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Polini Thor 200 - Carb settings


poz
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Can anyone with a Polini Thor 200 share any insight into whether they have been able to find the correct carb jet settings.

Unit ships with 52 low and 144 high. I have reduces to 38 and 142 respectively. Needle in the middle, as shipped. Airscrew 1 turn

Even with this reduction, it's running so rich on idle that it stops.

Will achieve 7400rpm up to around 5000ft, but very quickly dies off due to rich mixture. At 7000ft I could only achieve 6300rpm and hugely reduced power. Spark plug checked and very dark, although semi idle all the way down from 7000ft would or could cause this.

Will try 140 jet, but nervous about moving so far from manufacturers recs.

Interested to know other Polini Thor users settings.

Cheers

Dan

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I went for the Walbro carb as carb icing is an issue in Scotland and i d rather have a closed system for no leaks when in the car.

Can anyone with a Polini Thor 200 share any insight into whether they have been able to find the correct carb jet settings.

Unit ships with 52 low and 144 high. I have reduces to 38 and 142 respectively. Needle in the middle, as shipped. Airscrew 1 turn

Even with this reduction, it's running so rich on idle that it stops.

Will achieve 7400rpm up to around 5000ft, but very quickly dies off due to rich mixture. At 7000ft I could only achieve 6300rpm and hugely reduced power. Spark plug checked and very dark, although semi idle all the way down from 7000ft would or could cause this.

Will try 140 jet, but nervous about moving so far from manufacturers recs.

Interested to know other Polini Thor users settings.

Cheers

Dan

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The PWK Carb can be a bit of a mind job when it comes to figuring out what jets do what.

Attached is a SS that allows you to plug in the carb settings - sizes of jets, needles and clip positions, and see the relative effect of making a change.

I helped no end with my 2T dirtbike which also has a PWK carb.

Wicked!

Thanks for that Notch.

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  • 3 weeks later...
  • 1 month later...
Poz, what was the outcome did you go leaner ?

At the moment the engine is running (PWK )with jets: slow 38 and 140 main. It seems to be ok with this setting. One of the guys here has been to 5000 metres on this setting.

Testing with Walbro also seems to be OK, although it runs hotter (290 at one point). The engine is also harder to start with a Walbro. If it's switched of during flight for more than 10 minutes it's difficult to restart. Vapour lock suspected due to heat transfer from engine to carb.

Flash starter takes some getting used to as you don't get the same 'feel' as a normal pull starter.

Still not actually purchased my Thor yet. All this is from tests at the PAP factory.

More than a little disappointed with Polini at this stage. All of this stuff should all have been sorted out pre-production. Also, they aren't helping out at all with sorting their carb issues.

I'm still 50/50 on which carb to have fitted to my unit.

I'm definitely going to struggle selling my PA125. Its been so reliable. No issues in 150hrs, although I do have it serviced every 25hrs.

Dan

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with the thor 200/walboro I got ~4.5 lt/hour yesterday. Gentle flying. 120kg pilot (so ~155kg all up) on 30m paraglider wing.

I think this is still running a little rich generally. Could do with being a little leaner generally, but slightly richer at full power.

Starts easily from cold. Not much experience of hot starting. Still running in.

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  • 3 months later...

Hi Guys,

Just joined the forum having done a Google search for the Thor 200 engine. I hope you don't mind an "interloper" as I fly SSDR microlights, specifically a Flylight Dragon Lite.

I have recently fitted a Thor 200 after using a Simonini Mini 3 for two years. I read with interest all your “dealings” with carburetion. As an engineer who has spent a lot of time messing with carbs on microlight two strokes, I hope I can “bring something to the party” with regard to information.

I was very impressed with the Thor finish and the fact that it has a balancing shaft; it’s the only way to reduce vibration in a single cylinder engine.

I too have had issues with the carburetion “out of the box”. The engine was running so rich that it kept stopping. So I dropped the needle to its lowest position. After a day or so getting used to the engine with local flights, just to make sure it would behave, I had a day out with around four ours flight and a six mile crossing of the Forth estuary (I’m not that brave, I was at 6.5K ft! With my glide angle 2.5K would have been the safe min.)

You could be forgiven for thinking that with such a “course” carb adjustment then flying so far from home being a bit foolish, but I have a secret weapon with regard to engine monitoring that I am very surprised no one in the paramotor world appears to use. An exhaust gas temperature Gauge or EGT.

