HELP!! walbro carb? Velocity Stack?

[Micke Fredriksson]

Hi! I have a strange carb problem.
I´ve newly started my MT50 engine in a 25% yak54. The engine runns like a god in the whole registre up to 6800rpm with the xoar 20x10. It was really easy to tune in. Had a steady idle at 1800rpm- Awsome!! BUT... I hadn´t the cowl around the engine mounted. So now when I mounted the cowl- All hell broke lose. Nothing works. The engine doesn´t respond like it could. I had to decrease the fuel alot to get it run at all. The carb is about 35mm from the cowl. At idle it runs ok up to half throttle, after that something happens inside the cowl that I can´t tune. It just won´t rew over 6200rpm because it misses about 2 sparks every sec. In the air the engine runs even worse.
Is there someone that have experianced something like this?

I tryed to drill another 3mm hole in the membran cower if there was some pressure problems in there.- No differens.
Next thing I did was to make a plastic "wall" in front of the carb intake to make some difference in intake turbulence, just to try something... Now it revs 6300 after 20min of tuning Hmm.

According to this I think I have a pressure/turbulens problem in the cowl that interfears with the carbs normal "stock tuned" funktion.
Do anyone have tryed to mount a velocity stack on the intake to extend the intake OUTSIDE the cowl? It means that I have to extend the intake about 40mm to come out. How will this effekt my MT50? Is it a good idea?

[Zee]

I think you may have problem where you drill the extra hole. That plate should have one hole and you can attach nipple ( braze) and cover the drilled hole, than attach some fuel tubing to the nipple and route the tubing in to the cockpit area past the firewall. Different pressures in side of cowl will mess things up, by suplying static air presure without the cowl pressure should make it lot better.
Zee

[Micke Fredriksson]

Thanks! I will try it as soon as possible!

The velocity stack, is it a bad idea?

[erikv79]

Hi, I too have a problem with this engine. I cant get more than 6,300 rpm on a 20x10 master airscrew classic prop. I have tuned and tuned but no progress. I e-mailed lspro and met with no help whatsoever since i am not the original owner. It seems as though I have the same miss you speak of. Any suggestions?

[Micke Fredriksson]

Hi again! Now I have mounted a "vent" tubing on the carb. I haven´t tested it yet but I will tomorrow- I hope it will work

[smokin]

drawing that air from inside the fuse WILL help alot!

[aerolou2]

I would like to add that unless you have a sealed firewall running the diaphragm vent line into the fuselage will not correct your problem, In reading your post it appears that you are building static pressure inside the cowling and this will cause the fuel control needle to work outside its intended range, Investigate the cooling exit area and make sure that it is in the range of about 2.8/1 , (2.8 times the cowl inlet area), Even if you have a sealed firewall the pressure buildup inside the cowl will cause a nightmare of tuning issues as you will not be able account for the wide range of speeds that your model will experience , hope this helps

Lou

[erikv79]

I cant get these kind of numbers from my mt50. 6300 seems to be the best there is with a 20x10. Ive talked to Larry at lspro engines who pretty much refused to help me since my engine was purchased bfore he started his dealership. I was sent a form letter to the effect that i am on my own. I would be extatic to get 6800!! Milton from rcextremepower told me his experience with the motor was the same 63 to 6400 max. Now, I cant seem to find anyone talking about this motor so sorry to jump in but who can help me. Anyone else having similar problems with this motor?

[Dup]

