Topic: Cb750 runs in 1 km stretches, then dies, runs again after cool down.

[HondaMan]

It would be fine with an electrical fuel pump to ensure fuel without flowing over.
I got lean mixture on cyl 1-2 since those carbs fuel hose had a routing loop higher than the petcock pipe. My carbs have two fuel inlets, 1-2 and 3-4, fed by one hose each from the early CB750 petcock with 2 pipes.

Generally speaking, the 2-pipe petcocks work better at high speed than the 1-pipe version. When the transition from 2-to-1 pipe occurred, there was a brief time when the petcocks had the BODY of the 2-pipe, but only one exit. Here in the US, which was Honda's major CB750 market, the infamous 55 MPH speed limit had been forced upon us, so the 1-pipe bikes didn't suffer much (sales) for the performance loss: no one noticed! Those who did opted to get an earlier tank so they could use the 2-outlet feeds instead. Racers I knew even drilled out the pipes a little larger, saying they felt it made a difference (?).

[TwoTired]

Wait... How many gph does a racer need for full power?

[scottly]

Spark when hot on both coils should rule out electrical issues.
Try running the bike until it dies, then immediately turning off the petcock. Open the bowl drains and see how much gas comes out.
 

[Jimray23]

I'm not sure, but i don't think you can really rule out fuel level without doing a clear tube test on the floats.

--------------------------------------------------
Jim
1979 kz1000b ltd
    stage 3 cams
    pod air filters
    4-1 open exhaust(no muffler)
    full rewire
    dyna-s ignition
    dyna 3 ohm coils

[scottly]

Float level would not change within 1 km, unless there was insufficient flow from the tank.

[BomberMann650]

Wait... How many gph does a racer need for full power?

This much 

[PeWe]

It would be fine with an electrical fuel pump to ensure fuel without flowing over.
I got lean mixture on cyl 1-2 since those carbs fuel hose had a routing loop higher than the petcock pipe. My carbs have two fuel inlets, 1-2 and 3-4, fed by one hose each from the early CB750 petcock with 2 pipes.

Generally speaking, the 2-pipe petcocks work better at high speed than the 1-pipe version. When the transition from 2-to-1 pipe occurred, there was a brief time when the petcocks had the BODY of the 2-pipe, but only one exit. Here in the US, which was Honda's major CB750 market, we infamous 55 MPH speed limit had been forced upon us, so the 1-pipe bikes didn't suffer much (sales) for the performance loss: no one noticed! Those who did opted to get an earlier tank so they could use the 2-outlet feeds instead. Racers I knew even drilledout the pipes a little larger, saying they felt it made a difference (?).

My local tuning shop here sold high flow petcocks (Superflow) together with RC tuning parts to CB750, Z1-900/1000. I have their old catalog from 1979/1980 with the desired parts.

[b1jackson]

Spark when hot on both coils should rule out electrical issues.
Try running the bike until it dies, then immediately turning off the petcock. Open the bowl drains and see how much gas comes out.

Ahhh Scottly....my saviour from the past!  The keen eye that spotted my flipped around coil bracket.  Good to hear from you.  I hope to get some time in the evening this week to update further. I think the bowl drains are seized, so I'll just pop the whole bowl off and have a look.  The only thing different I did since its last 'fit' is top up the tank but given the petcock pipe was covered with fuel, it should have been ok.

[HondaMan]

Wait... How many gph does a racer need for full power?

Beer, or gas?

[pjlogue]

I would try rerouting your fuel tubing so it goes down from the spigot between #1 & 2 carb to engine boots then to the "T" and up to each inlet nipple between the carbs.  Your picture looks like it goes down from the fuel valve then up to the top of the carbs then down again.  I have had problems with getting air or vapor locks in the tubing and the symptom is the same as you describe.  Run for ~1km and then it's like running out of gas.  Letting it sit for a bit and it starts up again.  The tubing routing may be the problem.  It is more pronounced in hot weather and a down-up-down routing causes problems.

-P.

[b1jackson]

A quick update as I've been busy with other stuff this week and haven't looked at the bike. Did not do a full diagnosis with the options as some suggested as no time but   a top up with fuel was really all I did.  Bike started and idled great.  Let it warm up good before doing a short run (about 2km) af 80-100km/hr.  Did not want to go any further than that as is was getting dark.

In this short run, could not get the bike to act up.  I did turn it off, then turn the petcock off and popped a carb bowl off and it was full.  I did just remember that the PO put an aftermarket petcock in the bike.  It's one of the aftermarket ones from vintageCB750 I believe.

Although my test was brief, I wonder if it could be a fuel balance or pressure issue?  Given my current fuel hose routing (which looks correct but there could be different routes as Hondaman suggested) and perhaps weak flow petcock if the bike is somewhat finicky with fuel volume and it isn't enough to just have the petcock screen covered?  But it's weird that there isn't a buck or sputter like a fuel shortage issue.  Now that the tank is full, I'll do a flow test into a jar from that petcock.

[NobleHops]

I'm in the 'fuel flow' camp. Remove the bowls and floats, put 4 baby food jars under there and let 'er rip. Catch and measure it from each carb to be certain.

[b1jackson]

Float level would not change within 1 km, unless there was insufficient flow from the tank.

I have to think that this was indeed the cause.  I had a successful 10km rip with no issues.  Recall that originally when this started, my tank was low but the petcock screen was covered (just and that was all). I now filled the tank and shortened the fuel line by about 3/4 inch right at the petcock to remove a wee bit of the sag.

