Topic: Looking for troubleshooting Help on my 76 750F

[tofan]

Hi Everyone, I've been troubleshooting my 1976 750F trying to get it run top-notch.

Symptoms:
- It seems to be running rich according to my plugs. I've attached photos of each plug that I pulled this morning.
- Here's a video of idle fluttering after a 15min ride. https://www.dropbox.com/s/lncd3ifp2105sot/2023-06-06%2009.02.21.mov?dl=0 and someone mentioned idle should be set at 1100-1200rpm when warm. Is that correct for this bike?
- It sometimes dies when on kick-stand or when I come to a full stop. 

Here are the stats and what I've done so far:

- I'm at 2,500ft elevation (Tucson, AZ)
- 25,000 miles on the clock
- 120psi on all 4 cylinders
- Everything is stock except for the Dyno S ignition, new 5ohm coils, wires and caps, and cone engineering muffler. The muffler came with previous owner and I recently installed the electronic ignition and coils.
- The carbs have recently gone through 2 sonic showers
- Drain tubes have no cracks
- Recently replaced the float needle with NOS from Ebay
- Floats set at 26mm from the gasket mating service per manual, but will check them again for accuracy this weekend.
- Oil and filters replaced. Honda GN4 10W-40 and paper filter from 4-into-1.
- Checked tappets
- Checked timing
- Air screws are about 1.25 turns out
- Bench and vacuum sync'd carbs, but I will do this again this weekend.

What am I missing?

[robvangulik]

You say all is stock, so 105 main jets?

[tofan]

You say all is stock, so 105 main jets?

Correct

[RAFster122s]

Gonna need new plugs. Paper air filter, right? Not clogged up, right?
Is this a lot of idling in the shop?
What gas grade are you using?
Plug 2&3 appear to be older plugs than 1&4…

Is all the brass Keihin?

[tofan]

Gonna need new plugs. Paper air filter, right? Not clogged up, right?
Is this a lot of idling in the shop?
What gas grade are you using?
Plug 2&3 appear to be older plugs than 1&4…

Is all the brass Keihin?

Paper air filter. Brand new installed this past Saturday.
91 grade gas
Correct. The older plugs are about 2 months old. The 2 newer ones were installed this past weekend.
All Keihin

Stupid question: Is brushing off the carbon from the plugs not acceptable?

[scottly]

No, you can't just brush off the carbon, but sometimes you can burn the black off with a propane torch.
Check the actual fuel levels in the float bowls with the "clear tube" method.

[tofan]

No, you can't just brush off the carbon, but sometimes you can burn the black off with a propane torch.
Check the actual fuel levels in the float bowls with the "clear tube" method.

[RAFster122s]

Bike doesn’t need 91 octane, should run fine on 89 or 87 octane…regular gas.
Excess idling will foul plugs on these bikes.
Abrasive plug blaster is ideal, or burn off the carbon with propane torch…need to get insulator down in the plug body clean…

Warm up bike and carefully change to new or properly cleaned plugs (be careful not to cross thread new plugs) and then operate bike for normal riding at range of throttle you want to check. Hit kill switch after about 10-15 minutes at that operating speed throttle setting, coast to side of road or parking lot you can pull plugs on bike. Check plug color, photograph…

Either reinstall plugs or put earlier set back in for ride home or to your shop to adjust, clean set of plugs and retest…

WOT setting are tough to check because of the speeds you would attain are illegal.
Abandoned or lightly travelled area you can do this kind or runs would be ideal…
Interstate Highway would be closest to being able to operate for 5-10 minutes at-high speed, but likelihood of getting a ticket would be high….750 would be going 105-110 at WOT if in decent shape… not a speed the troopers would give you a pass on…

[scottly]

Those fuel levels are extremely low! The fuel should be about 3mm below the top of the float bowl. Have you been running the engine with the choke on?

[tofan]

Those fuel levels are extremely low! The fuel should be about 3mm below the top of the float bowl. Have you been running the engine with the choke on?

Choke is off.

I've been reading and hearing conflicting information about the fuel level requirements... some say as long as they're equal they should be fine. I set the floats at 26mm so I'm not sure what else to do here. Also, as I understand it, when lower it should be running leaner, not richer?

[tofan]

Bike doesn’t need 91 octane, should run fine on 89 or 87 octane…regular gas.
Excess idling will foul plugs on these bikes.
Abrasive plug blaster is ideal, or burn off the carbon with propane torch…need to get insulator down in the plug body clean…

Warm up bike and carefully change to new or properly cleaned plugs (be careful not to cross thread new plugs) and then operate bike for normal riding at range of throttle you want to check. Hit kill switch after about 10-15 minutes at that operating speed throttle setting, coast to side of road or parking lot you can pull plugs on bike. Check plug color, photograph…

Either reinstall plugs or put earlier set back in for ride home or to your shop to adjust, clean set of plugs and retest…

WOT setting are tough to check because of the speeds you would attain are illegal.
Abandoned or lightly travelled area you can do this kind or runs would be ideal…
Interstate Highway would be closest to being able to operate for 5-10 minutes at-high speed, but likelihood of getting a ticket would be high….750 would be going 105-110 at WOT if in decent shape… not a speed the troopers would give you a pass on…

How can I clean the plug body? Possible without taking the engine out of the frame and by just sticking a brush in there?

I will put in a new set today and report back results.

105-110 is not a speed I'm comfortable with... hah

[scottly]

I've been reading and hearing conflicting information about the fuel level requirements... some say as long as they're equal they should be fine. I set the floats at 26mm so I'm not sure what else to do here. Also, as I understand it, when lower it should be running leaner, not richer?

