Topic: '75 CB550 carb tuning - rich off idle

[7]

Hi everyone,

I’m looking for some advice on tuning my carbs while trying to not make yet "another carb tuning post". Specifically looking for advice for coming off idle up to 1/4 throttle. The issue I’m encountering is a very rich low end. I can’t start the bike without giving a throttle input. Once it starts it idles fine but is bogged down really struggles. If riding at low RMP it will blubber. At higher RPM it will just hesitate and lack power. Obviously its throttle dependent but wanted to provide context to the magnitude. Plugs show rich as well.

Thanks for taking time to read the forthcoming wall-o-text

1975 CB550k with the following modifications
-K&N air filter replacing the stock air filter (still using the stock plenum)
-cb650 cam swap
-mild porting on the head (mostly cleaned them up and balanced the pressure drop)
-Delkevic 4 into 1 exhaust
-I just rebuilt the engine where I honed cylinders, new rings, polished valve seats, new timing chain and tensioner. 

Current carb setup
-Slow jet size: 38
-Air screw: 2 1/2 turns out from bottom (see explanation below)
-Needle: D26 w/ clip in the 4th position
-Main jet size: 105
-float height set to stock (22mm)  and verified fuel level using a sight pipe to be ~2-3mm below the top of the bowls
-carbs are synced

Things I’ve tried so far…

I verified my spark advancer using a timing light. At idle all four cylinders are firing at the “F” mark. At ~3000 RPM they are fully advanced. It’s been difficult to see the in between due to the 1/8th throttle position tuning. A 1/4 throttle is enough to go full advance. I wanted to get this out of the way because of how much worse it felt at low engine speeds. More investigation has indicated that the advance snapping open is probably more a function of not being able to maintain a slow engine speed.

Last season I had a wide band O2 sensor on after the 4 into 1 collector. In the low throttle range it would peg out rich so I know the A/F ratio was below 10. I played around with the clip position and had it up to the top spot (most lean) and the problem persisted. Clip is back to stock. Keihin’s tuning guide says clip position isn’t likely my problem and performance past 1/4 throttle is great. Wide band is if off now but I’m getting it back on for continued tuning. I learned a lot over the winter about what affects A/F in different areas so I’m eager to use the wide band O2 sensor back on and check other throttle positions and really dial performance.

The bike came with a 40 slow jet but it really didn’t want to idle then. Last year I installed the 38 slow jet. This spring I turned the air screw out to 2 1/2 turns to lean out idle. Idle performance is now more what I expect. Brief choke is needed to start from cold. No throttle input is needed. No choke needed when warmed up. I suspect this is compensating for another issue and will need to be changed back if the root cause is corrected. Stock for this bike is a 40 slow jet so going as low as i have already seems odd.

Keihin’s carb tuning guide (link below) has lead me to believe my issue is probably needle diameter or throttle valve cut away. Which to me doesn’t make sense. I don’t see the modifications to the bike as enough to throw off these settings. No thread I've read has had such issues with similar modifications.

https://www.keihin-na.com/assets/1/7/slide_valve.pdf

Questions

What Keihin carb series are on the ’74-’76 CB550s? Everything I find says “type 1” but that doesn’t seem to correspond to any Keihin carb series. This has made looking for alternate needles difficult. Jets-r-us has a lot of Keihin needles available but I can’t figure out what series to be looking at.

What effect does the emulsification tube have on all this? The tubes were removed during my cleaning and reinstalled. I’ll probably just replace them as it would seem that damage (flared out) to the thin top end would effectively result in what would present as a small needle diameter. Does that make sense to anyone else?

Are there other items I should be looking at? To me, having to drop to a 36 idle jet or a larger diameter needle doesn’t pass the “make sense test” but I’m open to input or suggestions.

Thanks for reading!

Edit: Title changed for added clarity.

[TwoTired]

I'd guess, the aftermarket parts in your carb are the issue to correct.
D26 needles aren't Keihin parts.  I'd assume your pilot jets aren't, Keihin, either.

The emulsion tubes aerate the fuel.  And if the small pilot jets don't have original type tube and cross holes, it effects aeration and fuel delivery.

How do you know the D26 taper angle suits your set up?   The needle diameter is height adjusted for 1/2 throttle mixture.  It's then the taper angle than determines the mixture for 3/4 and 1/4 throttle.  I'd try the stock needles.

