Topic: CB550K started running like crap out of the blue. No power. Idles smoothly.

[painteddigital]

Hey guys! Here's my problem:
My '78 CB550K all of a sudden started feeling like it’s running on 2 cylinders when gas/load is applied. It jerks a little when in gear and will die if clutch is let out and revs are not above 3k. Seems to idle normally.

Diagnosing so far:

9/3
Replaced fouled spark plugs with working ones

9/8
Replaced dead battery with new LiFE battery

9/13
Gapped valve tappets
Replaced all spark plugs with Iridium plugs

9/20
Adjusted timing (had a Dyna S ignition from the time I bought it) Static and strobe light to be sure.
Double-checked the spark at each cap.

9/27
Compression tested engine
Dry: 115, 114, 115, 140
Wet: 122, 129, 131, 149

After each of these attempted solves, the bike was tested and found to have still have the problem. I'm suspecting the head gasket is bad, but also suspect the carbs. It's been running almost daily for over a year now. Only thing I know of that could be related is gas milage stinks at around 25mpg. Hoping to get someone else's opinion before I dive into something big. What do you guys think?

Thanks for any help.

[mikeelem]

This is a long try but... Something similar happened to me on my k1 cb750... Ended up being my gas tank cap that would't let the air in to compensate for the gas coming out... Which in turn would not feed the carbs right. So engine started bugging at same rpm, ran on 2 cylinders, couldn,t get rpm up and then the bike died... What worked for me once i figured it ou, was riding with the cap unclipped or open... Until i get a new one... Does make a mess on bumpy roads.... But only when it is full!

Hope this is it!

cheers!

[harisuluv]

Did you start by confirming it's not running on all cylinders?

[painteddigital]

@mikeelem I've heard of that happening. I have the style that has a little catch/drain hole so I don't think flow is restricted

@harisuluv I don't know how other than checking spark at each cylinder?

[evanphi]

@harisuluv I don't know how other than checking spark at each cylinder?

Check the heat of pipes after a bit of a warm-up.

[harisuluv]

@harisuluv I don't know how other than checking spark at each cylinder?

Check the heat of pipes after a bit of a warm-up.

this.  checking for spark is only evidence of spark.

[xtravbx]

By checking for heat - take a spray bottle and mist the header pipe. See if it boils off immediately, or another handy tool is a laser thermometer. Even if you're running on 4 cylinders, dramatic temp differences between cylinders lets you know something isn't right.

[eigenvector]

Fortunately you were designed with the proper equipment.  Take a finger and place it on each exhaust pipe.  The fingers that aren't hot dogs after a second indicate the cylinders which aren't running right

(Sadly, I made the mistake of doing this - ONCE.  Never use your fingers to test how hot the pipes are getting)

Yeah, best advice that's been given is to check the pipe temps, they should all be running about the same.  The plugs should also give you a good indication.  My guess is a gas flow problem, anything more serious would ruin the idle in my experience.

Personally I would invest in one of those infared thermometers, saves you the trouble using a spray bottle - which I've never had luck with.

[painteddigital]

Thanks guys. Can someone explain what this will tell me? If it's hot on all four what does that mean? I'll need to do some reassembly to do the test

[Mo]

Thanks guys. Can someone explain what this will tell me? If it's hot on all four what does that mean? I'll need to do some reassembly to do the test

Simply that all of the cylinders are operating. Meaning fuel and spark and compression are present and the problem is not likely with your carbs or ignition.

[painteddigital]

Alright. Thermometer purchased, bike reassembled, measurements taken. Here's what I got:
399, 305, 145, 375 (°F 1-4)

So it would appear something is going on with the 3rd cylinder. What do you guys think this means?

[calj737]

Both 2&3 are off. Since they are served by the same coil, I would start by verifying that the coil is producing the appropriate spark, and that the plug wires/caps for those cylinders are seeing the correct resistance.

You could also have 4 imbalanced fuel levels contributing to the disparity of temps faith in each pipe. 400/375 for 1/4 is off enough too that I'd check the float levels once I had determined the electrical status.

[evanphi]

Both 2&3 are off. Since they are served by the same coil, I would start by verifying that the coil is producing the appropriate spark, and that the plug wires/caps for those cylinders are seeing the correct resistance.

You could also have 4 imbalanced fuel levels contributing to the disparity of temps faith in each pipe. 400/375 for 1/4 is off enough too that I'd check the float levels once I had determined the electrical status.

Everything here. Also could be down to just bad plugs. How new are they? If old, get new, then go with calj's advice.

[TwoTired]

Alright. Thermometer purchased, bike reassembled, measurements taken. Here's what I got:
399, 305, 145, 375 (°F 1-4)

So it would appear something is going on with the 3rd cylinder. What do you guys think this means?

Don't know what year or style of carbs you have,  But maybe the main jet fell out of it's holder.   Might also need a carb vacuum sync.

[painteddigital]

This weekend I solved the main problem with simple solution. The bike came with a secondary inline filter. It looked a bit melted from the heat, so I replaced it and the existing fuel line with a new one and the bike sprang to life! So clearly it was indeed a fuel delivery problem. I'm so happy that it's back on the road! Thanks all for the advice.

I still get uneven temperature readings at the carbs, and a bit of backfiring though when letting off the throttle. Although I would like to sync the carbs and clean them, I also wonder how much an exhaust leak would contribute to these symptoms? The 3rd or 4th muffler has a pretty large hole in it but I like the extra noise it generates for letting drivers know I'm around.

Thanks again.

[calj737]

The temp measurements you are taking is from the exhaust and not the carbs right?

Theme should be fairly uniform between all cylinders. If not, and if you are certain the fuel level is accurate, then you have either an ignition problem (if all cylinders are fully hot, just too wide a spread) or you have an internal carb restriction. Is it possible one or more of your carbs is using different sized jets? Something to verify.

But focus on the ignition/spark for each cylinder. Bad wires, damaged cap, points/condensers, etc...