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tuning problems on old ST 61series x rear exh.

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tuning problems on old ST 61series x rear exh.

Old 02-21-2005, 07:27 PM
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cstewa
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Default tuning problems on old ST 61series x rear exh.

This engine is probably close to 20 years old if not a little older. It has not been run alot. It was bought specifically for an MK arrow pattern plane that I am recovering and trying to get back in the air. I do remember the engine not running great a 20 years ago when first put in the plane. Well the plane was retired after just a couple of flights because dad quit flying.

I have the engine on my bench and it starts in about two flips with the flip stick. Idles well. The high end will not tune. The needle valve is very insensitive. Generally when at full throttle it surges bad. While it is surging if I reach over and pinch the fuel line she (stops surging) and lights up and runs great for as long as I let her. But then I go back to idle and try to throttle up again it does the surging again. So pinch the fuel line and she lights up again. So I think she must be rich. But when I close down on the needle valve it really does not seem to do much. I do notice that the engine is very hot. so because it is getting hot and I would like to run it just a tad rich I back the needle valve out some and the surging returns. I will also say that it seems like it will do the surging at several needle positions and almost always stops surging if I pinch the fuel line just for a second. I probably should have stated that already. when I say pinch the fuel line I mean like only for a second and then let go and it stops surging. The surging is very violent.

It has a tuned piped , the headerc comes straight back out of the case and connects to a coupler and then the pipe. The muffler pressure tap is on the header. I have checked for leaks in the tank I am using and the fuel lines. The pipe definitly not to short.

any ideas? Would bad pressure on the tank cause surging and insensitive needle valve? The thing is I don't know where I would put a new pressure tap? I guess I could just enlarge the current pressure tap. But before I do that I would love to know from someone with more experience if the surging thing is a symptom of poor pressure.

I did clean the high end needle and valve. thanks.
Old 02-21-2005, 08:25 PM
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w8ye
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Default RE: tuning problems on old ST 61series x rear exh.

There's part of your explanation that does seem right?

Please go back and read it again and make it more clear to us? Start with "when I open up the....."

To make the engine more lean, you should close the needle?

Another thing that happens with the single chamber pipes is there is a big transition in mixture at high speed when the engine jumps on the pipe? Describe how the pipe is made?

Jim
Old 02-21-2005, 09:33 PM
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cstewa
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Default RE: tuning problems on old ST 61series x rear exh.


ORIGINAL: w8ye

There's part of your explanation that does seem right?

Please go back and read it again and make it more clear to us? Start with "when I open up the....."

To make the engine more lean, you should close the needle?

Another thing that happens with the single chamber pipes is there is a big transition in mixture at high speed when the engine jumps on the pipe? Describe how the pipe is made?

Jim
Sorry Jim, I did write the initiall question a little to quick. I made some modifications. Let me know what other info you need. I will try and post a pick of the pipe setup. Thanks, Colin
Old 02-21-2005, 10:18 PM
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ChuckN
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Default RE: tuning problems on old ST 61series x rear exh.

Your pipe length is simply too short for the prop size you are running. Either go down in prop size or lengthen your pipe. This engine was designed to spin an 11x7 prop fairly fast as it was designed before the "slow & quiet" era of pattern flying. If you aren't getting the prop up to at least 14,000 rpm then your combination is off.
Old 02-21-2005, 10:21 PM
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cstewa
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Default RE: tuning problems on old ST 61series x rear exh.

here are some pics of the engine and pipe setup. any help appreciated. Thanks
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Old 02-21-2005, 10:24 PM
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cstewa
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Default RE: tuning problems on old ST 61series x rear exh.


ORIGINAL: ChuckN

Your pipe length is simply too short for the prop size you are running. Either go down in prop size or lengthen your pipe. This engine was designed to spin an 11x7 prop fairly fast as it was designed before the "slow & quiet" era of pattern flying. If you aren't getting the prop up to at least 14,000 rpm then your combination is off.
I am running an 11x7. The pipe i think is really long. I may go measure it. Could the pipe be to long. I was always under the impression that long didn't hurt you only short.

