Topic: How sensitive are the '74 CB550 carbs to different air filters?

[uncle_fester]

New member. Glad to be here! 

I just bought a '74 CB550. It originally had two large pod filters on the inside carbs and two velocity stack filters on the outside. It started, but it was a very tough start when cold. I replaced the filters with smaller pod filters from 4into1 and could not get it to start at all. It'd "cough" but wouldn't stay started.

I'm in the process of a full carb clean and rebuild now. It looks like it needed it, but it wasn't horrible. I think the standard jets in there now.

With the smaller pod filters, do you think different jets would help? Are these carbs sensitive to different air filters? Are there any recommendations for after market filters that work good on this bike?

[uncle_fester]

I did some searching and found this from user dawdish:

FROM; DO THE TON...

Quote
MAIN FUEL JET SIZE CHANGES NEEDED PER TYPICAL MODIFICATION:


Typical Exhaust Changes:

+2 main fuel jet size for custom 4-into-2 exhaust

or

+4 main fuel jet sizes for 4-into-1 exhaust

or

+4 main jet sizes for no muffler (open headers)


Typical Intake Changes:

+2 main fuel jet sizes for single K&N filter (inside a stock airbox)

or

+2 main fuel jet size for drilling holes in the airbox with stock filter

or

+4 main fuel jet sizes for individual pod filters (no airbox)



Additional changes:

- Add up all the main fuel jet size increases and subtract 2 sizes.

- Decrease main fuel jet size by 2 sizes per every 2000' above sea level.

- Under a mis-match condition, such as when using pod filters with a 100% stock exhaust, or 4-into-1 header with stock filter and air box, then subtract 2 main fuel jet sizes.


PILOT FUEL JET SIZES CHANGES NEEDED PER TYPICAL MODIFICATION:

Pilot fuel jet size changes are related only to the change in main fuel jet sizes according to the main fuel jet size formula described above. Note that this pilot fuel jet rule is for the main fuel jet size change BEFORE any main fuel jet altitude compensation is factored in:

Increase the pilot fuel jet size +1 for every +3 main fuel jet size increases.

Additional changes:

- Decrease pilot fuel jet size by 1 for every 6000' above sea level.



PRECAUTIONS:

- Make sure your carbs are in perfect working order before making jet changes....meaning fully cleaned internally and rebuilt, operating properly in their stock configuration, proper sized air jets and needles, etc. Otherwise, you'll like find that all of your efforts are going to be a HUGE waste of time.

- Check plug color often and adjust as needed, 2 main fuel jet sizes at a time and 1 pilot fuel jet size at a time. Bright white plug insulators are a sign of an overly lean fuel mixture condition and WILL cause damage to your engine over time, up to and including engine seizure!

- Synch the carbs after each jet change.

- Make sure the floats are set correctly

- Seriously consider purchasing a Colortune Plug Tuning kit.

- You may find it necessary to make changes to the size or shimming of the main jet needle. There are no guidelines on what or how to do these changes, this is true trial-and-error tuning!



EXAMPLE:

A 1982 XJ550RJ Seca using an aftermarket Supertrapp 4-into-1 exhaust and a single K&N air filter in the stock, unmodified airbox. Bike is primarily operated at an altitude of 2600 feet above sea level.

XJ550 Seca Stock Mikuni BS28-series Carb Jetting:

#112.5 Main Fuel Jet
#35 Pilot Fuel Jet
#70 Main Air Jet
#170 Pilot Air Jet
4GZ11 Needle


MAIN FUEL JET SIZE CALCULATIONS:

Changes made:

Exhaust:
4 into 1 with Supertrapp = +4 Sizes Main Fuel Jet

Intake:
K&N Pod Filters = +4 sizes Main Fuel Jet
----------------------------
Equals: +8 main fuel jet sizes above baseline
Subtract: -2 main fuel jet size per formula above
----------------------------
Equals: +6 main fuel jet sizes due to modifications, thus:

Stock main fuel jet size is: #112.5
+ 6 additional sizes
= a #118.5 main fuel jet size
---------------------------
Subtract: -2 main fuel jet sizes for Altitude of 2500' Average

= #118.5 calculated from above
-2 jet sizes for altitude adjustment

= a #116.5 main fuel jet size.


PILOT FUEL JET SIZE CALCULATIONS:

The formula is: +1 pilot jet size increase for every +3 main jet sizes increased.

Stock pilot fuel jet size is: #35
+ 2 additional jet sizes (since we went up +6 main fuel jet sizes before the altitude compensation was factored in):

= a #37 pilot fuel jet size.

Note that no altitude compensation is needed on the pilot fuel jet since our elevation is less than 6000' a-s-l.

[TwoTired]

I just bought a '74 CB550. It originally had two large pod filters on the inside carbs and two velocity stack filters on the outside.

Disagree.   It originally had an air plenum filter box and well engineered ducting to provide laminar flow characteristics to the carb throat.
The carbs were tuned for the carb throat pressures this well engineered induction system provided.

Are these carbs sensitive to different air filters?

