I think I know the answer to my question but I want to make sure. The scenerio is this. I flew this jet with just two large 50oz Dubro tank and uat running amt280.
Then added two more 24oz saddle tank to add more flight time and they are in series configurations. Today I replaced the amt280 with 450 ran it did not noticed this problem until and shut down the engine and look what happened. Most of you guys probably know exactly what's wrong here. So what's wrong with this setup !!!.

Note: picture #4 electric starter mod... no more freaking scuba tank to log around

no vent?

There is vent on the last saddle tank it's not visible though.

HI, with 4 tanks, you have to run the 2 saddle tanks in parallel to the first main tank and then in series to the second main tank. The tank that is installed backwards , sould have the inside plumbing reversed.
Rcpete

It looks to me like Tank number 2 has the labels reversed. I'm not sure I agree that you have to have "two tanks in parallel". You can run four tanks in series no problem. In my megabar, I have one CG tank that is connected from the vent to two tanks in parallel and then out through a T to the UAT.

So from the UAT, I go to a T and then to two main tanks, then they go to a T into the CG tank, then out through the vent. To add another tank on top of the CG tank, in series would be no problem. At that point it becomes an issue of pump head, and loss through your system.

My clue to the reversed nomenclature is that the last tank is full, and then drains at the angle you have it into the next tank.

FWIW, I can not tell where the overflow line from Tank 2 goes, but I assume it goes into the one of the saddle tanks, or a T.

There's not enough vent. With a 450 the fuel draw exceeds the capacity of the vent you currently have. I recommend you run the 2 24oz extra tanks into 2 vent lines into the 50 oz tank. It would give you the effect of an additional vent. Big motors draw a lot of fuel. BV talks about this on his web site with the big motor birds.

Tom, I wanted to say it was not enough vent as well, but when I looked closer, the other tanks appear empty with the last tank full. Not enough vent would not have allowed fuel flow at some point, unless the suction of the last tank was such that it drew fuel back through the system after the pump was turned off, but that would still not explain the draining I'm seeing when he lifted the tank.

This is a little perplexing. Can't wait to hear the final results

Are the 2 saddle tanks T'd into one vent? Each one needs its own vent with each saddle plumbed with separate feeds into the main tank. The only thing that will collapse a tank is not enough air to replace the volume of fuel being pulled by the fuel pump. I'm surprised the motor didn't quit or at least not run at full power due to the lack of fuel.

Like I said I ran thesame setup on the 280 w/o any prob but when i changed to 450 that's when the problem occur.
Tom there's only one vent and its on the very last tank. I came on thesame conclusion that it's drawing more fuel and not enough vents. Another test I did was defuel from the uat and by the way I am using eight cells pack for my fuel pump and I can see that the uat was collappsing too so I think I have to change back to six cells pack.
And yes 450 sucks and powerful engine. I will remove all tanks from the plane and look for seversed plumb.

If there is reverse plumbing as Sean suggested, that of course is a problem. I agree with Tom though, that putting 4 tanks all in series is not ideal for this setup. Having the 2 saddle tanks (in parallel) feed a Tee going to a 50 oz tank would help a bit. Then, this 50 oz tank feeds the next 50 oz, which then feeds the UAT.

But I have a dumb question: where are the inlets on your model??? I'm sure they're there somewhere! [8D]

inlets are JUST visible fwd of the main gear mount......

I fly a Cermark ViperJet using a AMT 280 SP I use a UAT and 4 additonal tanks, I actually had to plumb it a few differant ways to get it to empty just right.

I plumbed UAT to largest center tank (1), Then I plumbed two saddles (2) (3) in parallel, to tank (1) using large T, Then I plumbed the last tank (4) into the the saddle vent lines with a T and a 6mm vent.

IMHO the more tanks used, the more concerned you must be of getting the vent lines larger sequentualy. You can actually hear the differance statically by filling the tanks full, with your fueler pump, then empty them out with same pump, and you can hear the fueler pump going quicker as it empty's.

FWIW, I use same diameter tubing and fittings in all my jets from the Mercury to the AMT 400 and they run perfectly, What I have noticed is the AMT uses same fuel pump on all mine, and what I observe from max volts of pump on ASU after flights is that volts increase.

I'm no expert, but here's my opinion of what's happening.

As you mentioned, your set up worked fine for the smaller engine. So it's not a question of how the plumbing is set up (what connects to what) but rather what is being used to do the plumbing (tube and fitting sizes.)

The tank that shows signs of collapsing is the last tank in the series, just before your UAT. You also mentioned that the UAT shows signs of collapsing when defueling. These two tanks have to suck fuel through all the other tanks and the tubing between them. The larger engine uses fuel at a faster rate. The tubing and fittings you are using are too small for that rate, at the distance the fuel has to flow from the first tank to the UAT, and at the beginning of your flight is providing too much resistance to that flow. The collapsing of the large tank probably occured during the first minute of flight. It has collapsed in such a way that it did not "spring" back after the flight.

In my opinion, you need to replumb the entire system with larger diameter tubing and fittings.

Regards,
...jim

I thanks everyone on providing me a valuable informations. I am realy glad that I conducted this test before heading to the field that morning otherwise it would have been a disaster results. By theway I am already using the large tygon fuel lines. Is there anything larger.

Im was just thinking the same thing [sm=thumbs_up.gif]

6MM tubing is what you need to use. large barbs too. make sure your vent line is also large and can flow the air

6 mm tubing, tanks in parallel, and double vent.

Make a test as follows : run the motor at full throttle for about 2-3 minutes and observe eventual collapsing. Only at sudden idle RPM you will notice the tendency to flame-out because of underpressure built up in the tanks vs the too low voltage of the fuel pump.

Note that the more empty the tanks are, the easier the fuel flow will be, thus less tendency to flame out or to collapse the upfront tank(s).

Nicolas.