Gas engine tuning for newbies....

[mfuess]

	Quote: Originally Posted by Pro1Foam
	Thought I would transfer this over from tha ****** (RIP). It is a couple of articals that Xipp wrote in response to my engine tuning question that got a great response from the RC Community. Xipp engine tuning method (Salsa) Petrol (Gas) Engine Tuning *** Carbs aren't too difficult to tune up if you know what you're doing. First of all, you need to know how the carb works and how the settings interact with each other. About 95% of all the gas airplanes I've seen at the field are somewhat out of tune. How can I tell this? Simple, at some point the engines "four cycle" in flight. Two Cycle engines are not supposed to "four cycle" PERIOD. This is caused by a rich mixture that is forcing the sparkplug to intermittently miss making it sound like a four stroke. This is not good. HOWEVER the good news is; gasoline two stroke engines are very tolerant of rich settings (most of the time) and will run fine. You'll just consume a little more gasoline than necessary, and create a little more oil mess on your plane. You may eventually foul your spark plug as well. So why do so many people leave their engines tuned like this? Simple answer, the engine will start much easier when it's cold AND there's little or no warm up time needed prior to flying. Those are pretty good reasons! But the fact is... the engine is not running like it's supposed to. (1) The low end needle on a Walbro carb is ALWAYS the one closest to the engine, the high end needle is the closest one to the intake/choke. (2) There is no fuel adjustment for idle fuel, only air feed set by the idle stop or servo. (3) Both low end AND high end needles feed the top end fuel supply. Let's tune up a Walbro! Set the low end & high end needles to about 1 to 1 1/2 turns each. Choke the carb or prime it, until the carb is wet. Fire up the engine and let it warm up. Let's set the top end first since it's the easier of the two. Go to full throttle. Adjust the top end needle for peak RPM. Leave it wide open for about a minute to see if it changes any. Should the engine go lean, open the low end needle slightly, if this doesn't work... you will have to adjust the needle valve inside the carb.( I will explain this later) If the top end runs OK, then slowly pull the throttle down until the engine begins to "four cycle" hold the throttle there. Adjust the low end needle until the "four cycling" stops. Now lower the throttle more until it "four cycles" again, and adjust the low end again. Keep doing this until you reach full idle. Now, from full idle begin to throttle up until the engine starts to bog or hesitate. Open up the top end needle just enough to eliminate the bog or hesitation. When this is done right, you will be able to set the throttle in any position and it won't four cycle, plus you will be able to transition from idle to full power without any hesitation at all. -Xipp, Flying******.com Member Understanding a Walbro Carb Ok this is for people who need a little insight into how a carburetor works in this case I will be using a Walbro for example. Starting from the gas tank, the fuel is pumped from the fuel tank and enters the carb through the inlet. The fuel works its way through the fuel pump through a little diaphragm pump that's controlled by 2 one way valves (little flaps). The fuel then passes through a needle & seat that is controlled by the "float" diaphragm. This "float" diaphragm manages how much fuel is available for the idle, low-speed, and high-speed throttle positions. The "float" diaphragm opens & closes the needle through a small lever attached to the needle. The fuel is then "standing by" waiting in the float area for a vacuum signal at the various jets. The lever setting is very critical since it controls the available fuel to the jets. If the lever is too low, the engine will run lean, if the lever is too high, the engine will run very rich and will likely flood out at idle The fuel starts its journey through the pump assembly first... Then the fuel is regulated by the float diaphragm that controls the needle & seat. All of these parts reside in the float cavity area as well as the fuel ready to be fed through the jets as needed. The amount of fuel available in the cavity is regulated by the lever and its relationship to the float diaphragm. So it's critical that the lever be set properly. Within the cavity, there are distribution holes that are managed by the low end and high end needles. Plus the idle circuit, which is a fixed size. (Note) All Walbro carbs will run in any position, but they tune "best" as a side draft carb. The down draft position tends to run a little rich at idle, and the updraft tends to run a little lean at idle. No big deal though, it's easily tuned none the less. This photo depicts one of Walbro's premier carbs, having a large bore and equipped with a high speed check valve and external fuel pump pulse inlet. Now let's talk about the pulse signal for the fuel pump. Your ENGINE will determine which pulse inlet type you need! If the engine "carb base" has a hole drilled into the crankcase you will use the STANDARD pulse port and the optional (if you have it) port must be closed off. If there's no hole drilled, you will find a fitting located somewhere on your crankcase. Use a piece of fuel line to connect the crankcase fitting to the fitting on the optional pulse inlet. There's no need to block off the standard port, as it's already blocked off by the engine mounting. The carb must get a pulse signal from the engine! This signal "pushes and pulls" on the pump diaphragm which feeds the carb fuel. Now let's check out the "float needle & seat" setting. This is the single most critical setting on a Walbro carb! Walbro offers a "setting gauge" to properly set the height of the lever for your particular carb. If you don't have one, the setting will be a trial & error adjustment and a real pain in the butt since you have to open up the carb to make the adjustment. For general purposes, the lever will be almost perfectly parallel to the carb base. This will get you close. If the lever is too high, your engine will tend to run a little erratic at idle. If the lever is too low, your idle will be OK but it will tend to run lean on midrange and high end. It may also run the float cavity "dry" at full throttle and die, regardless of your high speed /low speed needle settings. The needle valve seat is pressed into the carb base, and you should not remove this without having the correct tools and setting gauges. Do not remove it! Typical PROBLEMS The engine stalls when accelerated: POSSIBLE solution: High end needle way too lean, or low end needle slightly lean Engine goes rich in flight : Low end needle too rich, float diaphragm needle lever slightly too high Engine goes lean in flight : High end needle slightly lean AND low end needle is rich, float needle lever may be set too low Engine runs good, but no idle at all : There's crap in the idle jets, the carb will have to be removed and cleaned. You may also have an air leak at the base of the carb. The throttle butterfly could be damaged or worn out carb leaks fuel when not running : float needle is bad or has crap stuck in it, or the float lever setting may be too high, or the float diaphragm is bad. My engine four cycles momentarily when I back off the throttle, then runs normal : This is perfectly normal for carbs NOT equipped with a "check valve" high speed jet. If you do have the check valve, then your float needle setting is slightly too high, or your float needle is leaking a little the fuel leaks back into the gas tank when it isn't running : Bad fuel pump membrane, or an air leak in the fuel line at the carb NOTE 1: A very common problem with cowled in engines is, the air pressure in flight changes the "natural" pressure on the float diaphragm. This causes the engine to run rich in flight. There are several possible fixes available. Most of the time you can simply tune your engine for flight by trial & error. However, the easiest fix is to open up the cowling around the carb area to lower the air pressure. You may also rotate the cover to different positions to see if that works. The "BEST" fix is to solder a piece of brass tubing where the vent is, and route the vent line to a better location. I normally route it into the fuse going through the firewall. It works perfectly every time! Plus, your ground tune doesn't change in flight! NOTE 2 Carbs equipped with the high speed check valve are greatly superior for flying aerobatics, or flying whereby the throttle will be used extensively. The check valve prevents jet dripping when you back off the throttle. That's all it does... Straight through (non-check valved) jets always drip a little fuel while the throttle is being backed off, and causes a momentary four stroking of the engine until the jet stabilizes to the new air flow rate. This is completely normal. -Xipp, Flying******.com Member

