'75 CB550K losing power under load after a few seconds

Catburd

New Member
So I inherited this pretty badly neglected CB550K from a close family friend who had one too few spots in the garage. It had been sitting outside for at least two years, had some gremlins to clear out, was filthy, had several busted or missing panels or components, and after about a month of wrenching, I had it running perfectly.

Or so I thought.

I'd spent a long time buttoning everything up and testing it out to be sure it would be fine for longer rides, but the first time I rode it to work, the trouble started.
The trip out was wonderful. It ran like a Swiss watch, sounded great, pulled great from any speed, and was clean as could be.

But on lunch it started to get a little bit of breakup, like it was running a little lean. Then when I went to ride it home, the real trouble started. Right off the bat it was having trouble. You could accelerate hard for a split second, or accelerate gently for a few seconds, and it would very suddenly begin to break up, backfire, and run really short on power, lose speed. Distinct difference in engine tone, not quite sure if it was 8-stroking or what, as I'm unfamiliar with the sound. I stopped for a few minutes to have a look, found nothing obvious wrong, and it ran nigh on perfectly for the next 15 minutes, at which point it went back to the same situation. I was able to limp it home at about 45mph and pull over every few minutes to let the line of cars behind me go by whenever they were willing not to just pass anyway and try to run my ass off the road.

So I ran through the gamut of fixes, starting with seafoam at high concentrations, the Italian tune up, double checking my timing, double checking my sync, reading plugs (Slightly on the rich side at idle, perfectly clean and light tan after running under the error condition for 20 seconds and chopping the ignition), cleaning tank, replacing tank filter screen, cleaning carbs, completely rebuilding carbs after cleaning didn't work, drilling out the vent on the factory gas cap, and replacing the gas cap.

The issue persists and I am at wit's end. I nursed this thing back from the brink only to have it have an aneurysm on me the very same day I decide I'm satisfied with it. It was running better than fine for the better part of a month up until this issue reared its ugly head. I have no idea where else to look.

I do not have inline fuel filters, the bike has the stock airbox, and I do have a Delkevic 4-into-1 exhaust with full baffles, but it ran fine like that on stock jetting for weeks before having this problem.
 

Brodie

Gold Coast, Queensland
DTT SUPPORTER
Welcome mate. Looks like you have a bit of an idea about what you are doing having check over the carbs already.
With what you have done to the carbs I would turn my attention to the ignition system.
Have you checked the coils are good?
What is the battery health like?
 

pidjones

Over 1,000 Posts
In addition to Brodie's suggestions, I would revisit the fuel supply. Dirty tank, lines, petcocks can send crud into a freshly cleaned system. Just pulling one float bowl should give you an idea of that, though.

To help with the electrical troubleshooting, digital volt meters are cheap on ebay and easy to temporary wire in/mount.
 

Catburd

New Member
In addition to Brodie's suggestions, I would revisit the fuel supply. Dirty tank, lines, petcocks can send crud into a freshly cleaned system. Just pulling one float bowl should give you an idea of that, though.

To help with the electrical troubleshooting, digital volt meters are cheap on ebay and easy to temporary wire in/mount.
I already cleaned the tank and replaced the filter screen. Was one of the first things I did, since the tank had rather a significant amount of rust in it. Disassembled the the petcock too to inspect for contamination because it took all of 30 seconds while I had the thing out to replace the filter screen. This is what led me to believe I needed to clean the carbs initially.


Welcome mate. Looks like you have a bit of an idea about what you are doing having check over the carbs already.
With what you have done to the carbs I would turn my attention to the ignition system.
Have you checked the coils are good?
What is the battery health like?
Haven't checked the coils. Aside from grabbing the plug caps and getting a jolt while it was running during the initial outbreak of the problem, that is. If there's a specific method to use, I'm happy to hear it.

Battery is brand new. Put it in the day after I brought the bike home with me. I know that doesn't necessarily mean it's healthy, but I can certainly check voltage.
 

pidjones

Over 1,000 Posts
Good plug wires should keep you from the "significant jolt". Might check them. Helps to watch them at night in a dark area.
 

Catburd

New Member
Check over in the electrical section for a few great write ups from Sonreir.
This one should be what your after.
How to test your ignition coils
Good looking out. I'll check battery voltage and then try testing my coils.


Good plug wires should keep you from the "significant jolt". Might check them. Helps to watch them at night in a dark area.
Yeah, I was getting zapped clear through my gloves, too.
I had it in my mind that I just wasn't doing a good enough job hunting down sources of fuel delivery trouble. Maybe that's just not the issue.

Hopefully I get some time to do this stuff Friday night.
 

Maritime

Over 10,000 Posts
So Jimbonaught had a very similar issue on hos CB750, would run flawless for 45 min then would do what you describe, His turned out to be bad intake boot clamps that after the bike got warm they would go loose and let air in past, let it sit and cool and they'd re-seal and be ok again. Not sure but maybe the same thing if you used aftermarket clamps it could be the issue.
 

Catburd

New Member
Yeah, this happens right away. Like, I warm it up enough to run and it's already doing it. No warmup time or anything. Original boots and standoffs were in pretty bad shape, so I replaced everything but the clamps. They still looked to be in perfectly serviceable shape. Aside from that, I replaced them after the issue started and nothing changed. And I tried something I thought was unrelated that made me think it's not that anyway, more on that in a moment.

So. Spark was a little on the yellow side. Close enough to white that it didn't seem like it'd be an issue, but some of the wires were dryrotted and the plug boots were pretty rough, so I replaced everything because I figured even if it wasn't a problem now, it was going to be a problem soon enough. Checked my battery voltage, it was sitting at like ~13.5, but I threw it on the trickle charger overnight anyway just to be sure. Seemed like it idled a lot smoother and that had me excited, until it started doing the exact same thing.

