Rotax question from a newbie

[racegunz 1]

Hello all, I've read quite afew posts on here and thought I'd ask a question that no one has been able to really give me a good answer to. I have a rans s5 with a rotax 503 dcdi the tach looks like it is reading over revs from the engine, thus leading me to overpitch my prop, ect. The deal is if I run both mags the engine looks to over rev even with a fixed pitch Tenn. wood prop made for the 503.( they said no way it would over rev, same story with powerfin) If I run the left mag it still over revs (or it looks like it on the tach you really don't hear it over rev) Now comes the weird part if I run the right mag it looks normal at the high rpm range at idle it reads zero, literally, the tach starts to respond at about 2500-3000 rpm and then seems normal, then engine runs better with both mags though the rpm range and runs ok with either by themselves. if it's a bad connection (I've checked the tach itself) why only one mag? could the tach be bad? if so it sure is weird, I was leaning towards the left mag/CDI is creating dirty signals and screwing up the tach if so why does the other not read at idle? Any thoughts?

Edited 3 Mar 2010 by racegunz

[C5Engineer 1170]

What kind of Tach are you running??? Are you running a needle type tach off the grey wire in the wiring harness?? If so throw it in the trash and get a Tiny Tach http://www.tinytach.com/tinytach/index.php They are cheap and will give you an accurate reading. I'd about be willing to bet money that your NOT over revving your engine your tach just says it is. Everyone here has battled this issue. I was the last victim and almost landed off airport after I way overpitched my warpdrive prop trying to make my needle tach work right. My needle tach reads over 7000rpm when I'm really turning 6400 and it fluctuates when you do a mag check just like you describe.

[racegunz 1]

Yep sounds like you've been there, it is a needle type electronic VDO tach, if I change it I'll spend some bucks and get a MGL flight 2, the original intruments are used junk,(i'm not the original builder) it's time to get serious and make this thing right, and nice. Also I'm Looking at an Avid MKIV kit I hope to get it this spring!

I plan on using You guys as a resource, I'll let you know about the tach when I get the new one installed. thanks.

[akflyer 1427]

Yep sounds like you've been there, it is a needle type electronic VDO tach, if I change it I'll spend some bucks and get a MGL flight 2, the original intruments are used junk,(i'm not the original builder) it's time to get serious and make this thing right, and nice. Also I'm Looking at an Avid MKIV kit I hope to get it this spring!

I plan on using You guys as a resource, I'll let you know about the tach when I get the new one installed. thanks.

Joey is all over this one.. You should be running both mags at all times except for the mag check before takeoff. The needle tachs are notorious for being way off. You can use the Tiny Tach and wrap it around one plug wire on each mag so it will show both sides during a mag check. Alot of guys were trying to figure out why the performance of the plane was so bad.. Like taking 700' to get off the ground and 400 FPM climb etc. Turns out that they were only running 5500 RPM or so on take off.

What are your EGT's looking like. If you are over propped they will be reading low.

[racegunz 1]

Joey is all over this one.. You should be running both mags at all times except for the mag check before takeoff. The needle tachs are notorious for being way off. You can use the Tiny Tach and wrap it around one plug wire on each mag so it will show both sides during a mag check. Alot of guys were trying to figure out why the performance of the plane was so bad.. Like taking 700' to get off the ground and 400 FPM climb etc. Turns out that they were only running 5500 RPM or so on take off.

What are your EGT's looking like. If you are over propped they will be reading low.

I always run both mags while flying! All the tests were static run ups, and you are right my egt's were real nice although I never really thought about that being a symptom, thanks for the tip. I wish I had found this site months ago!!

[C5Engineer 1170]

When you order your Tiny Tach order it directly from them. There's a specific model for the two stroke. Lots of other places sell them but you want to make sure you get the right one or it won't work. They are the cheapest I found also. They also double as an hour meter when the engine is not running.

[akflyer 1427]

I always run both mags while flying! All the tests were static run ups, and you are right my egt's were real nice although I never really thought about that being a symptom, thanks for the tip. I wish I had found this site months ago!!

What do you consider real nice? At wide open throttle, straight and level you should be getting 1150 or so.. anything under 1100 and your mains are too rich, or your over propped. What is your static max RPM? ( on the ground, run up) What do you turn on climb out? What do the plugs look like?

[racegunz 1]

What do you consider real nice? At wide open throttle, straight and level you should be getting 1150 or so.. anything under 1100 and your mains are too rich, or your over propped. What is your static max RPM? ( on the ground, run up) What do you turn on climb out? What do the plugs look like?

I've never run "wot" straight and level because it (the tach) reads 8000 rpm on the climb out as soon as I get to 400 or so agl I back off to just under 6000 rpm indicated, God only knows what it's really turning. the egt's have never hit 1100 the cht's always under 300. plugs look good not hot and not fouled, changing/ re-gapping did not make any performance difference. I think the engine runs strong it's just I've been afraid of running it because of the tach readings. Static max fluctuates thus the reason I started the thread. I admit I'm a newbie to 2 stroke aircraft engines but I'm not a dumbass, i.e. I fly with both mags/ I monitor my egt cht readings,/ I even do a mag check during my run up. Realize I'm in phase 1 testing on this plane and it has 17 hrs, 8 of which I've put on it. With out a local resource to give me a what's normal I've just been trying to carefully test the characteristics of this experimental, the biggest hang-up has been the issue I started the thread about. I appreciate all the advice do you think there might be something else besides the tach that could be the problem?

