Topic: 1978 Honda CB750K Rich Condition

[njlantis]

Hello all! Looking for suggestions for my 78 CB750 K. I have been running into an issue since I bought the bike where it is running very rich, causing loss of engine power and lots of backfiring. The bike will idle and rev up in neutral but anytime I try to ride it down the road to test it after a carb sync I get the same backfiring loss of power and struggle to rev. I pulled the plugs and they are all very black (showing rich condition). I've taken the carbs off for adjustment countless times during this process. They are the PD42b carburetors. I have checked the main jet, and slow jet fuel circuits for blockages and I have checked that the accelerator pump is working correctly. I have the pilot jet set to 1.5 turns, and I have the float heights set to 14.5 mm. I have seen conflicting information on the correct settings for pilot jet and float heights with some saying 1 1/8 turns and 12.5 mm and others saying 1.5 turns and 14.5 mm. I have new float valves coming in just to make sure that leaky float valves are not causing the rich condition. Does anyone have any insight into what the correct settings are for this model bike and these carbs? Also any suggestions for fixing the rich condition if I do have the carbs set up right and my float valves don't end up to be the issue?

[smee]

Float: 14.5 mm
Main jet: 110
Slow jet: 35

Use original brass jets (clean and put them back in if you've switched them with a aftermarket kit) -> buy OEM if you don't have them.

All things aside, for help everyone here is going to want you to list out everything to get a baseline. Any mods to the bike? All stock? Syncing, timing, air filter clean, etc. If you provide an itemized list of where your bike is at, many will chime in including myself.

[scottly]

Verify the actual fuel level in the float bowls with the "clear tube" test.
Once the spark plugs are carbon fouled like yours, they either need to be replaced or have the black burned off the center electrode with a propane torch.

[njlantis]

Float: 14.5 mm
Main jet: 110
Slow jet: 35

Use original brass jets (clean and put them back in if you've switched them with a aftermarket kit) -> buy OEM if you don't have them.

All things aside, for help everyone here is going to want you to list out everything to get a baseline. Any mods to the bike? All stock? Syncing, timing, air filter clean, etc. If you provide an itemized list of where your bike is at, many will chime in including myself.

Thanks for the advice smee. The carburetors are all completely stock and I recently made sure that all the jets were not clogged. The bike has a 4 into 2 exhaust. I set the timing recently, checked the points gaps, and set the valve clearances on the bike. I have checked the carb and intake boots for vacuum leaks.

List Format:
- Carbs are stock with 110 main and 35 slow jets, float height set to 14.5 mm, idle screw 1.5 turns
- Points gaps in spec
- Timing is correct
- Valve clearances in spec
- Cam Chain Tension set
- The only mod to the bike is the 4 into 2 exhaust that was on it from the previous owner
- Carb synced using mercury tubes after completion of the above items
- Motor was still bogging after the carb sync and not delivering power
- Pulled plugs and saw rich condition, haven't run it with new plugs yet
- New float valves en route

[njlantis]

Verify the actual fuel level in the float bowls with the "clear tube" test.
Once the spark plugs are carbon fouled like yours, they either need to be replaced or have the black burned off the center electrode with a propane torch.

The clear tube test is a good idea, thanks for the suggestion this could give some insight into whether or not it is the float valves that are the issue. With the floats set to 14.5 mm if the clear tube fills past the top of the bowl then it would be safe to assume that the float valves aren't sealing?

[denward17]

Smell inside your oil tank for fuel, that would be a great indication.

Do you see any leaks out of the over flow tubes?

Could it be oil on the plugs?

[smee]

Float: 14.5 mm
Main jet: 110
Slow jet: 35

Use original brass jets (clean and put them back in if you've switched them with a aftermarket kit) -> buy OEM if you don't have them.

All things aside, for help everyone here is going to want you to list out everything to get a baseline. Any mods to the bike? All stock? Syncing, timing, air filter clean, etc. If you provide an itemized list of where your bike is at, many will chime in including myself.

Thanks for the advice smee. The carburetors are all completely stock and I recently made sure that all the jets were not clogged. The bike has a 4 into 2 exhaust. I set the timing recently, checked the points gaps, and set the valve clearances on the bike. I have checked the carb and intake boots for vacuum leaks.

List Format:
- Carbs are stock with 110 main and 35 slow jets, float height set to 14.5 mm, idle screw 1.5 turns
- Points gaps in spec
- Timing is correct
- Valve clearances in spec
- Cam Chain Tension set
- The only mod to the bike is the 4 into 2 exhaust that was on it from the previous owner
- Carb synced using mercury tubes after completion of the above items
- Motor was still bogging after the carb sync and not delivering power
- Pulled plugs and saw rich condition, haven't run it with new plugs yet
- New float valves en route

No pods right? Air filter clean?

[BenelliSEI]

Those carbs have a connection that slightly opens the throttles on full choke. I’ve seen it caught up so the choke does not fully ZERO. Check to see choke is fully backed off when you push in the knob.

[njlantis]

Float: 14.5 mm
Main jet: 110
Slow jet: 35

Use original brass jets (clean and put them back in if you've switched them with a aftermarket kit) -> buy OEM if you don't have them.

All things aside, for help everyone here is going to want you to list out everything to get a baseline. Any mods to the bike? All stock? Syncing, timing, air filter clean, etc. If you provide an itemized list of where your bike is at, many will chime in including myself.

Thanks for the advice smee. The carburetors are all completely stock and I recently made sure that all the jets were not clogged. The bike has a 4 into 2 exhaust. I set the timing recently, checked the points gaps, and set the valve clearances on the bike. I have checked the carb and intake boots for vacuum leaks.

