Four corner seizure

Taft Racing said:
Here's a topic that I'd like to throw out there, Talking with some of our 250 engine powered karts the issue of the remote fuel pump has been the center of attention. The theory being made is the length of the pulse tube from the cylinder to the pump, a couple drivers had issues with fuel starvation at high rpm, The cure they found was to shorten the pulse line to under 2". The thought being that rpms over 5000 the pulse becomes a constant low pressure zone with too long a tube length, causing the pump to stop pulsing. I'm not convinced that that happens, how ever the guys that have done the change have been pulsing off the transfer port and not in the intake track. Is it possible the wave reflection from the pipe is causing such a low pressure effect that the transfers become locked in a low pressure condition at high rpm?

I believe the pump should be mounted in a way that the pump sits as high or higher than the fuel bowl. preventing oil from puddling in the pump at a lower level.
Click to expand...

I would always run the pulse fitting in the case area. The tech reason behind the pulse fitting in the transfers althought I haven't tested it to prove it to you, is because the transfer duct has gas flow activity just like a carb throat. We all know what happens when the air stream passes over the fuel jet orifice in the carb, it creates a vacuum (low pressure zone) pulling fuel up. I'm pretty sure the same thing is happening in the transfer duct. When the piston travels down it forces the charge up the transfers which creates a vacuum over the pulse fitting and when the piston travels up, again it creates a vacuum in the total case area. At the low rpms the transfer duct is not flowing fast enough to create a long vacuum pulse but once the higher rpms come in the small amount of time diminishes leaving only vacuum in the pulse fitting. Basically one force over-rides the other force. The greater force is the high speed gas flow over the pulse fitting.

You can test it for yourself with a sensor hooked up to data recorder or oscilloscope. You don't need to run the fuel pump in the transfer duct at the time of the test, just a sensor so you can see the pulse length and magnitude. Obviously you do this on a dyno unless you are pretty savy with a mychron and the new data inputs that are available now. I don't think the mychron will give you fantastic data, but it would definitely tell you the pulse strength at what rpm and then you can compare this to the case pulse fitting strength.
 
And it's important to remember that; vacuums do not pull, the air, or fuel, gets pushed by the pressure differential.

I have an idea that the flow through the transfers becomes almost continuous as the rpm's increase. Meaning that, now there's a pressure, now there's a vacuum, becomes less noticeable. At some point, the flow might be almost continuous, with very little fluctuation in pressure. A column of mixture, like in the transfers, would need time to start and stop. I would guess that at some point, the starting and stopping, would have to occur so quickly, that the change in pressure would become unnoticeable. Just a theory mind you, but the data seems to support the theory. Read my sig.

From the desk of Al Nunley
Comments compliments criticisms and questions always welcome.
If the data does not support the theory, get a new theory.(Al Nunley
 
For what its worth, the kart specific motors that use external fuel pumps have the pulse line just behind the reed cage. Conclude from that what you will. I've always kept my pulse line as short as possible and try to user "harder" tubing when I can.
 
No one has asked what piston are you using? good name brand? or something on line for $20 and like I said 8oz. 6.5oz. of redline is not the ticket. What are you doing to the piston? anything to help oiling? and what pipe, have you changed the pipe and this is happening?.
 
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