Yep.  Change the carb's throat pressures and there is no automatic compensation device to perform fuel metering adjustments.  The turbulence introduced makes it unpredictable for certain rpm bands to mix the proper A/F ratio.

Are there any recommendations for after market filters that work good on this bike?

You mean filters that have man years of development time put into them for your specific bike?  No. 


I've also seen the attractive formulas for jet changes.  B.S. in my opinion.

Not all "Pods" work the same. "PODs" is a general style.  Not in any way a specification for restriction, flow, or turbulence introduction.  Suppliers won't even tell you how it differs from OEM.
Not all 4 into 1s work the same.  Again it's a general style.  Not in any way a specification for restriction, flow, or reverse pulse timing.

If you want a reasonable prediction of jet changes associated with a part change, the supplier of the filter or exhaust would provide recommended jet changes to make their product perform.

Most of the suppliers use a fence model of sales.  You throw money over the fence, and they throw product over the fence back to you.  Meeting your performance expectations is your problem, not theirs.

There is even no assurance, expectation, or guarantee an individual product functions the same, sample to sample, with a supplier product offering.  They reserve the right to change the product at any time, even though it may look the same.

How can a formula possibly have a finite outcome when all the elements of the formula are variables?

Want to use "PODs" anyway.  Invest in Dyno time. 
Or, learn to read spark plug deposits after each run on a test track, and have a supply of jets to test and try.  Either way, you'll find out how cheap your particular pods are (not).

Cheap, quick, good.   Choose only 2 for a solution.

Cheers,

[Bankerdanny]

Lloyd, I think the poster was saying the bike "originally" came to HIM with different filters on the inner and outer carbs, not that it came that way from the factory.

[uncle_fester]

Lloyd, I think the poster was saying the bike "originally" came to HIM with different filters on the inner and outer carbs, not that it came that way from the factory.

You are correct. It did not have the original air box and filter assembly when I bought it.

Unfortunately, I do not have the luxury of owning a dyno for the bike, so I'm just fishing for personal experiences here. I think I'll start adjusting the jets, monitor the spark plug wear, and go from there.

Thanks for the feedback, though.

[dave500]

just put a stock airbox back on.

[Bankerdanny]

The consensus is generally that the smaller SOHC4's are more sensitive to pods than the 750's. but there are lots of members here that have them and have been able to tune their bikes to run fine.

If you don't want to track down the original airbox setup, at least pick up a set of matching pods, two different styles just makes the job harder (and maybe impossible). You also need to settle on an exhaust. Also make sure the carbs are good and clean and that none of the jets are clogged and that the floats are set correctly.

Don't waste money on non-OEM carb kits with jets included. You simply cannot trust the generic jets and needles to be the sizes they actually say. Just get gasket kits, generic kits are fine for that.

Also, use only Honda float needles and seats. They will be much more expensive than generic, but they will be the correct length and size and are worth the extra money.

I would also go a generic internet search for "cb550 pods SOHC4.net" that will pull up the various threads on this subject (of which there are MANY) that will help you with baseline jet and needle settings.

[dave500]

make sure your ignition is perfectly set up aswell before you start,i notice that's not in the "do the ton" precautions list?

[uncle_fester]

just put a stock airbox back on.

I would, but I don't have one.

If you don't want to track down the original airbox setup, at least pick up a set of matching pods, two different styles just makes the job harder (and maybe impossible).

I currently have a matching set. Thanks for the info on the parts. I have lots of time left until spring to get things setup. Cold and snowy up here in Wyoming. 

make sure your ignition is perfectly set up aswell before you start,i notice that's not in the "do the ton" precautions list?

I've checked and am getting good spark on all four cylinders when cranking.


Thanks for all the input.

[scottly]

Which "pods" do you have now? The K&N are typically considered to be the best. What Danny said about the smaller bikes being more sensitive than 750's is true; the 750 has a generous air-box volume, while the smaller bikes have less volume, and a much smaller area of filter media. The 550 can run well with pods, once the jetting is sorted; member Mooshie has proven it time and time again.
http://forums.sohc4.net/index.php/topic,168263.0.html

[uncle_fester]

Which "pods" do you have now? The K&N are typically considered to be the best. What Danny said about the smaller bikes being more sensitive than 750's is true; the 750 has a generous air-box volume, while the smaller bikes have less volume, and a much smaller area of filter media. The 550 can run well with pods, once the jetting is sorted; member Mooshie has proven it time and time again.
http://forums.sohc4.net/index.php/topic,168263.0.html

These are pods I bought: http://4into1.com/set-of-4-small-chrome-pod-filters-39mm-honda-cb500-cb550-cb750/

I'll look through that thread.

[evinrude7]

from a k2 - https://www.ebay.ca/itm/Honda-cb550-cb-550-airbox-air-box-sohc/152865241361?hash=item23977a7911:g:k5UAAOSw7rdaWqzO&vxp=mtr

[dave500]

that's good value,you want the earlier one without that crankcase breather recirc affair,you can modify and block it off though,youll also need the plenum to carb rubbers easy to buy new,maybe missing a coupla frame mounting brackets also not hard to find or fabricate.