As a carb runs an engine through its rev range, the engine temp goes up & down like a yoyo. This is due to, as you will all know, different jets, air slide & needle all interacting with each other as it operates through its range. If you think it is a complicated process, try tuning a Mikuni diaphragm carb, that’s real fun, NOT!!

I am no expert on carb tuning, but the huge advantage that an EGT has over the commonly used CHT is that it gives you a near instant indication as to what is happening inside your engine at any given time. A CHT gauge has a bit of a lag as the heat has to heat up a lot of surrounding metal, i.e. the cylinder head. So unless you hold your carb open at an exact position for a time, the time it takes for the temp to stabilise at the CHT sender, you are not going to get a true indication of what is going on in the combustion chamber.

I phoned Parajet & spoke to their “engine expert” asking for ideal & max EGT’s & I was told that they had never used one! So I emailed Polini in Italy. I got a garbled reply & a picture of where they thought I should mount the sender. It showed it half way up the expansion pipe, about 300mm from the exhaust port. All my experience and examples on other engines, including EGT manufacturers say “as close to the exhaust port as possible”. Mine is 5mm past the machined ring at the engine end of the pipe, so around 25mm from the port? So when I do a carb adjustment, I can instantly see what is happening in the engine.

An EGT temp of around 600 deg. C is ideal, I have an alarm set on my EGT, it gives me a flashing LED at 650 deg. Above this temp you risk burning a hole in your piston. Below 580 deg. and you are wasting fuel & power.

There is only one small area of my rev band that takes the temp to 605 deg. the rest of the time its well below. I have other jets that were supplied with the engine, once I have ten hours on it I will start experimenting by swapping to smaller jets. This will probably cause me to raise the needle as there is an interaction with all adjustments.

A mate of mine has the same trike & engine and we have had identical results from the same changes. This is comforting as it proves consistency in manufacture.

Last December at the NEC air show I got talking to a paramotorist who had burnt a hole in his piston of his Thor twice. He reckoned that the carb “went out of adjustment” after a while. I don’t know it this was the case, but an EGT would have picked it up straight away.

You will not find a microlight without an EGT, that’s why I am so surprised you guys don’t use them, but they are a great device to get your engine running sweet right across its rev range.

There are some other mods I have done to the Polini, but I’ll bore you all later with another posting.

Regards,

Pete

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Hi Pete, and welcome to the forum.

I read your post with interest. What you say makes sense, although my piston blew at 219°. Many thought I was mistaken until louts more low temp holes started appearing, which is when the emphasis shifted to the squish and pre-ignition.

My question is would an EGT gauge have picked this up?

Cheers

Dan

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Hi Guys,

Thanks for the warm welcome.

Poz, 219 deg.!!!? Was that the 200 or 100 engine? That is a low temp to melt a piston, you have me worried now. How did you measure this temp? Was it with a CHT? If it was then my guess is that an EGT would have showed a much higher temp, i.e. 650+deg. C.

I based my 650 deg. C max on the Simonini which stated a max of 620, and Rotax two strokes who state a max of 675 or possibly 700 if I remember correctly. But at the end of the day, it will depend what type of alloy a manufacturer uses for the piston and how the gas burns, i.e. causing a hot spot on one area of the piston.

As I said, I’m no expert, but I trust my EGT. I might just set my alarm at 620 deg. C and adjust the jets to stay under that temp.

One interesting article I read on the complexities of using an EGT stated that; because the exhaust is designed to give back pressure and “hold” the charge in the combustion chamber, there is a possibility that at a certain rev setting as the back pressure changes, the “combustion flame” might protrude out of the exhaust port giving a false high reading on the EGT but as soon as you change the revs slightly it will go away.

The point being, don’t worry about small temp spikes.

Just got an email from Flylight, it appears that I am being sent a “squish kit”. Not sure what this is, but I think it might be a fat cylinder head gasket to reduce the compression. I get the feeling that Polini are happy for us all to have over rich engines to reduce the risk of a holed piston. But if you can only get max revs at ¾ throttle, something in the carb settings is wrong & that is the way mine is at the moment, so I’ll address this when it is run in. Also, my engine will cut out if idled for a long time and splutter as I try to pick the revs up. Another sign of an over rich engine.

Unfortunately, I’m in Baku, Azerbaijan waiting to go out to a rig for two or three weeks, so I can’t test and report back to you until I am home.

Regards,

Pete

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Hi Guys,

Thanks for the warm welcome.

