Topic: custom super sport tank

[wannabridin]

hey guys, i had a question for all the guru's out there. 

i just purchased a super sport tank from another member on here, thanks kevins750!!  well i love the length and lines of this tank, the main reason i bought it, but i don't quit like the gas cap and cover that's on there stock.  i love the gas gap on my K tank, and i wanted to incorporate the flip open gas cap with the super sport tank.  here's my question,

could i take off the gas lid and weld a piece of steel on there to cover it up and then install something like carpy's flip open gas cap, http://cb750cafe.com/parts.php?id=67.  if you guys believe this COULD work, let me know!!  i have a welder and i'm a pretty decent fabricator, and i think it will be a pretty badass look!  thanks guys!  i'll post a picture of the tank soon when i get down to the garage. 

[UnCrash]

Yep it certainly could be done. 

Just be careful with your welds to not blow through the metal.  The 750F tanks are notorious for being rusty buckets.  Good luck!

[wannabridin]

well the inside of the tank is pretty clean, at least thru the gas cap, kevin took good care of it!  and i would def. just stitch weld the sucker on the lowest setting.  also, my idea would be to weld up a collar on top of the stock opening to fill in the gap between the filler plate and the opening.  then cut a hole in the filler plate, weld in the collar and smooth it all down then install the gas cap.  does this sound like a good fabrication idea to all the masters??

[KB02]

I think I have seen someone on the forums do this before in the Project Shop. I can't remember who, though. So, Yes, it can be done, but like UnCrash says, be careful with your welding.

[wannabridin]

anyone know of whose done this before?  i've tried searching a bunch, but to no avail!!

[Geeto67]

have you thought about what you want to do about the vent?

[wannabridin]

is there a vent under the cover on the stock tank?  i've just been throwing around ideas, i haven't taken too much of a look at things yet, which is what this long weekend is for!  but if the vent is under there, then i don't know yet...

[Geeto67]

In 1975 Kawasaki got sued because of a fuel cap that popped opened in an accident and resulted in a fire that burned the rider severly (the bike was a 1973 H1 btw). The fall out was legislation requiring better fuel caps. As a result the big four began to make "safe caps", basically tighter sealing caps under hidden lids. If you look at all the new bikes for 1977 every one of them has a twist cap and a locking lid to cover it (cb750, gs750, kz900, etc).

The reason I tell you that story was to tell you this, the old flip up caps had vents built right into the cap so there was no need for an elaborate vent. However because the new laws required sealing caps, the vents became more complicated with a series of tubes venting underneath, and that is how they still do it today.

If you look at the 77-78 supersport tanks there is a fuel vent that comes out underneath in the tunnel that is the tank vent. I don't know how connected it is to the fuel door but if it is anything like the zx7R tank I cut apart it will have a connection to the overflow at the tank filler.

so your options are: buy a vented Monza cap and do away with the in tanke vent system. Find a way to connect the monza cap vent to the in tank vent, or use a different vented cap. One way or the other you will need to vent the tank.

On my zx7r tank, we cut the filler neck and vent system and ended up welding the entire top into my own 1976 kawasaki KH500 tank. the original KH tank did not have a vent system other than the cap so we just scavenged the entire setup from the zx7. works great.  

[Geeto67]

BTW, without a vent the drawing of fuel by the carbs will eventually create a vacuum in the tank and no more fuel will come out so a fuel tank vent is crucial to this.

[KB02]

BTW, without a vent the drawing of fuel by the carbs will eventually create a vacuum in the tank and no more fuel will come out so a fuel tank vent is crucial to this.

A lot of older style fuel caps have built in vents. That may be one way around the issue.

[eurban]

Geeto I am not sure you are quite right on this one.  The gas caps on both the Supersports and the 77/78K tanks (both have the flip lids) have vent holes built into them to allow air to enter the tank as fuel is dispensed.  The "vent" as you call it is actually a drain hole that is designed to allow any spilled fuel (during fill up or perhaps in an accident) to flow down to the street. There should be a hose that connects to the drain tube (that forms the drain hole underneath the lid) nub on the underside of the tank, and that is routed down by the center stand. . . .Wannab, if you want to do away with the flip lid you could either leave the section of the tank with the drain hole in place and simply cover it over or you could remove the tube and weld up the hole on the underside of the tank.  You will need a vented fuel cap of course. . .

