srp55

My Feedback: (54)

Gentlemen I have a PILOT SBACH 342 25% powered by a Mintor 33cc gas engine. My problem is I get a drop in RPMs when I fly inverted or go verticle or at the top of a loop. The engine is running great besides the situation I've described. The engine has 3 gallons of fuel run through it, using STIHL 2stroke oil. I have a 12 oz. dubro tank with the standard 2 line gasset up using a TEE for fueling. I have been using STAR MONTfuel stabilizer to help eliminate the problems with the ethonal. The engine performs great under normal flight, I don't see as much of a drop in RPMS if I am above 1/2 throttle. Thanks Steve.

dirtybird

My Feedback: (5)

With a two line system, when you are inverted the vent line is closed. Well not closed but it makes the tank harder to get air.This will change your needle setting and affect your rpm if the engine is sensitive. Suggest a three line system.

kmeyers

first guess for me is inside the cowl pressure changes. try a vent line on the regulator.

MTK

Geez how's that gonna help? A 3 line system has its ventfull of gasjust the same when inverted.

ahicks

My Feedback: (2)

With an Sbach, I'm struggling to believe there's going to be a lot of difference in cowl pressure inverted vs. right side up? The attitude/angle of attack should be nearly identical either way assuming the plane is trimmed for aerobatics.

I heard you say it's running great, but wondering if you have tried going any richer on the HS screw to see how that affected what it's doing? The fact it's dropping when going vertical is what leaves me wondering about that. Those are almost always lean - but could be air bubbles due to a leak as easy as it could be a carb adj.

Some guys are saying they're checking the lever height and spring tension inside the carb too. That the weight of the diaphram is affecting their mixture? I would leave that as a last place to look working from an easiest problems first perspective.

earlwb

Well the things that could affect the engine when the plane goes inverted are:

Fuel tank location changes affecting how the carb draws fuel, normally you wouldn't think that the fuel tank location woulod be a problem with a fuel pump equipped engine, but it is still there though.

The fuel tank vent is now covered with fuel and it affects how the carb can draw fuel too. That increases the resistance to drawing fuel then.

Internal cowl air pressure can change affecting the fuel pressure regulator diaphragm. The diaphragms can be real sensitive to this. it is hard to tell on the ground with the plane in the air as to whether the cowl angle has changed when right side up or inverted.

The fuel pressure regulator diaphragm can be affected by Gee forces and or its own weight when inverted. Yeah this has popped up as a problem every now ant then.

Sometimes the fuel tank and or the fuel lines move a little at different plane attitudes and the fuel line moves enough to get partially pinched or kinked even causing a fuel flow change.  I remember someone having a problem where the engine would flame out every time he went inverted. But on the ground in a static situation, it didn't happen as he couldn't simulate the Gee forces in the air.

You can rule out some by holding the plane inverted on the ground in a static situation and see what the engine does when the plane is at different attitudes such as right side up and inverted.  Maybe move the plane and engine around briskly too.  Fortunately your plane is still small enough to let you do that unlike those guys with the really big ones.

MTK

I've had both a DLE35 and an PTE36 that did something similar early in the break in cycle. I richened both HS and LS needles and ducted more cooling air over the fins. Both moves helped these engines perform better in all attitudes.

Let me suggest that you do the same. Inadequate cooling is almost always evidenced by engine sag at tops of loops and on verticals. Slight richening of needles until more break in also helps

dirtybird

My Feedback: (5)

With a three line system one vent is at the top of the fuel tank and one at the bottom. So one vent is always open to air. That is if you dont use one of the vents as a filler and plug it after filling.
The IMAC guys run the vent and fill vent back around the tank so fuel dont run out of it in a dive.

CK1

My Feedback: (60)

srp55 , Is it possible for you to post some pictures of your cowl setup showing us your air inlets and outlets and if possible pictures of your engine setup (cowl off) . The pictures will help to determine possible cooling problems and or possible airflow /pressure concerns.

propellantchem

I notice that the ethanol gas is making fuel lines stiffen up pretty quickly (i.e. a few months) is you klunk free to move? Might be sucking air/bubbles when inverted. Is their a primer bulb on the carb? I like the idea of testing the plane inverted.

srp55

My Feedback: (54)

I got to the field to fly this evening, 5 minutes into the 3rd flight the engine started to surge so I idled down and landed. I tried full throttle and it just started surging never going over what would be about 1/4 throttle. The field was cut today, so I may have picked up some grass. I plan on taking it apart tomorrow and set it up to do some bench testing. I will take some photos with the cowl on and off. Thanks for all the advice, Steve.

