Topic: 1978 Honda CB750K Rich Condition

[HondaMan]

Well I took some of your guys's advice and did the clear tube test which showed the fuel level to line up with the top of the float bowl (give or take a tiny bit) on every carb.

 I have no idea why the bike would be running rich like it is. See picture below showing the fuel level.

The level should be 2-4mm below the top of the float bowl. Your floats are set too high!

Or, maybe the float valves are not sealing well? Getting pretty deep in there!

[njlantis]

Are we absolutely certain the choke plates are not 'hanging up?'  Could it be as simple as a sticky or mis-routed choke cable, or a spring not working?

Yup, I'm positive that the choke is functioning properly. I have confirmed with the airbox off and pulling the choke cable.

[njlantis]

What about the cam timing being off a tooth? That will make it spit lol.

I'm not sure about that one. I haven't checked that, I'm assuming I would have to dissasemble the top end to check?

[njlantis]

Well I took some of your guys's advice and did the clear tube test which showed the fuel level to line up with the top of the float bowl (give or take a tiny bit) on every carb.

 I have no idea why the bike would be running rich like it is. See picture below showing the fuel level.

The level should be 2-4mm below the top of the float bowl. Your floats are set too high!

hmm okay. I have the floats set to 14.5 mm currently because I think that is the spec for this bike. Would you suggest going down to 12.5 mm then? I also just got new float valves delivered because I thought they weren't sealing before I did the clear tube test. I was going to  just return them sincemy current valves are sealing correctly but I suppose it couldnt hurt to just replace them while I have the carbs off.

[njlantis]

Well I took some of your guys's advice and did the clear tube test which showed the fuel level to line up with the top of the float bowl (give or take a tiny bit) on every carb.

 I have no idea why the bike would be running rich like it is. See picture below showing the fuel level.

The level should be 2-4mm below the top of the float bowl. Your floats are set too high!

Or, maybe the float valves are not sealing well? Getting pretty deep in there!

I have to new float valves so I'll replace the valves and keep the floats set the same and redo the clear tube test to see if anything changes.

[MauiK3]

One change at a time will make it take longer but at least you’ll end up knowing the culprit. I’m voting for bowl fuel levels too high.

[Airborne 82nd]

I have the same bike forget the 14.5 and 12.5 set them with the clear tube 3mm below the top of the bowl. Build a gig like mine to get them level and to tilt them to see when the float valve closes

edit no tool needed for clear tube on our carbs. A lot of people don't like these carbs. I like them but years ago there was a learning curve. Alsoo if your choke is correct it will activate your accl pump. You did remove your press in  slow jets and blow and see through all the little holes correct?

[scottly]

What about the cam timing being off a tooth? That will make it spit lol.

I'm not sure about that one. I haven't checked that, I'm assuming I would have to dissasemble the top end to check?

Rotortiller, why in the world would you suggest a problem with the cam timing when it's already been established the problem is with the fuel levels in the float bowls?  

[scottly]

Or, maybe the float valves are not sealing well? Getting pretty deep in there!

If the float valves were not sealing, the level would continue to rise until it reached the overflow tubes, pissing gas out.

[njlantis]

I have the same bike forget the 14.5 and 12.5 set them with the clear tube 3mm below the top of the bowl. Build a gig like mine to get them level and to tilt them to see when the float valve closes

edit no tool needed for clear tube on our carbs. A lot of people don't like these carbs. I like them but years ago there was a learning curve. Alsoo if your choke is correct it will activate your accl pump. You did remove your press in  slow jets and blow and see through all the little holes correct?

Okay sounds like the best next step is to lower the float heights to 3 mm below the top of the float bowl. And yes I have checked all the fuel circuits and pulled the jets to make sure there are no clogs in the system. I have also confirmed that the accel pump is functioning properly by turning fuel on and squeezing the throttle with the airbox off and visually checking that it is squiritng fuel.

[njlantis]

Or, maybe the float valves are not sealing well? Getting pretty deep in there!

If the float valves were not sealing, the level would continue to rise until it reached the overflow tubes, pissing gas out.

I thought the same thing. The fuel was sitting at a constant level during the clear tube test so I'm sure that they are sealing. I think I will adjust the float heights and go from there.

[scottly]

hmm okay. I have the floats set to 14.5 mm currently because I think that is the spec for this bike. Would you suggest going down to 12.5 mm then?