Poz, 219 deg.!!!? Was that the 200 or 100 engine? That is a low temp to melt a piston, you have me worried now. How did you measure this temp? Was it with a CHT? If it was then my guess is that an EGT would have showed a much higher temp, i.e. 650+deg. C.

I based my 650 deg. C max on the Simonini which stated a max of 620, and Rotax two strokes who state a max of 675 or possibly 700 if I remember correctly. But at the end of the day, it will depend what type of alloy a manufacturer uses for the piston and how the gas burns, i.e. causing a hot spot on one area of the piston.

As I said, I’m no expert, but I trust my EGT. I might just set my alarm at 620 deg. C and adjust the jets to stay under that temp.

One interesting article I read on the complexities of using an EGT stated that; because the exhaust is designed to give back pressure and “hold” the charge in the combustion chamber, there is a possibility that at a certain rev setting as the back pressure changes, the “combustion flame” might protrude out of the exhaust port giving a false high reading on the EGT but as soon as you change the revs slightly it will go away.

The point being, don’t worry about small temp spikes.

Just got an email from Flylight, it appears that I am being sent a “squish kit”. Not sure what this is, but I think it might be a fat cylinder head gasket to reduce the compression. I get the feeling that Polini are happy for us all to have over rich engines to reduce the risk of a holed piston. But if you can only get max revs at ¾ throttle, something in the carb settings is wrong & that is the way mine is at the moment, so I’ll address this when it is run in. Also, my engine will cut out if idled for a long time and splutter as I try to pick the revs up. Another sign of an over rich engine.

Unfortunately, I’m in Baku, Azerbaijan waiting to go out to a rig for two or three weeks, so I can’t test and report back to you until I am home.

Regards,

Pete

Pete, I highly recommend these 2 Thor 200 Facebook pages. There's some really good info there, and it looks as if you're going to be an excellent contributor :wink:

https://www.facebook.com/groups/174049336076750/?fref=ts

https://www.facebook.com/PoliniThor200Owners

Cheers

Dan

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  • 1 month later...

Hi Guys,

Here’s our latest findings with the Thor jetting, etc. i.e. My mate Pat & myself:

Done the squish kit & reed mod. I also chamfered the plug hole slightly to remove the sharp edge to discourage pre-ignition. I am using 95 octane fuel & Castrol TTS fully synthetic @ 50:1.

Have got a 35 idle and a 140 main jet fitted. I am using an exhaust gas temperature gauge with the sender just past the steel band on the exhaust pipe right at the manifold. I have set the float height to 21mm. Needle circlip is 2nd from the top.

The highest EGT I am getting is 630 deg. C. I consider 580 - 620 to be good, at 650+ I get nervous!

... Pat got the same results with the same settings, it is comforting to know that there is consistency with the carbs & engines.

On very close examination by Pat, yet to be confirmed by me, the high temp spot, which is in a very small rev range, is the point where the slide is opening but the needle is still parallel where it comes out of its "hole". So there is a constant flow from the idle jet at this point but an increase in air causing the temp to rise. If the needle is lifted one notch higher to counteract this, the mixture is too rich across the mid range & we get poor throttle response & low temps in the 520ish range on the EGT.

The problem as we see it is that the needle taper needs to start a bit earlier but not by lifting it. With some careful measuring with a Vernier gauge and some wet & dry paper, Pat is going to try to reduce the dia. of the needle at the point where it lifts out of its “hole” so the fuel volume goes up along with the air volume. We have looked for a suitable replacement needle with a taper starting higher up but can’t find one.

This hot spot is only at a very, very small range of the throttle movement but is close to the cruise, so we could be sitting on it for a long time, burning the piston.

The idle is still well rich, along with the mail jet if the EGT is anything to go by, so I am going to try a 30 idle & a 135 main once we have removed this high tem spot with the needle taper.

Ideally we are looking for the above stated temp from 1/3 to full throttle.

I haven’t paid much attention to the CHT as I/we consider the EGT to give a better picture as to what is going on in the engine, a bit like the oxygen meter thingy that I can’t remember the name of!

Interestingly, Polini say we could go to 670 on the EGT and they have their EGT sender 200mm further up the pipe away from the engine where the gas will be much cooler! Not on my (engine’s) life!!

Will let you know how it goes. Has anyone else found a small hot spot area & fixed it with a new needle?

Pete

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The problem as we see it is that the needle taper needs to start a bit earlier but not by lifting it. With some careful measuring with a Vernier gauge and some wet & dry paper, Pat is going to try to reduce the dia. of the needle at the point where it lifts out of its “hole” so the fuel volume goes up along with the air volume. We have looked for a suitable replacement needle with a taper starting higher up but can’t find one.