I re-read this and I come off testy, well I am at work and I haven't flown since dec.27.

Edited 3 Mar 2010 by racegunz

[akflyer 1427]

I've never run "wot" straight and level because it (the tach) reads 8000 rpm on the climb out as soon as I get to 400 or so agl I back off to just under 6000 rpm indicated, God only knows what it's really turning. the egt's have never hit 1100 the cht's always under 300. plugs look good not hot and not fouled, changing/ re-gapping did not make any performance difference. I think the engine runs strong it's just I've been afraid of running it because of the tach readings. Static max fluctuates thus the reason I started the thread. I admit I'm a newbie to 2 stroke aircraft engines but I'm not a dumbass, i.e. I fly with both mags/ I monitor my egt cht readings,/ I even do a mag check during my run up. Realize I'm in phase 1 testing on this plane and it has 17 hrs, 8 of which I've put on it. With out a local resource to give me a what's normal I've just been trying to carefully test the characteristics of this experimental, the biggest hang-up has been the issue I started the thread about. I appreciate all the advice do you think there might be something else besides the tach that could be the problem?

I re-read this and I come off testy, well I am at work and I haven't flown since dec.27.

The tach is getting the signal off the stator..I would lean towards the normal issue of the needle tach just being fully screwed. Tiny Tach, or an optical tach on the prop and you do the math for engine RPM.

[racegunz 1]

Okay I installed the tiny tach yesterday and re-pitched my powerfin prop to 11.5 deg. (pretty close to what I had before) I wired the tiny tach like the tech suggested and it read with the original up to about 4k then it started bouncing around,(just like the original) I then re-wrapped it in a different area with less turns and it read too fast? I had it set on 180 deg. which is what I thought was correct. I then wrapped only one plug wire (tiny tach suggested both from one mag) and made sure it was well clear of other wires, set it to 90 deg. and it is now smooth as silk and stays with the original tach until the original freaks out at about 5500. If the new tch is correct and I think it is I was way over pitched. With a static run of 58-59k on the tiny tach I was over 7500 on the original and it was bouncing around as ususal the tiny tach was steady. The only question is why did I have to put it on 90 deg.? a call to tiny tach is in order.

[C5Engineer 1170]

Okay I installed the tiny tach yesterday and re-pitched my powerfin prop to 11.5 deg. (pretty close to what I had before) I wired the tiny tach like the tech suggested and it read with the original up to about 4k then it started bouncing around,(just like the original) I then re-wrapped it in a different area with less turns and it read too fast? I had it set on 180 deg. which is what I thought was correct. I then wrapped only one plug wire (tiny tach suggested both from one mag) and made sure it was well clear of other wires, set it to 90 deg. and it is now smooth as silk and stays with the original tach until the original freaks out at about 5500. If the new tch is correct and I think it is I was way over pitched. With a static run of 58-59k on the tiny tach I was over 7500 on the original and it was bouncing around as ususal the tiny tach was steady. The only question is why did I have to put it on 90 deg.? a call to tiny tach is in order.

Not sure what you mean by 90 degrees. I only have my red wire wrapped around one plug wire per the instructions and it works fine. Just don't cut off the access wire. What did you use for a ground? On my airplane best climb is 6400 static and cruise is 6200 static. When I'm turning 6500 rpm on my tiny tach my needle tach is pegged at 8 grand! It's totally worthless. They are pretty helpful at Tiny Tach.

[racegunz 1]

Not sure what you mean by 90 degrees. I only have my red wire wrapped around one plug wire per the instructions and it works fine. Just don't cut off the access wire. What did you use for a ground? On my airplane best climb is 6400 static and cruise is 6200 static. When I'm turning 6500 rpm on my tiny tach my needle tach is pegged at 8 grand! It's totally worthless. They are pretty helpful at Tiny Tach.

The new model Tiny tach can be set up for different engines so only one model. The rotax 503 is supposed to fire every 180 deg so you would set the tiny tach to that, mine seems to be correct at the 90 deg setting. I flew today and the tach was consistant and about a grand lower reading than the original, still was reving a little high on climb-out but not erratic. I grounded it right where the stater ground wire is (big lug and copper ground wire) and I got the best reading with three wraps of the red wire on one plug wire. I had my eyes glued to the cht's/egt's and all seemed normal.

[C5Engineer 1170]

The new model Tiny tach can be set up for different engines so only one model. The rotax 503 is supposed to fire every 180 deg so you would set the tiny tach to that, mine seems to be correct at the 90 deg setting. I flew today and the tach was consistant and about a grand lower reading than the original, still was reving a little high on climb-out but not erratic. I grounded it right where the stater ground wire is (big lug and copper ground wire) and I got the best reading with three wraps of the red wire on one plug wire. I had my eyes glued to the cht's/egt's and all seemed normal.

