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GerardTC

Carb adjustments. Help needed

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Hi there,

Need some help to adjust the carburetor of my Thor 200 engine (recently upgraded with the HF kit).
After installing all the new parts (carb, air box, head, and fuel pump) the engine doesn't run very smooth. Here are the symptoms:

- Rough idle. Jumps from 1500 to 2500 rpm (1800 is ideal).
- Doesn't go higher than 7000 rpm. Should reach 7800-8000 rpm

I tried to unscrew 1/4 turn the air screw and the rpm reaches 7200 but the color of the spark plug indicates a lean mixture.

What would you recommend me to solve this issues?

The actual carburetor settings are:

Main jet: 142
Pilot jet: 45
Needle clip: middle
Air screw: 1 turn out.

 

Thanks in advance!

 

 

Edited by GerardTC

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Better check the reeds or replace them since it looks like you're doing a complete refresh.  If the reeds are good then sounds like maybe an air leak somewhere or your carb is not setup right.  Is there a pop-off valve in your carb?  Down the road you might need to pressurize the crankcase to check all the seals.

Also, is compression within specifications?

Edited by steelmesh

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1 hour ago, steelmesh said:

Better check the reeds or replace them since it looks like you're doing a complete refresh.  If the reeds are good then sounds like maybe an air leak somewhere or your carb is not setup right.  Is there a pop-off valve in your carb?  Down the road you might need to pressurize the crankcase to check all the seals.

Also, is compression within specifications?

Thanks for your response.

Today I replaced the reed valve and changed the main jet to a smaller one (140). Fortunately the engine revs up better and reaches 7800 rpm. Spark plug colour looks good.

However the idle is still very rough... between 1500 and 2500 rpm (is it normal?). I tried bigger and smaller pilot jets without noticing major differences.

Next I will measure the piston squish clearance and compression but I have no references. Does anyone know these values?

Thanks in advance!

BTW, thumbs down for Polini's information and documentation on their engines. I only found a couple of PDF with very little information. 

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The air screw should be used to set your idle fueling and the idle adjustment is used to crack open the barrel or butterfly.  Assuming your pilot/low jet is correctly sized you should be able to adjust the air screw until you get a smooth idle.  If you're air screw is way off then that means you might have to adjust the pilot jet.  

How many meters above sea level are you tuning your engine?  Is it hot out too?

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I am tuning the carb at about 200m altitude and most of the times I fly between 200 and 500m. Spanish climate, currently it's 20-25ºC but in winter season we fly at 5-15ºC.

Since this carburetor is very sensitive, in colder temperatures I will probably need to go for bigger jets. On the contrary, in higher altitude flights (+1000m) I'll need to go for smaller jets.

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On 20/09/2019 at 21:02, GerardTC said:

I am tuning the carb at about 200m altitude and most of the times I fly between 200 and 500m. Spanish climate, currently it's 20-25ºC but in winter season we fly at 5-15ºC.

Since this carburetor is very sensitive, in colder temperatures I will probably need to go for bigger jets. On the contrary, in higher altitude flights (+1000m) I'll need to go for smaller jets.

My experience with carbs is from tuning 2T ground vehicles and watercraft. I am pretty new to the 2T aircraft engines, in theory they are pretty much the same as any other low displacement 2T engine, but as you pointed out new variables are introduced from a relatively extreme change in operating conditions on-the-fly pun intended.

My only thought is to reference manufacturer settings then make adjustments from there for your application and location.  That sounds good on paper; however,  my experience with reality is that manufacturer specs aren't always right for your application or at least aren't always fully optmized (because emissions or dumb people for example).    

I hope to hear from some tuners who have experiences with these aircraft carbs, always something to learn.

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My experience is that some of the engines I have had (AC Nitro and Tornado) need virtually no tuning. Others needed, Polini (a lot) and Moster (a little). 

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For those who have problems like me to adjust a PWK carburetor, I found something interesting.

The image attached is the jetting chart provided by KTM for their 2 stroke bikes with a PWK carburetor (model PWK-S 36AG).
Take it as a reference for a smaller carb like the PWK 28.

 

ktm jetting chart.png

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Just for reference, I fly my 200 evo (no HF-kit, sealevel) with the following settings:

  • Main Jet: 146
  • Idle:42
  • Needle pos: 2.

I found the pwk carb very sensitive for float adjustment, however when adjusted correct it will operate super all-year around (I fly Aspen fuel).

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4 hours ago, coolhand said:

Just for reference, I fly my 200 evo (no HF-kit, sealevel) with the following settings:

  • Main Jet: 146
  • Idle:42
  • Needle pos: 2.

I found the pwk carb very sensitive for float adjustment, however when adjusted correct it will operate super all-year around (I fly Aspen fuel).

What is the max rpm of your engine? Do you change jets in hotter or cooler temps?

Regards

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No sure about the max. rpm. will check when the weather gets better (at the moment it is raining cats and dogs).

I never change my jets, flown between +35 and -15 (C) sealevel.

