Intake hoses. If the #4 is leaking badly enough, the #3 will foul the plug. This takes some explaining as to "why", but it is true on all of these bikes, seen it many times. Check the new intake boots for leaks: if you are using the normal OEM clamps on those new boots, they are not sealed because the new rubber is thinner, and the old clamps are also stretched. The 2 items together make for a poor, leaky seal.
The D8EA sparkplug is also too cold for these engines. The original plugs were D8ES-L, which is about halfway between the D8ES and D7ES plug heat range, and also has a longer, narrower ceramic tip to help burn off deposits: this sparkplug was created specifically for the CB750 by NGK at Honda's request in 1970. Today you can mimic that heatrange for in-town use with the D7EA or X22ES-U sparkplugs, and if you ride mostly in-town this might be the best all-around plug for your engine. Don't run Premium gas: it WILL foul the plugs today because our modern fuels burn far slower than the 1970s versions, with Midgrade (or even Regular in some areas) burning at the speed of 1970s premium-grade fuels. I only use Premium when riding our Out West interstates, and 2-up, at their 80-85 MPH speed limits on long trips. Otherwise I run Regular grade.
I'll check again for leaks, but also get new clamps.
The 22's seem to be a little harder to come by compared to the D7ES so I'll try the D7ES.
I was told X24ES-U were the way to go so I got some in stock. What's your take on those?
Also, I'm in Tucson, AZ and the weather is pretty hot here (if that matters).
The X24ES-U is my go-to plug for these engines, and I use it myself. In town I will run the XR24ES-U (resistor version) for a slightly longer spark and less unburned fuel buildup if I am going to be in heavy city traffic, or go one step hotter if I am spending more than a day or two in Denver proper (25 MPH speed limits there, lots of stoplights). Honda used to market this one as their own-boxed plugs for the CB750 engines until about 2004 or so when they quit handling privately-labelled plugs. I don't know if they are still doing it now?
One good way to tell if it is gas fouling or oil fouling is: as soon as the plug is pulled out, try to light it with a flame. If it burns it is gas, if not, oil.
Your hose clamps: put your screwdriver into the screw head and push hard to see if the hose clamp rotates. If it does, it is too big and the loss of vacuum causes the NEXT carb in the firing order to run richer because the engine stumbles slightly at the leaky one, so the next one's air intake speed is too slow. Since these carbs mix richer-per-air-volume as the air speed slows down, the next one gets a richer 'shot' of air-fuel mix, and after a few hundred thousand firings, the plug looks like yours. This leakage issue is 100% likely today if the hoses are new (and not NOS from before 2010 or so), and I run into it on every full-rebuild I do with these 750s. The reason: the new hoses are slightly thinner rubber, probably because we started yelling at Honda to make them again during their abandonment of us around 2006, but the old designs were on paper and long-gone. I suspect they took an old hose (2, actually) and made their best-guess at how they should be made because the engineer who originally designed them has died of old age and didn't pass on his notes to the new computer whiz kids at Honda who are designing the parts now. We are seeing this sort of lost-art happening a LOT these days, not just at Honda!
If the fouling is oil, you will likely also see some oil drooling around the exhaust header's clamp on the front of the engine. When Honda stopped using Stellite in the valve guides (circa mid-year 750K2 engines, 3/72 build dates) they switched to simple cast iron. The 750F0 engines once again received the fine Stellite valve guides, but they are expensive and vanished again in the 750F1 and all later versions. If yours is NOT a 750F0, and it has more than 10,000 miles on it, the exhaust valve guides are likely history if the bike was run during the 1990s when we suffered MTBE in our gasolines: that was especially good at dissolving cast iron rings and valve guides. I replace them with bronze guides today to prevent this from happening with our ethanol-laced gas, which strips all lubrication from the top end (it is a solvent) - this is done to extend the life of catalytic convertors in cars, but doesn't help us out one bit!.
