Timbers

My Feedback: (3)

is it the 7-90? Did you try replacing the two spark plugs? Did you try movinf those two plugs to different cylinders to see if the problem follows the plugs to the new cylinders or still exists on the previous two cylinders?

If you have the 7-90 then I’m not sure how you are seeing spark coming out of the glass because the boot covers up the whole plug.

partshunt

Timbers, it is a 50cc 7 cylinder. Yes, that is the first thing I did, no difference. I cleaned the plugs and reinstalled them. I see air bubbles in the fuel line originating at the pump. I will deal with the pump and let you know. It is still erratic, hard to adjust the carb...

Timbers

My Feedback: (3)

From my experience a cylinder that isn’t firing just lowers the top RPM of the engine. Erratic running can be the air you are talking about or maybe your pickup? Your pickup isn’t rubbing on the hub is it? I tightened the two screws for my pickup once and the plastic squished and the pickup was too close to the magnets and the engine ran really bad. Maybe check that too.

partshunt

Hello Timbers, You asked why I could see the spark, well only those arcing near the slott at the end. The slot is about 3/8 to 1/2 inches and those plugs arcing can seen. Others may arc hidden from the slot elswhere and cannot be seen, only snaping load enough to hear when turn over by hand, The engine speeds up when the plugs fire (cut in) for a second or so and engine slows down when the plugs cut out. I will check the pick up like you say. Find the airleak also... Thank you for the advice... Joe

partshunt

Got a couple of questions that I asked the supplier but no reply. Was asking what is the spark plug gap for the UMS 50cc- 7Cyl.??? Also, using a remote temp hand held sensor, what is the temperature expected on each cylinder head. I get a variation of 160 to 220 degree F.

Maxam

My Feedback: (62)

The temps should be within 20 degrees of each other at a given RPM. If you have that much variation some cylinders are not firing. -Tom

Hyjinx

Not untrue for sure. My experience weighs heavy on sound and rpm in conjunction with temp readings. I key in on exhaust header nuts within seconds of shut down to try to limit testing variations. Running tests with infrared meters seems very erratic. 160 to 220 would be typical of poor test quality in my humble opinion.
Doggy, less than full rpm acceleration and less than normal top end rpm, variation of sound are noticable indications of dropped or dead cylinders. Cylinders not running are usually just a small amount above ambient.
Lot of factors to consider. Fuel mixture and ignition consistency also may play a prominent role. Some of the ignition modules are finicky. Inconsistent at different voltages etc.

Edhamp

Hi, Just got a new 7-50 for a Skyraider. Just fits cowl but is tight and a prop shaft extension would help. I may get away with a 1/4" spacer bush and using either 20x8 or 18x8 props to start with.
Horizon used to do an extension for the 7-35 glow but I dont think this would work. Any ideas?

Also I was told I did not need a pump with the 7-50 latest engines from UMS, Just keep fuel line short and tank same level. Some engines I ve seen are being sold with a pump. I have not come across a UMS carb before, looks like a glow engine carb so presume tuning is same principle (last time I had a glow engine was 25 years ago!).

The only thing I don.t like so far are the plug leads are about twice as long as needed for a small engine like this.

Thanks Ed

partshunt

Ed, I have the same engine, 50cc-7 cyl UMS with UMS carb. Poor instruction book, I did not know the plug wires have to go on with a hard snap to make them seat, otherwise they misfire, Also, I cannot find the spark plug gap in the UMS instructions either. Anyone know the spark gap and the proper cylinder temperature??

Timbers

My Feedback: (3)

I wanted to post this awhile ago but I managed to Maiden my Phoenix Waco 60cc with the UMS 7-90. Everything went well, I broke the engine in according to the directions by running a full gallon of gas through it at 4 minutes per run, letting it cool off in between.

