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AndyB

Low power motors

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I flew this morning and it was my longest take-off run ever. Zero wind, grass droppings fell straight down. Grass was very wet, so wing got wet. But, the main issue was my motor was producing enough power when I tested it, but at full throttle, as I tried to take off it didn't have much umph. The climb was very, very slow. I take off in a 10 acre field, but it has a forest around 2 sides, power lines one side and housing the other. I cleared the trees by an uncomfortable amount ie for 20 seconds or so, not enough height to glide somewhere nicer.

For some reason the pop-off spring has decided to lift early causing loads of fuel to get through. I used 3.75 litres in 30 mins.....I circled round to gain lots of height before deciding to land.

I think I now know what a Top80 might be like with my weight! :)

Anyway, while at height I spotted these alternative style crop circles....more of a star sign I think.

1346894811_Starsigns.thumb.jpg.75319dfcb9f18ff313e735e3a170f05a.jpg

 

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I'm sure you know this Andy but for the benefit of the newbies:

I always have it in mind that when the temperature is high the power output is reduced a little due to less dense air.

It is noticeable given the same TO wing/configuration.

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Just spent 1 and 3/4 hours doing carb repair/rebuild. Never done this before. I read the previous thread on here about pop-off pressures and lever height (I already had a pop-off pressure tool bought after reading previous thread!), although I did not now how to use it.

I am a technical guy, so maybe bear that in mind. However, I thought this whole thing was really quite easy to do.

I measured the pop-off pressure before rebuild and it was 12 psi...however, once 'popped' it would then not re-seat and would not hold any pressure ie it kept letting more fuel through, causing my flooding issues. Because the pop-off seemed ok in terms of the pressure setting I decided to use the existing spring (I haven't got a new one, although I do have 3 spare carbs!).

I changed all the gaskets, the metering lever and shaft.

On fitting the new lever I checked the lever height using the Walbro gauge (bought after reading a previous thread on here!) and it was perfect without any adjustment.

Re-built everything, set jets, both at one turn out, and fired it up. It ran great straight off. Only adjustment needed was idle as it was way too high, having previously had trouble keeping it idling (because of too much fuel).

Here is my work bench after the re-build. All the gaskets and bits are the old ones.

Just for interest I measured the pop-off pressure on the spare carbs. These were new carbs that Parajet kept sending me when my new machine would not run correctly. One carb popped at 28 psi one at 10 psi and the other at 14 psi, but it would not re-seat (like my current issue). So the first one was flooding and one was so lean it would have destroyed the engine and the other was also flooding.

So, my advice to anyone with engine issues..... After you have ensured fuel is getting to the carb (no blocked filters, air ingress etc), then measure the pop-off pressure. It tells you so much.

 

20200917_125916.thumb.jpg.e498c99f8b5684cf749510e866e45b61.jpg

 

 

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Wow.  I had no idea that they had such variation right from the OEM.  That's... well, bad.  The popoff pressure can be tested while the carb is still mounted to the engine, right?  Seems like a thing might be done as part of a receive-in inspection.

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Yes you just have to pull the fuel line off and use another piece of fuel line to the pop-off meter. I also thought it was such a simple thing to do that the manufacturers ought to be testing as standard.

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3 hours ago, AndyB said:

Re-built everything, set jets, both at one turn out, and fired it up.

What engine is this on Andy, and is one turn out on both the factory spec?

One turn out on the low screw seems a lot, and one turn on the high screw (comparing to the settings on my Moster) would be on the lean side.

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AC Nitro. I usually find the NItro needs just under one turn on the high screw.

I flew twice this evening. Not quite right yet. All good except for trying to rev up after it has idled for a while. It wants to bog down rather than pick up quickly. Took a couple of throttle blips to make it go again. This likely means the low screw is not right....but not sure which way yet! Someone will hopefully tell me. 

CHT gauge showed a max of 130 with sustained full power. Cruise temp was 193 (5800 rpm trims full out). Max rpm same as always 7300 rpm.

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I tune my 2 stroke engines almost entirely off of sound so it's sort of hard to explain. Basically if your engine sounds smooth without very many exhaust poping sounds, then you are lean and the needle needs to turn counter clockwise. If there is a ton of poping sounds, then you are likely rich and you need to turn the needle clockwise. This way can be difficult for people learning to tune.

For your specific engine it sounds like the idle is slightly lean so I would try going like 1/8 turn CCW at a time until you get the best throttle response. If it gets worse reset the needle to where it was and try going the other way

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21 hours ago, AndyB said:

Not quite right yet. All good except for trying to rev up after it has idled for a while. It wants to bog down rather than pick up quickly. Took a couple of throttle blips to make it go again. This likely means the low screw is not right....but not sure which way yet! Someone will hopefully tell me. 

A good explanation in this video by Mitch might help you. He explains it well.

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4 hours ago, Bob27 said:

I tune my 2 stroke engines almost entirely off of sound so it's sort of hard to explain. Basically if your engine sounds smooth without very many exhaust poping sounds, then you are lean and the needle needs to turn counter clockwise. If there is a ton of poping sounds, then you are likely rich and you need to turn the needle clockwise. This way can be difficult for people learning to tune.

I know exactly what you mean, if the mix is lean the engine exhaust note sounds `faint` and the pulses regular, the idle speed will be smooth but on the high side, if the mix is rich it`ll be a `fuller` exhaust note and the `pops` will be random (4 stroking). The engine may shake more,too.

When i first got my Moster the factory settings were so rich and the engine shake so much it used to give me double vision with the motor on my back at idle.

Another tuning trick is to snap the throttle open from idle - if it bogs it`ll be lean, and this can happen if both the low or high screw is set too weak.

I like to adjust my low screw so that the `bogging` from a lean setting *just* disappears when the engine is warmed up - just about as lean as i can go safely. This gives a smooth idling engine and improved fuel consumption.

It`s also important on the Moster to set idle speed to 2200+ - your exhaust will appreciate a smooth and high`ish idle..

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ah that's good 7300 - exactly same as i get. with air conception saying 7700 i kept thinking i was down on power, but sounds like I'm same as you get.

yeh - no wind days - awesome once airborne - an utter sh1te to get airborne. when I had my top80 i remember once having to run maybe 150m - then climbing crazy slow. When you've so little power to start with, all it takes is a wee bit off on the carb or whatever and it's back to rad days.

I've never measures pop off pressure - what amazon purchases do i need to make ? :-)

stu

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I've never experienced the extended or fast run-out, I always seem to get enough lift to take off with a moderate throttle at easy jogging speeds. Might be because I'm 68kg and have 70kg + thrust.....

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