1984 Honda Nighthawk CB700SC | Fuel intake issue (?)

[rev.riveter]

Hey guys! First time posting here. I feel like I'm out of options, and the season is coming to a close and I'm doing everything I can to get this fixed before winter.
Recently I picked up a sweet red/black honda nighthawk, 1984. I also have the manual as well if you guys need it.
Seller told me it had no issues, but he also never rode it more than 30 minutes, up and down his block for 2 years. As for the other 38, I'm not sure.
THE PROBLEM:
Anyway, I'm riding around and I notice my fuel gauge going very quickly down from half, and dipping in and out of reserve. I panic and start trying to find a gas station, and I pull over to see if the issue is just with the gauge itself and the bike still has gas. I check, the bike still has plenty of gas but I'm cautious so I still venture to the gas station. The bike dies in an intersection, between a bus and a car- and I end up having to go for gas and haul it back to the bike parked on the sidewalk. I collect 3 gallons as the bike takes 4 when its totally empty.
The bike wasn't empty, it only took about 1.5 gallons of gas. Started good as new, and I was on my way.
Since then I have tried a few things, I checked  if the fuel gauge was faulty- and it didnt seem like it (but now I think it is, in the lower end). I drained the gas to check for rust or blockage- nothing, it's perfect.
I've had this issue since- the bike will say it's empty as soon as a little over 1 gallon is used, and once it's filled the fuel gauge will return to full and operate quite well until a gallon is used again.
I know many of you will say "carb issue" but the carbs are very well synched. I can check again, but I don't think that's the source of the problem.
What are your thoughts? Personally, I'm left thinking it's an issue with the vacuum fuel injector or something just to do with the injection of fuel. I was excited to use this as a sport-touring, but so far none of my rides have been able to exceed 50 miles, which is absolutely awful.
Thank you!

[Sterminare]

Is the petcock fuel filter screen clean? Maybe the bottom portion is clogged with sediment and so it only draws fuel from the tip, for the first gallon or so

[Larry Fine]

Welcome to the forum.

It sounds like the gauge float sending unit is dirty, so stop worrying about what it says for now. Go by the odometer for refueling.

You should probably drain the carb fuel bowls once or twice. Water from ethanol and condensation tends to sink to the lowest points.

Most Nighthawks have two fuel filters: a fine mesh sock over the petcock tube inside the tank, and the screen inside the petcock bowl.

[mollusc]

It could also be that your tank isn't venting properly.  Do you hear a suck of air when you open the cap soon after it stops?

Also, I noted your comment that you "also have the manual as well if you guys need it."  Per the user agreement and forum rules, copyrighted material (including manuals) is not to be shared on this site.

[rev.riveter]

To reply to the questions I've gotten;
- I do hear a bit of air- I have described it to others as a wind/breathy sound- but it's while im riding with everything closed and running so I'm unsure if it's related.

How would you go about systematically testing for issues? I am new to this, so I'm unsure of what would be my priority.

[mollusc]

One way you could test the tank vent is to not close the cap fully (i.e. don't lock it) and go for a ride.  If the behaviour doesn't reoccur, you can rule that out.
I'm not super confident that ventilation is your issue, because I would expect it to show up more quickly.

When checking for issues, only change one factor at a time.  Test, revert to the original settings, verify that the issue remains, and then re-test the fix to be sure of your result.  Being methodical is your best approach.

[slodoug]

i'd replace the petcock, its easy and cheap. i moved to a non-vacuum one so i know its not the issue.

[mollusc]

The 700S petcock is not cheap at all, and there aren't really any aftermarket options.

[draser]

If I read your post correctly, you have 2 issues: a) fuel gauge sending unit (see Larry's post) and b) fuel delivery stops after riding a while.

Now, to fix first issue, if the gauge does the same thing consistently, then you need to check the sending unit. If the gauge is erratic, shows full/empty/low with no apparent pattern, then it may be a loose connection between the sending unit and the gauge.

For engine stall after riding a while, can you restart without adding any gas? I'm thinking low fuel level in bowls causing starvation, then after a few seconds bowls will fill up with gas and can restart the engine.