Topic: CB550K3 Running Issue. Need help!!

[-CB-Jamie-]

Long time user on the forum but I mainly search and can usually resolve any problems I have from previous users posts. This forum has been a wealth of knowledge to me. I'm pretty stuck on this one though.

I'm looking for help with an issue on my dads CB550K3. He bought the bike just before Christmas in running condition. He test drove before he bought it and as far as he can remember it was running ok. I didn't test drive it myself when he bought it so I can't confirm this. He didn't do more than a few miles and I don't think he got got above 30-40 mph on it.

So since we had snow and ice and couldn't be riding I said i'd give it a once over as the wiring was in a bad enough way. The negative from the battery was just bolted onto a painted area of the frame near the rear mud guard, bare wires were wrapped around each other with just insulation tape around them etc. The positive from the battery had been "repaired" with a piece of copper pipe, see pic below.

Bike is completely stock (Intake exhaust etc.). No smoke of any sort from the exhaust when running.

So it starts and idles fine and sounds perfect. It drives perfectly up to about half throttle which indicates to me the pilot circuit is not an issue. No hanging idle and when idling it's very smooth. Anything above half throttle and it's like it has very little power or acceleration, I can get it up to about 50-60 mph and it'll move along ok at that but open the throttle and there's just no go. It will almost kind of surge I think, not sure how to describe it and it sounds loud through the airbox kind of?? I thought it may have been a fuel starvation issue.

Bike will rev and sound perfect when standing still, you wouldn't think there was an issue until you drive it.

So what I've done so far is the following:

• Wiring removed and all connectors cleaned and new connectors soldered on where they were missing.
• New positive and negative battery cables made up and fitted correctly, all areas of the frame where the grounds connect were sanded down to bare metal before putting back together.
• New o-rings between the intake manifolds and the head, the old ones were gone hard and somebody had pumped gasket sealant in around them.
• New carb insulators between the carbs and the engine.
• New air filter.
• Valve clearances checked and were fine.
• Carbs disassembled and ultrasonically cleaned, the vents were blocked up alright and they were hard to clear. All passages in the carbs were blown through with air and are clear. All jets were cleaned and inspected and appear fine. The emulsion tubes on the main jet were cleaned and appear fine. Original Keihein jets are in them #42 slow jet and #90 mains. Needles are correct type and are in the correct position per the workshop manual, 3rd position(Middle).
• New o-rings fitted on the idle mixture screws as they were missing.
• IMS are set at 3 turns out, 1.5 is stock but the idle was very rough at this and it's perfect at 3.
• Float levels set using the clear tube method. I checked and triple checked these since I've been having this issue.
• New electronic ignition fitted. Timing is set with a strobe timing light and is correct. Advance mechanism is working as it should checked with the timing light.
• New spark plugs (NGK D8EA). New spark plug boots (NGK 5k ohm) wires cut back 1/2" before fitting new boots.
• Coils checked, the secondary circuit on one coil was reading a bit low, 13.90k ohm should be 15k ohm I believe. So I swapped the coils out from another bike which I know is fine and this made no difference.
• Checked for a vacuum leak with WD40 and couldn't find any.
• Carbs balanced using a Morgan Carbtune.
• I had fitted an inline fuel filter and removed this and there is now one new piece of fuel hose between the tank and the carbs. I've checked the flow out of this and it's fine. The tank vent is also clear.
• Tank itself is clean.
• Compression test done when cold and got between 105 and 115 PSI across all four cylinders. I want to redo this when hot for comparison.

I can't think of anything else at this point that could be causing this issue besides mixture... I'm at the point of tearing my hair out on this one 

[strynboen]

i have the same model.
.thek the ignision advancer..evt give it a light oil..so the parts move free..thek the airfilter and be shure the top is there..its a snorkel vith a metal threds filter..it give just a bit more restriktion,,and thikken the air/fuel mix..specily at heigh rpm...intake leaks can also lean aut..

and fine rust in the tank can bloke the fine holes in the kaburettors..so make a fuel flow test..before go Deep in the carb nightmare..vaccum in the tank can also dirsturb the flov..and the famus inline filters...vho Blocks the flov...not on all bikes..but on mine it vas a big problem

hva thekked the Photo..the fuel line ...try to get a Falling flow in it..now it goes up..and then dovn..try to get a konstant fall dovn the line..and the spark plugscaps..is they the right ohm..if the ignisiom fails at heigher rpm..the feeling is the same as fuel starvasion..so thek both..direktions

edit..can see you have been through it most..but try once more..specilt the fuel line is sensitive on 550..the smallest vaccum in the line..and the fuel vill not flow

[calj737]

Does the bike rev up fully but not accelerate, or not even rev beyond half throttle?