It ran pretty well in the air. I have a problem getting it to get up to full RPM before the flight. Maybe I'm running it too rich, or maybe the bottom cylinders are just that flooded that it takes awhile to clean up, but it takes probably 3 or 4 minutes of warmup time on the ground before I can get it to spin to about 5400RPM and then I go for the flight. I'm running the APS Powerfuel RX pump at full speed when the engine is running.

Here is a video of the flight. Sorry its shaky and not very well done but I can't fly and video myself! I'm also linking a video of it running on the ground before flying.

Maxam

My Feedback: (62)

I tried to listen carefully. I did hear a cylinder come on as you throttled up. What drives me nuts is so many people blib the throttle or go from idle to full throttle and back a bunch of times. This tells you very little about how the engine runs. Throttle up slowly to full throttle and listen to hear a cylinder come on if it was not firing. If so , yes there is a problem. Did you tune the engine so it is not rich? Very definitely get a thermal temperature gun to find the cold cylinder and then check the plug to see if it is carboned up or not firing or both.

Timbers

My Feedback: (3)

Yes I have a laser gun that I have been playing with watching the cylinder temps. I had it clean up in the air really well one time and it was very powerful. In the Spring I will start experimenting with leaning the high speed needle a bit more to see if I can get it to clean up better on the ground.

Maxam

My Feedback: (62)

Oh and have you run the engine enough so that you can drop the oil content as the manual says. Less oil does help

Timbers

My Feedback: (3)

I've been keeping it at 32:1 in the air too just as a piece of mind safety measure. Do you think I should switch to 40:1? I can do that.

Maxam

My Feedback: (62)

Yes sir, but do not run lean! My 5-100 responded well to the decreased oil content. -Tom

Jaketab

My Feedback: (2)

Timbers. Be patient, your engine will come alive with more run time and proper tuning. The fuel pump enables you to lean the needles slightly more. Leaning the low needle will have more resolve to proper low rpm running. Adjust the radio to slow the speed of the throttle servo to at least 1 second. Radial engines don't like to be slammed open. Give it time to come up to speed. Install temperature telemetry to monitor you temps. And last, do as Maxum say's.

Regards - J Tab

flexjet604

My Feedback: (9)

Hi all was thinking about the 7-90cc or the 5-90cc, are these engine worth the money and easy to operate? I have a Blackhorse Gilmore I want to put it on. Thanks for any advice.

okcrcpilot

I do not believe UMS makes a 90cc 5-cylinder radial. They make a 75cc and 100cc 5-cylinder which is close to what you are looking for. And yes, these are good engines; well worth the money.

Maxam

My Feedback: (62)

I have one and it is excellent! Use a 25x12.

Timbers

My Feedback: (3)

They are worth it but more of a tinkerer’s engine. You will spend more time tuning, tweaking, making baffles, checking valves, breaking in, etc but it will be the coolest engine at your flying field when you are done.

Hyjinx

Been super happy with all mine. Minor field maintenance oiling the valve train and regular lash checks are all I experience.
Biggest issue is everyone gooning over the thing as I do it lol. Fire away at that 100 Cc target. You'll be happy you did!
Aaron-

Maxam

My Feedback: (62)

I have completed my Messerschmitt M 35. I purchased it very used in bad shape and worked hard getting it restored. I hope you guys enjoy the pictures. -Tom

karolh

Very nice looking model. Hope it gives you many happy moments of enjoyment.

Hinckley Bill

My Feedback: (569)

Just happened upon this thread while 'googlin' on UMS260 radials.....very interesting/informative to say the least.
This 1/3 BUSA Stearman is one I bought many years ago and am only now getting around to 'refreshing' for flight....it's modeled after the full scale one in the second picture
It currently has an older 3W120 twin for power, which is more than adequate, but it really deserves a radial.....so I'm thinking UMS260
Would love to hear folks comments on this proposed setup, the motor and any 'words of wisdom' on installation and / or flying.
Thanks in advance

cymaz

Excellent choice a radial. Just watch that too much power can spoil a Model....it sounds like your more than doubling the engine size.