Gas engine tuning for newbies....

[pe reivers]

	Quote: Originally Posted by Tired Old Man
	Pe, That's the best grade of Tygon for our applications. Saint-Gobain Performance Plastics, www.tygon.com for Tygon product specs.

It may be a bit more expensive, but no need to replace anything every season.

[ericb]

...

[JoeAirPort]

Hmmm....I should buy some "F" tygon from MC then.

I just ordered 25 feet of the 1/8" I.D.. That should hold me off for a while. Better than 3 feet for $4.50 or whatever I was paying for the Dubro.

FYI it says F-4040-A on the picture of the Dubro tygon on Tower's website. I thought I saw it on the stuff I had at home too. MC has it cheaper though.
http://www3.towerhobbies.com/cgi-bin...&I=LXD739&P=ML

[SuperCub Man]

Got this request from a friend and since I don't have a DA Engine thought this forum might provide an answer or two!! My DLE30 had this problem and I fixed it with a new Bowman ring and very careful tuning. He has put a few gallons through this plane but it lay dormant for five months while he was away. Think the carb needs re-building? I haven't played with it at the field but I'm not the best engine man here. Just helping a friend!! Rich in the LE?

1. In the air at mid throttle it sounds very blurby compared to the similar throttle position on the ground. This one isn't as much of a concern but more an annoyance and I think it may be related to #2.
2. On the ground and in the air, when going from mid or high throttle down to idle it runs at a fast idle for several seconds (it seems like forever) then slowly comes down. It seems to be worse when reducing the throttle slowly as opposed to punching the stick forward and back quickly.

I appreciate any direction you could provide.

[Al Lewis]

Bottom end is too lean.

[RTK]

As Al said, richen the bottom and to remove some of the mid throttle burble try leaning the high a little.

[ipsik]

Anyone know why 3W engines often show the diaphram vent line going to the velocity stack?

I have a 30cc plane where the engine runs lean upside down and rich right side up and I wonder if this might cure the issue.

[Al Lewis]

That tube is their answer to the metering plate mod. OS has done something similar on their GT 55.

[LouCetrangelo]

I have been reading all the posts and have not seen anything like what I am experiencing. I have several giant scale aircraft and they all run great. Here is one challenging problem that I have been working on for over a year. I would really like to get it fixed so I can fly it in next years Top Gun. Sorry for the long post.


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I have a 3W Inline twin cylinder engine (model 3W110iR2) with two Tillotson HS 25 carburetors. This engine is mounted in a 102” Comp ARF Spitfire with the cylinders inverted (plug at the bottom) and the carburetors drawing air up. My question is as follows: Is there a reason that the carburetors would go lean when the plane does a roll or goes up at the start of a loop? The engine will slow way down and usually recover. If I persist it will dead stick on me. On the ground it sounds great. Good idle, fast acceleration and a powerful top end. Ground max rpm is 6500 rpm with a 3 blade 25” x 12 prop. Level flight is rock solid. I have telemetry on-board and the max level flying rpm is 7,700.


Here are some of the things I did, all with no effect:
Diaphragm vents: Inside the cowl there is cooling air directed to cool the engine and supply fresh air to the carburetors. It is possible that the atmospheric pressure to the carburetors could see could change causing the engine to go rich or lean.

1. Tried it with open vents.
2. Ran vent hoses to the inside of the fuselage
3. Ran vent hoses that terminated on either side of the fuselage behind the cowl.
4. Operated the aircraft with the cowl removed and the engine completely exposed to the air with no vent hoses.

Fuel Tank: It is possible that the fuel supply could be interrupted due to the weighted fuel clunk inside of the tank drawing air for a short period of time.

1. First tank had a single large fuel line inside the tank with a weighted clunk it then went into a tee fitting to each carburetor.
2. I eliminated the tee fitting and added a second clunk with a separate line going to the second carburetor.
3. The tank has a separate fill line that is always plugged and an air vent that is always open. I moved the air vent hose from the rear of the aircraft to the front thinking that the length of the vent or the location was causing the problem.
4. I replaced the tank with a cylindrical tank that it recommended by the engine manufacture. I used a separate fuel line for each carburetor with a felt clunk. The felt clunk acts to eliminate air bubbles as well as being a filter.
5. Today I tested with a fuel pump mounted above the engine thinking it was going lean. The model is an APS Power Fuel 380 by Vogelsang. I made a great dead stick landing.


Needle valves: The low speed needle valves is set for good transition and acceleration from idle. The high speed needle valve is set for max rpm and then backed off by 200 rpm on the ground.

1. The effect is worse if I operate with a slightly rich setting (1/16 to 1/8 turn from optimum on the high speed needle valve).
2. The effect is much worse if I operate with a even richer setting (1/4 turn from optimum on the high speed needle valve).

I also replaced the reed valves rebuilt both carburetors with new fuel pump and float diaphragms. I replaced the diaphragm valve and set the diaphragm valve arm the same on both carburetors per factory specs.
The engine will continue to run (but slower) on one carburetor if I pinch off the fuel to the other one. What ever is happening appears to affect both carbs at the same time.
One thing that is a bit strange is that my setting for best operation (on the ground) is with the high speed needle valve set one half turn out and the low speed at one and one half turns out. The factory recommended setting for this engine is 1 turn out on each.