Please note, that this is my publication which was extracted and "Edited" from the Greater Southwest Areo Modelers TECH SITE. http://tech.flygsw.org/walbro_tuneup.htm

This is copyrighted material including the photographs, Copyright © 2007 M. B. Fuess

I certainly don't mind you guys using it, however I do require Permission, and please leave my NAME & COPYRIGHT intact. I would prefer you use a link to the original publication & website.

Best Regards,

Mark B. Fuess
Editor/Author
Greater Southwest Aero Modelers Tech Site
http://tech.flygsw.org

[JimC-MD]

Hi Mark,

Copyright is an important thing to note and if you are the original person that wrote the article, then credit is indeed due. I have to tell you that I am confused a little though. The original post in this thread is from 2006 and it was indicated that it was moved over to this forum from another place. This indicates that it was from even earlier than that. Your copyright is dated 2007 in the link you provided.

[Pro1Foam]

My apologies go out to you Mark. I originally copied the article from a past forum with no knowledge of a different author.

PRO1

[Dangerous Dan]

Great Thread

[mfuess]

	Quote: Originally Posted by JimC-MD
	Hi Mark, Copyright is an important thing to note and if you are the original person that wrote the article, then credit is indeed due. I have to tell you that I am confused a little though. The original post in this thread is from 2006 and it was indicated that it was moved over to this forum from another place. This indicates that it was from even earlier than that. Your copyright is dated 2007 in the link you provided.

ALL of my Tech Site Documents get updated regurlarly with new information and new illustrations. And, I copyright them for the current year they are UPDATED. This particular document was actually written in 2005, by me. I took all those Walbro photo illusrations as well. I have ALL the original documentation and illustrations dating back to its origin in 2005, as well as the UPDATED documents. I even have the original MS Word files I created this document in.

I spent 100's of personal hours creating this stuff for the benefit of all RC modelers. Especially those new to RC Modeling. I want you guys to benefit from my 37 years of RC modeling and my extensive gas engine expertise. But, I don't want anyone taking or editing my material without my permission. I don't think that's asking too much... do you?

All someone has to do is e-mail me for permission to link up to a particular Tech Page and I'll furnish the page link WITHOUT the Greater Southwest Areo Modelers advertising frames.

