Topic: cb750F fueling problem-baffling physics defying mystery!

[blue_roller]

Checking back in here with an update and to say thanks. Gentlemen, I greatly appreciate all the input, suggestions, and anecdotes. I'm very thankful to have a resource like this to turn to. Thank you.

I decided to get methodical with this and isolate potential causes. The tank was removed and a clear line run from the petcock directly into a gas jar below it, without the filter. I tried several conditions and took notes on each. Petcock on run and reserve, with filler cap open and closed. Same steady, full flow of fuel each way. It's not the petcock and it's not the tank vent.

In the same scenario of a clear line routed directly down to an open jar, BUT with an inline filter installed: Flow of fuel is normal, but a bubble is maintained in the top of the filter. There is fuel in the line above the filter. If the line is pinched below the filter to stop the flow, the bubble persists, even with tapping of the filter & petcock and with the filler cap open.

I re-installed the clear line with the inline filter between the petcock and the "T" to feed 1 & 2. Flipped the petcock to "run" and nothing flowed from the tank.

I reinstalled the clear line WITHOUT the filter. The line filled completely (after a bit of tapping). Upon starting the bike, ~3" of bubble developed below the petcock. This persisted. Eventually I found that revving the bike to higher RPM and holding it actually sucked the bubble downstream into the carburetors. Since this, under some moderate riding, the clear line has stayed full and there have been no stalls.

[blue_roller]

HondaMan, thanks for chiming in on this. I bought your book since you posted on this and it has really helped me conceptualize and understand how the fuel system and carburetors are supposed to work. It also made me fall much more deeply in love with my bike. All that stuff about laminar flow and carefully engineered areas of different pressure and airflow is pure beauty.

I'll be re-routing the vent hoses. They were down, just forward of the swingarm before. I'm curious, how does this make a difference?

Here's another thing fixed. The bike as I received it had the WRONG CARB BOOTS! They were too small, probably off a 550. They didn't fit around the carbs!! And they're hard as rocks. I had no idea, so getting the carb assembly on and off was a damned nightmare of frustration. Having the properly sized velocity stacks has got to make a performance difference. Can't wait to go for a spin when she's buttoned back up!

[pjlogue]

The gas has a surface tension and gas with ethanol has a higher surface tension.  If air gets in the line it will hit the inline filter element and stop dead.  If your filter is lower down and behind the jugs the heat can vaporize the fuel and this vapor gets trapped in the filter and blocks the flow.  Any overhead bends or high spots in the fuel line should be omitted.  They can trap air as well.  You are dealing with low static pressure because it is only several - ~8 inches of head pressure in the line. 

The OEM screen is pretty fine but I have had small particles get past the screen and clog up the very minute hole in the slow jet.  I have used filters and found if the filter is up higher and not directly behind the jugs it is less prone to vapor lock.  A 3D matrix filter is better than a thin paper element. 

Has anyone tried a strong magnet in line to trap rust.  As long as the rust has a trace of non oxidized iron in it the magnet should trap it.  It will not do anything for plain old dirt though.

-P.

[TwoTired]

I'll be re-routing the vent hoses. They were down, just forward of the swingarm before. I'm curious, how does this make a difference?

The carb vent lines supply atmospheric pressure to the fuel surface inside the bowl.  This is the push force for all the fuel metering jets inside the carb.
The air swirling about the bike while driving can add or subtract pressure on those vent lines.  You want your vent lines to present an opening at a point where the swirling air has neutral pressure influences.   Honda decided this was just ahead of the swing arm.  The stock air box internally has slightly lower pressure than what is all around the bike, and further it varies with the RPM of the bike as the inlet is generally smaller than the rest of the ductwork and creates a restriction related to velocity.  Any time you get the fuel bowl pressure lower than outside atmospheric, you reduce the fuel jet flow potential inside the carbs creating a leaning effect.  Of course, if you also open up the filter box inlet cross section area, the leaning effects, and pressure differential are less affected.

Here's another thing fixed. The bike as I received it had the WRONG CARB BOOTS! They were too small, probably off a 550. They didn't fit around the carbs!! And they're hard as rocks. I had no idea, so getting the carb assembly on and off was a damned nightmare of frustration. Having the properly sized velocity stacks has got to make a performance difference. Can't wait to go for a spin when she's buttoned back up!

Some casting rubber is made with a chemical solvent called a plasticizer.  This makes them flexible.  Depending on the formula used, the plasticizer out-gasses at a rate relative to the formula used, the heat and humidity it experiences.   This not only makes them harder and less flexible, but the material can lose volume, as well, noticed as shrinking.   You can actually unshrink them using wintergreen oil, in a soaking environment.  The process can be speeded up by diluting with Xylene.

I've done this and made original boots flexible again.  But, the process is time dependent.  I forgot about a soaking boot for a few days, and came back to find a nice very flexible boot expanded to twice the size of what is proper!  It took several months for it to eventually shrink back to proper size.

But, its never a permanent fix, just as a new one, it will eventually shrink and get hard again.  I've been toying with the idea of painting the boots for  encapsulation purposes, which would delay the out-gassing.  Haven't tried that yet, though.

Cheers,

[blue_roller]

PJ, everything you said jibes with what I experienced. This is making more sense now.

Two Tired, thanks for the info. I have heard of boots hardening, but i could not have imagined they would shrink this much. Before you mentioned this, I was certain they were just the wrong parts. This proves what they say about assumptions...