Many thanks

Lou Cetrangelo
Long Island, NY

[electricpaps]

Hi Lou,
I wonder if this might work :

Try opening the high end a little, say 1/8 from optimum, and leaning the low end at the same time.

I have experienced that this will allow you to lean out the low end without adversly affecting the idle and good fast transition.

I know some think that the two needles are independant (high needle does'nt affect idle, low needle does'nt affect top speed). My experience is that they are interacting. So there are many combinations of high and low which work pretty well on the ground, but not so well in the air.

I have had similar problems with a DLE 30 and once I found the right combination it was solid as a rock afterwards.

[LouCetrangelo]

Hi Thanks for the reply:

I actually have tried that before I had the fuel pump on. I opened up the high end on both carbs thinking it was going lean. As I remember it got worse in the air.

After I installed the fuel pump I expected it to be a little rich. I started it up and saw that the top end was at my max rpm already. The transition was fantastic. So I didn't play with the needle valves at all.

The thing is it only happens when I pull up or do a roll. I am not really sure if it is going lean or rich. It just slows down. I was just thinking that I have the choke on a dialed servo and could add a choke to see if it gets worse or better.

With the pump it's got to be getting a consistent fuel supply all the time. It seams to me that something related to the moment or possible the air flow is causing the problem.

As I mentioned I have the diaphragm vent line ported to just behind the cowl. If the change in attitude caused more air to be forced into the carbs then that would tend to close the flow valve due to a drop in ventui vacuum. The thing is it does it with the cowl off as well.

I am going to try and do a tune up using the steps in the first post and see what happens. It gets a little tricky with two carbs.

Lou

[Kimhoff43]

What is the ignition timing set at?

[LouCetrangelo]

Hi Kimhoff43:

I am not really sure. It was factory set and I never checked it. I could hook up a timing light and degree wheel to see. Do you think this would cause the engine to slow down when pulling up or doing a roll?

Below is a link to a video from the summer. Since then I redesigned the muffler to reduce the back pressure. I think I was also experimenting with a richer high speed needle valve settings which made it worse. You can see here that the even straight and level flight was erratic. Thank god for having a 1000 feet of sod farm at the end of our field. I dropped the gear and flaps right away and it was lucky to not hit the trees.

http://www.liskyhawks.org/video/Spit...ck_Landing.wmv

Any way I wanted you to see how well it runs on the ground.

Lou

[electricpaps]

Hi Lou,
How about the electronic ignition?
I once had engine stalls when rolling and tried to find a carburation solution.
Then the electronic ignition gave up completely a week or so later. So I realised that they can be sensitive to G forces when they are about to give up.

By the way, as gas engine carbs are fuel pumps, what made you go for an additional pump?

[LouCetrangelo]

Hi electricpaps:

That is an excellent suggestion. Let me answer the second part of your post first.

Early on I had some heat issues with the engine and found that I damaged the reed valves. I replaced them but the carburetors are mounted directly on the reed manifold with out an insulating block and must have suffered some heat damage. I rebuilt them both replacing the pump diaphragm, the float valve and its diaphragm. I noticed that the little flapper valves on the pump were a bit distorted. The next flight was good and I thought I fixed the problem. However it came back after two flights. I was thinking that since the tank is on the CG and about 18" of tubing is between the clunk and the carb that I might be right on the edge of having just enough pumping capacity to fly straight and level. When you add the extra effect of gravity I figured it might be going lean. The pump is sometimes used on the 5 cylinder Moki radial to eliminate some of the issues with the pulse pump that is driven off the cam. Anyway I was hopeful it would solve the problem. I only took one flight with it and did not make any carb adjustments but it did the same thing. It was actually much better on straight an level frying though. I did several rolls and pulled up like in a wing over and each time the engine slowed way down but would recover. I was really surprised when it died as I pulled up at the start of a loop being a little more aggressive.

With regards to the ignition early in the spring I had the same idea that there may be something going on in the ignition that is sensitive to G forces. It seamed logical that the effect I was seeing as the engine slowed down was due to one module quitting an it was running just the other cylinder. This engine being an in-line twin fires twice per revolution. There are two separate ignition modules, each with it's own crank sensor. I have them mounted on top of the engine (see attached early photo). To make a long story short Bobby from Cactus Aviation was kind enough to send me a new ignition module to try out. I mounted it in both positions with the same results. After that I have been focusing on fuel but It is still a possibility.

The latest thing that I am going to try is to tilt the fuselage vertical on the ground while it is running. Yesterday I made a frame that will bolt up to where the wing mounts and two people can tilt it up. I figure that either it the symptom will show up or it will not. If it does then I can experiment with different settings with out risking a dead stick. If it does not it means that gravity has no effect and it is something to do with G forces or air turbulence. Now I have to wait for a decent day to try it.

Lou