Topic: CB500 carb tuning for 4-2 exhaust

[jeremiah]

I have spent hours reading through everyone's posts about the 500/550 carb tuning and didn't come accross a setup like mine.  I have 500 with a 4 into 2 Mac short turn-out exhaust.  It has the stock airbox & filter. 

 Stock carbs (627B), jets (100) ran super rich, so I have since put in 98 jets and raised the clip on the needle to the full top position getting the leanest possible mixture.  It now runs TONS better, but still stalls when I give a quick stab on the throttle.  A slow, roll on of throttle and it's pretty smooth.javascript:void(0);
Huh  Plugs look better, but still eventually get carbon fouled so it's still running rich, right?.  Removing the air filter doesn't seem to make a difference.  However, when I close the choke 1/4, it smooths out quite a bit and I can give it full throttle no problem, why is this? 

  So, I am thinking that I need to try a different set of carbs off a 550k or 550F.  It seems they have a bigger size air jet drilled in them (where is this??) and different size holes in the emulsion tubes.   Anybody know the specifics on size differences between the carb models?  It seems that my 4-2 exhaust would be more similar to the 4-1 than the 4-4, so should I try the 550F 069A carbs?  Or should I first just use the emulsion tubes or needles from the 069A?  I have complete sets of all the types of different 550 carbs to experiment, I just don't know which ones would be best suited for my 4 into 2 exhaust.      Any help would be greatly appreciated,  it's seriously affecting my sleep & social life. 

[Pinhead]

Stock carbs (627B), jets (100) ran super rich, so I have since put in 98 jets and raised the clip on the needle to the full top position getting the leanest possible mixture.  It now runs TONS better, but still stalls when I give a quick stab on the throttle.  A slow, roll on of throttle and it's pretty smooth.javascript:void(0);
Huh  Plugs look better, but still eventually get carbon fouled so it's still running rich, right?.  Removing the air filter doesn't seem to make a difference.  However, when I close the choke 1/4, it smooths out quite a bit and I can give it full throttle no problem, why is this?

My bike does the same thing. Crank the throttle and it'll die unless I've got enough momentum to keep the engine turning. Slowly roll the throttle and it takes off like a shot.

At 5k rpm if I crank the throttle it'll miss out. I figured it was running lean. However, when I did this to try to check the spark plugs, I held the throttle wide open for a while (waiting for long enough to grab the clutch and hit the kill switch so I can check the plugs)... And it stopped missing out and cranked up to about 95mph where I let off the throttle for fear of a ticket. I thought maybe I'd just blew out the cobwebs, but it consistantly does that every time I ride. If I hold it WOT for a bit it'll clear out and accelerate. Can't hold it WOT at low RPM's, though. Anything below 4500 rpm and the engine will just die.

I've got stock airbox/filter/exhaust.

Does yours do the same?

[smashme33]

 Me too...I'm gonna follow this post and hopefully we will all figure out what it is. My '80 CB650c runs beautifully, idles reliably, but if you go and try to hammer WOT, it responds with a sound that sounds like an engine producing a question mark. Smooth advance to WOT, and we're in business. I feel like she's a little on the lean side, but there is no evidence on the plugs to back that up.

[jeremiah]

Doug - Yes, those are the same symptoms I'm having, If I hold WOT, it eventually kicks in, but not before totally bogging down first.  My plugs have been looking dark tan in center with a bit of carbony around the edges.   I've been riding around with 1/4 choke and it's very smooth but doesn't have as much top-end power as it should.  I am convinced that the 4-2 pipes are the culprit as I've read they are hard to tune.  But I'm confused because my carbs are leaned out to the max & the plugs still look rich....   

[Pinhead]

Hmm... I've been wondering if my pipes are clogged. When I first started it up, a few mud dobber nests came rocketing out the back. What's the best way to clean them out, other than taking them off the bike and running some kind of degreaser through 'em?

Also, less restrictive pipes would lean the mixture out, right? So if they are clogged, the carbs would be running rich.

[TwoTired]

Stock carbs (627B), jets (100) ran super rich, so I have since put in 98 jets and raised the clip on the needle to the full top position getting the leanest possible mixture.  It now runs TONS better, but still stalls when I give a quick stab on the throttle.  A slow, roll on of throttle and it's pretty smooth Plugs look better, but still eventually get carbon fouled so it's still running rich, right?.  Removing the air filter doesn't seem to make a difference.  However, when I close the choke 1/4, it smooths out quite a bit and I can give it full throttle no problem, why is this? 

What is your idle air bleed setting?
What was your needle setting before you went to extreme lean?

I'm thinking you made too many changes at one time.

Your needles are now too lean and you have to apply choke to force more draw from that system.

Your bike should have a rich idle. This and the fat part of the taper on the needle are what supply extra gas when you suddenly open the throttle.
You'll not be able to snap open the throttle on the Cb500 carbs, because it has no accelerator pump.  1/2 throttle position changes should result in good engine pick up, even under load.