Pretty dang fickle that aftermarket petcock if that was the case.  I do have a new OEM one that I should compare.

[b1jackson]

Another update:

The new OEM petcock I have doesn't have a screen.  The part # off the box is 16951-341-67.  It also is a one pipe out the bottom.  I don't know if I am going to use it.

I went for a drive to fill up the gas tank and just before I got to the gas station, it died.  In the same sudden sputter and loss of power as before.  This was as I slowed down from doing about 55mph and approached the stop sign. Coasted in and put about 9 liter in (2.5 gallons) in the tank (nearly filled it). Went in and paid for my fuel and it started and I drove away without issue.

Drove about another 15 km without missing a beat.

Should I shop for a higher flow petcock?  Where?  Should I ditch the tank screen on the current aftermarket one and just run an inline filter?  Recall this tank was professionally sealed so it's not a rust clogging issue.  I would have pulled a bowl at the gas station but when it fired right up, didn't see the need in getting fuel all over my hands.

[TwoTired]

Let's apply math and a little test.

Lets say your bike gets 30 MPG. (yes, that's poor mileage). If you drive 30 miles it would use one gallon.  15 miles; 1/2 gallon. 7.5 miles; 1 quart.

At 30 mph your fuel consumption is 1 gallon/per hour.  At 60mph it would be 2 gallons per hour (twice the distance).  That's 1 gallon per 1/2 hour, 1/2 gallon per 1/4 hour, 1 quart per 7.5 minutes.

Quart jars are pretty easy to come by.
Remove your fuel pipe for the fuel valve and aim it into a quart jar.  Time how long it takes to fill it.  If greater than 7.5 minutes you have a problem with fuel delivery from your fuel valve.  If it fills faster than that, you probably don't...Unless your fuel mileage is far worse that the assumptions above.
Then that's a different problem to solve.

Report back with your test results.

Cheers,

[Spanner 1]

I'm surprised no one is questioning the Pamco ignition  vs overvoltage from the alternator. It may be shutting down with say 15v from the old 'slow' electrical/mechanical regulator. The fact that the bike produces spark after it stops running does not prove the ign. unit is good with too high a supply voltage as in that case it's only receiving battery voltage ( button start or kick ). It wont run for 1km sometimes and quit and 15km sometimes and not quit and still be a fuel supply issue IMO. Also, how about the main fuse clips tarnished and getting hot with loss of conductivity, that's been reported many times here too 

[b1jackson]

Let's apply math and a little test

Report back with your test results.

Cheers,

1min, 40 seconds

[b1jackson]

Also, how about the main fuse clips tarnished and getting hot with loss of conductivity, that's been reported many times here too 

These were cleaned and polished last year.

I'm going to try Hondaman's over the carb rack bracket vs. through it as my fuel line route. And remove that white cover sheath on the fuel line.

[TwoTired]

Let's apply math and a little test

Report back with your test results.

Cheers,

1min, 40 seconds

I believe that's 36 quarts per hour, then. 36/4 = ~ 9 Gallons Per Hour flow rate.  An hour of riding at 60 MPH, 60 miles would be ~ 7 MPG consumption, If I did the math right.  That's what my 72 ford F100 gets with a 360 V8 engine hauling a 7000 lb trailer and a 1000 lb bed load, up and down grades.

If you are starving for fuel because of that fuel valve, you have the worst mileage I've ever heard of for an SOHC4. 

Cheers,

[b1jackson]

Dang...you love the math!

The over the bracket looked awkward and bendy, kinky and perhaps worse off.  I haven't ruled it out for the next trial but instead shortened each section that feeds the carbs by about 1/2 an inch.  This helped flatten the whole set up quite abit and took the rise the lines seemed to have once they left the "t" connection.

The line to the petcock goes underneath the choke rod between carb 1&2. Like I said, the setup looks flatter now so I'll try and report back.

[HondaMan]

I would try rerouting your fuel tubing so it goes down from the spigot between #1 & 2 carb to engine boots then to the "T" and up to each inlet nipple between the carbs.  Your picture looks like it goes down from the fuel valve then up to the top of the carbs then down again.  I have had problems with getting air or vapor locks in the tubing and the symptom is the same as you describe.  Run for ~1km and then it's like running out of gas.  Letting it sit for a bit and it starts up again.  The tubing routing may be the problem.  It is more pronounced in hot weather and a down-up-down routing causes problems.

-P.

The fuel lines should NEVER go below the carb's fuel level enroute to the tees. This will cause a strong hydraulic gravity 'lock' that can burn the valves on a CB750 when ridden at hiway speeds. Honda had a bulletin about this in 1970, after which they made holes in the carb brackets to 'guide' the average user to not do that again.

I've also replaced valves (exhaust) on CB750K0 engines that suffered this, in choppers.

[b1jackson]

I see photobucket is now being an ass when it comes to hosting pics.  So here is a re-post of the before pic.  Lines a little longer and some arc to them before they hit the carbs.




And shortened them abit.  Flattened things out.

[Airborne 82nd]

Maybe I over looked it but I didn't see if you ever did the clear tube test. It is easy to do and will tell you if you measured correctly.

[b1jackson]

Floats all measured with micrometer.  I consider that more accurate than a clear tube but I understand your theory. 

[scottly]

Drain the tank until the level over the screen is just above where the problem starts, and then check the flow again. See what the level is when it stops flowing compared to the screen. Once it stops, check the flow with the petcock switched to reserve.