Topic: Looking for troubleshooting Help on my 76 750F

[tofan]

069A carbs?
Slow jet size 38? OEM?
Jet needle at position 2? (position 5 is top slot) OEM?

Carbs - 064A
Slow jet - 40
Main jet - 105
Needle - #271307 and is set in middle (3rd) position

All Keihin and all to spec

Attached images from shop manual

Although I have seen other specs on the internet. For example: https://motorcycleproject.com/text/specs_revised.html

[smee]

069A carbs?
Slow jet size 38? OEM?
Jet needle at position 2? (position 5 is top slot) OEM?

Carbs - 064A
Slow jet - 40
Main jet - 105
Needle - #271307 and is set in middle (3rd) position

All Keihin's and all to spec

Although I have seen other specs on the internet. For example: https://motorcycleproject.com/text/specs_revised.html

Here is what I use: http://www.hondachopper.com/garage/carb_specs/carb_specs.html

Could be just a tad too rich on the slow jet. Are the carbs stamped 064A or 069A?

[tofan]

069A carbs?
Slow jet size 38? OEM?
Jet needle at position 2? (position 5 is top slot) OEM?

Carbs - 064A
Slow jet - 40
Main jet - 105
Needle - #271307 and is set in middle (3rd) position

All Keihin's and all to spec

Although I have seen other specs on the internet. For example: https://motorcycleproject.com/text/specs_revised.html

Here is what I use: http://www.hondachopper.com/garage/carb_specs/carb_specs.html

Could be just a tad too rich on the slow jet. Are the carbs stamped 064A or 069A?

064A

Found this interesting doc: https://www.dotheton.com/index.php?attachments/carburetor-specifications-doc.51991/  If you scroll down to page 3 there's a comment attached to the 76 CB750F that says "was 069A"

If changing the plugs and what doing what RAFster suggested doesn't help then I might try changing the slows to 38.

[grcamna2]

069A carbs?
Slow jet size 38? OEM?
Jet needle at position 2? (position 5 is top slot) OEM?

Carbs - 064A
Slow jet - 40
Main jet - 105
Needle - #271307 and is set in middle (3rd) position

All Keihin's and all to spec

Although I have seen other specs on the internet. For example: https://motorcycleproject.com/text/specs_revised.html

Here is what I use: http://www.hondachopper.com/garage/carb_specs/carb_specs.html

Could be just a tad too rich on the slow jet. Are the carbs stamped 064A or 069A?

064A

Found this interesting doc: https://www.dotheton.com/index.php?attachments/carburetor-specifications-doc.51991/  If you scroll down to page 3 there's a comment attached to the 76 CB750F that says "was 069A"

If changing the plugs and what doing what RAFster suggested doesn't help then I might try changing the slows to 38.

How worn are your 4) needle jets/nozzles ?
I keep an NOS to measure with to compare with the ones I have installed;when they wear too big,it can over-juice.
I use a #1-60 drill index and carefully try each drill bit on the smooth end starting with my NOS nozzle,then compare to the used ones to see how far they're worn.

[tofan]

How worn are your 4) needle jets/nozzles ?
I keep an NOS to measure with to compare with the ones I have installed;when they wear too big,it can over-juice.

Hmmmm... I think they looked ok? I didn't inspect them too closely and only recently got a couple NOS needles. But I can inspect them more closely the next time I open them up.

[smee]

069A carbs?
Slow jet size 38? OEM?
Jet needle at position 2? (position 5 is top slot) OEM?

Carbs - 064A
Slow jet - 40
Main jet - 105
Needle - #271307 and is set in middle (3rd) position

All Keihin's and all to spec

Although I have seen other specs on the internet. For example: https://motorcycleproject.com/text/specs_revised.html

Here is what I use: http://www.hondachopper.com/garage/carb_specs/carb_specs.html

Could be just a tad too rich on the slow jet. Are the carbs stamped 064A or 069A?

064A

Found this interesting doc: https://www.dotheton.com/index.php?attachments/carburetor-specifications-doc.51991/  If you scroll down to page 3 there's a comment attached to the 76 CB750F that says "was 069A"

If changing the plugs and what doing what RAFster suggested doesn't help then I might try changing the slows to 38.

That is an interesting doc! Yeah, good luck and let us know how it goes for you.

[grcamna2]

How worn are your 4) needle jets/nozzles ?
I keep an NOS to measure with to compare with the ones I have installed;when they wear too big,it can over-juice.

Hmmmm... I think they looked ok? I didn't inspect them too closely and only recently got a couple NOS needles. But I can inspect them more closely the next time I open them up.

