Mikuni Fuel pump issues

[C5Engineer 1170]

I'm having some serious issues after my engine swap. It will run all day long at 6500rpm in the chocks but everytime I try to take off it starts losing rpms at about 50ft agl. Three times I've had to set it back down on the runway. One of the only variables left is my fuel pump. I'm about positive this is a fuel starvation issue at WOT. It always picks right back up when I pull the power.

When I had things apart I rebuilt the pump with a Mikuni rebuild kit. I'm pretty damn sure I put it back together exactly how it came apart. I was trying to trouble shoot it and I have a few questions. Today I set the power at about 3500 rpms and then shut off my fuel at the shutoff valve which is on a little panel on the front of my seat. The engine would only run for about 30 seconds and I never saw any bubbles in the line nor did I notice a decrease in the flow on my fuel flow gauge. The lines from the pump to the carbs were dry but the line from the shut off valve to the pump was still full of fuel. When I turned the fuel back on you could watch it refill lines to the carbs (gravity). My theory is that it's only gravity feeding and at WOT with the nose in the climb attitude it's starving. Maybe I screwed something up when I rebuilt it? It has a brand new pulse line with hose clamps that are all tight.

Wouldn't a Mikuni pump suck the line dry from the shutoff valve up to the pump?? I ordered a brand new spankin pump from CPS today to eliminate it as a culprit. Anybody else have any theories?? I ran it wide open forever today tied down in the chocks and it never missed a beat and my EGT's were at about 1050. It idles smooth as glass and accelerates just fine on command. If this doesn't work I'm going to be at a total loss as to where to go next. This is getting pretty frustrating and I've now missed out on two awesome flying days.

[C5Engineer 1170]

Just got done talking to the guy that sold me the engine and his theory is the float bowl level is low. If the new pump doesn't solve it I guess that's where I'll go next.

[horsepower 24]

Just got done talking to the guy that sold me the engine and his theory is the float bowl level is low. If the new pump doesn't solve it I guess that's where I'll go next.

Are your carbs new or are you using your old ones?Maybe they are not jetted for your field elevation, but does sound more like a starve out condition than lean.Its too bad it wouldnt do it on run up so you wouldnt have to be a test pilot to see if its going to starve out.I would definately start at the beginning of your runway and hopefully its longer than Orofino's.Float levels would definately do that so your guy may be on track. Good Luck Randy

[C5Engineer 1170]

Randy the carbs are new but they were jetted per the Rotax jetting chart. I'm 20ft and the temp has been right at 60 so the conditions are perfect for the uncorrected chart. I have all the numbers for my old carbs. I was running a 158 main jet and now I'm at a 165. Even before it starts to quit it's not making the kind of power my old one was. It just doesn't accelerate like I used to. I'm a 4000x75 paved strip so more than enough real estate to land on. Like I said this is getting really frustrating. If it did it on the ground or I had high EGT's it would be one thing but to run perfect in the chocks and then crap out while climbing adds a bunch more variables to the equation.

While laying in bed thinking about it last night I thought of one other thing I changed. I have the aluminum tubes in my fuel caps per the Avid AD. I've been having issues with one side leaking. I used some grey RTV on the inside of the cap to cover up 4 little holes that surrounded where the aluminum tubes come through the inside of the cap. I think they were for the original vent portion of the cap. Does air have to be able to get back out of the fuel system??... That and the fuel pump are the only differences to the fuel system since it ran perfect with the other engine.

[horsepower 24]

Randy the carbs are new but they were jetted per the Rotax jetting chart. I'm 20ft and the temp has been right at 60 so the conditions are perfect for the uncorrected chart. I have all the numbers for my old carbs. I was running a 158 main jet and now I'm at a 165. Even before it starts to quit it's not making the kind of power my old one was. It just doesn't accelerate like I used to. I'm a 4000x75 paved strip so more than enough real estate to land on. Like I said this is getting really frustrating. If it did it on the ground or I had high EGT's it would be one thing but to run perfect in the chocks and then crap out while climbing adds a bunch more variables to the equation.

While laying in bed thinking about it last night I thought of one other thing I changed. I have the aluminum tubes in my fuel caps per the Avid AD. I've been having issues with one side leaking. I used some grey RTV on the inside of the cap to cover up 4 little holes that surrounded where the aluminum tubes come through the inside of the cap. I think they were for the original vent portion of the cap. Does air have to be able to get back out of the fuel system??... That and the fuel pump are the only differences to the fuel system since it ran perfect with the other engine.

Joey it sounds like the jetting is right,you say it doesnt accell like it used to, I would put the new pump on and run it on the chocks long enough to get a spark plug reading or maybe do it before the new pump so you can compare the two readings.Check your float levels and eliminate that problem.As far as your tank venting goes doesnt seem likely that one tank would cause that if you are running off of both.My Skyraider uses the plastic tanks that the newer Avids use. The caps are vented but you are supposed to epoxy in a 3 inch tall 1/4 inch alum. tubing.I havent got that far yet but Tim from Skyraider told me to only drill through the first part and epoxy from the out side.When the cap is off and your looking at the inside you should not see the tube.Dont know if yours is like that but you may want to check out.My cap is a Kelch brand. www.kelch.com

They have a website but dont know if they would know anything on aircraft venting.If the fuel pump change doesnt do it then I would get on the horn with the engine guy to eliminate any thing else such as a crankcase pressure leak or a rotary valve timing problem. Good Luck Randy