IRCantankerous

Have a gas engine that runs good on the ground, then goes rich at altitude. It will not clear out. Does not stop but loads up and loses power which makes for some hairy landings, having engine surging at low speeds. I have seen a post where the carb was vented into the fuselage, but can not find it again. The vent was off the 4 bolt/screw plate on the carb. My plane is a Yak55 with cowl. Any help thanks.

Truckracer

My Feedback: (19)

Many users here could write volumes of info but rather than repeating what has been written and discussed in detail before, I thought I'd suggest some links here. When I tried a search for the subject here on RCU the search function failed to work. So I went to Google and entered the following words "Walbro Carb Fuselage Vent" and pages of info came back. Rather than me repeating something here, try that yourself and I'm sure you will find what you're looking for.

Go read a bit and if you have any specific questions, come on back and I'm sure you can get them answered.

All Day Dan

My Feedback: (5)

Try flying it without the cowl and see what happens. You may need to vent the cowl or put that vent line in to the fuselage.
. Dan.

IRCantankerous

Thanks Truckracer, never occurred to me to just Google it! The solution was under "Equalizing your
Walbro Carb". I would post a link but do not know how. Thanks Dan.

Truckracer

My Feedback: (19)

As All Day Dan suggested, testing with the cowl on and off is an excellent way to know if you have the problem. You need to determine if the problem is caused by airflow directly striking the vent hole which can happen with front or side mounted carbs. Or is the problem caused by excessive pressure in the cowl? Is the problem constant or does it change with changes in airspeed. Many times the problem is actually a cooling airflow problem caused by excess pressure in the cowl. The fix might be as simple as adding additional cooling air exit area or decreasing the cooling airflow inlet area. On round cowl airplanes such as your YAK, I fabricate a baffle that blocks off all of that huge hole in the front of cowl except the small area just in front of the cylinder. I usually add baffles in the cowl to insure the cooling airflow has to pass through the engine cooling fins and not just around them.

speedracerntrixie

My Feedback: (29)

Good advice but let me add that we should all tune our engines according to what they do in the air and not on the ground.

Lifer

My Feedback: (1)

The vent method is simple and I use it on all my pump-equipped setups. No more issues as described by the OP.

av8tor1977

My Feedback: (6)

Double post. I wonder why that happens sometimes?
AV8TOR

av8tor1977

My Feedback: (6)

As Truckracer said, proper attention to cooling airflow is essential. And as Lifer said, I just vent all my carbs into the fuselage as a matter of course. I would rather just do that while I am building/setting up the plane, than to perhaps find out that I need to and have to go back and do it later.

Then, just as Speedracentrixie said, you have to do your final tuning based on how the engine acts in the air. It seems there is always at least a little tweaking that needs done on the needles based on how the engine acts in the air, even with the regulator vent installed. The engine is loaded completely differently in the air than it is on the ground, and loading makes a difference in how a two stroke needs tuned.

Lastly, if you are still have troubles with a mid throttle burble, try increasing the propellor size/load. A too small propeller will often cause an engine to run ok static on the ground, but will then go rich and burble in flight due to the propeller unloading.

AV8TOR

Truckracer

My Feedback: (19)

Likewise, I almost always just go ahead and install the vent line though I'm a fan of the balsa box termination rather than just running the line into the fuselage. On a couple of planes that had problems, just running the line back into the fuselage had almost zero effect on the problem so it was clear the whole fuselage had pressure issues. I fabricated a small balsa box with a single pinhole in it as RTK who use to be on here suggested and problem solved. The line just terminates to a piece of brass tubing glued into one corner of the box and the box is retained in a convenient location by Velcro.

I still feel it is necessary for a gas engine newbie to know the possible causes of why his engine is running so poorly in the air when it runs great on the ground.

I think the unique red regulator cover that DA uses on some of their engines is an interesting approach to resolving the problem. Likewise, the 3W vent line to the intake area.

av8tor1977

My Feedback: (6)

You know, I was going to add a caveat about the possibility of fuselage pressurization in my post, and then I forgot. My Giant Stik, for example will do that. I have a battery hatch with two four strong rare earth magnets holding it on just behind the wing. In a dive, that hatch will blow right off from the fuselage getting pressurized through some holes in the firewall for fuel lines, spark plug wire, etc. that are a little too big for what passes through them.

They are getting hard to find unless you know a professional photographer, but the small round plastic film containers for 35mm film work great for a static line setup too. Just add a hole for the tubing and a pin hole to allow pressure sensing.

AV8TOR

Truckracer

My Feedback: (19)

I always mention cooling in these threads as cooling airflow, or more clearly the airflow through the cowl is so closely tied to how the resulting air pressure affects the carburetor. The various forums are full of threads about engines that run hot and run rough. The P-51 being the worst offender due to its narrow and tight cowl and owners that refuse to acknowledge the need for proper airflow. The YAK or any plane that has a huge round cowl being a close second. Yup, two extremes from each other and both extremes present their own unique cooling and pressure challenges.

ron12901

If you know a diabetic the small plastic bottles that hold the blood sugar test strips should work for the box.

rstearman

My Feedback: (185)

Sounds like our on the right track, but in some cases engine surging is not a sign of a rich running engine. Suggest checking your engine temperature, check the color of your spark plug, and make sure your baffling your engine correctly . Your exit air flow from your engine cowl should be at least a third larger than the intake. With a round cowling you should baffle the front of the cowling leaving only a opening in front of the engine cylinders .

Venting the carburetor maybe a treatment to the symptom of improper engine ventilation and not a cure to the true problem.

AA5BY

One always has to allow for the strange. On a recent built, The Zenoah G26 surged like crazy at 1/3 throttle. The entire bottom of the engine cowl was open so it wasn't a matter of ram air pressuring the cowl. It was however a matter of the ram air being deflected so as to cause air fluid influence on the vent hole. Simply rotating the plate completely eliminated the surging.

Lifer

My Feedback: (1)

Good analysis and an easy solution. Thanks for sharing.

av8tor1977

My Feedback: (6)

I have a 62cc Stihl engine with a side mounted carb on my 29% Extra. There isn't room in the cowl for a velocity stack on the carb. While test running it, I couldn't get the carb to go rich enough. I had noticed this tendency of this particular engine on the test stand. But if I put my hand in front of the carb, it would go rich. The air stream blowing by the carb inlet was causing it to go lean. I figured that it would not be a problem with the cowl on. However, there was enough airflow going through the cowl that I had the same problem with the cowl on; couldn't get the carb to go rich enough. Finally I made an air dam out of balsa, and glued it into the cowl in front of the carb. Problem solved.

AV8TOR

Bcolici

Wish some of you knowledgeable gas engine folks lived in my area.. Im setting up my first gasser and the throttle linkage was driving me nuts.
Thanks for helping and sharing your knowledge.

Anyhow a plastic bottle of appropriate size seems to work well for the pressurization Mod.

Now Off to look for a video that shows what to Listen for and what needs to be done when tuning a 2 stroke gasser.

rye

My Feedback: (10)

wheres the video please

Truckracer

My Feedback: (19)

What questions do you have about throttle linkages? If specific questions, maybe we can post some photos or answer some questions.

Bcolici

Thanks for the offer Truckracer, i think i have it to my liking.
I will put some pics up here http://www.rcuniverse.com/forum/arf-...cc-thread.html for the setup and move my discussion there.

scoobiemario

My Feedback: (8)

Some good info here.
I wish I read some of that before last weekend.
My da50 overheated and cut off right after take off. And guess what plane was it in. Yup. Mustang