The mains emulsion tubes can also leak if the needle jet is worn or corroded.  Both the exit jet and the cross drilled holes meter fuel mixture, and unless the tube rattle about in the holder, mostly effect mixtures in the  1/4 to WOT throttle position.

What sort of idle vacuum does the cam make compared to the stock one? 
Does the exhaust config allow neighbor exhaust pulses to effect cylinder scavenge at low RPM.  Typical of 4 into 1 style with low back pressure in the pipe.

Race tuned engines often have a high idle and/or poor idle performance.  Because during racing, who cares about idle?  It's only those street bikes that have any concern about that.


How did you arrive at a 105 main jet size?   


Cheers,

[7]

Thanks for the reply TwoTired. Do you know what series of carbs were on this vintage? I'm having a hard time finding parts without that information. Answers to your questions below.

I'd guess, the aftermarket parts in your carb are the issue to correct.
D26 needles aren't Keihin parts.  I'd assume your pilot jets aren't, Keihin, either.

Correct. I got my pilot jets here https://4into1.com/replica-keihin-slow-jet-honda-cb125s-cb350f-cb400f-cb500-cb550-cb750/

The emulsion tubes aerate the fuel.  And if the small pilot jets don't have original type tube and cross holes, it effects aeration and fuel delivery.

Pilot jets have the same type of tube and cross holes as the originals i took out. The 40s had the Keihin stamp on them. If the top of the emulsion tube is damaged could it affect low throttle positions? It seems logical that it would effect how the throttle valve cut away works.

How do you know the D26 taper angle suits your set up?   The needle diameter is height adjusted for 1/2 throttle mixture.  It's then the taper angle than determines the mixture for 3/4 and 1/4 throttle.  I'd try the stock needles.

I don't. These are the needles that it had when i purchased it. Where do you recommend getting a stock needle? I've only seen D26 listed as the stock needle. 1/4 to 3/4 throttle is not an issue. Only below 1/4 throttle.

What sort of idle vacuum does the cam make compared to the stock one? 
Does the exhaust config allow neighbor exhaust pulses to effect cylinder scavenge at low RPM.  Typical of 4 into 1 style with low back pressure in the pipe.

I don't have a pre cam swap vacuum value. I can measure it now though. Do you know what it is stock? As for the exhaust question, not sure how to answer it.

How did you arrive at a 105 main jet size?   

Changed based on input from others. This was done a while ago before i knew much about carbs. They have performed well enough though.

[TwoTired]

Pilot jets have the same type of tube and cross holes as the originals i took out. The 40s had the Keihin stamp on them. If the top of the emulsion tube is damaged could it affect low throttle positions? It seems logical that it would effect how the throttle valve cut away works.

The pilot jet "bleeds" some over the entire throttle position range.  As long as there is vacuum in the carb throat, fuel gets metered.  The exit port for the pilot system is just in front of the slide, and has the most dominant effect when the slide is lowest.

So, you are saying the pilot emulsion tube has the exact same configuration, hole size and count, as the Keihin jets?  How is the tube damaged?

I don't. These are the needles that it had when i purchased it. Where do you recommend getting a stock needle? I've only seen D26 listed as the stock needle. 1/4 to 3/4 throttle is not an issue. Only below 1/4 throttle.

The stock needles from Keihin are stamped 272304 for the K model and 273004 for the F model.  Have you tried getting them from Honda or Jets r us?

The slide needles also bleed a bit at just above idle.  So, if they have too much taper, they will begin the slide lift with too much fuel delivery.  If you made the slide needle adjustment for mixture at 1/2 throttle position only, then you don't know if the needle taper angle is proper for your set up.   You have to read the plug deposit for a specific throttle setting under full load to determine if the mixtures are correct for that setting.  Reading plugs after running with all throttle settings will not let you dial in the carbs.  A dyno exhaust sniffer would be an even better way to determine mixtures at particular throttle settings.   Mechanical slide carbs are not self adaptive, mixture wise to variations from stock in engine vacuum or cylinder needs reflected from changes in pipe characteristics.  The exhaust manufacturer should give you pointers on what their design requires in the way of carb jetting changes, assuming they did any testing besides mechanical fit.

I don't have a pre cam swap vacuum value. I can measure it now though. Do you know what it is stock? As for the exhaust question, not sure how to answer it.