The OEM Honda 'needle sets' come with the needle and needle jet/nozzle all inside the package as a set.

[scottly]

069A carbs?
Slow jet size 38? OEM?
Jet needle at position 2? (position 5 is top slot) OEM?

Carbs - 064A
Slow jet - 40
Main jet - 105
Needle - #271307 and is set in middle (3rd) position

All Keihin and all to spec

Attached images from shop manual

I just noticed the float level spec on the bottom pic is 26mm, or .866 inch. 26mm is actually 1.02 inch, and .866 inch is actually 22mm. I would go with the 22mm setting, as your clear tube levels are too low.

[Tracksnblades1]

Have you verified your 105 Keihin jets are 1.05mm bore…
If you’re not the original owner…?  They may have been cleaned with a drill or wire..

My 76f runs the spark plugs on the white side of light brown with OEM 105 mains and OEM needles and clip positions.
It requires the choke to start cold…

[willbird]

069A carbs?
Slow jet size 38? OEM?
Jet needle at position 2? (position 5 is top slot) OEM?

Carbs - 064A
Slow jet - 40
Main jet - 105
Needle - #271307 and is set in middle (3rd) position

All Keihin's and all to spec

Although I have seen other specs on the internet. For example: https://motorcycleproject.com/text/specs_revised.html

Here is what I use: http://www.hondachopper.com/garage/carb_specs/carb_specs.html

Could be just a tad too rich on the slow jet. Are the carbs stamped 064A or 069A?

064A

Found this interesting doc: https://www.dotheton.com/index.php?attachments/carburetor-specifications-doc.51991/  If you scroll down to page 3 there's a comment attached to the 76 CB750F that says "was 069A"

If changing the plugs and what doing what RAFster suggested doesn't help then I might try changing the slows to 38.

How worn are your 4) needle jets/nozzles ?
I keep an NOS to measure with to compare with the ones I have installed;when they wear too big,it can over-juice.
I use a #1-60 drill index and carefully try each drill bit on the smooth end starting with my NOS nozzle,then compare to the used ones to see how far they're worn.

What would wear the float needle valve body and increase the dia the fuel flows through ??

Bill

[Tracksnblades1]

069A carbs?
Slow jet size 38? OEM?
Jet needle at position 2? (position 5 is top slot) OEM?

Carbs - 064A
Slow jet - 40
Main jet - 105
Needle - #271307 and is set in middle (3rd) position

All Keihin's and all to spec

Although I have seen other specs on the internet. For example: https://motorcycleproject.com/text/specs_revised.html

Here is what I use: http://www.hondachopper.com/garage/carb_specs/carb_specs.html

Could be just a tad too rich on the slow jet. Are the carbs stamped 064A or 069A?

064A

Found this interesting doc: https://www.dotheton.com/index.php?attachments/carburetor-specifications-doc.51991/  If you scroll down to page 3 there's a comment attached to the 76 CB750F that says "was 069A"

If changing the plugs and what doing what RAFster suggested doesn't help then I might try changing the slows to 38.

How worn are your 4) needle jets/nozzles ?
I keep an NOS to measure with to compare with the ones I have installed;when they wear too big,it can over-juice.
I use a #1-60 drill index and carefully try each drill bit on the smooth end starting with my NOS nozzle,then compare to the used ones to see how far they're worn.

What would wear the float needle valve body and increase the dia the fuel flows through ??

Bill

Isn’t he referring to the slide’s needle jet? 

[willbird]

069A carbs?
Slow jet size 38? OEM?
Jet needle at position 2? (position 5 is top slot) OEM?

Carbs - 064A
Slow jet - 40
Main jet - 105
Needle - #271307 and is set in middle (3rd) position

All Keihin's and all to spec

Although I have seen other specs on the internet. For example: https://motorcycleproject.com/text/specs_revised.html

Here is what I use: http://www.hondachopper.com/garage/carb_specs/carb_specs.html

Could be just a tad too rich on the slow jet. Are the carbs stamped 064A or 069A?

064A

Found this interesting doc: https://www.dotheton.com/index.php?attachments/carburetor-specifications-doc.51991/  If you scroll down to page 3 there's a comment attached to the 76 CB750F that says "was 069A"

If changing the plugs and what doing what RAFster suggested doesn't help then I might try changing the slows to 38.

How worn are your 4) needle jets/nozzles ?
I keep an NOS to measure with to compare with the ones I have installed;when they wear too big,it can over-juice.
I use a #1-60 drill index and carefully try each drill bit on the smooth end starting with my NOS nozzle,then compare to the used ones to see how far they're worn.

