Topic: K1 ran well last night, now running poorly.

[JohnK3]

I rebuilt the carbs on my all stock 1971 CB 750 4 years ago. It starts easily, idles nicely, and pulls strongly up to redline. I've been using ethanol free fuel. Saturday night I started it up to make sure it was OK for the next morning's club ride, and all was well. I topped off the tank with more ethanol free fuel and shined it up.
Sunday morning it was very hard to start, would not idle, and felt like it was running on 3 cylinders. I rode about 200 miles but it did not get any better.
The #4 exhaust pipe is now a little bluer than the other 3 pipes.
Any ideas on how to proceed with trouble shooting?

[scottly]

I would start by inspecting the spark plugs.
Edit: Before pulling the plugs, start the engine and to a "spit sizzle" test on the exhaust pipes: if one pipe is significantly cooler than the other 3, pull that cylinder's plug first.
Was there some kind of event in Prescott today??

[BenelliSEI]

+1 to plugs. I bet you’ve fouled one. NGK D8EA? Screw in four new ones and see if your problem is solved. Do it when the engine is cold and put some anti seize paste on the threads of the new set.

[HondaMan]

I might suggest using the D7EA (or X22ES-U) if you're running 'ethanol free' gas. It is probably real high octane, which today is much too 'rich' for these engines.

[scottly]

if you're running 'ethanol free' gas. It is probably real high octane,

No, the ethanol free gas around here is not high octane, only 87 IIRC.

[newday777]

John
Has it been exposed to higher ambient moisture air, as in heavy dew or rain? Moisture can get into old plug wires/caps with age.
How long has it been since the plug wires were cut back 1/4" to give fresh wire for the plug caps to screw into?

[MauiK3]

Ethanol free here is also 87.
I run it in all my vehicles, old and new. I know, not necessary in new stuff but I do it because I've convinced myself that I get better fuel mileage. When I had my 2009 Ruckus it really helped the little 49cc banger.

+1 on plugs

[HondaMan]

if you're running 'ethanol free' gas. It is probably real high octane,

No, the ethanol free gas around here is not high octane, only 87 IIRC.

Around this neighborhood it is 99 or 102 octane at the 2 different stations. It is also nearly $5/gallon(!).