Here is the main GSWAM Tech Site address: http://tech.flygsw.org/

You can link up to my Tech Site and ALL the Tech pages YOU want! That's what its there for...

But please allow me due credit for my VOLUNTARY HARD WORK for you guys.

[mfuess]

	Quote: Originally Posted by Pro1Foam
	My apologies go out to you Mark. I originally copied the article from a past forum with no knowledge of a different author. PRO1

Pro1Foam,

It's really no big deal. I just want my Tech stuff to remain intact so the information being used by you guys, is accurate, high-quality, and first-hand experience.

[mfuess]

	Quote: Originally Posted by carlosigl
	Guys When you say tuning for peak RPM. Does this mean we do not back down 2 to 300 rpm from peak? Just learning how to deal with these gasser and want to be sure.. Thanks Carlos

Carlos,

Remember, this isn't a glow engine! You peak a gasser to its maximum RPM. You will also likely have to adjust the lowend too. Both lowend and highend supply fuel at full throttle. It takes a little practice to get a good balance between the two.

[mfuess]

	Quote: Originally Posted by ifixairplanes
	I recently rebuilt the walbro crab on my brison 2.4 . I replaced: pump diaphram, pump check valves, fuel screen and gasket. Also diaphram, lever and float needle. The old float needle I noticed had a soft rubber type tip, new one is all metal. Reason for rebuild was engine would go lean and die after a while of nose up running at WOT. After rebuild, I noticed much better fuel draw and no more running lean at WOT. However I did notice, that no matter how I set the low needle, it would still load up on me. It was fine in the air, never had a lag. but on the ground, during starting I noticed fuel dripping out of the carb. It would get severly flooded to the point all I could do was pinch off the fuel line and flip till she started. Then I would unclip the line and she would stay running fine. After reading the article and noting my 2 symptoms after the rebuild I took apart the diaphram area and bent the arm down a hair. Hopefully next weekend I will notice the change! Comments? Suggestions? Sean

Sean,

The loading up is caused directly by the float valve not holding fuel pressure from the pump. By your bending the lever down, it now won't supply enough fuel at continous WOT. To correct this, you really need to replace the float valve with one that doesn't leak and re-set the lever to its level position. The float valve must be able to hold 5 PSI minimum, I generally test carbs to 16 PSI just to be sure it works correctly. http://tech.flygsw.org/carb_pressure_testing.htm

[mfuess]

	Quote: Originally Posted by Kiwi
	cforcht. Try soldering a small brass tube onto the vent hole on your carby. That is the vent hole over the top of the diaphram. Remove the four screws, take off the plate and neatly solder the tube to it. When its cool blow through it to be sure you did not block the hole in the process. If it's clear refit it then run a fuel tube to a location inside the fuselage. Some place where you think the air will be static. See if that helps. Kiwi

Kiwi, here is how its done: http://tech.flygsw.org/walbro_carbvent.htm

Mfuess
Greater Southwest Aero Modelers

[mfuess]

	Quote: Originally Posted by cforcht
	OK guys I tuned my 3W70 as per instructions found here. it runs flawlessly on the ground with good transition. the idle might be a tad high but otherwise runs great. In the air however its another story. it runs fine for a little while then acts like its choking on fuel and loading up. I have tried to lean out the high side a bit to remedy it but then it runs worse. same goes for richening it. but yet it runs fine on the ground. what am I missing here. fortunatley it hasnt died in flight even though now and then it sounds like it might. but it has died on the ground after landing. any ideas whats wrong?

Do this and your in-flight tuning troubles will go away: http://tech.flygsw.org/walbro_carbvent.htm

But in order for this to work, all the holes in your firewall must be closed off.

[mfuess]

	Quote: Originally Posted by cforcht
	Well I found something quite interesting. I went to remove the carb so I could solder the line to the diaphragm cover and discovered the high needle valve is now missing. after the last flight today I didnt even look at the engine I just packed it up and went home. So.... in light of that I now wonder if the needle valve was moving around the whole time making it run erratic. sounds like a good possibility to me since its now MIA. now that I have the carb in hand. where the heck is the model number found on these things. I do not have a walbro carb. its a tillotson carb. the next question. is this a good thing or a bad thing. would a walbro do better or should I stick with the tillotson carb?

There's nothing wrong with Tillotson carbs, but finding a replacement needle could be tough. IF you decide to replace the carb, you really need to go with an HDA Walbro instead of the "normal" WT carb. Your Tillotson carb is designed for bigger engines operating under a heavy load. An HDA Walbro is almost identical in performance.

[mfuess]

	Quote: Originally Posted by israelg48
	So do you tune the carb before the very first flight? do i put it on the stand, run it and tune it, fuel it up again, and fly?

No, once your engine is suitably tuned it should be OK for a very long time.