The metering systems are throttle position specific per the chart below.  Make adjusment to the the device per the throttle position you are trying to correct.   For example, changing the main jet for idle mixture or midrange correction is the wrong approach.

If the Cb650s also have slide carbs without accelerator pumps the above is applicable to them, too.

Cheers,

[jeremiah]

What is your idle air bleed setting?
What was your needle setting before you went to extreme lean?

I'm thinking you made too many changes at one time.

Your needles are now too lean and you have to apply choke to force more draw from that system.

Your bike should have a rich idle. This and the fat part of the taper on the needle are what supply extra gas when you suddenly open the throttle.
You'll not be able to snap open the throttle on the Cb500 carbs, because it has no accelerator pump.  1/2 throttle position changes should result in good engine pick up, even under load.

The metering systems are throttle position specific per the chart below.  Make adjusment to the the device per the throttle position you are trying to correct.   For example, changing the main jet for idle mixture or midrange correction is the wrong approach.

If the Cb650s also have slide carbs without accelerator pumps the above is applicable to them, too.

Cheers,

Air-bleed doesn't really affect the problems I'm having.  the low throttle area always is good.

I didn't make all the changes at once.  First I started with clip in middle, 100 jets.  Smooth power with low throttle, low rpms, but otherwise unrideable.  So I put in the 98 jets.  It got alot better, but still boggy through midrange and WOT.  So, I raised the clip 1 notch.  Got even better but still boggy & fouling plugs so I raised it all the way to the top, ran better yet.   I am wondering why it runs better when I give it a little choke??  the plugs make me think it's still too rich, which would mean it needs more air, but choking gives less air right?  Or is it giving it more air suction and therefore sucking more gas??   What would the bigger "air jets" & "emulsion tubes" from the 550F carbs do to my situation?  Can anyone explain to me what the 4-2 pipes are doing, or how they affect mixtures & flow?  Anyone run a 500/550 with smaller than 98 jets?  thanks

RE: accelerator pump...  I have 3 550's that will all take off when you give it sudden WOT so I don't think it's a lack of accelerator pump...

[Pinhead]

I did a test the other day when I was riding around, trying to think of what could be causing my problem... After the engine was good and warmed-up, I pulled into a parking lot and put it up on the center stand. With the tranny in neutral, and at a smooth idle around 1350 rpm, I cranked the throttle wide open. It died. Started it back up, and rolled on the throttle a few times up to about 6k. I noticed that no matter where I put the throttle, it never went above 7. So finally when I rolled it to about 5500 I held it WOT. It hovered around 7k, or just a little over. With the throttle held wide open, I moderately pulled the choke cable and it instantly revved to redline.

It seems that we've got the exact same problem, maybe the same carbs, IDK.

I tried the same test going down the road. When the engine started to miss out (always around 5500 when going down the road) I cranked it WOT and pulled the choke. This time, I lost power. I don't know why I'm getting the different results...

According to the chart, the most effective adjustment would be the needle clip position for 1/2 to 3/4 throttle (since we can't change the taper without replacing the needle) and then the next most effective would be the main air jet size.

On my bike, I'd be inclined to believe that I've got stock everything. Stock carbs, carb parts, exhaust, everything should be stock. So in my situation, I shouldn't have to change anything from stock settings considering these bikes didn't have this problem from the factory (other than cleaning, etc.). Things that could've changed since '79 would be engine wear and the mufflers could've "blown out" since they were made (though they're very quiet now). Other than that, not much should be different.

One issue that I may be fighting... My charging system isn't fully functional. I can drive around all day before the battery runs down, though, and the engine doesn't seem to lose horsepower until the extreme end of the battery's charge. I can then pull the fuses and at least make it another few hours if need be (only in emergency situations). Since my ignition system isn't getting the "full voltage" that it would with a running alternator, my spark wouldn't be quite as hot as normal. Since it's harder to fire a leaner mixture, this may be my problem entirely. I would tend to doubt it, though, because I can hold the throttle down the road at WOT and it'll eventually "clear out."

I added "Sea Foam" to this tank of gas and I'll see if it helps anything once it's all run through. I could still have a clogged jet somewhere that I didn't find the 5 times I pulled the carbs apart... Hopefully one of us will find the problem and the other will benefit, too!

[jeremiah]

1 case of beer, 1 sack of weed & many frustrating hours later I've finally found a winning carb setup for my 4-2 exhaust.  Thought I'd share my results incase someone else ever comes accross this setup. 

CB500 Four
Mac 4-2 exhaust
stock airbox/filters
stock carb bodies
550F main jets (#98) & emulsion tubles (has smaller holes up the sides)
550F needles on 2nd clip from the top
550F slow jets (#38) & air bleed screws (no holes on them). 

It now runs & sounds fabulous at any throttle or rpm range.