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How Does Neoprene Tubing On Exhaust Affect Carb Settings?

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Old 09-25-2017, 11:13 PM
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Rooster353
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Default How Does Neoprene Tubing On Exhaust Affect Carb Settings?

How would you expect adding 3 inches of neoprene tubing to two exhaust pipes to affect your carb settings,
Engine is a 30cc Homelite with Walbro carb.

1) Would you expect adding the tubing to make the high end richer, leaner, or no effect?

2) Would you expect adding the tubing to make the low end richer, leaner, or no effect?

Exhaust length without tubing is about 1 1/8";
Exhaust length with tubing is about 3 1/2"

The carb is set to run without the tubing.
After installing the tubing, the engine did not run as well.

Thanks.
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Old 09-26-2017, 09:18 AM
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My OS32SX with a Pitts muffler ran better with the tubing.
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Old 09-26-2017, 01:24 PM
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I wouldn't expect that tubing to make much of a difference. However, in general, an increase in exhaust back pressure generally causes an engine to run a bit richer.

You mentioned tuning problems running with the cowl on. This can be caused by three things. The most common is air pressure in the cowl acting on the carb diaphragm causing the carb to go rich in flight. The cure is to solder a piece of 1/8" tubing to that hole in the regulator cover, and run a hose from there into the fuselage to an area of static air. If you drill out that hole to better accept the 1/8" tubing, be sure to take a Dremel and dress up the inside of the cover after soldering, for clearance and safety of the fragile diaphragm.

Secondly, the cowl could be causing heating issues. You may have to add baffles to direct all air over the head and cylinder, and possibly increase the air exit area and/or add a lip to the exit to increase air extraction.

Lastly, and less common, is the possibility of the cowl directing a high speed flow of air directly across the carb opening. This can carry away the "spit back", or reversion, common to all two stroke engines, especially piston ported models. (Non reed valve nor rotary valve engines.) This reversion is normally drawn back into the carb/engine. If a high speed flow of air blows this reversion away, your tune will go lean. I have cured this one before with a simple air dam glued into the cowl in front of the carb. If you have room for a velocity stack on the carb, that can help as well.

Hope these tips help,
AV8TOR

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Old 09-26-2017, 01:39 PM
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On closer inspection of your plane picture, I had one more thought. It looks like the side of the cowl might pass quite closely to the carb inlet. If it does, that could possibly choke the airflow, causing power loss and a richening effect both. I've run into that one too. The cure was to make a tube extension to mount on the carb and go through the cowl. I know, a fellow hates to cut a hole in the cowl, but.....

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Old 09-27-2017, 09:31 AM
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Thanks Av8tor. As my Dear Old Dad used to say, "You know some stuff!"

Yesterday I tached the uncowled engine with and without the neoprene tubing on the exhaust. With no change to the carb needles, adding the tubing added 150-200 rpm to the top end.

Very pleased with that.

I have not yet flown with the engine cowled. The engine seemed to overheat on the ground. I'm going to test it again to verify that overheating could be the problem.

A PT19 has a very long cowling, with the engine sitting in the front half of the cowled area. Before I start hacking away on the cowling, I may try to baffle off the back half of the cowled area behind the engine to keep the air flow through the front where the engine sits.

Someone recommended Creative foam from Michaels as a good source for baffle material.

(The photo is not my airplane, just something I found online to show the front of a PT19.)
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Old 09-29-2017, 01:06 PM
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Wow!!! I got a surprising new present today! A new in box Homelite 30cc. Thank you so much! I really appreciate it. And hey, if I have some Homelite 25cc engines lying around as I think I might, don't worry about us cannibalizing them for parts. I have converted and hopped up quite a few of the 25's, but the 30 is much better and weighs exactly the same, so I don't bother with the 25's anymore. (See the "Sticky" above for a show and tell about me hopping up a Homelite 25cc.)

Here's some pics of the last Homelite 30cc I did. It's "in the wings" waiting for a plane to go on. I built it to put in my big Giles, but I ended up giving the Giles to my brother. That's ok, I have other planes it would "like".

I will try to post a video of it too, but I don't know if that is possible. At any rate, I know the pics will post. Most people just cut off the easy part of the front housing on the Homelites. I go to the extra work to trim everything possible away, both for weight savings and nice looks. I think you'll like how they look and run on CDI instead of the magneto. Super easy starting too!

EDIT: Well heck. It wouldn't let me upload the video. So I created a Youtube account, and put it there. I don't know how to embed the Youtube video here, so I'll just post the link:

https://www.youtube.com/watch?v=knJb...ature=youtu.be

AV8TOR
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Last edited by av8tor1977; 09-29-2017 at 01:37 PM.
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Old 09-29-2017, 08:49 PM
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Wow Av8tor, I'm stunned by the beauty of your craftsmanship on these engines.

