Topic: CB750K4-K6 Fuel Hose Routing

[HondaMan]

This one can stymie your assembly and slow down your 750 on the freeway if not done correctly. A LONG time ago I had a long post about this, but it got lost back when our site had some problems, and I never rebuilt the thread. So, here is sort of the same thing again, given I have a late K4 in the shop that happens to have the same system on it.

Background: Honda had 2 fuel hoses feeding the 1-2 and the 3-4 roundtop carbs on this bike. In midyear-1974 (about 5/1974) Honda was feeling the pinch of sales lost to the new Kawasaki Four, but was determined to hold the line on price. This it did by reducing the cost of building the 750, in many ways - none beneficial, as it later turned out, but that's another topic - and one of those ways was to reduce the fuel spigots on the K4-style fuel tank to just 1 spigot and install a (nice) brass fuel tee (#16235-657-000 in 750K5 listings today) to split the fuel feed between carbs #1-#2 and #3-#4. Since we all enjoyed riding at 55 MPH in those days, even on long tours, this was adequate fuel feed (until about 95 MPH), so it largely went unnoticed (those of you who have a CB550K3 will recognize these parts, too).

This worked fine until around 2005 when we started losing those nice brass tees (5mm size) with cafe' tanks and other such mods, and nor we're back to trying to feed 4 carbs with 1 fuel line during restos. So, here's some pix of what that looks like, a few pix at a time.

[HondaMan]

Once up and over the 1-2 carbs, the fuel hose MUST be kept as flat as possible: don't let it loop way up and back down again. Shorten it until it will fit in front of the airbox, as shown below. If you don't do these things, it will trap an air bubble inside which will cause 2 annoyances:
1. In hot, slow city traffic, the fuel will "pump" erratically to the 3-4 carb side and cause, alternately, low-bowls there and then overflowing (momentarily) bowls there. This makes the idle erratic at best.
2. Lack of top speed, sometimes even 'surging', at modern hiway speeds above 70 MPH. Top speed will also drop to about 90 MPH and it will 'feel' weak up there: it should not. The top speed should be well in excess of the ton. I've sometimes experienced that the bike will reach a higher top speed, then slows down and can't reach it again. Same trouble.

An aside: note the carb vent hoses in picture 4 below: they MUST be there, and the longer, the better. Honda's only reached behind the airbox, fine at 55 MPH. Above 70 MPH you will find things better if they reach further up, like behind the battery, snaking alongside it until they can fall behind it. I have also sometimes drilled 3 holes into a small pill bottle lid and stuck the 2 hoses into those holes, then assembled the lid onto the pill bottle and set it behind or below the battery (much longer bowl vent hoses): in roadracing this helped noticeably. I didn't get to do it with a CB750K0 with its 4 bowl vents, only with the 2-vent versions after the K1 roundtop carb linkage setup came out. The K0 would require a bigger pill bottle...

Note also: the PD carbs have a wholly different arrangement. When I next get one of those in for work I'll try to make this post longer, showing those carbs. Some of them don't even have vent holes in their bodies, a casting flaw that wasn't policed very well. Those bikes pay a price above 60 MPH.

[HondaMan]

A final note about these petcocks: in the 750K4 there were 2 different fuel tanks as Honda transitioned the system. The first version used the earlier screws-to-the-tank-bottom petcock like in the earlier bikes, just on the wrong side of the tank(!). Later the bung was added and the 'new' petcock came out, but with 2 spigots on it. This only lasted about 3 months. The spigot become singular with the fuel tee somewhere around the 4/1974 or so builds. The earliest 1-outlet petcocks were used with the carb racks that have the idle speed setting screw on the left side (like the bike shown above), making idle adjustments on a hot engine almost impossible without gloves. The idle-speed screw then transitioned to the right side of the rack, with a slide lifter arm that has 2(!) holes for the screw - one on each side - and the carb's mounting rack has holes on BOTH sides for the fuel hose to come into the carbs on either the 1-2 and/or the 3-4 side. These racks are rare, but VERY useful to have kicking around in the shop. I've seen where riders have then run a fuel hose for the "far" side carbs across the front of the rack: note that this will only work well in hiway situations, but if fitted behind a Vetter with Lowers, not very well at all. That long rubber hose will boil today's gasolines with their light condensates, right in that line. This will overflow the far carbs when in slow city traffic, but work fine when sitting in the garage.

[HondaMan]

Now, all that ^^^ said: if you don't mind aftermarket and want 2 fuel outlets like Honda briefly did on this bike, there is an aftermarket copy of that petcock. Here's one of them:
https://www.common-motor.com/honda-cb500t-cb550f-petcock?srsltid=AfmBOoqq6rLMZ1bu6tQ0OArLWscAlycJZMbfMdQe7-1M7aEyNN7_7o-h

I've found this one on a cafe'd 750 before and while the owner was a city rider (not a mountain racer) he hadn't experienced troubles with it. Having the 2 fuel lines fully running between the petcock and the carbs will help insure you won't get run over on the Interstate by a robot-controlled 18-wheeler...

[newday777]

Now, all that ^^^ said: if you don't mind aftermarket and want 2 fuel outlets like Honda briefly did on this bike, there is an aftermarket copy of that petcock. Here's one of them:
https://www.common-motor.com/honda-cb500t-cb550f-petcock?srsltid=AfmBOoqq6rLMZ1bu6tQ0OArLWscAlycJZMbfMdQe7-1M7aEyNN7_7o-h

I've found this one on a cafe'd 750 before and while the owner was a city rider (not a mountain racer) he hadn't experienced troubles with it. Having the 2 fuel lines fully running between the petcock and the carbs will help insure you won't get run over on the Interstate by a robot-controlled 18-wheeler...

A great writeup (and with pictures!) on fuel starvation fix on the K5/K6, F0/F1 Mark.

Well the petcock at the link is out of stock.....hopefully that is just a temporary problem. I got on the wait list.
How about Pingle? Do they offer a 2 outlet for these?