av8tor1977

My Feedback: (6)

I was running up my latest plane; a Wild Hare 28% Extra with a Stihl 62cc that I converted. With the cowl off, I couldn't get the mixture to go too rich. So I wasn't sure it was going rich enough. I could just keep opening the needle and nothing would happen. There is no room for a velocity stack in the cowl, so I figured the prop blast was blowing away the reversion, if not actually sucking fuel out of the throat of the carb by venturi effect when the prop blast went by the open carb throat. So, I thought, "Well, when the cowl is on, the prop blast won't hit the carb directly", so I installed the cowling.

Running it with the cowling on, it right away went too rich. Even with the high speed needle closed down to 1/2 open, it was still pretty rich. I thought "Well, we know what causes that!" So, I took the cowl back off and ran a static pressure line from the carb regulator cover back into the fuselage so that the carb wouldn't receive cowl pressurized air. Back on with the cowl.

Now, unbelievably, the engine wouldn't go rich enough again. Hard to believe but enough prop blast was getting to the carb to affect it. So since I had noticed when running it with the cowl off that when I placed my hand in front of the carb that it would richen up, I decided to make a baffle to do the same thing with the cowling on.

The pics explain it. Now it tunes perfectly with the cowl on. I wonder how many of the "Mixture Changes in Flight" problems we all have diagnosed as being the static line, might also have had a problem like mine; the airflow past the carb throat disturbing carburetion.....

AV8TOR

Truckracer

My Feedback: (19)

I have a Lanier Laser that has a FPE 4.2 on it with a very similar deflector! Side intake, piston port engines do sometimes present problems that rear intake engines are immune from just because of the location of the carb. But of course rear intake engines can present other problems. I always thought air blast and air pressure problems were related but still separate issues that have to analyzed and dealt exactly like you did on the Extra.

av8tor1977

My Feedback: (6)

It was aggravated in this case because the Stihl has a fairly high intake port timing for a piston ported engine. It is nearly 160 degrees. Some day, if I like the plane enough, I am going to make a 90 degree elbow to mount the carb, and incorporate a reed valve in the process. Another reason to do so, is that with the engine mounted cylinder down, the carb is angled downwards and it wants a good bit of choke, sometimes repeatedly, to get started when it is cold. I'll mount the carb more level if/when I make an elbow manifold for it.

I'll answer your e-mail, probably this evening. I am rushing around getting a bunch of things ready for a big visit from my brother this coming weekend. Hopefully while he is here we'll have time to maiden the Extra, because he can help me hold the plane for the range test, etc. He also needs some "dual" via the buddy box with a plane he left here; a World Models Mach 2. So I need to get it out of mothballs and make it ready as well.

AV8TOR

kmeyers

Same problem with a piston ported 26cc I put in a .60 sized extra that was converted from glow. there wasn't enough original cowl left to fix it so I had to scab on a piece of plastic using ducktape as a "temp" fix at the field. It ran with that temp fix for two years until I crashed it.

Blocking plates can do wonders.

Nothing is as permanent as a temporary fix!

rexbirk

Same issue with a Yak with a 3W 55I Us Edition with a side carb. Could set it perfectly on the ground but when it got moving on the takeoff run it would go rich. I had to block airflow over the carb throat. I learned this from Gearhart at Aircraft International (3W Distributor Supreme). He described this as blowing across the top of a pop bottle. It causes a reduction in pressure over the carb throat. Since the carb doesnt sense pressure there it puts in the fuel it thinks is needed where it is sensing air pressure hence a rich mixture.

av8tor1977

My Feedback: (6)

Duck Tape, Duct Tape, 200 mph Tape, Racers Tape, whatever, it's a miracle product! Have you guys seen the different Mythbusters antics with duct tape?

AV8TOR