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Poor Pickup.....


Roger Dyke
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Hi All,

I have an SC61A which I purchased back in the 90's and ran it until 2004 when I left the hobby. In 2018 I took up the hobby again and decided to resurect my model with the SC61A in it. I stripped and reassembled the engine and bench tested it before installing back in the plane. It as always been a reliable runner and easy to start which is still the case. The engine is mounted with the cylinder head at about 230 degrees (about 8 o-clock) in between horizontal and vertical (looking from the front).

 

Problem: The carb on the engine is the two needle type. I have set the the high needle for peak revs then backed it off two or three clicks to the rich side. I then did the "blow through the tube" test to roughly set up the low needle at idle gap. Running the engine, it works and flies perfectly at middle to max revs. At idle position, it idles fine following adjusting it a few clicks leaner. The problem is pickup from idle to max revs or from about 1/4 throttle to max in the air. At idle I have also tried pinching the fuel pipe for cut out. when pinching the pipe, the revs stay the same (no increase) and eventually fade and die. The pickup is slow and a gut feeling is that after it has been at the lower revs for a while, there is a build up of fuel in the crankcase which leads to a rich pickup. I know that I've droned on a bit here but does anyone have any idea why the pickup should be so poor. 

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Hi Martin,

Many thanks for your reply. I have tried that and also leaned the high needle right off to peak. Any more and the revs start dropping off. It's still exactly the same. The effect is that it seems stifled when trying to pick up. I am not snapping the throttle open from idle to to full. I am taking about two to three seconds from idle to full. The engine is in good order. Strange........

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You appear to have diagnosed it yourself re fuel collecting in the crankcase. You say you have set the high running needle three clicks rich , why ? One click from max is enough the re adjust the bottom end for idle and a good pickup .

If the pick up is slow ,splutters and smokey its rich. If engine hunts or has to be coaxed to rev up then its a weak mixture. All this is easy to adjust provided that there are no air leaks and crank case comp is good ie backplate gasket and crank seal is OK. Also if your engine has a large seal on the carb barrel check it's not cracked or split.

 

Just seen your last post. Siffled,,,,,,,, , sounds like a weak idle setting. While an engine will idle OK it needs a little more fuel for transition to full throttle. Its a fine balance but engine e should idle for twenty seconds or more and pick up smoothly when throttle opened smartly ( not snapped open )

Again if it hunts then dies its weak . If it splutters , is smokey but eventually picks up then its rich. Adjusting either high or low speed settings will affect the other very slightly as the older ASP/SC etc cards were not the best bits of engineering.

Edited by Engine Doctor
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Engine Doctor,

Thanks for you reply/advice.

 

1) It needed three clicks on the high needle to show a slight decrease in the rpm.

2) It definitely faulters when I start increasing the throttle, and then slowly picks up to get to maximum revs. 

3) All crankcase gaskets are good with no leaks. The barrel seal and main needle 'O' ring seal are also good. Engine seems generally in good condition. Fuel is Lasor 5 (5% nitro).

 

I understand the SC carbs are not the best in the world but I think it should operate better than this.

 

I'm going to strip the carb to see if there is any debris in the fuel tube/main needle aperture.

 

Edited by Roger Dyke
spelling errors.....
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Hi Denis,

I think that you might have something there.....

 

It's doubtful that there is a leak as all the plumbing was renewed when I came back into the hobby and resurected the model a short time ago. I have also checked the plumbing out since.

 

(It could be this)

The prop I am using is an APC 13x6 which I know is a little over-propped for a 61 but am using this to help keep the revs/noise down. Up until I gave up the hobby in 2004, I used an APC 12x6. I also use the SC quiet silencer (and always have).

 

Roger

 

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I would not be too worried about 13x6.

 

It ran in the past, it’s got a new plug. The fuel is different, so the mixture settings are changed. Different fuel = different settings.
 

If you are confident you have a leak free system, mixture setting are the problem. 

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The fact that you have poor pickup from idle added to the fact that the engine doesn't stop immediately when pinching the fuel tube at idle sounds exactly like the slow running needle is set too rich. You say that you have already leaned it, but I would be leaning it some more.

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Roger, I notice you are using 4 stroke fuel in a 2 stroke engine, may be it needs a higher oil content. Also upping the nitro content can improve throttling although there should be ways to make it better without this. Can you try a different carb as Chinese engines are not known for having quality carbs.

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1 hour ago, Andy Stephenson said:

Roger, I notice you are using 4 stroke fuel in a 2 stroke engine, 

 

This is a common misconception created by fuel manufacturers. There is no such thing as 2stroke or 4 stroke fuel. All model glow fuel is oil, methanol, and nitro and only the % of each varies. The Laser 5 fuel in use is fine for a 60 2 stroke. 

 

To pick up on one point raised earlier 

21 hours ago, Roger Dyke said:

It needed three clicks on the high needle to show a slight decrease in the rpm.

 

If you see an RPM drop you are too rich. As ED rightly points out, one click back from drop off is plenty so tune for peak revs and either leave it alone, or back one click only. All this stuff you see about 300 revs rich of peak is nonsense. 

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Hi Jon,

 

Thanks for your reply. Point taken about tuning for peak. I suppose that I have got it ingrained into me from the distant past about peaking the engine , then a couple of clicks  towards rich to guard against a flame-out when the engine unloads in the air. Sounds like I need a culture change. I will give it a go as you suggest. Thanks for that.

 

Roger

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42 minutes ago, Roger Dyke said:

Hi Jon,

 

Thanks for your reply. Point taken about tuning for peak. I suppose that I have got it ingrained into me from the distant past about peaking the engine , then a couple of clicks  towards rich to guard against a flame-out when the engine unloads in the air. Sounds like I need a culture change. I will give it a go as you suggest. Thanks for that.

 

Roger

 

 

When they unload (within reason) they go rich so its all fine. 

 

Most 'lean' engine stoppages are caused by poor cooling and/or poor tank placement. 

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Hi All,

Managed to do a lot of engine messing today. Good news and bad news.

 

I manged to get a decent pickup now by leaning the high needle and getting a good setting on the low one as advised.

 

Another problem: When tuned for peak rpm and then I hold the model nose up at about 10 deg from vertical, the engine runs at peak rpm for about 8 seconds or so then dies. It does this both if I richen it up or lean it out. Years ago when I used to fly models, modellers used to do this to see if it was too lean and then give it a click or two rich to prevent quitting if it started to die. Is this the correct procedure?

 

The system has no plumbing leaks whatsoever. The tank is directly behind the firewall  and the centre of the tank is about levei with the engine spraybar.

 

Also, how long should an engine be able to successfully idle for (at about 2800 rpm)? I hear a lot talk about 20 seconds or so. Would it be expected to eventually die after an extended period due to crankcase cooling?

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47 minutes ago, Roger Dyke said:

Also, how long should an engine be able to successfully idle for (at about 2800 rpm)? 

Indefinitely, I would suggest.

 

Temperature conditions stabilise after around 30 seconds, I think, hence the test of leaving it to idle for that long, then checking the pickup. 

 

 

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dont bother with waving its nose around in the air. It is not representative of the conditions the engine will see in flight. Tune as you have and then just fly it. 

 

As for idle, engines should idle for a good long while, but dont go looking for a problem yo dont need to solve. In reality, it will idle long enough to get from the pits to the runway you have all you need. 

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