Topic: Cb550 PD carbs question

[Kcanela]

Hello all, virgin here to the cb550 world.

Bike: 1978 cb550 four
Upon pick up bike started and ran perfectly fine, all stock with ripped intake boots started and idled perfect.

Of course upon my arrival I typically had to change out to pod filters and went 4-1 exhaust (why would you do that) I know I know I was deff dropped as a baby let’s move past that.

Fast forward to me tearing the bike apart and installing new parts such as filters and exhaust I also added some 110 main jets, that is all and adjusted the mixture screw to 2 out that seems to be the only setting where the bike wants to start on its own and idle.

My issue: sputtering and breaking up under load.

Another problem is that there is literally no visual reference on these carbs on how to bench sync from what I’ve checked, either the picture are outdated or there just isn’t any info. All my carbs have for adjustments are these pins on the bottom of the bowls and that’s it, I don’t even know where I’d plug in an actual vacuum sync tool if I had it

Any suggestions towards these carbs would help greatly

[DaveBarbier]

These are PD46C carbs, right? There are sync ports that are capped with aluminum flat head screws on the engine side of the carbs. But first bench sync with a drill bit. Search the forum for info on how to do that. The #2 carb is your master carb so it can’t be adjusted. Set the 3 other carbs to that one.

The best way to get rid of your rideability issues is to put the air box back on. If you don’t want to do that then you probably need to raise the needles one notch. The stock position of the clip is in the middle so you want to move the clip to the next notch down.

I had good luck with 110 mains, needle clip one notch lower than stock, stock pilot jets with pods and 4-1 exhaust.

[Kcanela]

Thank you good sir, will be #$%*ing down the needle in the am hopefully that fixes this issue * fingers crossed*

[DaveBarbier]

Just to be clear, you are raising the needle by lowering the needle clip. The lower you move the clip the higher the needle is raised. By adding pods and, what I assume is, a freer flowing exhaust you have leaned out the overall a/f ratio. Jumping up to 110 mains only addresses about 3/4 - WOT. The needle clip affects more of the mid range of throttle position. You can test this by slightly closing the choke when you feel the bike stuttering and losing power. Adding some choke will richen the mixture and hopefully will smooth out that throttle position.

I wasn’t joking about putting the air box back on. Honda’s engineers did a lot of work to make the bike run well. Making drastic changes means you will have to redo this carburetor tuning work which most people don’t know how to do or don’t have the patience for.

The best thing you can do is find a dyno that can tune old bikes. I brought my CB674 hybrid up to Rick Stetson at Harry’s Machined Parts in Northborough Mass. The guy knows his stuff.

Maybe this picture will help.

[Kcanela]

Now is your recommendation with the needle drop for my stock motor? Cause I see your build is a hybrid maybe what worked for you won’t work for me since it’s completely stock?

[Flyin900]

One thing you must understand is there is no specific set up that anyone can offer you. It is trial and error period. Honda and Kehein likely did extensive testing to reach the combination of air box volume, exhaust flow and many other parameters.

You have thrown that all out and as you correctly stated what works for someone else may not work for you. These bikes run best with the original set up period. Anything else will be a compromise likely with some issues, such as what your already seeing now.

[Kcanela]

I am fully aware that I threw out years worth of engineering to make these motors run perfect, I get that part.

[DaveBarbier]

Now is your recommendation with the needle drop for my stock motor? Cause I see your build is a hybrid maybe what worked for you won’t work for me since it’s completely stock?

Sorry, I should have said I did that with my ‘78 CB550. Pretty much same bike as yours. Stock motor, pods and 4-1 exhaust.

And yes, my exact settings might not work as well for you depending on the type of pods, exhaust and wear in the motor. I did find that the PD46C carbs were pretty forgiving though. I didn’t notice much change going from UNI pods to velocity stacks or even plain open mouthed carbs.

[DaveBarbier]

Now is your recommendation with the needle drop for my stock motor? Cause I see your build is a hybrid maybe what worked for you won’t work for me since it’s completely stock?

Oh and again, we’re raising the needle, not dropping it. Just want to be sure you go the right direction.

[Kcanela]

So I lowered the needle clip 1 so now it’s in the middle, adjusted my IMS to 1.5 turns out and bike doesn’t even want to start now just cranks and garbles if I hit it wot with choke fully on I’m stumped

[Kcanela]

Welp, of course as I suspected rookie mistake. With taking the carbs out more than 10 times and fiddling I forgot adding gas might help start the bike...

With lowering the needle pin to the middle clip bike strarts right up no choke and no more breaking up 😈

[DaveBarbier]

I’ve done that before so no worries. And glad it’s working out for you. It would be good to get to a dyno in the future if you really wanted to tune the carbs and ignition. But more importantly it always helps to go through and do the 3k mile maintenance checklist in the manual.

[DaveBarbier]

Oh and interesting you say the clip is now in the middle position. I believe the stock setting IS middle position. If you’re happy with it then fine but I would want to double check you’re still not lean. You can still try to apply a bit of choke as you’re riding and see if that gives better response.

[Little_Phil]

Oh and interesting you say the clip is now in the middle position. I believe the stock setting IS middle position.

AFAIK stock position of E2350F needle is 2nd from top. That's what mine is.