Can everyone who is running a gas engine (rear carb) like my ZDZ40 please tell me how you have your engine box setup (do you have the top on it? Do you have holes drilled in the box for air? ) Also, is anyone having trouble with air not circulating properly once the plane is in the air.

I have a ZDZ40 in mine with no top on the engine box, the front of the cowl is wide open (no Cowling baffles) and the head of the engine sticks out the bottom. I think I am having air supply problems especially in the air and I'm wondering what setup other people have.

Thanks ahead of time for any help on this.

I have the top on my engine box. Also two large holes drilled top and bottom plus each side.
I cut out the small black rectangle area beneath the spinner so more air could hit the cylinder fins, doesn't look bad at all.
With the head hanging out of the cowl it should get plenty of air for cooling.

I would think you get more airflow flying than you would tied to a stake. I haven't noticed any overheating with what I've got. Doesn't seem hot at all after flying. I think it would be tough to baffle the cowl for an inverted engine.

Why do you think you have air supply problems? I will try to send you a couple of pic's of mine.

Dave

I think I have air supply probs because my engine gets very rich in flight. I can't really think of anything else. With the top off I figured it would get plenty of air but with the cowl on there is actually very little space for air to get in through the top. I think I am creating a low presure area in the engine box and the engine is not getting enough air.

Pics would be great...thanks.

Terry,

I run the top on the engine box. I have two 1 1/2" holes drilled through the bottom of the box for the engine air supply. I cut the front of the box so it is very close/tight to the carb. I also installed a removable 1/8" light ply divider at the firewall so fuel doesn't spit into the fuselage. To remove the divider for fuel line/throttle servo access just remove the top of the box and slide the divider up and out of it's balsa wood guides. My lower cowl is cut out like yours which exposes the head and some of the cylinder fins, I did not cut out the small the small rectangle area. I do not have any baffles installed for cooling. The ZDZ40 runs great all the time and has never had an overheating problem. I have approx.170 flights now and love it. Perhaps by running without a top on the box you are getting turbulent airflow and pressure differentials in the engine box causing your mixture to change and run rich sometimes and lean sometimes with the lean causing heating problems. JMO

Jim,

Thats what I was thinking. I am very new to gas but it sort of made sense to me that I might be creating a vacumm in the engine box or turbulent air that was causing the rich symptoms. I am going to try adding a few holes in the bottom and see if that helps. It also seems much worse when its very hot out. Not sure what that has to do with it?

Thanks

Terry,

The hotter it gets the more you need lean down and the cooler it is the more you should richen up. The reason is simple, cold air is more dense packing more oxygen into the same amount of air so you richen up the mixture to keep the oxygen/fuel mixture the same ratio and just the reverse with warm air. I find it only takes a small adjustment, like less than 3/8 turn on the high speed needle from very cool 40 degree temps to high 90 degree temps.
The ZDZ with it's rotary valve and higher compression is a bit fussier than most engines and the balance between low and high speed jets is a bit harder to attain. You might try since the low speed jet dumps fuel through the entire RPM range but the high speed only affects from about 1/4 to full throttle you might try richening up the low and leaning down the high. Check for a reliable idle and smooth transition from idle to wide open. The engine should be able to idle for a minute then transition to wide open without any burbles (to rich) or hesitating/cutting out (to lean on bottom). Once you have that balance correct you just tweak the high speed depending on the temp/barometer/humidity of the day to max RPM minus a couple hundred for a safety factor.