Topic: Terrible gas mileage ... like 40 miles to the TANK ...

[weekend_junkie]

On the topic, I had an '81 CB900F that would only leak out of the overflow lines when it was running due to the vacuum petcock.  It was impossible to diagnose until I started the bike while checking it out.

[larryblevinson]

Well, it is not the tank. I pulled off the rear rubber mount and lifted the tank to check underneath. I did not take it all the way off because the fuel gauge sender unit wires were attached, but I practically stood the thing on its head and it did not leak from the front. Oh, eventually it did, but that was only because the gas was coming out of the filler cap panel. The point is, I shook it and rolled it around and felt all the way on the underside and looked at the frame and head directly under the tank and there was no evidence of leakage that I could see. Sigh. Back to the board de drawing ...

[w1sa]

Try leaving the tank off and/or connect a temporary fuel source and run the bike. While the bike is running, see if you find the source of the leak by following the fuel/ line routing and closely check/inspect  every fuel line junction, fitting and component between fuel reservoir and the intake/engine side of the carbs.

[joerizzo0]

kreem your tank even it its not causing this problem.. its pretty simple and well worth it.. it excludes rust in the tank as a variable for other issues that you may have down the road..

[scottly]

Pull the #3 spark plug; is it wet with gas?

[scondon]

Judging from your report, and where your gas is dripping, I would guess that gas is somehow squirting from your carbs, running underneath your tank and dripping from the front right corner.

I just replaced an engine on a 750 Hondamatic and was puzzled by all the extra stuff on the carbs compared to the other cb750's. A maze of hoses and wires connected to a solenoid and one other component not found on the cb750's. Also the accelerator pump that is common on all the 77-78 750's.

Try removing the rear of the tank from the rubber strap so you can lift it up while the engine is running. Blip the throttle and see if you see gas coming from top of your carbs.

We have an online Honda Service manual for your bike too. Love this site

http://www.sohc4.net/index.php?q=content/technical-0

Check section 4 for your carbs.

[Spanner 1]

Makes sense to me!, as Scondon says ..... I'd be looking at the acc. pump set-up, particularily the hoses that link the carbs and that carry the acc. pump gas, maybe hose 'tween 3 and 4 carb is holed = squirt up and under tank and may only be apparent when throttle twisted ( not idle ) to activate the pump OR hoses from idle increaser circuit ( unique to the A ) connected all wrong and mixed-in with the acc. pump circuit.....??

[larryblevinson]

I think I forgot to mention that I tracked down the leak. As far as I can tell, it is beading up on the head cooling fin adjacent to the No. 3 cylinder's exhaust port, where the header pipe enters the top of the block. Perhaps I loosened something when I was twisting off the muffler?

[TwoTired]

The only gas supply on the exhaust side of the engine is the gas tank.

[larryblevinson]

oh, and should I mention I found a small screw that must have come from the bike on the driveway, and no obvious place to return it?

[scottly]

I think I forgot to mention that I tracked down the leak. As far as I can tell, it is beading up on the head cooling fin adjacent to the No. 3 cylinder's exhaust port, where the header pipe enters the top of the block. Perhaps I loosened something when I was twisting off the muffler?

My guess is that the #3 carb is pissing fuel, and the gas is leaking out the exhaust port. Check the spark plug, check if the #3 exhaust pipe is cooler than the others...

[Spanner 1]

Yeah, I'm with the #3 carb overfilling theory too..... if the float valve doesn't close the access fuel will run into the motor and fill-up the cylinder and drip out at the header to cyl. head junction........ only possible explaination of gas in that spot unless it's dripping down from a hole in the tank ( you say not ).  Blocked #3 overflow tube would cause this......

[TwoTired]

Yeah, I'm with the #3 carb overfilling theory too..... if the float valve doesn't close the access fuel will run into the motor and fill-up the cylinder and drip out at the header to cyl. head junction........ only possible explaination of gas in that spot unless it's dripping down from a hole in the tank ( you say not ).  Blocked #3 overflow tube would cause this......

If that were the case, then there would be gas in the oil, and likely the air box, too.

[scottly]

The OP has reported the leakage only happens when the motor is running. Need input from the OP about plug condition, and pipe temp.

[larryblevinson]

onworking the plug and the pipe temp. run the bike BEFORE checking the plug or after? Obviously, we have to get the bike at operating temperature to check the pipe temp.

[larryblevinson]

Youse guys are great, btw.

[Spanner 1]

#3's not running = cold pipe and the engine vacuum on #3 is sucking the overflowing #3 fuel into the cylinder.... but NOT the fault of the float vavle sticking, but the Main Jet ( or even the Slow Jet ) backed-out and  laying in the carb bowl..... that's the best I got ...!

[Retro Rocket]

I would be careful and check the oil for evidence of fuel contamination, if fuel is pouring through number 3 then some of  it will end up in the crank cases.......not good....

Mick

[Spanner 1]

Agreed Retro, but if my theory is right, the fuel getting sucked into the cyl. is also getting ejected out the exhaust port by compression and would minimise leaking past the rings into the 'sump'....

[scottly]

onworking the plug and the pipe temp. run the bike BEFORE checking the plug or after? Obviously, we have to get the bike at operating temperature to check the pipe temp.

I personally don't like to pull the plugs on a hot motor; tend to get burned fingers. The pipes will get hot rather quickly on cylinders that are firing. I use the spit test: spit on my finger and dab it on the pipe. If it sizzles, its hot. Compare the temps of all four pipes.

[Rosinante]

There are people who are intrigued by these mysteries.....and those who are not.  You guys are the curious type.  Smart, too.

I do not believe that the only two possible sources of the gasoline are the tank, and the exhaust port.  I believe the gas could still be coming from the carb area, getting on the fins, moving around the cylinders and then dripping down the breast of the engine.  These engines are tilted slightly forward, which tilts the fins downward in the front.

The OP should, in my view, continue his research.  If he has indeed eliminated the tank as a source, then he needs to either follow the gasoline route from the tank downstream through the carbs......or upstream from the fins back to the carbs.  I would first try tracking the fuel upstream from the breast of the engine.  I'm guessing the use of paper towels, twisted, or Q-Tips could "find" the gasoline stream on the fuel-side of the engine. 

Another tactic might be to carefully remove the float bowls, making sure none is spilled, and check volume.  One of them might be too full.  But I don't think this is a better tactic than gasoline-tracking.  It might have nothing to do with floats.  I do not think the engine is injesting this fuel.  Perhaps this is as simple as a fuel line connection, or o-ring.  My guess?  O-ring.

[TwoTired]

If there wasn't an obvious rust band visible at the bottom of the tank seen at 45 seconds into the video on reply #9, I'd look for leakage elsewhere.


Now, if you can operate the bike with a different fuel tank, or with a long line from the tank on a stand next to the bike and still get leakage on the exhaust, then I'll buy the carbs or the fuel line to the carbs.  ( I KNOW the wires to the sending unit can be disconnected.)

Cheers,

[KingCustomCycles.com]

ok, thanks. Off with the tank ... anyone know where I can get a new one if this one is toast?

I have 2 or 3 Hondamatic tanks I am not using.  Leasurem@purdue.edu

[Spanner 1]

The 'leak' only occurs when the bike is running according to the OP so it's not the tank and supports my theory.... now how about dropping the #3 float bowl ?

[scottly]

I agree that it may well be a main jet that has dropped into the bowl. I would first do a spit test on the pipes, as it is quick and non-invasive. Also, in the video, it appears that maybe gas is also dripping on the #2 pipe, and it appears that it is not very hot.