[Geeto67]

Geeto I am not sure you are quite right on this one.  The gas caps on both the Supersports and the 77/78K tanks (both have the flip lids) have vent holes built into them to allow air to enter the tank as fuel is dispensed.  The "vent" as you call it is actually a drain hole that is designed to allow any spilled fuel (during fill up or perhaps in an accident) to flow down to the street. There should be a hose that connects to the drain tube (that forms the drain hole underneath the lid) nub on the underside of the tank, and that is routed down by the center stand. . . .Wannab, if you want to do away with the flip lid you could either leave the section of the tank with the drain hole in place and simply cover it over or you could remove the tube and weld up the hole on the underside of the tank.  You will need a vented fuel cap of course. . .

I am pretty sure the vent and the drain hole are connected. It has been a while since i looked at a 78 F tank closely and I have never cut one apart, but that being said the way it works on most bikes is that the vent hole is in a chamber so if overfilled fuel goes in it drains right out as well. I do not think the 77/78 caps have vents in them however, i will have to look. They seal pretty tight to the tank.

[eurban]

I have cut my 78k tank apart in anticipation of doing a similar cap modification.  The tube goes straight thru the tank with no provision / chamber for venting the tank's contents.  On the gas cap there is a hole in the center that is for venting.  The supersport's cap is indentical although the drain hole and tube under the cover and tube is tiny in comparison with the k setup.  I haven't cut an f tank apart but the tube runs straight thru the tank with no apparent chamber . . . . .

[Geeto67]

the chamber is usually a little stamped lip in the tank where the cap fits on, the overfil hole should also be in this area. There are vent to air vented fuel caps and then there are vent to chamber vented caps - the difference is the position of the outter seal. If you look at a 69-76K cap the seal has an airgap between it and the housing meaning that it is meant to draw air from around the seal and though the housing. On the SS, the cap draws it's air from in a channel that runs around the seal, and where the vent hole is. I think.

[eurban]

Geeto, time to look at an actual tank and stop with the speculation which IMHO is adding imaginary complication to the proposed project.  I have both tanks to look at and have done so.  As I have already said, the gas tank caps for both the supersport tanks and the and the 77/78K tanks are vented through the center of the cap to a hole in the center of the raised lip that you use to grab the cap with.   The rubber tank seal is a solid flat washer that seals all 360 degrees to the raised flange on the tank.  On the supersports there is a metal ring "gutter" (no such piece on the 77/78K tanks which have the drain hole at the back way away from the cap) around the cap flange with an opening at the front that is intended to direct any spilled fuel or rain water to the front drain opening and down to the street via tubing. The gutter keeps fuel etc from finding its way to the back of the area under the tank flap which might otherwise trap liquids since it isn't enough of a high spot to allow gravity to do the job of dirrecting liquid to the drain. . .Want a reality check??   I have experienced carb fuel flow issues due to cap vent holes that got crudded up with Kreem liner. Problem was fixed once the crud was remvove from the cap . . . .This ain't no Kawasaki sir!

[bikebitzofvt]

OP - Here's a link that sounds a lot like what you're interested in doing, stumbled across it here in a fuel injection thread:

http://www.orleanscycle.com/Fuel%20injection.htm

I'm thinking of doing the same, with a weld-in vented pop - up cap from Biker's Choice, something like this:

[Geeto67]

Geeto, time to look at an actual tank and stop with the speculation which IMHO is adding imaginary complication to the proposed project.  I have both tanks to look at and have done so.  As I have already said, the gas tank caps for both the supersport tanks and the and the 77/78K tanks are vented through the center of the cap to a hole in the center of the raised lip that you use to grab the cap with.   The rubber tank seal is a solid flat washer that seals all 360 degrees to the raised flange on the tank.  On the supersports there is a metal ring "gutter" (no such piece on the 77/78K tanks which have the drain hole at the back way away from the cap) around the cap flange with an opening at the front that is intended to direct any spilled fuel or rain water to the front drain opening and down to the street via tubing. The gutter keeps fuel etc from finding its way to the back of the area under the tank flap which might otherwise trap liquids since it isn't enough of a high spot to allow gravity to do the job of dirrecting liquid to the drain. . .Want a reality check??   I have experienced carb fuel flow issues due to cap vent holes that got crudded up with Kreem liner. Problem was fixed once the crud was remvove from the cap . . . .This ain't no Kawasaki sir!