What would wear the float needle valve body and increase the dia the fuel flows through ??

Bill

Isn’t he referring to the slide’s needle jet?

Yes I think so, my misunderstanding.

[grcamna2]

069A carbs?
Slow jet size 38? OEM?
Jet needle at position 2? (position 5 is top slot) OEM?

Carbs - 064A
Slow jet - 40
Main jet - 105
Needle - #271307 and is set in middle (3rd) position

All Keihin's and all to spec

Although I have seen other specs on the internet. For example: https://motorcycleproject.com/text/specs_revised.html

Here is what I use: http://www.hondachopper.com/garage/carb_specs/carb_specs.html

Could be just a tad too rich on the slow jet. Are the carbs stamped 064A or 069A?

064A

Found this interesting doc: https://www.dotheton.com/index.php?attachments/carburetor-specifications-doc.51991/  If you scroll down to page 3 there's a comment attached to the 76 CB750F that says "was 069A"

If changing the plugs and what doing what RAFster suggested doesn't help then I might try changing the slows to 38.

How worn are your 4) needle jets/nozzles ?
I keep an NOS to measure with to compare with the ones I have installed;when they wear too big,it can over-juice.
I use a #1-60 drill index and carefully try each drill bit on the smooth end starting with my NOS nozzle,then compare to the used ones to see how far they're worn.

What would wear the float needle valve body and increase the dia the fuel flows through ??

Bill

The slide needles moving up and down do wear them after thousands of miles;the needle jets/nozzles are just soft brass.

[tofan]

069A carbs?
Slow jet size 38? OEM?
Jet needle at position 2? (position 5 is top slot) OEM?

Carbs - 064A
Slow jet - 40
Main jet - 105
Needle - #271307 and is set in middle (3rd) position

All Keihin and all to spec

Attached images from shop manual

I just noticed the float level spec on the bottom pic is 26mm, or .866 inch. 26mm is actually 1.02 inch, and .866 inch is actually 22mm. I would go with the 22mm setting, as your clear tube levels are too low.

Huh. I will add it to my list of things to try. Good catch. That would mean this product is wrong: https://jokermachine.com/products/cb750-carb-float-level-setting-gauge

[tofan]

Have you verified your 105 Keihin jets are 1.05mm bore…
If you’re not the original owner…?  They may have been cleaned with a drill or wire..

My 76f runs the spark plugs on the white side of light brown with OEM 105 mains and OEM needles and clip positions.
It requires the choke to start cold…

I haven't, but I'll add it to my list of things to check.

[tofan]

Just for reference. This is what was inside when I first opened up the carbs. Since this photo, I have thoroughly cleaned all parts and replaced the float needle and valve with NOS.

[HondaMan]

It is entirely possible that the problem is coming from the new intake boots. Reason: the OD of the new ones (even from Honda/Yamiya) are a little bit too small, so the OEM clamps do not usually seal the hoses at the head, even when tight. I have variously made thin strips of brass (0.010" thick) and laid them inside the boot's grooves, under the clamps, or just used different clamps like the Oetiker 050-9 (available at Aircraft Spruce) to fully solve the problem without shims or such trickery.

The OEM clamps usually are 'pulled' where the head meets the clamping bracket, making it impossible to even approach a snug fit. I always re-flatten them with [near new] needle-nose pliers and make oval-shaped washers to fit under the screw heads, as the very least fixup.

If the hose leaks at the inlet port, the mix goes rich because the airflow is slower than the engine at that intake. That carb then mixes at a slow air speed, which is richer than the actual desired mix.

[scottly]

Have you replaced the spark plugs yet? I had a problem once with a carb on a car that ran extremely rich, fouling the plugs. I swapped in new plugs, and they fouled within minutes. I rebuilt the carb, and brushed the carbon off the plugs, but the car still ran like crap. I even swapped a different carb on, and the engine ran the same. My roommate at the time, who was a few years older and wiser than I, came out to the barn and stated flatly "Your plugs are fouled" I told him I had replaced the original plugs, and cleaned the new ones. He said "Your plugs are fouled" I started to protest, but he interrupted me and stated once again "Your plugs are fouled", then turned and went back in the house. I gave in, bought new plugs, and the problem was solved.
The moral of this lesson is that you can fix the carb problem that caused the plugs to be fouled, but once fouled, they tend to stay fouled, even after you have corrected the original problem. Your #3 plug is wet; that cylinder wasn't even firing, so there is no way it could ever burn off the carbon.

