US 35cc - Carb continually going rich... need advice
08-16-2006, 09:34 AM
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US 35cc - Carb continually going rich... need advice
I have a US Engines 35cc on my Dynaflight Super Decathlon. I bought the engine used off eBay about 6 months before I finished the plane... so other than mounting it to a stand and running it for 1 minute just to make sure it would start when I bought it... I didn't really get to run it much until lately.
The engine seems to run well... my first flight will be great... into the second one it will start going rich on me. I will lean it out before the next flight (I have gotten to the point that I just leave the cowling off now because I am doing so much adjusting)... and the 3rd flight will be good... but it starts to go rich again on the next flight.
The needle is not moving while running... it is always in the spot I left it after last adjusting. I was told this may be a sign of the engine still breaking in. Could that be the case? The seller didn't specify how much it has been used, just that it was used. I have about a gallon of gas through it now myself.
Any help would be appreciated. I am afraid of continually leaning it out... it already throws my prop faster than I thought it would... I was expecting 7K+ RPM at the top... I am getting just under 9K with an 18x10 Master Airscrew Classic Series prop.
As you may have guessed... this is my first gasser. So I just am not sure what to expect here.
Thanks,
Jason
The engine seems to run well... my first flight will be great... into the second one it will start going rich on me. I will lean it out before the next flight (I have gotten to the point that I just leave the cowling off now because I am doing so much adjusting)... and the 3rd flight will be good... but it starts to go rich again on the next flight.
The needle is not moving while running... it is always in the spot I left it after last adjusting. I was told this may be a sign of the engine still breaking in. Could that be the case? The seller didn't specify how much it has been used, just that it was used. I have about a gallon of gas through it now myself.
Any help would be appreciated. I am afraid of continually leaning it out... it already throws my prop faster than I thought it would... I was expecting 7K+ RPM at the top... I am getting just under 9K with an 18x10 Master Airscrew Classic Series prop.
As you may have guessed... this is my first gasser. So I just am not sure what to expect here.
Thanks,
Jason
08-16-2006, 10:35 AM
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RE: US 35cc - Carb continually going rich... need advice
Hey Jason,
Why don't you come to the house some night and I'll tune it for you!
Seriously though, it could be breaking in. I would worry more if it was leaning out than getting rich. Keep tuning it until it runs right. Use your tach and find the highest RPM by leaning the high end needle. then back it off (open) about 1/16 to 1/8 of a turn.
Bob Michaud
Why don't you come to the house some night and I'll tune it for you!
Seriously though, it could be breaking in. I would worry more if it was leaning out than getting rich. Keep tuning it until it runs right. Use your tach and find the highest RPM by leaning the high end needle. then back it off (open) about 1/16 to 1/8 of a turn.
Bob Michaud
08-16-2006, 10:49 AM
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RE: US 35cc - Carb continually going rich... need advice
If you have the clearence--try a 19-8 prop. Master Airscrew makes them in a wood schimitar.
I think 9000RPM is too much. Prop for 7400--7800RPM on the bench. Thats plenty. Your tip speeds have got to be going supersonic with an 18-10 at 9000RPM. I'll bet thats loud. You may even need a 20-8 prop to slow it down.
If you don't have the clearence to swing that prop, then go to a 3-blade. If your getting 9000RPM from an 18-10 2-blade, then you can go to an 18-10 3-blade and probably slow it down 1200--1500RPM.
It could be going rich because the carb is oriented into the airstream and there is pressure on the diaphram. That pressure will force the main jet open and let it run too rich. You might have to do the little fix that all the DA guys are doing. Solder a brass tube into the coverplate and then seal the tiny hole on the bottom of the plate. Connect a piece of fuel tubing to the brass tube and run it inside the plane so that it will just see normal atmospheric pressure.
Do a search in this forum and you'll be able to read about it. Maybe someone will post a link to one of the popular threads that shows pictures of how to do it.
I think 9000RPM is too much. Prop for 7400--7800RPM on the bench. Thats plenty. Your tip speeds have got to be going supersonic with an 18-10 at 9000RPM. I'll bet thats loud. You may even need a 20-8 prop to slow it down.
If you don't have the clearence to swing that prop, then go to a 3-blade. If your getting 9000RPM from an 18-10 2-blade, then you can go to an 18-10 3-blade and probably slow it down 1200--1500RPM.
It could be going rich because the carb is oriented into the airstream and there is pressure on the diaphram. That pressure will force the main jet open and let it run too rich. You might have to do the little fix that all the DA guys are doing. Solder a brass tube into the coverplate and then seal the tiny hole on the bottom of the plate. Connect a piece of fuel tubing to the brass tube and run it inside the plane so that it will just see normal atmospheric pressure.
Do a search in this forum and you'll be able to read about it. Maybe someone will post a link to one of the popular threads that shows pictures of how to do it.
08-16-2006, 11:17 AM
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RE: US 35cc - Carb continually going rich... need advice
Thanks for the advice...and yes, that prop is getting LOUD. I don't think I have the clearance for a 20 inch prop... I will start looking for a 3-blade... good thing I have a good handle on this plane... bumping a 3-blade into the ground could get expensive!
As for the airflow issue... that is DEFINATELY an issue with the cowling on... the cowling wouldn't fit around the carb... so I cut a hole out for the carb so it got directly exposed (makes it easy to choke with my thumb, though). I taped a dome over that carb with only an opening in the back so the air wasn't getting forced down the carb... and that helped a lot... but still have the problem with it getting richer on subsequent flights... with or without the cowling.
As for the airflow issue... that is DEFINATELY an issue with the cowling on... the cowling wouldn't fit around the carb... so I cut a hole out for the carb so it got directly exposed (makes it easy to choke with my thumb, though). I taped a dome over that carb with only an opening in the back so the air wasn't getting forced down the carb... and that helped a lot... but still have the problem with it getting richer on subsequent flights... with or without the cowling.