How were you measuring the 14.5mm? Was it with the carbs on their side, with the float tang just touching the end of the spring loaded needle? The problem with this method is that it takes some pressure on the needle to push it closed, so the level will be higher by the time the float floats and presses the needle against the seat hard enough.
Do the static measurement with the carbs upside down, with the weight of the floats pressing down on the needles. After the static adjustment, verify the level with the clear tube method.   

[Airborne 82nd]

hmm okay. I have the floats set to 14.5 mm currently because I think that is the spec for this bike. Would you suggest going down to 12.5 mm then?

How were you measuring the 14.5mm? Was it with the carbs on their side, with the float tang just touching the end of the spring loaded needle? The problem with this method is that it takes some pressure on the needle to push it closed, so the level will be higher by the time the float floats and presses the needle against the seat hard enough.
Do the static measurement with the carbs upside down, with the weight of the floats pressing down on the needles. After the static adjustment, verify the level with the clear tube method.

+1
All this can be done on the bench with a aux fuel tank do it correct one time on the gig. Everything can be done on the bench except idle and vac. sync...

[njlantis]

hmm okay. I have the floats set to 14.5 mm currently because I think that is the spec for this bike. Would you suggest going down to 12.5 mm then?

How were you measuring the 14.5mm? Was it with the carbs on their side, with the float tang just touching the end of the spring loaded needle? The problem with this method is that it takes some pressure on the needle to push it closed, so the level will be higher by the time the float floats and presses the needle against the seat hard enough.
Do the static measurement with the carbs upside down, with the weight of the floats pressing down on the needles. After the static adjustment, verify the level with the clear tube method.

Ahh, I see! I read in a lot of different places that you were supposed to do it with the carbs on their side so that was how I was setting the float heights. Maybe setting to 14.5 upside down will be enough to take the fuel levels to 3 mm below the top of the bowls.

[madmtnmotors]

hmm okay. I have the floats set to 14.5 mm currently because I think that is the spec for this bike. Would you suggest going down to 12.5 mm then?

How were you measuring the 14.5mm? Was it with the carbs on their side, with the float tang just touching the end of the spring loaded needle? The problem with this method is that it takes some pressure on the needle to push it closed, so the level will be higher by the time the float floats and presses the needle against the seat hard enough.
Do the static measurement with the carbs upside down, with the weight of the floats pressing down on the needles. After the static adjustment, verify the level with the clear tube method.

Ahh, I see! I read in a lot of different places that you were supposed to do it with the carbs on their side so that was how I was setting the float heights. Maybe setting to 14.5 upside down will be enough to take the fuel levels to 3 mm below the top of the bowls.

Clear tube method is the only sure-fire method to get the floats spot on. The float settings are at the mercy of the little springs inside the float needles, buoyancy of each individual float, irregularities (dimples) on the float needle tang, the amount of force required for each individual float needle to seal properly, etc...

Each of these are overcome with the clear tube method, with the exception of dimples on the float needle tang which can cause inconsistent results.

EZPZ

[Airborne 82nd]

From Lucky on 2012
OK....I have made another discovery.
On the 1978 CB750 PD carbs with accelerator pump the floats are rectangular plastic.
The Clymer's manual does NOT show this style at all in chapter 3 or in chapter 6.

The measurement is 14.5mm above the carb body to the top of the float when the carb is upside down. OR .570 thousandths of an inch.

But you do not even need to measure. Just place a straight edge over the top of the idle jet (push in style) and you will see that when the float is .570 above the carb body surface that the top of the float and slow jet are the same height,when the carb is upside down. Do not push down on the floats to make this happen.
You do not have to worry about the float pressing on the float spring. The spring is so stiff that you would have to push it down with your finger to make a difference.

[njlantis]

Update for everyone.

I replaced the float valves with new OE ones and confirmed the fuel levels were 3 mm below the top of the float bowl using the clear tube test. I also confirmed that jet needles and pilot screws were in good condition, I did not find any wear or abnormalities in any of them. I also replaced the spark plugs with new NGK D8EA plugs. I just got the carbs back on the bike and took it for a test ride and it seems that nothing has changed from before. I fired the bike up and it started with no problems however struggles to rev up anytime I give it any throttle and and once it does reach higher rpm it starts backfiring and the exhaust pops, if i twist the throttle very quickly then the bike immediately start die and the engine almost cuts off. I rode the bike up and down the street and I couldn't even get out of first gear, the engine was very weak and struggled to increase RPM. I'm honestly at a loss at this point, any advice is welcome.

