Jetting my Polini Thor (200 Evo) on my F2 Flyer

[LA F2 Flyer]

I thought I'd open a conversation about jetting, since I'm at that point in my build.

My Polini came with a PWK 028 carb out of the box. It had a 148 main jet and a 45 pilot jet installed. On initial run (not break-in; this is a used engine and had already been broken in and flown but with a different carb) my static RPM reached around 5000. From what I was able to deduce online (there is unfortunately limited data available on the Polini engines) the static RPM with my e-prop should be just over 7000RPM. The engine also had an idle hang on deceleration. (Note that the max static RPM will be affected by the pitch of the prop; research what it should be for your configuration.)

My (relatively uneducated) deduction was a rich condition at the top end keeping me from reaching max RPM, and a lean condition on the bottom end causing the idle hang. The idle hang seems to have been solved by bumping up to a 50 pilot jet. I moved to the top end and worked my way down in main jet sizes (from a 148 to a 144, then down to a 140 main) and the RPM has been increasing slowly to 5400, so I feel I'm moving in the right direction. I've since installed a 136 but haven't had a chance to test it. Hot weather has kept me from tuning further - once the temperature drops I'll be able to get back to it.

Some notes - feedback, constructive criticism, additional notes and tips etc appreciated!

Jet for the temperature and elevation you intend to fly your UL in. These 2-strokes tend to need constant attention to mixture, and often can require a rejet for temperature changes. Don't tune in 105 degrees, when you plan to fly in 75 degrees. (There are plenty of sources for density altitude charts and calculators online - use them if you need to.)

Beware the tanks with the "venting caps". I have one that came with my kit; I'm sure you do too. They rarely actually vent. I noticed a severe vacuum the first time I ran the engine. This WILL affect your jetting and create an artificial lean condition. For now I have been loosening the cap while tuning, but have bought a small breather valve which I need to install before I get back to tuning.

The Polini choke - it has proved to be pretty much ineffective (something I wish I had known prior to fabricating a remote choke with a knob on the instrument panel). Apparently this is not unusual with some of the Polini models, as it has been mentioned a few times in the Polini forums. My cold starts are much easier with no choke and the throttle cracked. I tested the choke with the engine cold - no change at all between choke engaged or disengaged. This may be worth testing on an engine stand before you start fabricating or modifying.

At the very least, have an accurate CHT while tuning. 2-strokes are somewhat delicate and temperamental, so watch that gauge. An EGT is often considered better than a CHT because the results are almost instantaneous, whereas the CHT takes a bit longer to register. Having both is ideal.

I intend to get back to tuning soon, as the temps here in Los Angeles drop. More to come!

[LA F2 Flyer]

CALLING ALL 2-STROKE ENTHUSIASTS

It's been a while since I've touched this thread, but the testing has continued, and I hate to admit that I may be stumped. Since my original post, I've dropped the main jet all the way down to a 120 (from 158 originally).

I've moved the needle up and down to try to adjust the mid range.
I've gone through the rest of the engine and made sure I've clamped all lines, fixed potential air leaks (I was missing one of the two gaskets on the intake manifold and have since added it).
I've checked compression (throttle wide open and airbox off) and it showed 120psi or so - a new engine should be 130-150, so I assume I am within spec or close to it.

The result of all this? The static RPM never really changed all that much - it stayed between 5200 and 5500 (roughly). The top end doesn;t stumble, doesn't burble, doesn't seem to sound anything but normal. The engine reaches 5400 RPM or so and just stays there with little variation.

Aside from the 5400 RPM top end, I had a pretty severe idle hang at decelleration - the engine would hang at about 3000 RPM for 10-15 seconds before dropping back down to idle (1500-1800 RPM). I MAY have corrected the idle hang by bumping the pilot jet back down to a 45 (from a 50). I have a feeling the 50 pilot jet required the throttle slide to be open further than it should have been, maintaining a higher than normal airflow when the throttle was "closed". We will see - further testing is required at this time.

But...back to the static RPM. I had heard that is SHOULD be around 7100 RPM, but that seems high considering the engine redline is 7500 RPM or so. (Or am I mistaken about this?)

Also, the "7100 static RPM" spec seems to be repeated on the paramotor forums, but they are turning a smaller diameter prop than I am, by about nine inches (insert size joke here)...how much would this affect a difference in static RPM?

Finally, I removed the prop and ran the engine again (briefly) up to 7500 RPM without any noticeable issues.

What does all of this tell us? I don't know, exactly - that's why I am posting here!

All questions and suggestions are welcome (and needed at this point).

[LA F2 Flyer]

After a very long delay, I finally managed to return to my engine conundrum today. (To recap, I can't get my engine to rev over 5200 RPS or so.)

Air temp here is about 80 degrees, elevation is 600MSL.

The first step was to move my EGT sensor closer to the exhaust port, since I couldn't get it to register a reading at all during my past tuning attempts. It's an EGT purchased from Aircraft Spruce - a dual (CHT/EGT) gauge, and reads the EGT from 700F up to 1500F (or maybe 1600F? I don't actually recall).

