Missing parts on my carburettors..? WITH PICTURES

[andy6693]

Hi there, I am new here and have just bought my first CB750. It has been stood for 10 years or so and had the carbs rebuilt just before i bought it and a full service. Was running pretty rough so i pulled the carbs to clean and blow them through. Now they are back on the bike and I'm struggling to get the float bowls to fill with fuel. I connect the fuel line an open the fuel tap and no.1 carb seems to be getting fuel but 2,3 and 4 have no fuel out of the drain screws... I have a clymer manual which shows a accelerator pump on carb 2 but mine don't seem to have this... is this correct?

Where should the vacuum line from carb 2 go? mine was connected but blanked off.

The fuel balance pipe has a T piece in it but nothing connected to it. is this correct?

What is the correct method to fill the carbs up with fuel after they have been off or should they just gravity feed?

 

[amc49]

US spec carbs have to meet EPA lean emissions settings and those carbs get the accel pump. Euro spec ones are richer at idle and don't use it or need it.

The vacuum line off #2 carb goes to a vacuum fuel demand valve usually mounted toward right side of carb bank. Petcock line goes to it then another line to carb fuel supply. Engine has to be running to open that valve up or no fuel. You can bypass that valve to run without it, simply run line from petcock to carbs and turn fuel off every time you stop bike. The valve stopped people who do not turn petcock off from flooding engine with fuel with bike on sidestand, it can hydrolock the engine to break rod or such. Honda had warranty complaints over it.

The fuel inlet is the brass colored straight up vertical tube in the dark in last pic between 1 and 2 carb toward back of carb.

That last pic seems to show carbs not all the way in the manifolds, you show too much aluminum at the front of #1 carb.

 

[andy6693]

US spec carbs have to meet EPA lean emissions settings and those carbs get the accel pump. Euro spec ones are richer at idle and don't use it or need it.

The vacuum line off #2 carb goes to a vacuum fuel demand valve usually mounted toward right side of carb bank. Petcock line goes to it then another line to carb fuel supply. Engine has to be running to open that valve up or no fuel. You can bypass that valve to run without it, simply run line from petcock to carbs and turn fuel off every time you stop bike. The valve stopped people who do not turn petcock off from flooding engine with fuel with bike on sidestand, it can hydrolock the engine to break rod or such. Honda had warranty complaints over it.

The fuel inlet is the brass colored straight up vertical tube in the dark in last pic between 1 and 2 carb toward back of carb.

That last pic seems to show carbs not all the way in the manifolds, you show too much aluminum at the front of #1 carb.

Hi, thanks for your reply. So from what you're saying it sounds like my bike does not have this vacuum fuel demand valve as the vacuum line was blanked off and the fuel line ran straight from the fuel tap to that brass pipe between carb 1 and 2.

Yes the carbs aren't all the way in, i had just pushed them on to the the picture.

So how exactly should the float bowls fill up, are they just gravity fed?

The black plastic T that is in the centre often has a pipe coming off it in pictures I've seen but mine doesn't and i don't know where it should go.

Any more help gratefully received as i just cant seem to get the fuel to run across into all 3 carbs at the moment

thanks for you help

Andy

 

[amc49]

Yes but the carbs are from a valve equipped bike though, maybe somebody already removed it. No-valve carb banks have no hole or fitting in the #2 carb.

Gravity fed with or without valve, most bikes are.

Black plastic tee is the common carb vent, the tube simply droops down low to collect with the missing ones off the bottom of each bowl, all 5 dump at the back of the engine, there may be a hook piece to collect them toward the right side of engine.

All passages obviously have to be open to flow fuel. The floats must be properly clipped to the needles too, float drops and the clip pulls needle open. You can pull hose at petcock to check for fuel flow with petcock open. Fuel tank vent must be open. You can leave cap off to check that. The drain screw passage commonly plugs to not drain even if carbs getting fuel.

 

[andy6693]

Thanks again for your help. Ok so I got it running last night, better than it has ever run before but still not perfect. Turns out I had been an idiot and fitted 3 of the floats upside down... it was late one night I rebuilt them.

The problem I'm getting now is one exhaust is cold and the plug is wet so it must be getting fuel but I don't think the spark is consistent. When I pull the plug and rest the spark it is there but I think it is weaker than the others. All plugs are new but the HT leads all look a bit old.

I was considering upgrading the ignition system to better coils mainly because I don't like the connections of HT leads to coils, I think that is where my misfire is coming from.

Any recommendations on good upgrades? I've seen the dyna 2000 system but looks a little pricey