Yes, with the fuel level that low, the motor should be extremely lean, possibly to the point that it won't run without the choke on, which is why I asked. Try using 14mm for setting the floats, then verify the level with the clear tube. 

[RAFster122s]

Download the factory shop manual for your 750F from the Manuals section of the forums, go by what they say, not Facebook experts or the like. 26mm measured to top of the floats when carb rack on its side and pin just touching float valve?
Clear float method of measuring the levels from the float bowl seam takes guess work out...

Excess running at idle will load up the plugs with soot, nature of the beast to get off throttle response needed...

See if you can burn them clean...

Yeah, WIde Open Throttle (WOT) sustained is not for sissies...or bikes that are not sorted out for good braking, etc...

Ask your supplier if they will cut you a break in a dozen plug order...
Or go crazy and get 24...
Denso plugs are another option for spark plugs...
X24...forget number

[smee]

Do you have O-rings around the air screws? I don't think the stock set up has this. May just be a case where you are not letting any air in the pilot circuit.

[scottly]

Were the emulsion tubes removed from the carbs and thoroughly cleaned? If the air holes in the sides of the tubes are plugged up it will cause a rich mixture.

[RAFster122s]

Plug body, as in spark plug body...the ceramic insulator needs to be clean, that why Scott and I suggested you try burning off the sooty deposits first with propane torch. Sometimes it works.

[grcamna2]

subscribed

[tofan]

Download the factory shop manual for your 750F from the Manuals section of the forums, go by what they say, not Facebook experts or the like. 26mm measured to top of the floats when carb rack on its side and pin just touching float valve?

Got it already and yep, but I will check them again.

Clear float method of measuring the levels from the float bowl seam takes guess work out...

Excess running at idle will load up the plugs with soot, nature of the beast to get off throttle response needed...

See if you can burn them clean...

Yeah, WIde Open Throttle (WOT) sustained is not for sissies...or bikes that are not sorted out for good braking, etc...

Understood. Who you callin a sissy? 

Ask your supplier if they will cut you a break in a dozen plug order...
Or go crazy and get 24...
Denso plugs are another option for spark plugs...
X24...forget number

Just bought 2 dozen NGK's DEA8, but will get some Denso's as well.

[tofan]

Do you have O-rings around the air screws? I don't think the stock set up has this. May just be a case where you are not letting any air in the pilot circuit.

No, just springs. Picture below is not mine. For reference only.

[tofan]

Were the emulsion tubes removed from the carbs and thoroughly cleaned? If the air holes in the sides of the tubes are plugged up it will cause a rich mixture.

Yeah, thoroughly cleaned and inspected multiple times before assembly.

[tofan]

Plug body, as in spark plug body...the ceramic insulator needs to be clean, that why Scott and I suggested you try burning off the sooty deposits first with propane torch. Sometimes it works.

Gotcha!

[grcamna2]

Download the factory shop manual for your 750F from the Manuals section of the forums, go by what they say, not Facebook experts or the like. 26mm measured to top of the floats when carb rack on its side and pin just touching float valve?

Got it already and yep, but I will check them again.

Clear float method of measuring the levels from the float bowl seam takes guess work out...

Excess running at idle will load up the plugs with soot, nature of the beast to get off throttle response needed...

See if you can burn them clean...

Yeah, WIde Open Throttle (WOT) sustained is not for sissies...or bikes that are not sorted out for good braking, etc...

Understood. Who you callin a sissy? 

Ask your supplier if they will cut you a break in a dozen plug order...
Or go crazy and get 24...
Denso plugs are another option for spark plugs...
X24...forget number

Just bought 2 dozen NGK's DEA8, but will get some Denso's as well.

I like the original Nippon Denso X24ES-U plugs(very difficult to find them NOS)as they were made from some type of nickle metal and last a long time.

[Jerry Rxman Griffin aka MuthaF'er]

Honda air screws? I once put in aftermarket screws. TOTALLY screwed it up. Plugs looked like yours. One set had the holes and the other did not. Can't remember which.

New plug wires?

New plug caps? Or at least you've verified the resistance across all 4?

New HONDA coils?

New intake boots with new clamps that are not overly tightened?

Your 105 mains are correct and should be fine at 2500', maybe even better than at sea level. However the mains aren't coming into play until ~ 5000++ and probably aren't creating those nasty plugs. New set of plugs and do a plug chop at about 7500rpm after a short burst. You'll probably have to do that in 3rd at ~ 75mph since you don't properly exercise a good F.   

What exhaust are you running? Stock muffler plugged up from all that #$%* you're running through it? Take it off and see what you get. It will sound much better anyway     

[tofan]

Honda air screws? I once put in aftermarket screws. TOTALLY screwed it up. Plugs looked like yours. One set had the holes and the other did not. Can't remember which.
New plug wires?

Air screws are original or stock (99.5% certain) and the wires are new. Came with the ignition kit.

New plug caps? Or at least you've verified the resistance across all 4?

Yes and I will add testing resistance to my list.

New HONDA coils?

This is what I got. https://4into1.com/electronic-ignition-kit-dynatek-honda-cb750-1969-1978/

New intake boots with new clamps that are not overly tightened?

Intake boots are about 4 months old now. Still malleable. Clamps are not new, but in good shape. I made sure not to overtighten, but will check again.

~ 75mph since you don't properly exercise a good F.   

Hah, it's true. I underutilize the poor thing.

What exhaust are you running? Stock muffler plugged up from all that #$%* you're running through it? Take it off and see what you get. It will sound much better anyway

Exhaust is stock, but muffler was changed by previous owner. I think it's cone engineering. I think it sounds pretty good! (Attached photo is not most recent, but that's the exhaust)

[smee]

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