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Old 04-24-2007, 07:16 AM
mfuess is offline
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Re: Gas engine tuning for newbies....

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Originally Posted by RappyMan View Post
Hi All
I have an old G-38 on my modified Roadrunner. It's not a hovering aerobat, but it is a lot of fun & can do quite a bit. The engine is old & my friend put a carb kit in it last summer and used the setting guage to get it just right. It ran really nice, but after a few flights, it got hard to start. You really had to choke it & then it would die after it ran out of the fuel you just choked in it. If I put my hand over the carb intake while it was running & choke it a few times, it would start running fine and run great through the flight. We just had a nice day here, so I got it out and couldn't get any fuel to choke into the carb. I finally squirted some fuel in the intake and did the hand over the intake quickly a few times after it started to get it running. I tweaked on the needle valves to get a smooth slow idle and a good top end. There was just a tiny bit of hesatation on idle to full speed & if the motor sat idleing for quite awhile, it would lag worse. I'm getting 8640 rpms with a APC 18-8 and no cowl around it.
Any ideas on what to try? About a month ago I had it out, choked it & it started on the first flip - gotta get that back somehow!

Thanks
Terry Peterson
Terry,

ANYTIME your engine changes in how it starts IE: just like you describbed, there's a problem lurking within your carb. The most common problem is the internal filter screen. Remove it, clean it, or replace it. Also, check the float diaphragm. If its stiff OR if its wavy looking, replace it too.
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Old 04-24-2007, 07:22 AM
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Re: Gas engine tuning for newbies....

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Originally Posted by rcflyer View Post
For my glow engines, I tune the low end by pinching the fuel line (close to the carb) and listen for the engine to speed up. I keep tightening the needle valve a little at a time until the engine no longer speeds up when the fuel line is pinched. Then I open the needle back up just a little. Just wondering if this method works on gas engines too?
NO!

Glow engine carbs are muffler pressure/gravity fed. Your carb is pump driven. You need to remove all GLOW FUEL methodology from your thinking. Especially regarding tuning...
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Old 04-24-2007, 07:44 AM
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Re: Gas engine tuning for newbies....

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Originally Posted by JoeAirPort View Post
To those that have this method down. I need to run my confusion by you and see if you can help me here.

It says to start the high end by adusting to peak RPM. Does that mean adusting the high end until it holds the RPM without sag? Because then he says to open the low end slightly if it does sag. That's my first point of confusion. Seems like both of the needles are trying to achieve the same result (peak RPM without sag). Is this correct? Or am I reading this wrong? How exactly do I adjust my high needle for peak RPM (micro steps please)?

Excuse my thick headedness but I'd like to get the end result he talks about. I tried this once but I didn't get it right. It's a new DA 50 with less than a gallon through it.
Before you begin your tuneup, reset your needles to factory settings to start from gouund zero.

It's best to set the topend first. Start your engine, let it warm up. Transition to full throttle. Adjust the HIGHEND NEEDLE to peak RPM. Then, SLOWLY throttle back until the engine begins to FOUR STROKE. At the point, adjust the LOWEND needle until the FOUR STROKING stops. Pull the throttle down more until it FOUR STROKES again and repeat this until you reach full IDLE. NOW, transition up rapidly. IF it sags or bogs, open the LOWEND needle SLIGHTLY until the sagging is gone... YOU"RE DONE!

One big MISTAKE people make (when they tune up a gasser) is to peak the engine then back off the needle to richen it up slightly just like a glow fuel engine. DO NOT DO THAT! Once your carb is properly tuned to peak it will maintain the CORRECT air to fuel balance. Richening it up only causes the engine to run wet and inefficient. IF you are not sure if you have a good tune, look inside your muffler or pull your spark plug. It'll tell you real quick. If it's black & wet its time to re-do your tune.
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Old 04-24-2007, 11:23 AM
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Re: Gas engine tuning for newbies....

Quote:
Originally Posted by mfuess View Post

It's best to set the topend first. Start your engine, let it warm up. Transition to full throttle. Adjust the HIGHEND NEEDLE to peak RPM. Then, SLOWLY throttle back until the engine begins to FOUR STROKE. At the point, adjust the LOWEND needle until the FOUR STROKING stops. Pull the throttle down more until it FOUR STROKES again and repeat this until you reach full IDLE. NOW, transition up rapidly. IF it sags or bogs, open the LOWEND needle SLIGHTLY until the sagging is gone... YOU"RE DONE!
tune.
Sorry mfuess i spotted a contradiction, in the purple section i highlighted there, do you actually mean the High end needle, i was looking at your article and this info is in conflict with that. im just second guessing myself now. and thanks for the articles really do appreciate the info
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Old 04-24-2007, 03:01 PM
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Re: Gas engine tuning for newbies....