[tofan]

It is entirely possible that the problem is coming from the new intake boots. Reason: the OD of the new ones (even from Honda/Yamiya) are a little bit too small, so the OEM clamps do not usually seal the hoses at the head, even when tight. I have variously made thin strips of brass (0.010" thick) and laid them inside the boot's grooves, under the clamps, or just used different clamps like the Oetiker 050-9 (available at Aircraft Spruce) to fully solve the problem without shims or such trickery.

The OEM clamps usually are 'pulled' where the head meets the clamping bracket, making it impossible to even approach a snug fit. I always re-flatten them with [near new] needle-nose pliers and make oval-shaped washers to fit under the screw heads, as the very least fixup.

If the hose leaks at the inlet port, the mix goes rich because the airflow is slower than the engine at that intake. That carb then mixes at a slow air speed, which is richer than the actual desired mix.

Interesting. Is there a way to verify whether there's an air leak in the intake boots?

[tofan]

Have you replaced the spark plugs yet? I had a problem once with a carb on a car that ran extremely rich, fouling the plugs. I swapped in new plugs, and they fouled within minutes. I rebuilt the carb, and brushed the carbon off the plugs, but the car still ran like crap. I even swapped a different carb on, and the engine ran the same. My roommate at the time, who was a few years older and wiser than I, came out to the barn and stated flatly "Your plugs are fouled" I told him I had replaced the original plugs, and cleaned the new ones. He said "Your plugs are fouled" I started to protest, but he interrupted me and stated once again "Your plugs are fouled", then turned and went back in the house. I gave in, bought new plugs, and the problem was solved.
The moral of this lesson is that you can fix the carb problem that caused the plugs to be fouled, but once fouled, they tend to stay fouled, even after you have corrected the original problem. Your #3 plug is wet; that cylinder wasn't even firing, so there is no way it could ever burn off the carbon.

Not yet. I will hopefully have some time this weekend after the Denso plugs arrive today.

[newday777]

It is entirely possible that the problem is coming from the new intake boots. Reason: the OD of the new ones (even from Honda/Yamiya) are a little bit too small, so the OEM clamps do not usually seal the hoses at the head, even when tight. I have variously made thin strips of brass (0.010" thick) and laid them inside the boot's grooves, under the clamps, or just used different clamps like the Oetiker 050-9 (available at Aircraft Spruce) to fully solve the problem without shims or such trickery.

The OEM clamps usually are 'pulled' where the head meets the clamping bracket, making it impossible to even approach a snug fit. I always re-flatten them with [near new] needle-nose pliers and make oval-shaped washers to fit under the screw heads, as the very least fixup.

If the hose leaks at the inlet port, the mix goes rich because the airflow is slower than the engine at that intake. That carb then mixes at a slow air speed, which is richer than the actual desired mix.

Interesting. Is there a way to verify whether there's an air leak in the intake boots?

Lite squirts of carb cleaner or use an unlit propane torch on, very low, so you can pin point where the leak is.

[tofan]

I only received one of the Denso plugs yesterday so I went ahead and installed the NGKs. It's obviously very happy, idling good at about 1100rpm with great throttle response. After I get some miles in I'll report back.

I think what RAFster said about it not liking too much idling is probably the biggest issue as I mostly take it through the town. Lots of stop and go, unfortunately. I just remembered there's a decent stretch of road with not too many cars I can push it to see how it does.

I appreciate everyone's help with this and will update you.

[grcamna2]

I only received one of the Denso plugs yesterday so I went ahead and installed the NGKs. It's obviously very happy, idle running good pretty good at about 1100rpm. with great throttle response. After I get some miles in I'll report back.

I think what RAFster said about it not liking too much idling is probably the biggest issue as I mostly take it through the town. Lots of stop and go, unfortunately. I just remembered there's a decent stretch of road with not too many cars I can push it to see how it does.

I appreciate everyone's help with this and will update you.

The battery charge rate goes down and discharges if it idles;when I sit in traffic,I usually have a light touch on the throttle to keep the rpms(rolling between 1350-1500,sometimes I just turn off the engine if I'm sitting too long)above idle enough to keep it overcoming 'discharge mode'. I'm careful not to run the rpms up too much while sitting for fear of overheating the engine/bluing the headpipes.
I Hate red lights/sitting in traffic and watching everyone stare at their phones.. 

[bryanj]

If you have the lights on all the time switch it wont charge till about 3000 rpm

[Jerry Rxman Griffin aka MuthaF'er]

Needle in middle position.

Food for thought. The needle jet/Jet needle matched set is a -392 part number, ie -392 designates a 75/76 F part. To me this would indicate the set has a different taper for 75/76 F's.