DaddySam

My Feedback: (9)

You might want to check the fuel screen on the carburetor. I was having issues with my DLE 30 in a Yak when folks here made two great suggestions to me:
1. Check the screen under the carb cover. My filter screen had dirt and varnish in it (from old fuel).
2. Ditch the two line system and go to a three line system. It is a bit more hassle to set up (especially in a small tank) but with a two line system any crud in your gas can is getting pumped into the tank through the filter, and then it goes from the filter to the carb( because the dirt is on the carb side of the filter).
After I tended to these two items my engine has run great!

ahicks

My Feedback: (2)

The fact it ran fine the first 2 flights makes me believe this issue could also be a low ign. battery? When the voltage drops down below a certain point the module will try to tell you that rather than quit completely. It will idle just fine, but run progressively worse as you add throttle. Checking batt. voltage will tell you nothing if there's no load on the batt. Suggest you try a different battery altogether?

darrolair

"With a two line system, when you are inverted the vent line is closed. Well not closed but it makes the tank harder to get air.This will change your needle setting and affect your rpm if the engine is sensitive. Suggest a three line system."

Dirtybird,

I've read many of your posts and agree with most. The above statement makes no sense. I've been around 2 cycle engines for 30 years and have never heard of that. So what you are saying is, when an 2 cycle engine drops RPMs, I need to make another hole for a vent. My weedeaters and chainsaws run just fine inverted or so the fuel is against the vent in the caps. Hummm.

Darrolair

dirtybird

My Feedback: (5)

Cutting off the air supply(vent) and changing the mixture does not make sense? I think it does.
Weed eaters and chain saws are usually not set to deliver the last ounce of power so you would not notice a difference. The ventis not closed but its more difficult for the tank to get air. There has to be enough vacuum created in the tank to suck the air down the vent line.
Our engines are usually tuned to get maximum performance and more sensitive to changes.
Years and years age I flew a controlline stunt A/C. If you did not have a bottom and top vent the engine would quit when inverted. But those engines were very sensitive and there was no pump.
Note that all controlline tanks were made with essentially a three line system.
BTW when do you use a weed eater or chain saw upside down? It seems to me that would be hard to do.

darrolair

Dirtybird,
It's hogwash with these 2 cycle engines. Maybe so with Controline engines. If you aren't running a chainsaw for maximum power, you'll be wasting time and money. Chainsaws and weedeaters have a vented fuel cap. The cap will be on it's side meaning upside down for the vent. While having it dogged into a tree, a chainsaw will run just fine in that position. Or edging with a weedeater same result.
I agree a 3 line setup has it's advantages, but what you suggested doesn't.

Darrolair

dirtybird

My Feedback: (5)

He didnt say it wouldnt run. He just said it was loosing RPM. You probably would not even notice that in a chain saw or weedeater.
Chainsaws and weedeaters are designed to be reliable and easy operating. You dont get that if you want the maximum power out of an engine. Engines like Zenoah are just converted commercial engines so they probably would not be bothered. But he is using a Mintor. That is a purpose designed engine for RC. You can bet they designed it to get every ounce of power out of they can get. That will make things much more critical.

ahicks

My Feedback: (2)

On the bold, I think this is what we're really talking about, right? When considering this as part of a gravity feed system, or even a muffler pressured system, I would think the restriction would be more of a factor? The pumped system we're talking about can pull fuel straight up a good 18" or so when vertical without issue on some of my planes with the tank mounted on the CG. With that kind of suction available, I would think that pump would not struggle much pulling air through a fuel covered vent when inverted?

That's my opinion though. FWIW, -Al

srp55

My Feedback: (54)

Gentlemen, I pulled the cowl off my Sbach, started removing the engine when I noticed the carb was loose. A further inspection reveal that one of the carb bolts broke off flush with the crank case housing, I believe this may be the problem I been having when putting the SBACH in a negative attitude. No help from Mintor or the North American supplier TOPDAWG AVIATION. So I had to split the cases and use an easy out to remove the broken bolt. Craftsman 2 step easy outs work great. I will give her a try tomorrow and see if this was the problem.

ahicks

My Feedback: (2)

That was going to be my next guess! ;^)

Sounds like you found it. My fingers are crossed...
-Al

dirtybird

My Feedback: (5)

OK I give up. Go ahead and use your two line system

speedracerntrixie

My Feedback: (29)

I have used 2 line setups for over 15 years with no issues. This includes over 100 IMAC contests and thousands of practice flights.

lopflyers

My Feedback: (12)

2 line, 3 line if the clunk line is cutted too short or too long this is going to happen.

When inverted the clunk line needs to be fully submerged in fuel or is going to draw less fuel. Less RPMs or dead stick.

Although it looks like you found your problem take a look at the tank

dirtybird

My Feedback: (5)

Good for you.
Have you used a Mintor engine?
Do we even know a Mintor has a walbro carb?
I too have used a two line system with good results. I went to a three line system because I wanted to use a filter in the tank.
My comment was just a suggestion. Its just too bad all of you had to waste so many comments proving me wrong.
By the way a pump just ads pressure to an existing pressure. So if you change the existing pressure that CHANGE is transmitted to the output.
There is an old cliche "Comparing apples to oranges"
I think that applies when comparing chainsaws to RC engines.
Or comparing large expensive IMAC engines to a little Mintor.

ahicks

My Feedback: (2)

dirtybird -
Please don't let this get to you!
I for one, was just offering my thoughts on the topic, and I'm sure most others were as well? Wasn't meant to be taken personally or be antagonistic at all!

If that's the way it was taken, you have my apologies for sure.
-Al