Before I saw your photos and video, I had pretty much decided to stay with the magnetos since my current Homelite starts so easily and runs so strong.

Now I'm not so sure.

What is the difference in weight between your CDI and magneto Homelite 30s?

Also, I'm thinking my magneto Homelite 30's break-in might be about complete.
After running about a gallon of gas through it on the test stand and then having about two hours of flight time on it, RPM's now seem to be increasing a couple hundred RPM's after each flight.

5 flights ago with a Master Airscrew 18-6, it was taching about 7400.
After my 3rd flight today, it was taching about 8250.

Or maybe my tach is mafunctioning.

Steve

Last edited by Rooster353; 09-30-2017 at 08:37 AM. Reason: Software won't allow the word "m a x i n g"
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Old 09-29-2017, 09:00 PM
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Deleted. Duplicate post.

Last edited by Rooster353; 09-30-2017 at 08:32 AM.
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Old 10-04-2017, 06:09 PM
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Default Another engine I'm proud of...

Hi, and thanks so much for the compliments. It did turn out nice. I figure I save at least a pound of weight by going to CDI and getting rid of that heavy magneto. Possibly more. And then the engine makes more power too, because it doesn't have the drag of the magnets, the force needed to turn the heavy flywheel and those useless air fins, and you can optimize the timing. Many of the "scooter" guys buy special offset keys to advance the mag timing for this reason. The CDI automatically retards for starting, and at idle, then advances to whatever you set it at. Normally 28 to 30 degrees.

Then there is the cooling benefit. As you have noted, the ignition on a mag motor blocks a bunch of needed cooling airflow. Another thing many don't think about, is that those flywheel fins are throwing (blowing) a disk of useless air outwards that can also interfere with cooling airflow to the engine.

For me, CDI is the only way to go!

With stock rings, it generally takes at least two gallons for an engine to break in; to answer your question about that. Sometimes more. Keep flying in cycles that load the engine, then unload it. That's best for break in. It's also good to do complete heat cycles. Let it cool down completely between flights if you can.

Here's a pic and Youtube video of another engine I did that I am proud of: https://www.youtube.com/watch?v=5Y9T...ature=youtu.be

I made it out of two Echo 25cc weedeater engines, and a bare crankcase I got from Keith at BME engines. (Unfortunately, the crankcases are no longer available.) I built two for myself; one basically stock as proof of concept so to speak, and one with all my trick hop up mods. They are really cool and I have had one flying on my Giant Stik for years now. I also built several of these for my customers. I wish those crankcases were still available!! I do have one more crankcase, but it is to make a 62cc twin. I'll get around to that one one of these days....

AV8TOR
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Old 10-05-2017, 05:43 AM
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Are you going to weld on the VW beetle to the top of the van first?
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Old 10-05-2017, 11:37 AM
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I thought about it!

Actually that was in the middle of a chopped top VW Rat Rod project I was working on for a customer. Unfortunately he lost interest about mid build, and he sold off the pieces. The old VW van I was using for storage, and I sold it too. I now wish I hadn't, because I have a cool idea for using one to make a unique custom bodied hot rod using a VW van as part of it.

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Old 10-06-2017, 12:48 PM
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Av8tor, that twin is awesome!

Your 30cc looked rock solid on the work bench with the bar bell counterweight. The twin seems to move more on the work bench.
Is that just a function of how the twin's test stand was secured to the work bench, or does the twin vibrate more than the 30cc?

And not to change the subject, but today I had the cowling off of the PT19 with the Homelite 30cc.
After tuning the engine with a good top end and good transition, I shut it down and re-installed the tubing.

The ID of the muffler exhaust tubes is 1/2". The ID of the neoprene hose extensions is 5/8" to fit the OD of the muffler exhaust tubes.

After reinstalling the neoprene tubing, the engine started and idled normally, but would not transition to full throttle no matter how slowly I tried it.
Increasing throttle caused the engine to start to kill, and reducing throttle allowed the engine to continue to run.I solved the problem by richening both the high and low needles about 1/8 turn. Now it has a good top end and transition again with the neoprene tubing.

Since this is a different result from the last before and after testing, I am not drawing any general conclusions from today's adjustments.
Just passing along the info for your general amusement.


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Old 10-06-2017, 02:53 PM
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The twin runs much smoother than the 30cc single. With it running in the plane, you can sit a glass of water on top! (SCRATCH THAT; a beer!!) The test stand was so weathered and worn, the plywood was coming apart. That was the last run on it, then I built another. It was so wobbly that I shouldn't have used it that last time, but I was anxious to test run that engine and didn't want to stop to build a new stand.

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