Topic: 1978 CB550K carbs bogging BAD

[trentonswartz]

Hey guys! This is my first post, so go easy on me.

I have 78 CB550K that I just bought in "running" condition. First bike. It came with K&N pods, 4-1 exhaust, carbs have been rejetted to 108 mains and I've meticulously cleaned them 3 different times trying to figure this out. Float heights set to 14.5mm, as it has PD46C carbs. Fuel mixture screws are 2.5 turns. Basically, I can get the idle down around 1500 comfortably, but is a bear to start at 45ºF temps and lower...kicking and kicking once the battery dies from trying to electric start it. I probably need a new battery.

The biggest problem is that in 1st gear at 1/4-3/4 throttle, at 4500RPM it loses all power and bogs down. If I give it WOT, it slowly powers through and can get up to redline. It pretty much does it in gears 1-3, and tops out at 55mph in 5th...not ideal. The bike is firing on all cylinders. I'm trying to get my hands on a carb sync kit right now, but haven't been able to sync the carbs yet.

Where should I start in my checklist? I had one guy tell me that a carb sync would fix it, but I'm not 100% sold because it bogs BAD. Any advice on every possible thing I can check would be greatly appreciated.

[JamesM]

Bad battery can make it bog down.

[trentonswartz]

Bad battery can make it bog down.

I've tried it with two different batteries fully charged, and it does the same thing, so I'm fairly sure it's more of a carb/timing issue.

[magnus72]

pods? turn up your idle.

[TwoTired]

Let me get this straight.  You are complaining that if you snap the throttle open to WOT when the engine is at idle and under load, the engine doesn't give you full power, right?

Do you have any idea at what position the slide needle clips are in? 

... And, you don't believe that carb vacuum syncing has any value?

How do you feel about performing the complete and routine 3000 mile tune up checklist?
This could help with cold starting.
Have you ever checked/verified that the choke plate close completely when attempting to start the bike cold?

Have you measured the voltage arriving at the coils while the starter is engaged?

[DustyRags]

Everything TwoTired said. Also, set timing. It takes 10 minutes, and can make a world of difference.

[seanbarney41]

check that your ignition advancer is working...then, when you find that it is, or that it doesn't entirely solve your problem, find yourself the factory airbox

[K3Owner]

My first post as well since I shared this same problem with my K3 for a couple of years. The problem is that it can't tolerate a 14.5 mm float height. I was also measuring at the lower point of the float which meant even leaner. Hard starting and lean spots in the rev band. Weak mid-range - just not happy.

I have a cheap micrometer to make it easy and measured 12-12.5mm at the highest float point. Beats the cutout cereal box I used to use  . $10 at Harbor Freight and it can do double duty COAL on the reloading bench.

All the other advice is spot-on, if I may say so. A good sync and high speed timing and she'll be the bike she was again. Check the regulation to be sure she's charging at speed.

Good luck! Still plenty of time to ride here in the South.

Marc

[trentonswartz]

Thanks to everyone for the replies. I'll get right to work on everything as soon as possible!

[DustyRags]

Report back when you're done!

Oh, and you know what'd be cool? A video with sound before and after so others can compare this later. If you happen to have a cam or a phone with a decent cam handy.

[trentonswartz]

Quick update:

I pulled off the carbs and re-set the float heights to 12mm, and I took a micrometer and bench-synced the carbs to match carb 2 (they were way off). I also did a 3000 mile tune-up and I realized that the electric start drains my battery too quickly, so I'm using the kickstart.

Once I put the carbs back on, it started in 2 kicks! Then I took it for a ride, and immediately noticed a power difference. It still has a weak mid-range and a dead spot at 5000RPM, but it's not nearly as bad. I got it up to 65mph in 4th and it seemed to even have more power left.

Once I can get with a buddy of mine, we'll have to reset ignition timing and vacuum sync the carbs, and she should be good! It also backfires just a little bit when it's idling. Does that mean I need to richen my fuel mixture screws? They're already at 2.5 turns.

[DustyRags]

Awesome! Sounds like it's just a tuning issue

Can't help you on the rich/lean issue, I haven't been there yet.

Get a timing light, and the timing is easy peasy. Be easy with the points gap adjustment screws, they're soft as putty.

As for the sync, it's a bit fiddly, but really not hard. I made my own manometer with 25 feet of clear vinyl tubing from the hardware store and some plastic barbed T's from the garden section. Filled it with the same oil as in my bike (new, not the filthy used stuff) and it worked a treat.

One important trick for syncing: the adjustment screws have lock nuts. Those lock nuts work by pulling the screw up against the threads. When you adjust them, you push down into the threads. That bit of wiggle room is enough to cause issues. If you leave the lock nuts partially tightened, that doesn't happen. Basically, you don't want them so tight that it's really hard to turn, but just tight enough to exert a constant upwards pressure so there's no up-and-down play in the screws. It's much, much easier this way. And don't forget to blip the throttle after each adjustment.

[flybox1]

applause for following directions and suggestions from the knowledgeable group here.
3000mi tuneup to the rescue.....again!    

[TwoTired]

Once I can get with a buddy of mine, we'll have to reset ignition timing and vacuum sync the carbs, and she should be good! It also backfires just a little bit when it's idling. Does that mean I need to richen my fuel mixture screws? They're already at 2.5 turns.