just got on ebay and looked at the supersport tanks, didn't even realize that the 75-76 and 77-78 f tanks had different stampings around the fuel cap hole.  look at this auction:

http://cgi.ebay.com/ebaymotors/Used-Honda-supersport-CB750F-tank-1976-77_W0QQitemZ130324114716QQcmdZViewItemQQptZMotorcycles_Parts_Accessories?hash=item1e57ec2d1c&_trksid=p4506.m20.l1116

And you can see the stamped chamber from which the cap draws its air and the pronounced channel where the tube goes. But as to the 77-78 tanks you are correct

[Geeto67]

OP - Here's a link that sounds a lot like what you're interested in doing, stumbled across it here in a fuel injection thread:

http://www.orleanscycle.com/Fuel%20injection.htm

I'm thinking of doing the same, with a weld-in vented pop - up cap from Biker's Choice, something like this:

I have one of these on a kz900 I bought. The only thing I can warn you about is that they were designed for harley style tanks with small tunnels, on my KZ you can't actually get a fuel nozzle in there because the bottom of the cap channel is practically resting on the top of the tunnel.

[wannabridin]

I'm just going to take some close up pictures of each of my tanks, the F tank is a 75-76 tank and I have my 76 K tank too, I'll take a look at both and snap some pics to help settle this debate!

[eurban]

just got on ebay and looked at the supersport tanks, didn't even realize that the 75-76 and 77-78 f tanks had different stampings around the fuel cap hole.  look at this auction:

http://cgi.ebay.com/ebaymotors/Used-Honda-supersport-CB750F-tank-1976-77_W0QQitemZ130324114716QQcmdZViewItemQQptZMotorcycles_Parts_Accessories?hash=item1e57ec2d1c&_trksid=p4506.m20.l1116

And you can see the stamped chamber from which the cap draws its air and the pronounced channel where the tube goes. But as to the 77-78 tanks you are correct

Geeto, the supersport tank on ebay is identical to the one I have in my basement.  To beat a dead horse here, you are still mistaken on how the venting system works on these tanks.  The cap rubber seals all the way around (the rubber seal is the same part number for all years of the supersports) to the outer lip on raised flange on the tank.  The "gutter" as I called it or the circular dam / baffle (whatever) around the cap is for directing liquids. There is no Honda engineered way for air to enter except through the gas cap it self. There is no seal around the tank flip lid so air is free to enter and be sucked into the tank though the gas cap. Sure, some air can be drawn up from the street through the drain tube but that isn't it's primary purpose nor is it required.  Also the drain tube is solid all the way down meaning that it doesn't T off and open up inside the tank. . . .How does all this affect the proposed project??  Any cap installed on the bike should be of the vented variety regardless of whether or not the metal underneath the tank lid is removed.    

[wannabridin]

OP - Here's a link that sounds a lot like what you're interested in doing, stumbled across it here in a fuel injection thread:

http://www.orleanscycle.com/Fuel%20injection.htm

I'm thinking of doing the same, with a weld-in vented pop - up cap from Biker's Choice, something like this:

I have one of these on a kz900 I bought. The only thing I can warn you about is that they were designed for harley style tanks with small tunnels, on my KZ you can't actually get a fuel nozzle in there because the bottom of the cap channel is practically resting on the top of the tunnel.

congrats on the 2000th post, keep it up!!

[wannabridin]

ok, i just got an e-mail back from carpy, and he said the caps are indeed vented.  so this is pretty much the selling point for me, since this will help the tank breathe properly.  pictures of my tanks to come soon so y'all can continue arguing with pictures to prove your points!  :-D

[wannabridin]

ok guys, here are the pics.  the first three are from my 76 K and the last 3 are from my 75 or 76 F tank.  you can clearly see the vent hole in the K cap, directly in the center.  on the F tank i took a close up of the drain under the flap to show where it was.  i can take some other pics, but i think these will clear up some speculation.  the carpy flip up cap is vented, like the K cap, so i think all will work just fine!

[wannabridin]

here's the last pic, wouldn't load more than 5

[Geeto67]

the caps carpy sells are monza caps - he doesn't make them (though all his stuff he makes it seem like he produces it).

http://cgi.ebay.com/ebaymotors/Flip-Top-Gas-Cap-Halibrand-Bobber-Chopper-Hot-Rat-Rod_W0QQitemZ220473839642QQcmdZViewItemQQptZMotors_Car_Truck_Parts_Accessories?hash=item335543d01a&_trksid=p4506.c0.m245

I have a supplier in NY who gets them as well. Shop around this stuff is out there.

either way, if you are going to cut and fill the top of the tank you may want to do something with the tube.