If it's sat for a few seconds it runs perfectly fine. At least until I've revved it for another few seconds again.

So I hooked up my sync gauges, double checked it when it was running well, and then revved it until I was certain the neighbors were going to come out and try to kill me, just to see if maybe it was an issue localized to one cylinder or two cylinders, because I figured that might tell me something more helpful. (This is also what makes me think it's not an air leak, because I would tend to think it would need a nearly identical air leak in each cylinder all at the same time to create this sort of issue. Please correct me if I'm mistaken.)
My thought process was that, if 1-2 were behaving differently than 3-4, maybe it's an issue with a fuel line. If 1-4 were behaving differently than 2-3, maybe a wiring thing with the coils, or a timing thing. If it was just one of the carbs dragging the others down, like I'd just lost one cylinder, I'd put it down to maybe a bad plug boot, bad plug, bad carb, or damaged cylinder. So, like... No matter what I do, all four are almost *perfectly* in sync all the time.

So, correct me if I'm wrong, but that means whatever is wrong is affecting all four cylinders pretty much perfectly equally.
I'm running out of things to check. So I'm trying to use my diagnostic skills to try to pinpoint everything that *might* still be the cause of the problem, even if it's something I've done already to hedge against the distinct the possibility I'm actually an idiot and don't know what I'm doing, or what I'd prefer, that I'd simply gotten a defective part during my testing and troubleshooting.

Like, in my mind I keep coming back to fuel, but if I knew what I was doing this thing would be running already, so I'm trying not to project any assertions onto the issue and just trying to take feedback from people who know a little more about bikes than I do. I grew up with cars, so the really gross level fundamentals are there. Fuel, air, spark, timing, it stats and runs, so I'm at the end of where my automotive experience is necessarily relevant.

Please help. I'm out of hair to pull out.
 
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teazer

Over 10,000 Posts
DTT BOTM WINNER
New plug wires and boots is a great place to start (pardon the pun). The problem sounds like fuel supply, which usually means it's electrical, so let's try one more thing. Hook up a cheap voltage meter to the coil feed (other side to ground) and run the bike to see if voltage drops off at all as it dies. As revs drop, so will voltage, but if voltage drops significantly the chances are that it isn't charging and that the battery is not holding a charge.

When the bike is running at low revs, voltage should be somewhere around 12.5 to 13.2 volts and at higher revs it should be somewhere around 13.5 - 14.5 volts if the charging circuit is OK. If the battery is good, voltage will still be 12-13v after it dies.

If the electrics all check out, look at the fuel system. The fuel tap may be partly clogged or the filler cap breather may be blocked. Either of those situations will cause the symptoms you describe. You can pull the fuel feed tube off the carbs and turn on the fuel to measure how much fuel flows. You can use whatever container you have available but no sparks and have good ventilation. If flow is weak it's probably the fuel tap. If it starts off OK and soon drops off to a trickle it's more likely to be the filler cap breather, so open the cap and repeat and see if flow is better.

It sounds like there may be a rust flake blocking the fuel tap.
 

Catburd

New Member
Thanks man, I'll check out the voltage tonight. It's a new battery and I left it on the charger overnight before testing last night just to be extra certain (figured if charging the battery fixed the issue, then either it's an issue with the charging system or the battery, as many have told me low battery causes similar issues), so I'm not super confident, but at this point I'm willing to try almost anything.

It's got a fresh fuel cap, a newly rebuilt petcock and filter tube, and I de-rusted the tank, was the first thing I tried actually.
 

Catburd

New Member
It's on the lower end with each, around 12.6 at lower revs and around 13.6 in upper revs, but definitely within the range. Battery is sitting right around 13.5. It should, I just bought it about six weeks ago.

I'm gonna run out later in the morning to find a couple of fuel bottles from Harbor Freight so I can test for more than a minute or so without the tank on.

EDIT -
I swear I thought I saw them at Harbor Freight before. I guess Amazon next-day it is.
 
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moto_mec

New Member
Hi,have you checked the points and condensers? Or was it changed to electronic ignition? I had a 77 cb550 and it had points and condenser (stock)
 

Catburd

New Member
I just replaced them a few weeks ago, but I replaced them with the cheapie rebuy kit from 4-into-1 so I'm strongly considering swapping them out for a Hall effect drop in unit, as there is a greater than zero chance that they're already toast.
 
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Catburd

New Member
Ordered the Dyna Hall-effect ignition module yesterday and it should be here by the weekend. It should take the points and condensers out of the equation, and if nothing else, it eliminates a future failure point. But if this doesn't do it, I might have to find a shop that'll have a look at it.
 

Catburd

New Member
So I threw the electronic ignition unit into the bike mostly out of spite, to keep people from asking me to keep checking my points and condensers over and over again.
Needless to say I wasn't confident it would fix the issue, I just didn't want to put a third set of points and condensers in the bike.

Apparently that was the issue.

Near as I can gather, my points and condensers went bad magically a week and a half after I installed them, and then the set I'd replaced them with were defective in the exact same way.

Absolutely nuts.

Thanks for the support, guys. We did it.
 

pidjones

Over 1,000 Posts
Probably bad condensers to begin with. Few new ones are any good these days. Take out a set of points very quick.
 

Catburd

New Member
Probably bad condensers to begin with. Few new ones are any good these days. Take out a set of points very quick.
That's terrible. Is it just so unusual to run points and condensers anymore that just nobody bothers to make a decent set?
 

Brodie

Gold Coast, Queensland
DTT SUPPORTER
Glad to hear you got it sorted out.
On the bright side at least you are now very familiar with the ignition and fuel systems on your bike.
 

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