Sired by Fire,

Nope, LOWEND needle. Anytime you transition off of idle and it sags, the lowend is lean. The highend setting doesn't kick in until your engine hits a good 3500 RPM or more.
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Old 04-24-2007, 03:54 PM
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Re: Gas engine tuning for newbies....

So what is a good head temp?
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Old 04-25-2007, 04:17 AM
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Re: Gas engine tuning for newbies....

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Originally Posted by Pro1Foam View Post
So what is a good head temp?
This varies greatly from engine to engine. Most of my gassers operate around 190 to 225f. But I have a few that run best at 235f. If you do a lot of 3D, your engine will run a bit hotter.
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Old 04-25-2007, 10:05 AM
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Re: Gas engine tuning for newbies....

Any tips on telling if you high end is to lean other than a white plug?
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Old 04-25-2007, 12:33 PM
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Re: Gas engine tuning for newbies....

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Originally Posted by Pro1Foam View Post
Any tips on telling if you high end is to lean other than a white plug?
RPM is your best bet.
If it's too lean your RPM goes down and it runs hotter than normal. If it's too rich your RPM goes down and it runs cooler than normal. But you have to know what "normal" is. I recommend you peak it using a Tach, and leave well enough alone.
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Old 04-25-2007, 01:04 PM
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Re: Gas engine tuning for newbies....

peak and back off a hundred rpm? I think thats what you said in the artical.
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Old 04-25-2007, 01:26 PM
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Re: Gas engine tuning for newbies....

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Originally Posted by mfuess View Post
Sired by Fire,

Nope, LOWEND needle. Anytime you transition off of idle and it sags, the lowend is lean. The highend setting doesn't kick in until your engine hits a good 3500 RPM or more.
please clarify. Your original article says to open the highend not the lowend. Thanks!
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Old 04-25-2007, 04:53 PM
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Re: Gas engine tuning for newbies....

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Originally Posted by Pro1Foam View Post
peak and back off a hundred rpm? I think thats what you said in the artical.
Where did you see that???
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Old 04-25-2007, 05:11 PM
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Re: Gas engine tuning for newbies....

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Originally Posted by khan View Post
please clarify. Your original article says to open the highend not the lowend. Thanks!
Khan,

I found where you saw the reference to highend and that is incorrect. I pulled up my original article and it is correct, lowend. Sorry for the confusion.

Tuning a gas engine isn't "rocket science", but there's a method to the madness. If your engine is equipped with a Walbro or similar diaphragm carb there are a few things that are consistant. You can start and run your engine with the highend needle completely closed. It won't run great, but I'll run. Of the two needles, the lowend does most of the work because it operates all the time even at full throttle. The topend only operates at just above mid-throttle and above. So, once the topend needle is peaked, it's done... the rest of the tuning will be on the lowend, which includes transition, and mid-range.
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Old 04-25-2007, 06:19 PM
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Re: Gas engine tuning for newbies....

I recently went out to tune my engine using your method.

I let the engine warm up, set the high end and then ran it at WOT for 20 seconds and the rpm did not drop. This all took a little over a minute. Then I let the engine cool.

I then continuously leaned the low end bit by bit following the 4 stroke popping sound all the way down to just above idle. Then I let the engine cool.

The engine transitioned from idle to WOT perfectly and ran very smooth all the way down to just above idle but still four strokes at 2000-3000 and maybe even a little higher rpm. Just above idle.

I am afraid to lean the low end more because I may have already leaned it in a whole turn.

Other than that everything else is factory settings.

How do you get this 4 stroking right above the idle to go away? Do you keep leaning the low end?
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Old 04-25-2007, 06:21 PM
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Re: Gas engine tuning for newbies....

Oh BTW, The engine still holds RPM and full throttle for 20 secconds when it is warmed up. So the low end has not leaned the high too much, yet. The "yet" is what I am worried about and is what is keeping me from leaning it more.

The engine runs great in the air, it just 4 strokes right above idle.
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