How should I vent my tank?

Ok so I picked up a 1982 kz 750. It was a train wreck. I believe the whole build thread is on here. I have completely rebuilt everything and fixed all the problems. My last issue is this. It has an aftermarket petcock on it and it seems to be very hard to source an oem one. Not to mention the price if I can find one. So without an oem one the gas is not being pushed into the carbs. The bike runs great but only for a short period of time. The gas is not going into the carbs bc the tank is not vented. The complete air suction system has been deleted. Any ideas on how to vent it? My current idea is to drill a small sized hole somewhere on the top is a somewhat hidden location and put a hose in there. The idea came from how a dirt bike gas tank is vented. I have some very good gas resistant sealant that I would use to seal the hose in the hole.

The bike before and after.

Don't drill any holes in anything!

It sounds like you are confusing the fuel tank vent (which is in the gas cap, not the petcock) with the vacuum operated petcock. If the vent is clogged you should be able to clear it by disassembling and cleaning the vent in the gas cap. One easy way to see if this is the problem is to run the engine with the gas cap open. With it open the tank is vented without question; so if the engine is starved for fuel with the cap open you have a different issue.

The suction to the petcock from carb #2 isn't used for venting anything, it is used to open the vacuum operated petcock so fuel can flow to the carbs. If your aftermarket petcock is the type that is vacuum operated you will need to connect the line from its vacuum port to the #2 carb vacuum port. As a test, if it is that type of petcock, you can run the engine with the petcock in the "prime" or "reserve" position. In thos positions no vacuum is required fo rfuel to flow.

If fuel still doesn't flow with the gas cap open and the petcock in the "prime" or "reserve" position you may have a clogged petcock screen, clogged fuel line, or clogged inline filter (if installed). One easy test is to pull the fuel line off the carbs and direct it into a catch can to see if fuel flows with the petcock in the various positions. Ed

Thanks for all the info. Didn't realize the vent was in the gas cap. I'll check and clean that first. Next as far as my petcock it doesn't seem to flow while in the on position. Only in the prime and off position. My buddies xs1100 does this also. He has no problems tho. Their is no where to run a hose to any vacuum port on the petcock. It seems to flow fine but ill check for clogs anyway. I have new fuel lines and filter so I doubt they are clogged. Any more info based on this Ed?

Can you show a picture of the petcock? The manual ones normally have "OFF, "ON", and "RESERVE." (Below is an image of a stock KZ650-C1 petcock pulled apart, but you can see the position markings.) The vacuum operated petcocks are normally marked "RUN", "PRIME", and "RESERVE." The vacuum operated petcocks have a second rubber hose that runs from the petcock to a vacuum port on the #2 carb. If yours is that type, and if fuel doesn't flow when it is in the "RUN" position, either the vacuum line to the carb is not connected (or is clogged), or the petcock has an internal problem. Often the vacuum operated diaphragm inside the petcock fails. Also, if your petcock is the vacuum operated type make sure the vacuum hose is attached to the correct port on the #2 carb (see second image below). Ed

I cleaned the gas cap. Wasn't too dirty. Looked like normal dirt build up. I don't feel as tho it was the problem. It's clean now tho. Here are some pics of my petcock. It's labeled as follows: on, off, and r ( what I believe can only mean reserve) I feel like an idiot. Just pulled the tank of and tested to see the flow of each setting. And they do as they are supposed to. I have only had small amounts of gas in the tank to test the running of the bike. I guess it wasn't enough to make it to the petcock or it was on one side of the tank. Shook the tank around and it started to come out. I'm gonna fill the tank and test it out and see what the results are. Now next question. There is an air bubble in the fuel line. Do I need to bleed the line in any way or is it of no concern?

Upon inspection. I can see what's happening. Unscrewed the petcock from its mounting plate. In the off position it's closed of course. On reserve the hole opens up allowing what little gas is left in the tank to pass through. See pic. And on the on position with the gas tank filled the gas enters from the top. See pic.

My problem was I believed what little gas I had in the tank would be enough but it wasn't. So let me try a full tank and see what happens. Now as far as the air bubble in the line is that a problem?

Air in the fuel line is no problem at all. That is normal and will not affect the flow of fuel. That's one of the great things about gravity fed carbs. Ed

Splendid. Now I'll have to test the full gas tank thing tomorrow but in the mean time I have another question. As I had mentioned the air suction system has been deleted. The valve or cam cover has two flat plates to delete the two reed or suction valves. The crank case breather is just open with no hose on it. When the bike runs air is just blowing out. I want to cap it off but can I do that? Doesn't the engine need to be vented to release pressure and this is the only other vent. If I cap it then the pressure will build and cause damage. Am I correct in my thinking?

The bike idles fine but when the throttle is revd it kinda has rev hang and the idle then is a little funny. Is this the reason why?

Could I fab up a catch can of some sort with a filter? I'm trying to apply my knowledge of cars to this. Just ranting over here haha

No you should not cap it off. It needs to vent. You could connect a hose from and run the hose to the back of the engine. Or you can buy a filter from you local auto parts store and connectit right to the vent.