Hi try this as an alternative solution!
Carb balance
The carb has both a pump and a regulator. The pump is driven by crankcase pressure and is under the carb cover with one screw in the middle. The regulator is on the other side of the carb under the cover which is secured by four screws.
The regulator monitors the air pressure that the engine is drawing air from and compares it with the carb throat depression. As the throttle is opened the vacuum created in the carb throat pulls the diaphragm in opening a valve to increase fuel flow. This is simple and very effective. Until…..
If you have a trumpet drawing air from outside the cowl while the regulator monitors air pressure inside the cowl the regulator may become a little confused!
You may find that the motor runs OK on the ground but then struggles to take off. This is due to a pressure build up inside the cowl as the model accelerates. This pressure causes the regulator to richen the mixture.
Alternatively you may find it runs OK on the ground and in the air until you apply a lot of rudder. The engine goes sick when you feed in the rudder. This is because the air pressure on the outside of the cowl is changing but the regulator doesn’t see this. This causes the motor to go rich with one rudder input and lean with the opposite input!
OK the fix…
On the cover held with four screws you will see a small hole. Remove the cover and block the hole with soft solder. Now drill a hole on the flat part of the cover that is a nice tight fit for a short length of brass tube, the kind of tube found in small glow tanks. Push the brass tube into the hole and soft solder it. The tube should protrude about 10mm. Now clean up the inside of the cover before refitting it to the carb.
I now add a piece of fuel tube to the brass tube and run this out along the trumpet. I secure it to the trumpet with some insulating tape and cut it off flush with the face of the trumpet.
I position the tube at approx. 8 o’clock when looking down the trumpet from outside the model. This position will be close to ideal but a little experimentation with the exact location may help when you start using a lot of rudder!
This way the regulator is monitoring the air the engine is drawing…

[Micke Fredriksson]

Hello everyone! Thanks for all the answers!
Now I have tested the vented regulator cover. I sealed the firewall and mounted the venttube though it in to the fuse.
Yes! The engine runs now much better. But it runs not perfekt. There is some bludder at full throttle, it seems to go rich at full rev. The bludder stopps if I lean it really much. But then I haven´t any throttle respons left- almost goes dead when acc fast from idle to full throttle. (I changed the prop to a 20x8 and I got it to rev 7320rpm but reving is nothing without good respons and control.)
If I rich it in highranch the response is ok and revs up ok. But it wouldn´t rev upto max and starts to bludder. I think the air runns past the carb inlet so fast that it makes a vaccum and make the carb go rich= less air.

Ivé woundered about making a wall in front of the carb from the coal and out "sealed agains the coal" so the air can´t get past the carb inlet and force the carb to suck it from the backside.....

"DUB". The "trumpet" you talking about is it the same as a velosity stack like this: http://www.fullkontroll.se/images/GPMG2100.jpg ?

I think putting a extended "funnel/trumpet" out through the coal could be the only way to get the problem under control! what do you think?

[erikv79]

I think you may be chasing your tail. I have the same problems with the same motor. I cant get a straight answer from anyone who deals with this motor as to what the cause could be. I have replaced the cylinder,carb,reeds, and carb block with no change in performance! I am beginning to believe that the problem is in the design, this is a converted scooter motor, parts can be bought online at many scooter sites. Everyone I have been able to talk to have had the same problems you and I are having. This motor just will not perform, and lspro is unwilling to help other than to refer me to their service dept. (cunningham) Their web site claims 7000 with a 20x10 but i have yet to hear of anyone getting these numbers. If you have not had yours apart heres some pics of the cyl. it looks as though a blind two year old did the porting! If it is under warranty I would take full advantage of it and send it in! The high end blurber you have is also present in mine and others i have talked to. I am at a loss as to what to do next, All that being said if anyone has one of these that does perform id love to hear about it or if you know of someone let me know.

[Micke Fredriksson]

Hello! ErikV79, I´m doing everything I can to get it run. I´ve modefied the "reed intake valve" (don´t know if you call it that in english?) so it would get tighter and seal ok- it didn´t from factory.
Tonight I´ve made a "velocity stack" that I welded togetger in alu. I couldn´t fint anyone who had one in store so to save time I made it my self. It looks like this but is 50mm long http://www.fullkontroll.se/images/GPMG2100.jpg with 20mm inside. I haven´t tested it yet but hopefully I will tomorrow.I designt it to go aprox 20mm outside "through" the coal. I have big hopes for this because my engine ran flawless without the coal. I will start with the venttube inside the fuse but if I still get bugs I´ll try to mount the tube "on" the velocitystack outside the coal, like Mr DUP said