Carb pressure balance problems
04-19-2004, 06:15 AM
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Carb pressure balance problems
Since fitting the cowl to my Homelite 30cc powered Lazer 200, the motor coughs and loses power on the knife-edge. Im thinking its a pressure differential formed between the carb diaphram pressure sensing inside the cowl and the carb inlet which is outside the cowl. On knife-edge right to left (when it happens), the carb maybe sits in a lower pressure pocket developed behind the front of the cowl edge and then with less air entering the carb runs rich, causing the spluttering power loss.
Has anyone else had this problem, if so, whats the cure? I was thinking of tapping an exhaust pressure nipple in to the carb diaphram plate hole and running a pipe out to the carb so that the pressure detected is the same as carb pressure. IF this could work, the carb will automatically compensate for the reduced pressure and keep the mixture constant.
Speaking of pressure, I have to get this sorted out before the end of the month because the plane is due to feature in an advert shoot at that time, AND Ive got to keep the day job going!! - HELP!!!
Has anyone else had this problem, if so, whats the cure? I was thinking of tapping an exhaust pressure nipple in to the carb diaphram plate hole and running a pipe out to the carb so that the pressure detected is the same as carb pressure. IF this could work, the carb will automatically compensate for the reduced pressure and keep the mixture constant.
Speaking of pressure, I have to get this sorted out before the end of the month because the plane is due to feature in an advert shoot at that time, AND Ive got to keep the day job going!! - HELP!!!
04-19-2004, 08:45 AM
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RE: Carb pressure balance problems
The carb diaphragm works from atmospheric pressure, no more or less..If you put a fitting in the cover plate and run a tube into the fuselage through the firewall you will have done all you can do...
04-19-2004, 08:57 AM
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RE: Carb pressure balance problems
Has anyone else had to do this?
I read in RCME (I think?) recently that the guy who put the 'Big Stuff' plane together had to run the pressure monitoring pipe into the fuz because thats where the carb was and the carb needed to measure the static pressure in the fuz not to overcompensate the carb.
Thats why I think Ive got a similar problem by measuring pressure at the carb in the cowl vs drawing air from the slipstream outside the cowl. Ive tuned the motor for straight ahead running, however if the slipstream changes by sideslipping or knife edgeing, there will be a pressure difference between the carb diaphram and the intake. What I need to know now, is where to position the sensor pipe. I was thinking of running it along and behind the velocity stack, terminating at the entrace to the stack.
BTW, and NB, after reading this forum a bit more, I see that everyone is facing their velocity stack AWAY from the slipstream. Im facing mine forward. Maybe this is contributing to the problem...? I would have thought a slight ram effect would be better than a slight vacuum effect, but maybe not, as the mixture will run lean if the ram effect is removed in a slide slip away from the stack.
Anyone fiddled with this with any success??
I read in RCME (I think?) recently that the guy who put the 'Big Stuff' plane together had to run the pressure monitoring pipe into the fuz because thats where the carb was and the carb needed to measure the static pressure in the fuz not to overcompensate the carb.
Thats why I think Ive got a similar problem by measuring pressure at the carb in the cowl vs drawing air from the slipstream outside the cowl. Ive tuned the motor for straight ahead running, however if the slipstream changes by sideslipping or knife edgeing, there will be a pressure difference between the carb diaphram and the intake. What I need to know now, is where to position the sensor pipe. I was thinking of running it along and behind the velocity stack, terminating at the entrace to the stack.
BTW, and NB, after reading this forum a bit more, I see that everyone is facing their velocity stack AWAY from the slipstream. Im facing mine forward. Maybe this is contributing to the problem...? I would have thought a slight ram effect would be better than a slight vacuum effect, but maybe not, as the mixture will run lean if the ram effect is removed in a slide slip away from the stack.
Anyone fiddled with this with any success??
