1978 CB750 Carbs - Accelerator Pump Fuel line mystery parts

[alexm91]

I took apart and rebuilt my 1978 CB750f carbs. When I took off the accelerator pump fuel rail lines I found brass bushings inside the fuel line.

I do not see anything like that on the carb schematics. Are these barrels needed? I'm guessing it is for increased fuel velocity?

Please let me know if anyone has imput on these mystery barrels in the fuel rail.

 

[amc49]

Normally the restriction is at the nozzle exit...................Someone else may chime in, I for one cannot say.

 

[madmtnmotors]

Yes, they are required. Save these and place them inside the new crossover rubber hose. They are NLA from Honda, but new ones can be fabricated from simple metal tubing of proper inner and outer diameter. I could find the proper dimensions if necessary, but it sounds like you retrieved the originals before discarding the old hoses.

 

[alexm91]

Thank you. The brass barrels have an ID of 2mm and OD of 4mm.

The original rubber tubing has an ID of about 3.9 and OD of 8mm.

Do you know the best place to get replacement tubing? I'd like to stick with original metric tubing.

 

[madmtnmotors]

You can still get bulk 3.5mm tubing of the correct size directly from Honda. It's a little pricey but I like the way the stock tubing fits properly:

https://www.hondapartsnow.com/genuine/honda~bulk~hose~fuel~95001-35003-60m.html

The 3.5mm tubing is also used for the overflow tubing underneath the carbs.

 

[amc49]

The DOHC while using the same style and design of pump does not use anything there at all but the pipe ID is much smaller there. Probably a volume stuffer thing going on there, there could be a couple of reasons for it.

 

[dirtdigger]

Somewhat interesting, my set does not have those inserts yet accel pump does work.

 

[amc49]

If like on a car the squirters feed a certain amount of fuel at high vacuum like full open throttle then they may minimize the volume there to make sure even the very first application of the pump squirts solid fuel instead of partial air like if the tube was half empty and had to purge air for part of the first stroke. Accelerator pump passages should always be as small in diameter as possible so long as the size does not impact the squirter restriction at the very end, it minimizes system lagging for a millisecond. Why on car carbs they put a weight like a steel BB at some point close to the squirter, it stops vacuum from clearing out fuel deeper in the passage.

Common on circle track dirt cars, the decel going into the turn sucks the accelerator passage dry and then driver stomps on the carb coming out of the turn and the car just spits its' guts out because it takes two throttle stabs to get pump shot there.