Author Topic: CB550 high rpm power loss  (Read 338 times)

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Offline Wyatt565

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CB550 high rpm power loss
« on: January 21, 2019, 11:18:48 am »
Hi all, you may have seen my other post about trying to get this 1977 CB550f to idle with any semblance of consistency. Well I took a break from this project for awhile and finally came back to it this past weekend. I took some advice from a coworker and leak down tested it, the results came back perfect, 98 to 100 percent holding in the chamber. So I took apart the carburetor and cleaned it for the billionth time. This time I moved the needle clip position down to the last spot, and when I put it all back together I decided this time to run the velocity stacks instead of the pods I've had on since I bought it. Well to my surprise it not only idles great with no choke but also rides! (Sort of) I was able to take it down the driveway and I had no mid range. I came back and cut out some cloth to slip over the stacks and when I test rode it after that it was doing really well. The only issue was that at 5000 rpm it had no power, just bogs down. Also the 4Th float bowl Lets  glass out the overflow when at a stop sometimes, but not while riding. So now I come to you with a new problem to work out. Ill label below the specs and what's been done, I appreciate any advice guys.

Cb550F with velocity stack intake with cloth over stacks on stock carbs and a 4into1 exhaust.
Carb setup: 115 mains and 40 pilots
Needle clipped at last position at top of needle
Floats set to 22mil awhile back but carbs have been messed taken apart a few times since.
Carbs thoroughly cleaned and slides set with 1/8 drillbit  a few times
Ignition system has been changed to Dyna S ignition coils and wires.
Timing has been set following dyna s instruction.
Valve clearance has been adjusted to recommended spec

Offline hsas.69

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Re: CB550 high rpm power loss
« Reply #1 on: January 21, 2019, 12:11:19 pm »
Vaccum sync the carbs and then do a plug chop.

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78 CB750 F3
78 CB550 K4

Offline Wyatt565

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Re: CB550 high rpm power loss
« Reply #2 on: January 21, 2019, 12:23:02 pm »
Head.69 Will do, hopefully I can get this thing tuned up and completely road worthy. Thanks

Offline Tracksnblades1

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Re: CB550 high rpm power loss
« Reply #3 on: January 21, 2019, 02:44:00 pm »
Can you breathe normally through the cloth you used to cover your velocity stacks..?
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Offline Tracksnblades1

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Re: CB550 high rpm power loss
« Reply #4 on: January 21, 2019, 02:47:53 pm »
Clip at the top of needle is as lean as you can go without making changes....
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Offline Wyatt565

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Re: CB550 high rpm power loss
« Reply #5 on: January 22, 2019, 07:21:15 am »
Can you breathe normally through the cloth you used to cover your velocity stacks..?

From what I’ve seen yes it seems to breath pretty normally, it’s not very thick cloth.

Offline PeWe

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Re: CB550 high rpm power loss
« Reply #6 on: January 22, 2019, 10:54:04 am »
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 stock looking project started 2017

Offline sixthwisconsin

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Re: CB550 high rpm power loss
« Reply #7 on: January 22, 2019, 12:11:34 pm »
You are leaking gas at stop signs? Sounds like you need to do the clear tube method for float bowl levels. You can measure all you want but it will never be right until you do the clear tube method. (ask me how I know!) imbalance also causes poor idle.

You have leaned out your needles for less fuel AND added high flowing velocity stacks for more air. At higher RPMs you are probably running out of fuel. So if you really want to run the stacks you'll need bigger mains.

Also, where are your air screws set?
1974 cb550k0 with 1978 motor resto-mod
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1974 cb550k0 Custom
1975 cb750f0 Custom Cafe
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http://forums.sohc4.net/index.php?topic=101126.msg1134620#msg1134620
2009 Kawasaki Concours 14

Offline Wyatt565

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Re: CB550 high rpm power loss
« Reply #8 on: January 25, 2019, 06:10:48 am »
Sixthwisconsin, this bike has been giving me a hard time since I got it, now that I have set it up where it runs halfway decent I really don’t want to mess with carbs again in fear of messing it all up, but I am about to order new mains, I also have a vacuum synchronizer on the way to get them dialed in. The mix screws are out half a turn

Online seanbarney41

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Re: CB550 high rpm power loss
« Reply #9 on: January 25, 2019, 09:36:20 am »
If your 550 only runs below 5000, you are really missing out. 5-9000+ is where the 550 makes acceptable power.  Your carb troubles are not over until this is fixed.
If it works good, it looks good...

