Gas engine tuning for newbies....

[CRUISER]

Mfuess,
I have read most of the pages here and have seen "No Where" any referance to the setting of pop off ! I have recently found issue with an eng i purchased used. It ran faily well but required the needles to be twisted out too far, and the high end was off the spring and i nearly lost it. I have a homemade pop gauge setup so i checked it using syringe and load of fuel for wet pop reading. it was over 18.5 lbs. upon looking at the spring it looked like it had been stretched & clipped till it wasn't much of a spring anymore. I went to the saw shop and they kindly gave me a freebee spring to check. I knew it was too short but it had plenty to work with. I put it in and tested it as is with a reading of 3.5 lbs. I then proceeded to carefully increase the length/tension of spring and testing each time till i got to 11 lbs at which point i stopped. I haven't been able to retune yet as i also had an ignition error and needed to purchase new hall sensor. I've done many serches to find some rule to pop off pressure and only found two bits of info of which one stated he liked 8 lbs and another liked 11/12 lbs pop. neither stated what eng or use but i assumed gocarting. Now i must assume also the the regulation of the fuel is imperitive to the eng's performance and longevity. I would then also have to assume that the pop off is the first step to setting any other adjustments especially the fulcrum lever at needle, barring of course any obvious other problems like air leaks. I read your article and I know it is highly accurate and fully useable but i feel also it might be incomplete, or we are missing another section of it somewhere.
I for one would very much like some clarefication on the popoff and some helpfull guidelines/info so that i might have a fighting chance of getting to that point of having the whole story ! What settings would be best concidered ?

[Mithrandir]

I think this has been linked to already.. Mark is the Original author.....

http://tech.flygsw.org/walbro_tuneup.htm

[CRUISER]

Mithrandir,
Thanks for the link, I read it a while back and if you scan it as i did again there is no reference that i could find to the "POP OFF setting" I've been searching for solid info on this as i believe that if the pop off is out of range then the other settings will be bogus. I have talked to anybody that will listen and one person from ch ign stated he'd seen em as high as 30 lbs which would be quite lean in my book. Mine, as stated earlier was at 18.5 lbs and quite lean at transfer and required the high needle to be out close to 4 to 5 turns and were off the tension spring that keeps the needle from backing out and being lost. I have mine set to 11 now and plan to run it tomarrow and see how it performs and tunes in. My point in all this is this ! The pop off pressure is much the same as the dwell in point ignitions, and sets the duration that there is a flow of fuel into the carb. If the pop is too high then the duration will be short and lean as is a low pop would produce a long duration and a rich flow which would be much easier to adj with high/low needles than a lean flow ! Bending the fulcrum would have some effect on increasing and decreasing the voluum of flow, but the pop should always be set proper first ! My need is for more pertenant info as to the actual pop off settings that these motors in this situation require for that optimum or pristine setting ! So far i have two inputs one stateing 11-12 lbs and another of 8 lbs and i'm going with 11 for my next run.
Don R

[bob_nj]

Don

Not sure if you've seen these two links yet, but they are informative.

http://www.aerocorsair.com/id28.htm

http://www.geistware.com/rcmodeling/...n_pressure.htm


Take care
Bob

[rcign]

Pop off pressure is way over rated...The only REAL expert on this is Mr Luis Salas at the Walbro factoryin Cass City, Michigan....
Anyone that knows more than he does should maybe apply for a job

[RTK]

I have to agree Ralph, If it is extremely high or low it seems to cause problems, but if you change it a couple of lbs one way or the other it only seems to affect needle adjustment and not run-ability.

[babsvdmerwe]

Hi name is Andre and i would like to know if altitude plays a role as to the amount of oil used in the fuel mixture. Elevation is 1085 metres.

[CRUISER]

Bob mj,
Thank you very much for the links ! they were reinforcing to say the least.
Grumpy,
we met before, you were a problem then and you are a problem now. if you can't be a part of a solution, why do you persist in being a problem !
Thanks Bob

[RTK]

	Quote: Originally Posted by babsvdmerwe
	Hi name is Andre and i would like to know if altitude plays a role as to the amount of oil used in the fuel mixture. Elevation is 1085 metres.

Not to any great extent.

[pe reivers]

In reading all the posts here I feel that many are very hesitant to adjust their carbs for fear of doing something wrong, that may damage their engine, or crash their plane.
Rule number one:
The most reliable engine is the engine that has the carb tuned well!!!
Rule number two:
pop-off pressure is the last to worry about.

Here is rule #1 in tuning, with Walbro manuals as needed.
http://prme.nl/forum/viewtopic.php?t=10

Keep in mind, that at idle only the idle needle is active. The H-needle does not influence the idle micture at all.
The idle needle may, or may not be active at WOT (wide Open Throttle).

This implies, that idle mixture needs to be set first. This is well described in the walbro maintenance manual, on how to find the lean and rich cut-off settings for L- and H-needle.
Erring should not be necessary, but when, do it on the rich side

[rcign]

Any gasser engine will run with the high speed needle completely closed, just not as well..
If you want to set the low needle first be sure the high needle is open at least 1 1/2 turns, or 4 turns on the new EPA spec carbs with superfine threads...Somewhere on the Walbro website it explains this 4 turn thing, do a search...

[RTK]

Hell Ralph, the last two enviro-mental yard tools I bought had the needles capped off, you could not get to them with out destroying the carb. One ran so rich it four cycled from idle to WOT horridly, the other was so lean it would not run at WOT. I Replaced those carbs with some I had lying around, all is good now
Big Brother is telling us we are not smart enough to set our own needles, so we now can't, and thus not wreck the environment, right. You wouldn't believe how much that one spewed.

[pe reivers]

	Quote: Originally Posted by rcign
	Any gasser engine will run with the high speed needle completely closed, just not as well.. If you want to set the low needle first be sure the high needle is open at least 1 1/2 turns, or 4 turns on the new EPA spec carbs with superfine threads...Somewhere on the Walbro website it explains this 4 turn thing, do a search...

True.
Starting point always is with both needles 1,5 - 2 turns open. Except of course with the very fine thread needles.
I had one carb without knowing this, but soon found out how much I could adjest the needle for just a little mixture change. Removing the needle to check the taper then revealed the very fine thread pitch. of about 0.4mm

[RTK]

	Quote: Originally Posted by rcign
	Any gasser engine will run with the high speed needle completely closed,

A long time ago I tried just that, and adjusted the low needle for very smooth running on the low end. Then I started opening the high, as I did this I discovered how soon the high starts feeding and richening the low/mid range. Not all carbs are the same, but the high kicks in really soon on some.

[flynz4me]

Symptoms of a diaphragm carb having a float valve leak:

(1) Rough idle, and engine loading up.
(2) Fuel dripping from the carb when its not running
(3) Lowend needle does not tune well.
SOLUTION:
Replace the float valve and adjust the lever to its flush position OR use a "W" gauge to set it.



Mark - What is the "Float valve??" Is it the needle valve?