I believe the carb screws turned beyond 2x is pointless on your model. If it takes that much, there's a jetting issue. As Stryn said, check for an obstruction on the snorkel under the seat for the airbox intake.

[-CB-Jamie-]

Yes it does rev up fully but it's just extremely dead. Very slow to accelerate.
The snorkel under the seat is clear also.

Strynboen I've changed the fuel line since then so it does come straight out of the tank and downhill the whole way to the carbs. Good spotting!

[calj737]

If it revs up fully but doesn’t accelerate, I’d begin to suspect the clutch is slipping. What gear are you accelerating in, and what RPM range? These motors like to be revved up above 5-6,000 before you shift gears.

[uksparky]

Lack of fuel yes..did you check the in tank filter its on top of the fuel turn on valve, putting the engine under load will need more fuel, not like rev standing still......... how do the spark plugs look after run out..........  white plug tip lack of fuel, or big vacuum leak

[-CB-Jamie-]

Calj737 - Definitely not a slipping clutch, I've experienced this before and I know what it feels like, the engine sounds almost bogged down when trying to accelerate and this would be bewtween 3-6000 rpm range. I haven't looked at the spark plugs after running either, another thing to check.

uksparky - I might revisit the in tank filter, it was stuck in the tank so I didn't try pulling it out to check it. It looked pretty clean looking into the tank with a torch. I might try get it out to check it.

[calj737]

Excellent. Then I am with Sparky, fuel issue.

[strynboen]

make a fuel flow test...i did vhen i bayed a china fuel tap..but cant renember the liter in auer flow.(.think it vas o,2 dl in 60 sekunds)..but the point is that it stayes the same heigh flow in more minuts..id there is komming a vaccum it dryes up/slowes dovn.. after some minuts run..i have a fuel symptom..at full speed,,at more then a few minuts..then one cylender missing fuel..and i run on3..after to habe sloved dovn..it komes back on 4..and NeXT time i run for longer times full trottel it cut aut..but that is a different..one..think its a vaccum in the tank..and the one carb gets less fuel..but it gives a idea of the small optimal funktion vorld ve have to run those bikes..they are simpel bikes..but can still be hard to get running perfekt

[Deltarider]

Do what Strynboen suggested and also check carb vent tube(s) is/are open and not clogged by dirt at the bottom or even pinched closed. If necessary shorten an inch.

[-CB-Jamie-]

Haven't had much time this evening to look at it but  I did a quick flow test there, it filled a one litre in approximately 1 minute 10 seconds, not sure if this is good or bad but I'd think that's ok??

Deltarider - carb vents are definitely clear, I couldn't do the clear tube method of checking the float levels because these were blocked at first, the fuel would just rise up the tube and overflow, took me a while to figure out what was wrong but it was the vents in the end!!

Sent from my VTR-L09 using Tapatalk

[seanbarney41]

I don't know much about 550's but 42 pilot jet is bigger than on a 750.  Is that correct?  I would expect a 38 or smaller.

[cb_n00b]

I don't know much about 550's but 42 pilot jet is bigger than on a 750.  Is that correct?  I would expect a 38 or smaller.

42 is stock on the K3 and K4. I've had two sets of PD46A carbs off a 1977 CB550K3 and both had the stock Keihin 42 pilots in them. For some reason the manual says the '77s have a 38, but I have yet to find a '77 that actually ever had 38s in it.

Regardless, I had 42s in my stock carbs and they worked very well with that setting, it would've been too lean moving down to 38s (speaking to how it was setup stock, before any mods I made).