If this is at 0- 3/8 throttle, a different throttle slide may help. A lower number slide will richen the mixture.

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Hi Notch,

You may well be right, but my understanding of the problem is that as the needle is still parallel at the hot spot any slide will increase the air with no extra fuel getting in until the needle comes off the parallel.

The mid range is generaly quite cool, an EGT 0f 540 - 570 so I would like to drop the needle down one more notch to the lowest notch. I have done this before and the engine temps and response is good but the hot spot covers a slightly longer area and goes up to 640+ on the EGT. But it is very flyable now, just trying to get my fuel burn & carbon deposits down.

Pete

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Changing the slide changes the vacuum placed at needle, so can increase or decrease the fuel at 0-3/8 throttle.

Changing the diameter of the needle can also help.

Have you seen the jetting spreadsheet posted on the FB group?

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  • 1 month later...

Hi All,

I have at last got my Thor 200 working just the way I want it!

Here is the story regarding the carb adjustments on my engine. Much credit should go to Pat Gardner, my flying buddy for this, it was his idea.

135 main jet.

30 idle jet.

23mm for the float height, measured as per the way to do it that someone put on Facebook!

Idle mixture 1 turn out.

Needle in its lowest position, i.e. the circlip on the top groove.

Needle re-profiling;

I found that with the needle at its lowest position, I got very good throttle response right up to full throttle, no bogging down after a long idle. The only problem was a hot spot near 1/3 -1/2 opening.

Using the hand throttle once the engine is warm very slowly increase the revs watching the EGT carefully. As soon as the EGT shows a temp above 630 deg. C., don’t move the hand throttle but kill the engine.

Remove the air filter box and mark the needle at the point that it exits the brass tube that is fixed in the venturi.

If you measure from the top of this little brass tube down to the restriction, this is where the needle interacts with the “jet” to control the fuel exiting the jet. I measured it at 3.2mm lower down the brass tube.

So the theory is that 3.2mm down from the mark, as the slide lifts letting more air in, the needle, at the 630 deg. C. EGT is not letting enough fuel in, hence the hot spot.

If you make a mark on the needle 3.2mm down from your original mark, you will see that the needle is still parallel. So the slide is letting more air in & the carb is still effectively only using the idle jet for fuel delivery.

This is where it gets a bit un-scientific!

You need to reduce the diameter of the needle at the lower mark and “blend it in” to the taper lower down.

Place the needle in an electric drill and with fine wet & dry, remove some brass!

Be careful doing this, it is easier to remove too little, do the test again & then remove some more brass going through the marking procedure again.

I must have repeated this seven or eight times before I got my EGT reading 580 – 610 deg. C. right through the rev range.

At the first 0 to ¼ throttle openings, the temps will be much lower, so I did the idle jetting changes (fitting smaller & smaller idle jets) until I got a fast pick-up from idle, even after the engine had been idling for five min.

Some people will be shocked/surprised at so small an idle jet, and the 30 is the smallest Polini supply. But my last engine, the Simonini, was 270cc and its idle jet was a 28. Bigger Rotax engines have smaller idle jets, so I think Polini are well off the mark with this one!

I have mounted my EGT sender as close to the engine as possible, it's 10 mm after the welded ring on the exaust at the engine, a dight fit under the springs. Any further away & the same engine EGT will obviously give you a slightly lower reading. How much lower I haven’t got a clue, so with your EGT further up the pipe & getting the same or higher readings might be risky, I am not sure. I also don’t know at what EGT, with the sender where I have it, that the piston will melt. If I was Polini I would be finding this out, but I’m not going to do it for them!

Finally, most people using this engine (the paramotoring world) don’t use EGT’s in microlighting, they all have EGT's. Because the tiniest position move on the throttle can hit the one little hot spot area, using a CHT to figure out engine temps will give too slow an indication & you will may miss seeing the hot spot until it is too late. That’s why Polini say “don’t use constant throttle positions for any length of time”. Bollocks! Just get rid of the hot spots and you can keep the throttle at one position all day!

A note on the idle screw adjustment; set the idle to slightly faster than normal. Adjust the screw to give you max revs. Slow it down again if the revs have gone up a bit then repeat. Simple as that!

Hope this makes sense.

Pete

57336420a98f0_EGTsender.jpg.c41a3407a43d

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