Basic engine tuning.

[Notorious B.E.N.]

Couple of things come to mind but it seems there might be an issue with the GP61.

1) Take the carb to a small engine repair shop that can sonic clean it. Might help, might not.
2) Try changing the timing. There was a similar issue with the DA 60 and that was one of the fixes.
3) I had a similar problem with trying to tune out midrange burble with my DA 60. It didn't matter what I tried, I could never get it to go away without affecting the low or high range. Someone told me to try stretching the diaphragm spring (another alternative) but I put a different spring in that felt a little stiffer. I had to richen the needles up but now the motor is clean from idle to full power.

May help, may not but if you tried all 3, you're looking at less than $50 in parts and labor.

 

[Jetpainter]

It's never really had a mid range burble, and up until recently it's run really well, but I've always wondered why the needle settings were so strange.

 

[Notorious B.E.N.]

Yeah, gas engines are great until they start doing some weird things.

That's one reason why I was thinking of going to electric last year. I might still eventually do it for other reasons but it's hard to beat a gasser when it's running right.

 

[gyro]

In my experience, the GP-61 has a "mid-range burble" that is not able to be tuned out entirely, due to the design of the stock side dump muffler. I've had a GP since they first came out, and long ago stopped worrying about the sound, and just tuned it for performance. It really runs well, even with it's unique sound.

On another note, I have a relatively new DLE35, (bowman ring/reedblock checked) that i'm scratching my head with now. After about a 4 or 5 flights for about an hour of flight time (break in flights, heating/cooling cycles), it had two in flight flame outs. After the second one, I tried holding the plane on the ground while running, and noticed that when I flipped it inverted, RPM increased audibly (maybe leaned some?) and after I flipped it back upright it died. So I took the engine off disassembled the carb and inspected everything, no problems. Changed the tank-to-carb line. Tank is brand new Fourtitude v2 16oz.

Next, I went outside and re-tuned form scratch. I got the transition exactly where I want it, and bout 7000 or so rpm on a 20x8 Mej carbon prop. I can leave it at idle for 20-30 seconds, and advance throttle to full with out hesitation.
However, when I do the same test holding the plane inverted, it will stumble on quick full throttle application, and I have to back down and advance slower or it will stall out. Normal inverted smooth application of full throttle is fine.

so... I took it back inside, and changed the clunk line. Not sure what else to do? Ideas?

 

[Notorious B.E.N.]

Fuel flow problem for sure. Check to see that the carb pulse hole (if there is one in the carb block) is lined up properly and then start checking out carb stuff.

Can you see about getting a genuine Walbro for that motor? I don't remember if VVRC sells them or not.

 

[ThePledge]

I probably sound like a broken record

When a rear carb engine runs leaner with the carb diaphragm plate facing the ground and richer facing away I'd suspect weak pop off pressure.

There is a little spring under the level under the diaphragm plate. It's likely too weak causing low pop off pressure.

A small engine shop could set it to 25 ish PSI for you. You could try adding a small spacer under the spring. The spring could be lengthened and tempered a little at a time until the tuning differential goes away. A new stronger spring could be installed.

This all fits your situation in that you are tuning it with the plane upright on the ground which is the richest orientation. So it runs great upright, but flip it inverted with the plate facing the ground and it goes lean.

 

[gyro]

I probably sound like a broken record

When a rear carb engine runs leaner with the carb diaphragm plate facing the ground and richer facing away I'd suspect weak pop off pressure.

There is a little spring under the level under the diaphragm plate. It's likely too weak causing low pop off pressure.

A small engine shop could set it to 25 ish PSI for you. You could try adding a small spacer under the spring. The spring could be lengthened and tempered a little at a time until the tuning differential goes away. A new stronger spring could be installed.

This all fits your situation in that you are tuning it with the plane upright on the ground which is the richest orientation. So it runs great upright, but flip it inverted with the plate facing the ground and it goes lean.

I'm not familiar with these parts you mention, but I'll study up and take a look at the spring. Thank you

 

[acerc]

I probably sound like a broken record

Don't you know no one listens to records any more, try something digital.

 

[Notorious B.E.N.]

I'm not familiar with these parts you mention, but I'll study up and take a look at the spring. Thank you

There's a spring that meters fuel for the motor. Sounds like your spring is too weak and not allowing enough fuel to flow.

I had a similar problem with my DA 60. Changed out the spring and it's mostly ran great until here recently. I've decided most of my problem is using pump gas and not flying enough. I can't even get through a gallon in a month so I'm going to go back to SEF. Yes, it's more money but to me it's not too much considering my engine runs better and I'm not fighting with it. Of course, I may just sell the DA 60 and get an electric motor...

That would be for different reasons none of which related to being dissatisfied with gas engines or fuel sources.​

 

[acerc]

@Notorious B.E.N., not to be a PITA but an arrow to where the spring is hidden may be helpful to some.