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On 28/09/2019 at 13:54, GerardTC said:

What is the max rpm of your engine? Do you change jets in hotter or cooler temps?

Regards

Finally had one flying day yesterday. Checked the max. RPM => 8200 RPM.

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My guess first off if you've fitted the new carb that comes with the kit it's probably the best place to start.... However, don't be shy at checking the stator and your earth's.... Stators are an increasingly common issue on these. And for the sake of a 2 second ohms test could be the first easiest test to try even before even touching the carb.... A nightmare to do however.... Especially since you've probably just done all the work you need to do putting the kit on.

Let us know how ye get on.

 

Ps if the stator is fine.... It might be worth just trying the old carb on first just to see how it runs with it before stripping down new carb to tune it up and change jets etc

 

Edited by FlexFlying
To switch on replies

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3 hours ago, FlexFlying said:

My guess first off if you've fitted the new carb that comes with the kit it's probably the best place to start.... However, don't be shy at checking the stator and your earth's.... Stators are an increasingly common issue on these. And for the sake of a 2 second ohms test could be the first easiest test to try even before even touching the carb.... A nightmare to do however.... Especially since you've probably just done all the work you need to do putting the kit on.

Let us know how ye get on.

 

Ps if the stator is fine.... It might be worth just trying the old carb on first just to see how it runs with it before stripping down new carb to tune it up and change jets etc

 

Thanks for your suggestion,

After few tests I've finally found the correct adjustment for my carb.

Hight jet: 140
Low jet: 42
Needle clip: middle position
Air screw. 1+ 1/8 turn out.

Engine runs very smooth on the medium range. No more jumps from 5000 rpm to +6000rpm. Tops at 7850. Drastic reduction in CH temp (20-30 ºC in medium range and 40-50 ºC in max range)

However I still have some issues...

  • Rough idle. Jumps constantly from 1400 to 2500 rpm. Not a problem when flying.
  • I have some leaks from the spark plug. I've realized that I don't have the small washer that should be placed between the CH and the spark plug cooler (see pic 1). I've tried to replace it with the temp gauge but it still leaks.

Does anyone have the thickness measurements of this washer? And should I remove the washer that all spark plugs have at the top (pic 2)?

Thanks in advance

 

 

 

 

Captura de Pantalla 2019-10-01 a les 23.26.03.png

Captura de Pantalla 2019-10-12 a les 17.35.13.png

Edited by GerardTC

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why are you considering removing the crush washer on the plug? the function of it is to help keep the cylinder head pressures from bypassing the threads as when you tighten it down the flat part of the plug crushes it down to conform to the shape in the outside of the cylinder head (or in your case the plug cooler) also depending on cylinder head/combustion chamber design if you do that the plug will then sit quite a few MM deeper into the combustion chamber possibly increasing chance of piston to plug contact depending on how shallow the CC's dome is....i don't know much about paramotor engines to know what the head or combustion chamber/squish band sizing's are like but i'd probably avoid doing the crush washer removal unless its specified in whatever modifications you have done.

Have you done some full throttle pulls to see what the plug colour is like? if you've had a big reduction in temps then you've most likely been too lean and you now have it in the area when some smaller adjustments are required

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On 19/09/2019 at 18:39, GerardTC said:

Thanks for your response.

Today I replaced the reed valve and changed the main jet to a smaller one (140). Fortunately the engine revs up better and reaches 7800 rpm. Spark plug colour looks good.

However the idle is still very rough... between 1500 and 2500 rpm (is it normal?). I tried bigger and smaller pilot jets without noticing major differences.

Next I will measure the piston squish clearance and compression but I have no references. Does anyone know these values?

Thanks in advance!

BTW, thumbs down for Polini's information and documentation on their engines. I only found a couple of PDF with very little information. 

Do you have a pop-off valve and if so have you checked it?  I know pop-off in mikuni carbs works with the low speed jet, so if you're pop-off is way off then your changes to the low speed jet might not have enough impact to compensate for an out-of-spec pop-off pressure.

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On 12/10/2019 at 14:25, FlexFlying said:

My guess first off if you've fitted the new carb that comes with the kit it's probably the best place to start.... However, don't be shy at checking the stator and your earth's.... Stators are an increasingly common issue on these. And for the sake of a 2 second ohms test could be the first easiest test to try even before even touching the carb.... A nightmare to do however.... Especially since you've probably just done all the work you need to do putting the kit on.

Let us know how ye get on.

 

Ps if the stator is fine.... It might be worth just trying the old carb on first just to see how it runs with it before stripping down new carb to tune it up and change jets etc

 

You were right! Last day I couldn't start my engine. Checked the spark plug and the spark was very weak and inconsistent during the start pulls. A mechanic (who is also a pilot) tested the stator with a multimeter and confirmed it is fried...

 

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Stator should read 300ohm's + or - 10%  if your running a regulator bin it there crap!! most failures i see are due to regulator malfunction. should be max 14.8v some i've seen tested are pumping out 19volts.. 

idle air screw 1-1.5 turns out.. Have you fitted the correct new prop with the HF kit.. 

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