"Backfire" as in out the carbs or out the exhaust?  Makes a big difference toward diagnosis.

[Bankerdanny]

If the electric start is draining your battery then you still have a problem. My '77 didn't turn over for more than a couple seconds before firing, warm or cold.

[trentonswartz]

"Backfire" as in out the carbs or out the exhaust?  Makes a big difference toward diagnosis.

Backfire out of the exhaust...sorry. I did get a little spit-back from the carbs before I made these changes, but it hasn't done that since I adjusted the float bowls. It just quietly puffs out of my exhaust a little sparaticly when I'm idling.

[TwoTired]

Exhaust explosions can be due to leaking header gaskets, or lean idle mixtures.  Lean idle mixtures will also make throttle response wheezy.  As in, twist the throttle and it breathes but there is no power until the throttle is backed off.  Some call it "bog".
With pod style filters, you will probably have to turn out the IMS 3 or 4 turns (perhaps even stretch the retainer springs so the screws don't fall out), as they really screw with the carb throat pressures that draw the fuel up from the bowl.  There is a reason why pods are "cheap" in every sense of the descriptor.  They really are a "kludge" addition to the bike.  Some like the fact their bike looks like an assemblage of afterthoughts.  I can't explain why.

Cheers,

[harisuluv]

"Some like the fact their bike looks like an assemblage of afterthoughts."

LOL!

[trentonswartz]

Update....

I got with my buddy and adjusted the valves, set timing (was a little off), and adjusted the cam chain. Took it back out, and it had the same issues, but the engine sounded better and felt a little smoother. Still, the bog at 5000rpm was terrible. Then, I decided to try one last thing and back out the fuel mixture screws 4 turns from 2.5 turns like TwoTired said, and then put tape over the top and outer sides of the pods to restrict air flow, since I figured it was getting too much air and not enough fuel.

She sang right past 5000 and had tons of power! Still a slight little bit of a burp on the midrange, but if you crank the throttle she'll power up for days. I figured once I'm able to sync them, they'll be golden.

So the conclusion was that apparently she doesn't like the pods (I know I know....people rant about them hourly on here). I'm just on a budget, and that's what came on the bike when I bought it, so I'm trying to make due with what I have and see if it works.

[DustyRags]

Cool! Check around a bit, I seem to recall someone on here creating some sort of cover for his pods out of PVC pipe that did a great job restricting air flow.

Edit: Found it! http://forums.sohc4.net/index.php?topic=7041

[RFogelsong]

After playing with my carbs last week I'll take a crack at it.

Everything you posted points to lean (imo) as others have mentioned.  In reading countless 550 carb setup threads I'd say you might want to go up on both jets (maybe needles and other stuff, but that's above me).  I seem to recall somewhere in the neighborhood of 115-120 mains and maybe 40-42 pilots for a decently open 4-1 and pods on a 550 (again, from examples on here).  Don't quote me on that, but I don't think it would be a bad place to go.  This would also explain why it's hard to start when cold, lower temps=leaner burn, take out a plug and check.

On the battery issue, if it's not turning the starter over long enough to start the bike and you're sure the battery's decent it could be the starter motor.  Get a local small engine shop to rebuild it.  I had mine rebuilt for $25 locally and it was amazing how much slower it was turning over than it was supposed to before I had it rebuilt.

 

[TwoTired]

While all factory cb550 engines were the same, they came with two different style carbs  Being of different design, the ideal jetting choices and predictions for one will not apply to the other style.  For one thing, the venturi shape and ramp angles are different between the two.  So, they make different pressures for the same air velocity going through them.

They also have different air jet selections,  So, the fuel jetting for one style will simply not apply to the other style of carb body/ set up.

Because the stock engines are all the same, induction changes will NEVER provide "more air" to the engine.  However, shortening the intake duct (as "pods" do), changes (raises) the carb throat pressures and make each and every fuel metering device in the carb provide less fuel than with the stock induction arrangement.

You can choke off the pods to raise the carb throat pressures.  But, this will negate any possible air volume increase at or above red line RPMs, which is the only operating regime where pods "might" help.

I will say that a 115 -120 main jet on any stock CB550 engine is ridiculous.  It can only be "tolerated" if the engine is seldom run near red line and other fuel metering paths are restricted to compensate.  Show me a dyno fuel map where the mixture is correct with jets that large and I'll admit there may be exceptions.

The CB550 K3 needs a carb jetting increase just from removing the stock exhaust restriction.  Change the induction path design too, and you have carbs that have nowhere near the proper jetting for the engine's needs.

Mechanical slide carbs do not care about engine RPM.  They care about throttle position and carb throat pressure, whether generated by induction pressure drops and/or venturi pressure drops.  Those factors determine which jet is dominant and how much fuel it provides.  There is no feedback loop from the engine to change the fuel delivery ratio based on demand.  The jetting choices must anticipate what is needed.

Honda engineers figured out these needs.  Change what they set up for you and YOU must re-engineer how the carbs anticipate what the engine is going to need at each throttle position.  This process usually makes the pods a not-so-cheap choice.  (Unless you are looking for a quick sell to the ignorant/naive.)

[RFogelsong]

Thanks for the info two tired, sorry about that