So it's a pressurised fuel system, which explains why the engine doesn't go lean as the fuel level drops.

No more pressure than a regular tank. It just puts the pressure where it is needed. At the clunk/fuel pickup. In many CL models, the tanks just use the ram air into the Uniflow vent. Many of these engines did not use mufflers. Ram air, or muffler pressure will work.

Ahh, I think I understand now. Hence the need for the vent/pressure line to have a clunk. I am thinking the pressure and engine lines would need to be pretty close to the same length as well for it to work the best. And with a clunk on both lines, it keeps them close to each other.

Some have Soldered a tube to the clunk and put both lines directly on the clunk. It seems that just getting the pressure closer to the clunk is all that it takes. I usually just run a long length of brass tube to the back bottom of the tank as the Uniflow line.

Yep, no worries. I've learnt something. Thanks.

Flew it, really awesome plane, engine did ery well too thanks guys