The sensor was originally about 6 inches from the exhaust port, but has been moved to about 1.5 inches from the port.

I pulled the plane out, chocked it, and fired it up. I let it warm for a bit, and hit the throttle. Once again it revved to about 5200 or just under. The EGT started to register at around 4500 RPM and leveled off at 800 degrees at 5100 RPM or so.

Based on the EGT, I seem to be running VERY rich, and it's why I can't seem to get over 5200 RPM. However, it makes me a bit nervous because my main jet is currently a 136 and the unit shipped with a 148 main. I feel like that's a pretty significant drop in jet sizes, or am I mistaken?

So should I just trust the EGT and keep going smaller, or what?

And is there another reason my EGT reading might be so low, aside from being rich?

On a side note I am happy that the gauge finally shows a reading...at least it's a small step in the right direction!

Thoughts? Questions? Anyone?

[Badland-F5 Pilot]

After a very long delay, I finally managed to return to my engine conundrum today. (To recap, I can't get my engine to rev over 5200 RPS or so.)

Air temp here is about 80 degrees, elevation is 600MSL.

The first step was to move my EGT sensor closer to the exhaust port, since I couldn't get it to register a reading at all during my past tuning attempts. It's an EGT purchased from Aircraft Spruce - a dual (CHT/EGT) gauge, and reads the EGT from 700F up to 1500F (or maybe 1600F? I don't actually recall).

The sensor was originally about 6 inches from the exhaust port, but has been moved to about 1.5 inches from the port.

I pulled the plane out, chocked it, and fired it up. I let it warm for a bit, and hit the throttle. Once again it revved to about 5200 or just under. The EGT started to register at around 4500 RPM and leveled off at 800 degrees at 5100 RPM or so.

Based on the EGT, I seem to be running VERY rich, and it's why I can't seem to get over 5200 RPM. However, it makes me a bit nervous because my main jet is currently a 136 and the unit shipped with a 148 main. I feel like that's a pretty significant drop in jet sizes, or am I mistaken?

So should I just trust the EGT and keep going smaller, or what?

And is there another reason my EGT reading might be so low, aside from being rich?

On a side note I am happy that the gauge finally shows a reading...at least it's a small step in the right direction!

Thoughts? Questions? Anyone?

Congrats on this step. Good to hear the EGT is showing something that's usable. My suggestion would be to fire it up, run for about 15 minutes at idle, shut it down, check the plug. See what it looks like at idle. If good, then go to a higher RPM with the same process and see what the plug comes out like at that RPM. Pretty much repeat at 1000 RPM increments or half that and see how things look. That should give you a really nice look at what the air/fuel ratio is and when it changes through 5000+ RPM, then start making adjustments ad needed. Hopefully this'll resolve the low top out RPM issue.

[Mountain Cat]

I am going through the same process now with my 303DS. Switching everything over from the Polini gauge to standard steam gauges. Trying to use the Polini senders since they are the smaller metric size. Polini doesn't post what the CHT should be since it's water cooled, but I want to know what it is. It shouldn't be over 450 deg and probably more near 350. My main jet is a 122 since it is the USA version. May have to drop to a 120 to bring the EGT up. But I have had experience with these issues before with Rotax. The Polini is a different beast all around. But still the same principal.

[Badland-F5 Pilot]

I am going through the same process now with my 303DS. Switching everything over from the Polini gauge to standard steam gauges. Trying to use the Polini senders since they are the smaller metric size. Polini doesn't post what the CHT should be since it's water cooled, but I want to know what it is. It shouldn't be over 450 deg and probably more near 350. My main jet is a 122 since it is the USA version. May have to drop to a 120 to bring the EGT up. But I have had experience with these issues before with Rotax. The Polini is a different beast all around. But still the same principal.

So does Pollini tell you what the normal water temp is? I would guess 185 ~ 195, but it would be nice to know both. It would be nice if all the specs and information was easily accessible.

Interesting about the jet size being so small. That might be the answer for Peer. Now that he's successful with getting a working EGT, hopefully that'll help in getting those carb mixtures adjusted.

Hopefully we'll also be getting (praying for) some cooler weather soon, which should provide a more enjoyable environment to work on aircraft.

[Mountain Cat]

Normal water temp for the 303 is 130-194 degrees. But I did order the thermostat kit which has a 134 deg thermostat. From all reports the Polini runs on the cooler side. A CHT, EGT and water temp gauge will warn you of pending problems of the engine ahead of failure. The USA version 303 has a different head, jet and needle to allow running of 91 or 93 oct pump gas. The early Polini's got a bad rap due to burning holes in pistons because of people running pump gas in engines designed for 100LL. Polini made the US version to eliminate this. Though it's a high maintenance engine, its still a wonderful little engine.

[Mountain Cat]

Just got all the gauges and probes I ordered Monday. Now, get everything hooked up and fire it up.

[LA F2 Flyer]

You might be able to find CHT parameters on the Southwest Airsports page. I'll post them if I am able to find info for the 303.