Offline Bodi

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Re: CB550 high rpm power loss
« Reply #10 on: January 26, 2019, 07:52:24 pm »
The carbs are a likely source of problems, yes. A plug chop will tell that tale (or a dyno with EGA...). I also wonder about the cloth, have you tried without that?
Ignition and valve timing affect performance progressively more as RPM increases, there is less time for intake/exhaust and spark events to happen so a few degrees out can make a big difference. I worked on a 550 with no top end power, wouldn't even rev up in neutral, but it started and idled just fine. Tracked it down to a cam chain one or two teeth out, plus the ignition was set a bit retarded. Fixed those problems and it was "a new bike!" according to the owner.

Offline Wyatt565

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Re: CB550 high rpm power loss
« Reply #11 on: February 09, 2019, 11:42:44 pm »
Hello, sorry for the delayed reply, I have been a bit busy lately. But I did have a chance to do a little fiddling, I moved the needle clip down two spots and that made it a lot better still had a flat spot around 3000 rpm so I think I may be too lean still, but before chasing that down I’m going to check timing again and make sure everything is good to go, If so then I’ll go for the plug chop method to see where it’s running.

A bunch of parts came in for the front end that I ordered so the forks have been torn off and rebuilt, the new headlight assembly and clip on handlebars with new controls are in. So for the foreseeable future I’ll be figuring out the mess of wires I’ve got hanging off my front end. Speaking of which I need to make another post to answer a question regarding that. But once it’s all back together I’ll be back to getting it tuned in perfect and rollin through Seattle’s new viaduct before you know it!

Offline BomberMann650

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Re: CB550 high rpm power loss
« Reply #12 on: February 10, 2019, 03:59:01 pm »
Sweet seattle!

Greetings from the south puget sound.

When snowpocalypse is over maybe I could help.
可燃自転車 Kanen Jitensha Combustible Bicycle
75 CB550F codename Bloo
76/78 XS750 triple; trash can.
07 FJR1300; Horse

Offline Wyatt565

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Re: CB550 high rpm power loss
« Reply #13 on: February 10, 2019, 07:37:43 pm »
Ahh I’m about an hour north of the city, the assistance may be needed with how deep of a hole I’ve dug myself  :o

Offline BomberMann650

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Re: CB550 high rpm power loss
« Reply #14 on: February 11, 2019, 04:20:33 pm »
Tuning modified setups is a trial and error process if you don't have all the fancy gadgets a dyno shop does.  Went down lots of dead ends with the 750 triple project.  It was flat in the middle too.  It would rev to a certain point then begin making a blubbery sound with no throttle response.  Went up in jet size, got worse.  Went down in jet size,  Saw some improvement.  Got to a point that things were real close, but no adjustment would smooth the trouble spots.  That's when plug chop readings came to the rescue.  Even with the K&N pods the bike was over fueling at WOT.  Thanks to the forum I understood a bit about manifold air pressures.  A couple of minor mods to the mikuni CV carbs helped to deliver the extra air and keep the intake pressures up.   The bike runs smooth. 

Well, as smooth as a non-counterbalanced 3 cylinder can.

On the other end, my 550/4 used to run up to about 2/3 of the throttle before there was no more response.  It didn't blubber out, just wouldn't go any further.  That's a sign of under-fueling.  Rebuilt the motor and restored some carbs with bigger 105 main jets, and it's got a little more spunk now.  This is with a stock airbox, 650 cam and port contoured intakes.
« Last Edit: February 11, 2019, 04:22:58 pm by BomberMann650 »
可燃自転車 Kanen Jitensha Combustible Bicycle
75 CB550F codename Bloo
76/78 XS750 triple; trash can.
07 FJR1300; Horse

 

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