The shop manual states 16-25 CM Hg  On the Honda Gauge H/C 39340.

[calj737]

Might help you-

[7]

Thanks calj737. I used that for the initial carb changes i made.

TwoTired, i found the stamp number you provided in another post but can't correlate that to a Honda PN or anything from Jets-r-us. The Honda PN for the needle and emulsification tube is 16012-323-004. Seems like for NOS i'll be paying $30-$40 a carb. Ouch! I've been trying to find a set of needles on Jets-r-us but they are broken down by carb series and the "Type 1", listed on the diagram linked by calj737, doesn't seem to mean anything outside of that manual.

I emailed Mike from Jets-R-Us and i'm awaiting a reply. They have a complete rebuild kit so I'm thinking he'll know the PNs I need.


And Happy Father's Day everyone!

[calj737]

Your carbs should have their type ID stamped on the backside of #4, look between the carb and the intake. There’s a flat spot where the number is. Order jets/needles/emulsion tubes by that type, when Mike gets back to you.

[7]

Calj737, the carbs are stamped as 069A but set up as a 022A. The later being appropriate for this motorcycle. I covered that in another post. Likely not stock parts as covered previously.

[TwoTired]

The needle numbers I gave you is what was stamped on original needles.  Likely, a Keihin number.  Honda probably has a cross for their own number system.  Honda bought assembled carbs from Keihin to install on production bikes.  They didn't assemble the carbs themselves.  Still, the numbers in their parts catalog would have been for dealer/field replacements.   They will be package renumbered Keihin parts, with Keihin numbers on the the parts themselves.

I don't know the history of Keihin as a company.  For all I know, the carbs that first appeared on CB500s were the first model carb delivered.  If you only produce one carb type, why bother attaching a model number?   The numbers stamped on these carbs that I've seen, refer to an internal set up configuration, presumably so that factory assemblers can take the correct carb off the self to install in the production bike.

Nice to have a solid paper trail.  But, Honda didn't always provide one easily followed by those outside the company.  And, that was 50 years ago.  Think all those nuances were passed on to modern day employees?

Cheers,

[7]

Well I found a set of used 272304 needles on ebay and ordered some actual Keihin brand #38 slow jets. We'll see at the end of the week if anything changes.

[7]

Is there any identification on the Needle jet / emulsion tube? Not sure how to tell if they are stock or not.

[TwoTired]

Is there any identification on the Needle jet / emulsion tube? Not sure how to tell if they are stock or not.

I've not seen markings on them.  Have to measure them and compare to originals.

Cheers,

[7]

Update time!

I've been using a wide band O2 sensor on the exhaust for tuning. If you have a 4 into 1 exhaust i highly recommend this. It makes tuning much easier. Since not all jets are created equal then not having this can make you go crazy.

I received the Keihin 38 slow jets and the stock 272304 needles and those are installed. The current set up is...

Air screw 3/4 turn out from bottom
Needle clip in 2nd from the top
Main jet is a 105 (not a Keihin brand)

Currently the idle A/F ratio is about 13.
From 1/4 to full throttle the A/F ratio is about 11.

First off... WOW, the idle jets were so different between the Keihin and off brand. I can see how people pull their hair out trying to tune with off brand jets. With the off brand #38, the leanest A/F ratio i could get was 12. With the Keihin, i had to richen it up from the stock air needle setting as it was originally a big leaned out. The throttle response of idle is night and day. I will be dropping the needle one more level (last available) and swapping to a 100 main jet to lean out 1/4 throttle and up.

I suspect the needle jet tube is not stock. Nor do I know how to tell. Once i make the final adjustments I will be very close to actually stock aside from the clip position. If the tube is not right then the clip may be whats compensating for it. In the end if the A/F ratio is right then who cares. Still frustrating though.

[TwoTired]

Curious, were those A/F ratios read during engine load tests?  Or, unloaded?

Glad you've made good progress.

Cheers,

[7]

Curious, were those A/F ratios read during engine load tests?  Or, unloaded?

All readings were engine loaded while out for a ride.

[7]

Final update...

The additional modifications corrected the reading on the O2 sensor but the it would occasionally miss on the acceleration and the top end felt sluggish. Plugs read a little lean also. I went back and all is well again. I have good acceleration across the throttle ranges so I'm leaving it there.