How are you justifying good vs bad readings when you have nothing to compare it to? The tiny tach only updates every few seconds so unless you've been at one power setting for a few seconds it is going to be erratic until it catches up.

[racegunz 1]

How are you justifying good vs bad readings when you have nothing to compare it to? The tiny tach only updates every few seconds so unless you've been at one power setting for a few seconds it is going to be erratic until it catches up.

Actually the update rate is .5 sec and I am comparing it to the needle tach that I left in for now. Also I would call erratic an up and down reading not trending in a coherent fashion, i.e. swinging 100's of rpm's bothways THAT would be erratic.

[C5Engineer 1170]

Actually the update rate is .5 sec and I am comparing it to the needle tach that I left in for now. Also I would call erratic an up and down reading not trending in a coherent fashion, i.e. swinging 100's of rpm's bothways THAT would be erratic.

Got ya.... Didn't mean to sound snooty was just curious

[racegunz 1]

Got ya.... Didn't mean to sound snooty was just curious

No prob. called tiny tach today, the tech had no explaination he just said it's weird that it would be correct at the 90deg setting and if it starts acting up send it back for a replacement.

[racegunz 1]

Just in case anyone else ever has this problem here's the scoop. I never could get the tachs reading consistantly enough to keep flying it so I took the engine off and took the charging system out to inspect and I found.... nothing

nothing wrong at all, no bad ohm readings no rubbed wires it all was like new (it practically is). So I started rewiring the entire plane and changed quite a few things mostly running sheilded wire and keeping the charging system wiring clear of the rest of the harness. I even went to the length of sheilding the plug wires. I am not sure which of these worked but both tachs are now steady although as you all figured the needle tach reads about 1k higher than the tiny tach. A very good thing came out of this though, i found while checking the jets on the carbs to make sure they were the right ones that the main jets were inside the plastic cup instead of under it causing the surging I was getting at cruise and the low egt's. Thanks once again for the suggestions and ideas.

[akflyer 1427]

Just in case anyone else ever has this problem here's the scoop. I never could get the tachs reading consistantly enough to keep flying it so I took the engine off and took the charging system out to inspect and I found.... nothing

nothing wrong at all, no bad ohm readings no rubbed wires it all was like new (it practically is). So I started rewiring the entire plane and changed quite a few things mostly running sheilded wire and keeping the charging system wiring clear of the rest of the harness. I even went to the length of sheilding the plug wires. I am not sure which of these worked but both tachs are now steady although as you all figured the needle tach reads about 1k higher than the tiny tach. A very good thing came out of this though, i found while checking the jets on the carbs to make sure they were the right ones that the main jets were inside the plastic cup instead of under it causing the surging I was getting at cruise and the low egt's. Thanks once again for the suggestions and ideas.

Nice that you got it sorted out, and that extra 1000 RPM will make a new bird out of it! Can you elaborate on the main in the plastic cup thing? Are you talking about the plastic screen that slips over it

[racegunz 1]

Nice that you got it sorted out, and that extra 1000 RPM will make a new bird out of it! Can you elaborate on the main in the plastic cup thing? Are you talking about the plastic screen that slips over it

I have Bing 54 carbs, inside the slide is a white plastic cup it locks the throttle cable inplace with the tension from the return spring, the main needle (not the main jet) has 4 places for an e-clip and small o-ring to set the mid-range leaner or richer, that clip is supposed to be under the cup but it fits nicely inside and if you don't know any better it looks like that's where it goes. The side effect is the midrange will be crazy rich, mine was actually surging at cruise I had to jockey the throttle every couple of minutes to try and keep it where it should be. The moral of the story is even if you are a newbie check the other guys work, all of it. With both issued fixed I now can cruise at 70 + mph at 5500 rpm. and the tach reads steady, it's a real relief to see every guage reading what it should the entire flight.

[akflyer 1427]

I have Bing 54 carbs, inside the slide is a white plastic cup it locks the throttle cable inplace with the tension from the return spring, the main needle (not the main jet) has 4 places for an e-clip and small o-ring to set the mid-range leaner or richer, that clip is supposed to be under the cup but it fits nicely inside and if you don't know any better it looks like that's where it goes. The side effect is the midrange will be crazy rich, mine was actually surging at cruise I had to jockey the throttle every couple of minutes to try and keep it where it should be. The moral of the story is even if you are a newbie check the other guys work, all of it. With both issued fixed I now can cruise at 70 + mph at 5500 rpm. and the tach reads steady, it's a real relief to see every guage reading what it should the entire flight.

OK I got you now... holy crap, that would have made the midrange WAY rich and allow the needle to move up and down. It is a wonder that you got the darn thing to run at all at anything other than an idle and WOT. I agree, it is nice when all instruments are as they should be and you are getting full TO power and a good midrange burn!