By the way I am running 15% nitro. Could that be to high for this engine?
Old 02-22-2005, 08:28 AM
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pe reivers
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Default RE: tuning problems on old ST 61series x rear exh.

The problem you describe is in line with a tuned pipe having the header very short, so it depends on plane speed to come on song. It may be troublesome to get the plane to fly in the first place.
Once on the pipe, the engine goes leaner, and you can tweek the needle a lot before something happens.
Before going on the pipe, you <must > be rich, otherwise the engine will be too lean when on the pipe. There lies the problem of high power tuned pipes. In order to get all that power, you must find a way to rev up the engine without leaning the needles.
On the test stand, just add about one inch to the header in order to compensate for static running.
Old 02-22-2005, 09:59 AM
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cstewa
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Default RE: tuning problems on old ST 61series x rear exh.

ok.....I am going to pick up a longer header and see if that will fix it....thanks..
Old 02-22-2005, 11:13 AM
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Default RE: tuning problems on old ST 61series x rear exh.

Try removing the pipe from the header and see if you can tune it. If you can, then your difficulty is with the pipe.

Enjoy,

Jim
Old 02-23-2005, 07:47 PM
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cstewa
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Default RE: tuning problems on old ST 61series x rear exh.

Well I tried a tuned pipe that I had on my webra 61 and the engine seems to be running better...still not completely tuned as when I let it idle for a maybe 15 seconds and then go full throttle it dies.....

but it is not surging like it was....this pipe is considerably shorter...probably by 2 or 3 inches....

So either the other pipe was to long or the pipe is bad....has any else had this experience?
Old 02-23-2005, 09:52 PM
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Default RE: tuning problems on old ST 61series x rear exh.

Try running it without the tapered pipe part. And see if you can tune the engine?

A shorter pipe is for a higher rpm than a longer pipe.

When it gets to a point where the pipe is becoming effective, the engine will typically run leaner. You will need to have the engine running slightly rich when it is below the effective rpm of the pipe.

Enjoy,

Jim
Old 02-24-2005, 07:39 AM
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Flyer95
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Default RE: tuning problems on old ST 61series x rear exh.

cstewa,
The lenght of the pipe is not the same thing as the tuning lenght. The tuning lenght is measured from the glowplug to the end of the rear cone in the pipe.

So try to measure the lenght from the pipe-inlet to the end of the rear cone and then add the header lenght to it. Thats called tuned lenght. If the pipe is as old as the engine is maybe some good old castor oil is blocking of the holes in the silencer chamber and making your pipe restrictive. Too smal or restrictive tuned pipes causes all kind of trouble for your setup and should always be avoided. Change your tuned pipe to a bigger one, or to one that you know for sure is made for your size of engine.

A tuned lenght of about 50cm( 20") should be a good starting point. The wooden prop in your pictures gives less rpm than a plastic prop like the APC, or master airscrew. My old Webra 61 turned a APC 11x6 about 13500rpm so your engine should turn your 11x7 wooden prop about 13000rpm.

Good luck, Amir
Old 02-27-2005, 07:01 PM
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cstewa
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Default RE: tuning problems on old ST 61series x rear exh.

Ok I have made some progress. The high end is steady with no surging and I have it running a little rich. Finally seem to have gotten the low end correct. It will idle for at least 2.5 minutes. The transition is ok after it warms up.

It seems like a totally different engine after it is warm. For example when I first crank it I can not transition to WOT. After about two or three restarts it will transition to WOT. Then I let it warm up and can transition fairly well. However if I stop her for just a couple of minutes then I have to go through the whole start up process a couple of times to get it to WOT. Does this make sense?

I am not sure if it just this engine or if it is not tuned correctly. It has one of the old carbs that the instructions tell you to set the high needle and then the low needle and then for the transition/midrange you angle the spray bar. It doesn't tell you wich way to angle it. I also don't know if more angle makes it more or less rich or lean. Maybe it is supposed to be trial and error?
Old 02-27-2005, 07:27 PM
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cstewa
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Default RE: tuning problems on old ST 61series x rear exh.

I just found Pe' Reivers' website. It is awesome and very informative. spray bar clockwise richens transition!

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