List Format:
- Carbs are stock with 110 main and 35 slow jets, float height set to 14.5 mm, idle screw 1.5 turns
- Points gaps in spec
- Timing is correct
- Valve clearances in spec
- Cam Chain Tension set
- The only mod to the bike is the 4 into 2 exhaust that was on it from the previous owner
- Carb synced using mercury tubes after completion of the above items
- Motor was still bogging after the carb sync and not delivering power
- Pulled plugs and saw rich condition, haven't run it with new plugs yet
- New float valves en route

No pods right? Air filter clean?

Nope, stock airbox and the filter looks like it's brand new

[njlantis]

Smell inside your oil tank for fuel, that would be a great indication.

Do you see any leaks out of the over flow tubes?

Could it be oil on the plugs?

Hmm.. I'm not sure if its oil on the plugs or not. How would I check for leaks, I'm not sure where the overflow tubes are on these carbs actually

[denward17]

^Carb overflow tubes are on the bottom of each carb.  Should have a hose connected to each and routed behind the engine and drains just above the center stand.

If fuel is leaking out you should see it dripping in the center of the bike on the ground, assuming the hoses are not clogged up.

[jaytee-nz]

Carbon fouled plugs caused by a rich condition are sooty looking with no "shine" to them. Oil fouled have that oily sheen on them.
There is no mention of your needle and needle jet condition. If they are original, they are likely worn and will cause rich running. Don't use aftermarket replacements if at all possible.

[newday777]

Welcome aboard the forum njlantis

How many miles on the bike?
Have you had the head off to check the valve guides for wear/replacement and replaced the valve seals?
What plugs are you using?
Did you buy it as is or have you been doing the work??

[njlantis]

Carbon fouled plugs caused by a rich condition are sooty looking with no "shine" to them. Oil fouled have that oily sheen on them.
There is no mention of your needle and needle jet condition. If they are original, they are likely worn and will cause rich running. Don't use aftermarket replacements if at all possible.

The plugs were definitely not shiny. They seemed more carbon fouled than oil foiled by your description.

[njlantis]

Welcome aboard the forum njlantis

How many miles on the bike?
Have you had the head off to check the valve guides for wear/replacement and replaced the valve seals?
What plugs are you using?
Did you buy it as is or have you been doing the work??

Thanks for the welcome. The bike has 16 k miles. it had the ngk DE8 plugs in before I pulled them and I plan to replace them with the same ones. I bought the bike from the previous owner right after he did a carb rebuild and cleaned the carbs. I can confirm that the carbs were well cleaned since I have had them apart multiple times. The previous owner provided me with paperwork from a professional shop that goes over work they did including new valves, guides, and springs along with some sort of resurfacing of the piston heads. He also outlined that during the top end work they also installed/gapped new piston rings.

When I bought the bike th owner stated that he only rebuilt the carbs and slapped them back on the bike, he didn't confirm float heights or do a carb sync.

[Rosinante]

Sounds like the way my K8 runs when the choke is left on.  Idles great but won't rev above about 4 or 5K under load. 

These guys are suggesting there is a carb problem and since this does appear to be a rich running condition, we pretty much know this is a carb problem.  What if you took them off and meticulously went through the carb clean/adjust procedure step-by-step? 

My carbs were rebuilt by a supposed professional and I did not do this, thinking he had already done that.  I still have not done it, but I found at least one major problem he overlooked.  The set screws holding the throttle mechanism to the long throttle shaft were loose!!!!!!!!!!  I still wonder if the choke mechanism was configured properly on the bench.  I have not fully listened to the wisdom here on this Board.  That's on me.

[njlantis]

Sounds like the way my K8 runs when the choke is left on.  Idles great but won't rev above about 4 or 5K under load. 

These guys are suggesting there is a carb problem and since this does appear to be a rich running condition, we pretty much know this is a carb problem.  What if you took them off and meticulously went through the carb clean/adjust procedure step-by-step? 

My carbs were rebuilt by a supposed professional and I did not do this, thinking he had already done that.  I still have not done it, but I found at least one major problem he overlooked.  The set screws holding the throttle mechanism to the long throttle shaft were loose!!!!!!!!!!  I still wonder if the choke mechanism was configured properly on the bench.  I have not fully listened to the wisdom here on this Board.  That's on me.

Well I have had the carbs off a ton of times at this point and I would say that from all the things I've checked I have basically gone through a rebuild process already other than the slide mechanisms, I just haven't replaced any parts because I haven't found any issues yet. I know for a fact my choke is working properly and the slides open and close correctly with the throttle cable.

[denward17]

Does your choke flaps open and close correctly just by using the choke pull cable?

[njlantis]

Does your choke flaps open and close correctly just by using the choke pull cable?

Yup, I've confirmed functional choke by testing the cable with the airbox off and making sure the flaps open and close completely.

[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. So I know the float valves are sealing correctly, which confuses me even more now. I guess the only other thing I have left to check is wear on the needles, other than that I have no idea why the bike would be running rich like it is. See picture below showing the fuel level.

[RAFster122s]

High fuel levels in your carb bowls can cause rich running. Additionally, Excess idling of the SOHC4 motor leads to plug fouling, just inherent in motor design with these carbs.

[rotortiller]

Have you looked for an exhaust restriction and taken the air filter out for close air-box inspection? I know it's a long shot. See if it runs better with no air filter maybe?

[Rosinante]

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?

[scottly]

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!

[rotortiller]

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