[Mountain Cat]

With most alum cyl's and pistons, 500 deg is about the max. And that doesn't leave any room for gauge error or anything else. I like to see 350-400 deg on CHT's. Then you have a little wiggle room for error. Plus, if something happens (water leak) fan belt breaks (Rotax) you have about a minute to cut power, shut it down, before it gets real quiet on it's own. Plus you might save your engine if you act quick.So it does pay to have as much info available to you about the engine operating conditions. But you have to be scanning the gauges on occasion.

[LA F2 Flyer]

Looking for thoughts on this plug color - this is with the 134 main jet and the float lowered to 23mm.

It may be just slightly darker in the picture than in real life, but it's close. I find the metal ring being so clean to be very strange, but it's been a while since I had to inspect a 2 stroke plug.

This is after about ten minutes of running at various RPM.

[LA F2 Flyer]

Just ran the engine again with the 130 main jet (down from 134). Pilot jet still 45. Revs up to 5460 now (from 5100-5200 prior), so that seems promising.

EGT is up to 825 from 800. Seems like I am moving in the right direction.

Will go down to a 126 main jet and see what happens. More to come!

Still hoping for some feedback on the plug color.

[Badland-F5 Pilot]

Just ran the engine again with the 130 main jet (down from 134). Pilot jet still 45. Revs up to 5460 now (from 5100-5200 prior), so that seems promising.

EGT is up to 825 from 800. Seems like I am moving in the right direction.

Will go down to a 126 main jet and see what happens. More to come!

Still hoping for some feedback on the plug color.

YEAH! Sounding good and yes, the right direction. Good confirmation out of the EGT too and not that much of an increase in heat for such a nice jump in RPM. Very nicely done!

[LA F2 Flyer]

So I ran the engine again tonight, this time with a 126 main jet. Pilot jet is still a 45. Needle clip in the top position (leanest setting). The results were both confusing and disappointing. Revs came up to 5430 (+ or - 30) so same as before.

EGT was now up to 900 degrees F, but the revs just still aren't there. The plug is a lighter color - more of a cream color than a dark brown.

I am considering dropping the float down another 2mm or so without changing anything else, to see what happens.

Thoughts?

[LA F2 Flyer]

Okay, so I've had a chance to run the engine again (with 45/126 jets) now that the float bowl has been dropped to 25mm from 23mm.

The results were promising, after an initial brain malfunction on start-up. (I was pulling and pulling the starter rope before I realized I hadn't hit the main switch yet.) After doing so it started on the first pull.

I let it warm up a bit and hit the throttle. Initial run-up was to 5850 or so (touching 5900 for a brief moment)... this seems promising. It dropped slowly from there and ended up leveling off at at 5490. Slightly higher than before - a good sign (I think). Any thoughts on why it would drop slowly after running for a few seconds like it did?

EGT remained oddly low - 800-825 but I didn't run it for very long...not as long as last time.

So, next steps...do I go smaller on the main jet or try lowering the float a bit further? (I am inclined to drop the float another 1mm, but looking for feedback in the meantime.)

[Badland-F5 Pilot]

I wonder if the initial rev up was because of the fuel bowl being at the right (or better) level, but the slow drop in RPM is because there's not enough fuel going into the bowl? Maybe get a bottle with a taper tip (https://a.co/d/huIoGxo), fill it with gas and temporarily mount it above the carburetor for gravity flow. Then fire up the engine again and see if it'll hold the higher RPM you saw.

At least you know the engine is capable of hitting the higher RPM, and it sounds like the float adjustment made that improvement. The next thing it sounds like is fuel starvation. I think you checked the fuel pressure from the crank vacuum fuel pump sometime back, but I don't recall what PSI you saw, or am I thinking about something else?

I think your plan to drop the float again is a good idea. I would suggest getting the higher RPM consistent would be the next step, followed by adjusting the float again.

[LA F2 Flyer]

So, in spite of the heat here (99 degrees or so) I headed back to the garage for some tuning.

I went down to 45/124 jets - the result was about the same. It revs to about 5490 after hitting 5900 briefly...it's a slow drop to 5490. Tried idle and back up - no change. Tried pumping the primer bulb - no change.

Then I put the original needle back in...no change in revs but the idle hang was back. At least I learned where the hang is coming from - the top of the stock needle is too thick, clearly.

I went back to the old needle but moved the clip down one notch (one notch richer) - results were the same.

Went to 122 main jet - same result.

The top RPM fluctuates maybe 30 RPM between these jet sizes (130 - 122)

Just for giggles, I threw in a 140 main jet - the RPM got up to 5400 and then took a sudden and noticeable drop to 5000. Almost like a burp and drop. I think it's safe to say it was too rich and choking itself out.

Next step might be to drop the float just a little more and start over with the 130 jet and work my way down again. The plug was noticeably white with the 122 main jet, but I am never sure how much the ethanol fuel affects the color. Opinions seem to vary on this.

Here are pics of the plug at its lightest...the color varies from one side to the other so I took a pic of both sides.