[scottly]

I fired the bike up and it started with no problems however struggles to rev up anytime I give it any throttle and and once it does reach higher rpm it starts backfiring and the exhaust pops, if i twist the throttle very quickly then the bike immediately start die and the engine almost cuts off. I rode the bike up and down the street and I couldn't even get out of first gear, the engine was very weak and struggled to increase RPM.

What do the new spark plugs look like? Twisting the throttle quickly causing a bog sounds like a lean condition. Is there any black smoke coming out of the exhaust when you twist the throttle? Does it backfire, or "sneeze", out of the carbs?

[Jerry Rxman Griffin aka MuthaF'er]

Simple question I don't see info on - what is the condition of your battery? Voltage before/after charge/after hitting starter. Load test and not just voltage.

[madmtnmotors]

Alright, so if the carbs are right and the battery voltage holds under load, then all I can figure is a vacuum leak or low compression.

[smee]

Update for everyone.

I replaced the float valves with new OE ones and confirmed the fuel levels were 3 mm below the top of the float bowl using the clear tube test. I also confirmed that jet needles and pilot screws were in good condition, I did not find any wear or abnormalities in any of them. I also replaced the spark plugs with new NGK D8EA plugs. I just got the carbs back on the bike and took it for a test ride and it seems that nothing has changed from before. I fired the bike up and it started with no problems however struggles to rev up anytime I give it any throttle and and once it does reach higher rpm it starts backfiring and the exhaust pops, if i twist the throttle very quickly then the bike immediately start die and the engine almost cuts off. I rode the bike up and down the street and I couldn't even get out of first gear, the engine was very weak and struggled to increase RPM. I'm honestly at a loss at this point, any advice is welcome.

What happens if you start it and go for a ride with the choke on? Does it get any better?

[njlantis]

I fired the bike up and it started with no problems however struggles to rev up anytime I give it any throttle and and once it does reach higher rpm it starts backfiring and the exhaust pops, if i twist the throttle very quickly then the bike immediately start die and the engine almost cuts off. I rode the bike up and down the street and I couldn't even get out of first gear, the engine was very weak and struggled to increase RPM.

What do the new spark plugs look like? Twisting the throttle quickly causing a bog sounds like a lean condition. Is there any black smoke coming out of the exhaust when you twist the throttle? Does it backfire, or "sneeze", out of the carbs?

I haven't pulled the new plugs yet but will be doing that tonight and I'll post some pictures. There is no black smoke out the exhaust, there are definitly sneezing or popping sound when trying to rev up but I assumed they were coming from the exhaust.

[njlantis]

Simple question I don't see info on - what is the condition of your battery? Voltage before/after charge/after hitting starter. Load test and not just voltage.

I'm not sure about this. I did just put a new battery in it but I suppose that doesn't mean it is staying at the correct voltage while the bike is running. I haven't had any issues with having to charge the battery after running the bike and It cranks over with no problem every time. But I will do some voltage tests and post them

[njlantis]

Update for everyone.

I replaced the float valves with new OE ones and confirmed the fuel levels were 3 mm below the top of the float bowl using the clear tube test. I also confirmed that jet needles and pilot screws were in good condition, I did not find any wear or abnormalities in any of them. I also replaced the spark plugs with new NGK D8EA plugs. I just got the carbs back on the bike and took it for a test ride and it seems that nothing has changed from before. I fired the bike up and it started with no problems however struggles to rev up anytime I give it any throttle and and once it does reach higher rpm it starts backfiring and the exhaust pops, if i twist the throttle very quickly then the bike immediately start die and the engine almost cuts off. I rode the bike up and down the street and I couldn't even get out of first gear, the engine was very weak and struggled to increase RPM. I'm honestly at a loss at this point, any advice is welcome.

What happens if you start it and go for a ride with the choke on? Does it get any better?

I just took it for a ride up and down the street and running it with the choke on showed no improvements to the engine performance, if anything it made things worse.

[njlantis]

Alright, so if the carbs are right and the battery voltage holds under load, then all I can figure is a vacuum leak or low compression.

I plan to be running a compression test tonight, just need to go get a tester. If I do have low compression in any cylinders this could be a cause for how rich the bike runs? Obviously I know that the bike loses great amounts of power and isn't rideable with low compression but would this affect the air/fuel mixture? I guess if there's low compression it probably would struggle to pull air into the cylinder as well as compress the mixture.