Why do you need a petcock?

Hi All,

This post relates to my 1983 Honda VT250F, but might apply to other bikes...

The VT has a vacuum petcock where a hose (26) runs from the rear cylinder vacuum pipe to the petcock (6) and creates a vacuum when the engine is running. The petcock has three setting, ON, OFF and RES. ON gives you fuel from the main (15) inlet; RES gives you fuel from the reserve inlet (14); and OFF lets the fuel balance between the two inlets, but no fuel will flow from the tube that feeds the front carb (16). The tube shown as 24 is an atmosphere tube.

When the vacuum diaphragm (6) isn't working properly (for whatever reason) the carbs don't get fuel. My question to those more learned than I is why do I need a petcock anyway? The bike seems to run fine when I plug the vacuum tube (26) and just feed fuel into the carbs directly (bypassing the petcock altogether. A simple fuel tap between the tank fuel lines and the front carb seems to work fine. I assume there might be some issues with venting the tank maybe? Any comments would be greatly appreciated. Also, this bike is for track days only.

Cheers

Frank

 

So if the fuel is only flowing (turned on) when the engine is running you should never experience hydraulic lock?

 

Ok, so based on this I can only assume my floats and needles are working properly as I've had the engine running for 15-20 min bursts without a petcock and gravity feeding fuel into the carbs. As the floats rise they're cutting fuel entering the bowels until the floats drop enough for the needle and seat to let more fuel in. Is that right?

 

Thanks

 

I have to say the VT250F runs great without the vacuum petcock (seems to rev quicker), but I was concerned that I might be going to damage something by modifying the fuel delivery system and removing the petcock from the equation. I wonder if the vacuum petcock is used to meter fuel into the bowls and help fuel economy more than it is to aid performance. As a track bike I don't care about fuel economy, but I also don't want to be blowing engines up.

 

Hi Guys,

I hate to keep banging on about this, but I'm curious about a few things...

My ZXR250A (with a C engine) drag bike has a complex series of vacuum devices (fuel tap & vacuum valve). There are hoses from some carbs (2/3) going to the fuel tap and others (1/4) going to the vacuum valve. The fuel petcock has 3 settings...ON, RES & PRI. When I run the bike I normally run it on RES. I'm wondering what will happen if I throw all this vacuum stuff away an just run the bike on PRI when the engine's running? As long as all the vacuum points are sealed properly, I'm guessing it will be fine.

Comments/thoughts?

Frank

 

I'm thinking gravity feed is enough. I can't see the benefit of vacuums when I'm holding throttle wide open.

 

Can someone explain to me what the Vacuum Valve does on a ZXR250C? The ZXR250A does not have this valve on its engine.

As per the attached diagram, the vacuum from cylinders 1 and 4 seems to affect the flow rate from the fuel tank (via the Fuel tap; the vacuum from cylinders 2 and 3 goes to the vacuum valve. The vacuum valve has 4 inlets/outlets...one for the hose from cylinders 2 and 3; one that goes to the airbox (?); one that goes to atmosphere and my personal favourite, one that goes back to the carbs (shown as 6 in the attached diagram).

I'm thinking of throwing all this away and just running my (drag) bike on PRI (prime) fuel setting. I assume plugging all the vacuum spots on the carbs will be fine, but pipe 6 will need to remain to allow the carbs to have atmosphere? Yes/No/Maybe?

 

Thank you Blair. That's exactly what I was hoping to hear.

Cheers

 

Hey Greggie,

Thanks for details. I've actually seen this happen on my original ZXR engine. I bought the bike as a non-runner and when i got it home I did the normal check before trying to diagnose the problem...when I dumped the oil there was so much fluid in there that my oil tray overflowed! You could tell by the smell and viscosity that there was more fuel than oil.

Blair from LiteTek mentioned possible hydraulic lock issues as well, but the rings on my original engine let most of the fuel through to the bottom of the engine.

Thanks to everyone for your help. I have ordered a couple of straight through fuel taps and some rubber plugs. I'll fit them next week.

Cheers