Fuel tank vent location (1984 750s)

[chris]

Hello,

Just trying to locate the vent for the fuel tank.

Having an issue where running approximately 10 minutes the fuel supply runs out. This mostly happens at about 4kRPM in 4th gear. Anything lower it seems to do okay, hasnt died on me in 1,2, or 3 but then i feel good and push it to 4th and it dies after a few KM.

I looks at my inline fuel filter and see it dry. I open the fuel tank lid and fuel starts rushing into the filter right away. Bike starts up and i get on my way.

I have a vacuum hose from the petcock to the intake manifold on cylinder 2 but im not sure if this is a vacuum assisted petcock or if that is the breather.

Also appears that i have an amazon fueltank cap installed as it looks identical one for sale.

My XL250 has a hose coming right out of the fuel cap for a vent but when i had the tank off for another issue i could not see a vent plug or anything like that.

https://imgur.com/a/aOdd9fk

Id be looking for a proper way to vent the tank or if there is an option to have a fuel cap installed with a vent valve in it if anyone has experienced this problem.

Any help would be appreciated.

[Bob H]

Here's a photo of '84 NH gas tank (I don't have your model).
I think this is similar to my 1993 NH, a small vacuum tube fitting vents to atmosphere.
Mine had tons of emission garbage, I think the stock hose went to a charcoal canister.
I saw where others with Honda shadow would just cut a 12" piece of hose and let it dangle. That is what I did.
I think there is a metal tube that must run up near the top of the inside of the tank, the hose just lets air in as fuel is used up or pressure changes with temperature changing.

[mollusc]

I don't think the "California" 700S (750S) with the extra emissions equipment was sold in Canada, so there should not be a tank vent.  Instead, the tank vents through the cap.  If you've got an aftermarket cap, my guess is that there's where the problem lies.
The original petcocks were vacuum actuated, so that sounds correct.  Definitely not a breather.
There should already be two filter screens built into the fuel system.  Inline filters can often cause starvation issues.

[chris]

Here's a photo of '84 NH gas tank (I don't have your model).
I think this is similar to my 1993 NH, a small vacuum tube fitting vents to atmosphere.
Mine had tons of emission garbage, I think the stock hose went to a charcoal canister.
I saw where others with Honda shadow would just cut a 12" piece of hose and let it dangle. That is what I did.
I think there is a metal tube that must run up near the top of the inside of the tank, the hose just lets air in as fuel is used up or pressure changes with temperature changing.

I was just feeling around under there and cant find any sort of vent.

I don't think the "California" 700S (750S) with the extra emissions equipment was sold in Canada, so there should not be a tank vent.  Instead, the tank vents through the cap.  If you've got an aftermarket cap, my guess is that there's where the problem lies.
The original petcocks were vacuum actuated, so that sounds correct.  Definitely not a breather.
There should already be two filter screens built into the fuel system.  Inline filters can often cause starvation issues.

I guess i could try removing the filter and get a union or new line made up. Or just remove the filter because the housing did help me indicate the issue.

The aftermarket cap i have appears identical to the stock one which i see for sale for about 70USD with shipping to here and im not sure i can notice a vent location or mechanism through the pictures on eBay.

I put about 80km on it just now with the cap cracked open and didnt have any issues. Just went to fill up fuel and got the level a little too high so some dripped out the fill hole when the nose dove down in one braking instance. Made sure it didnt leak down off the tank and onto the exhaust and took it easy. Half tank is my new full tank for now pending proper venting.

[chris]

Haven't yet been able to find out a diagram that shows a vented cap or alternative vent location. Ive been trying to examine my cap to see where it could possibly vent through but cannot identify it. The OEM ones look identical to after market ones top and bottom, no vent to be seen on either side.

I do not know exactly where to drill a hole through my cap to make a vent can be done without ruining the latch mechanism nor do I want to drill a hole in the tank and install a vent.

[Bob H]

There's an OEM new cap for your bike on Ebay.
Considering I spent almost $1,000 on carb diaphragms, OEM gasket sets etc. This $87 isn't so bad.
Some delay shipping from Japan, but no charge for shipping.

His listing shows a lot of other Honda's that use the same part (Magna, Ascot, Shadow) so that may open the door to getting a less expensive used one from the correct year donor.

[chris]

Thanks for the reply, i have had that item in my cart for a bit but am unable to notice how different it is from the one I have. I cannot visually see a vent on the cap and cant find pictures of the bottom side. I am afraid to order as I wouldn't like to pay about 100 dollars for a cap if the issue is still the same. Also cannot find a diagram or information in the manual about tank venting so it does sound like its supposed to all be handled by the cap.

https://imgur.com/a/ANXxARI

This is the bottom of my cap. I could not notice where the vent could possibly be and if its the same as the one NOS OEM one I may just have a blocked vent port which I cannot locate yet. I have yet to pull back the gasket though. 

[mollusc]

Maybe you could try removing your current cap and dropping it in an ultrasonic cleaner for a while?

[mollusc]

Scratch that.  I don't think this is a vented cap.  Someone may have replaced it.  The vented ones are longer underneath: https://www.ebay.com/itm/226135782020

[chris]

Thank you for that. That does look like a possible vent location and the part number does match with Partzilla. I was searching P/N 17620-MB1-023 and not -033 as it has listed both. I cant tell from the picture of the one you linked but it does look like it has to vent from somewhere and is the same price as the other one which no snout on the bottom.

Box in the picture lists it as 17620-MB1-043 though. Reverse searching the PN on partzilla offers no results for bike models, but the pictures it looks identical from every other way. And it does look like it will barley just clear going into the tank from the was the vent is positioned.

https://imgur.com/a/qVPzxTg

[chris]

So i was looking around a way to not have to spend 100 dollars just yet and kept searching for info.

Came across this thread for Honda Rebels https://www.hondarebelforum.com/threads/is-this-a-vented-gas-cap.12879/page-2 which is essentially described the same thing. Looking through the thread and some grainy pictures i managed to get a sense of where to drill a small hole (~1mm) while still maintaining a seal of sorts. Managed to drill a hole about 1inch deep just behind the latches from the bottom side. Had the bike idling and could see fuel coming into the line. I took it for a short spin around (5min) the neighbour hood and it did not starve of fuel and when I went to park it or crack the cap open while I was at stop signs, it did not hiss or appear to have a vacuum as it did before.

Will continue testing this vent possibility as it was too late and dark to get a good test drive in.

Update:
This worked temporarily or at low speed/rpm. Rode about 20Km with no issue going about 4th gear 5krpm highest. When i got to about about 6k rpm and shifted up to 5th, rpm dropped to about 3k and bogged trying to rise. Downshift to 4th, was about 3k rpm, no power just bogged. Clutch in and pulled over, cracked the tank open quickly and bike started to idle smoothly again.

So I will be the new owner of a cap suggested earlier and will then hope this will be solved.

Update:
Got new gas cap in mail. Tested function of vent which was correct. Installed, ran no issues at any RPM. Highway driving okay and high RPM okay. Now I have gotten an actual full tank of gas,