Topic: Carb tuning and where to buy Keihin carb jets?

[HawaiiMike]

I just picked up a nice 1976 CB550F.  It has the stock airbox and a MAC exhaust.  The former owner's receipts show that a Honda mechanic rebuilt and synched the carbs.  The bike runs but it's not so good idling and just off idle. 

When idling, I have to adjust it upwards when the engine is cold and downwards when the engine is hot.  If I don't adjust it when the engine is hot it will idle just over 2,000 rpm.

In the first 1/4 throttle or so the engine is very weak.  I asked a mechanic if I can raise the needles, he said he doesn't think the needles are adjustable.  I never took CB550 carbs apart but it seems like the needles must be adjustable.

If I need pilots or any other jets of non-standard size, where can I buy them?  And how do I refer to them?  For example, on E-bay somebody is selling Keihin N424 21 series jets.  Do these fit the CB550? 

thanks from Honolulu!
Mike

[KB02]

I know the needles are adjustable on my 76 750.

As for the jets, just stop into your local Honda shop. Bring in the jets you have now and tell them you need some in a different size. My local shop has even swapped me even sometimes for jets.

[Jonesy]

Welcome to the forums, Mike!

It is normal for the idle speed to increase as the engine warms up. I'm not sure if the '76 model 550 had it, but later versions had a fast idle cam connected to the choke. This would keep the throttle propped open during warm-up, and then when the choke knob was pressed in, the idle would drop to the proper speed. You may have to hold the throttle open a bit while the bike warms up (I think by '76 they removed the adjustable throttle brake, for "safety" reasons), but in the nice, warm climate of Hawaii it shouldn't take too long...

The needles are adjustable in the 550 carbs. the needles have 5 clip positions.

As for jets, in the FAQ section there is a list of companies who sell carburetor parts. On the side of the carbs there will be a stamping with a couple letters and numbers. (I'll need to look up the nomeclature) This will tell you what type of carbs you have. Once you have this info, the jets themselves have a number on them, and you would order accordingly. Example: If the main jet was labeled "100" and you needed a slightly bigger one, you would order a 105.

Hope this helps!

[KB02]

The Keihin's are sized by 5's right?  100-105-110-115-120... etc...

[Jonesy]

I've seen them offered in half sizes too (mains, anyway).. 100, 102.5, 105, 107.5, 110...etc

[.RJ]

Bikebandit has a good selection of jets -

http://www.bikebandit.com/product/A6548771

Those prices are for 4 jets.  I just ordered a 'few' sizes last week

[rlarkin70]

In the first 1/4 throttle or so the engine is very weak.  I asked a mechanic if I can raise the needles, he said he doesn't think the needles are adjustable.  I never took CB550 carbs apart but it seems like the needles must be adjustable.

If I need pilots or any other jets of non-standard size, where can I buy them?  And how do I refer to them?  For example, on E-bay somebody is selling Keihin N424 21 series jets.  Do these fit the CB550? 

I have a 76 CB550 too. I put K&N filters on the carbs, so I needed to rejet. You can adjust the needle (clip) position. I left mine as is initially, but I think I need to move it a couple positions.  The bike is great above 6K RPM, pulls HARD....but lower RPMs, especially if I have been cruising at a set speed for a while, the bike gurgles and hesitates as I give it gas, then after a second or two it sounds OK.

The good part is you don't have to de-rack the carbs, or take off the float bowls, or even touch any tubing. Just remove carbs from the intakes, and work on the top.

As for jets, I got mine from these guys. They have sizes 98-150 and are 3.25 each plus shipping. works out to $19 for 4 jets shipped to my door.

http://www.crc2onlinecatalog.com/misc_carb_parts.htm

-Ron

[Jonesy]

If you work carefully, you don't even have to take the carbs off the bike to change the needle clip positions.

[rlarkin70]

If you work carefully, you don't even have to take the carbs off the bike to change the needle clip positions.

oh yeah? if you just take off the tank? good idea...would save some time. I am riding around tuning my carbs today, and about to dive into the needles now. just following some notes from the FAQ.

[fmctm1sw]

If you work carefully, you don't even have to take the carbs off the bike to change the needle clip positions.

+1 on the carefully.  I dropped a screw down into the middle of the carb rack.  Took a while but fished it out with a flat tip screwdriver...

[TwoTired]

The bike runs but it's not so good idling and just off idle. 

When idling, I have to adjust it upwards when the engine is cold and downwards when the engine is hot.  If I don't adjust it when the engine is hot it will idle just over 2,000 rpm.

Get used to it.  The engine is more efficient when warm and the fuel atomizes better.  The computer above the seat addresses the changing needs of the engine during warm up like the computer under the hood for a modern car/bike.  I don't imagine you'd have to hold the throttle open by hand for ore than 5 blocks of travel in the temps you normally encounter in Hawaii.

In the first 1/4 throttle or so the engine is very weak.  I asked a mechanic if I can raise the needles, he said he doesn't think the needles are adjustable.  I never took CB550 carbs apart but it seems like the needles must be adjustable.

The slide needles are adjustable.  But, that generally addresses 1/4 to 3/4 throttle positions.  Idle and pick up from low RPM is the domain of the idle adjustment (pilot jets,/air screw) and slide cutaway shape.  It's not very common to reshape throttle slides, but it is common to adjust the Idle Air Bleed Screws.  First thing you need to know is where they are currently set.  Mechanics used to setting modern engine will likely set these screws too lean in order to reduce emissions, and that ruins low speed throttle response on these bikes.  The stock setting is 1 1/2 turns out from lightly seated (+/- 1/2 turn).  If that still results in a too lean setting, then turn them in, in 1/8 turn increments, and test run until throttle pickup results are satisfactory.
All carb adjustments are addressed for an engine at full operating temperature.  Address the easy stuff first.

User expectation note:
Since the slides are mechanically linked to the throttle, and these carbs have no accelerator pump enrichment feature, you will not be able to snap open the throttle from idle and expect good pick up, as the sudden opening causes vacuum loss, which prevents fuel draw through the jets.  The engine should respond predictably and positively from throttle changes of 1/2 total twist grip changes.

Mahalo,

[patpollin]

I have my air screws (also a '76 550k) set at 1.5 turns out and it is a bit choppy at 0-1/8 throttle but it sounds like that is the nature of the bike-what I'm not sure about is if I ride around a bit at low speeds, 1500-3000 rpms, with some intermittent stops,  I check the plugs and they're completely sooted black.  is this normal? I've tried turning the screws to leaner positions as far as 2.5 turns out, but the plugs still come out done black. 

[TwoTired]

I have my air screws (also a '76 550k) set at 1.5 turns out and it is a bit choppy at 0-1/8 throttle but it sounds like that is the nature of the bike-what I'm not sure about is if I ride around a bit at low speeds, 1500-3000 rpms, with some intermittent stops,  I check the plugs and they're completely sooted black.  is this normal? I've tried turning the screws to leaner positions as far as 2.5 turns out, but the plugs still come out done black. 

How clean is your air filter?

What heat range plugs are you using?

[patpollin]

my air cleaner is one thats been sitting in the bike during a 10-15 year sit.  It looked clean but I'm beginning to doubt its cleanliness, I'm going to try a new one and see if it helps.  My plugs are what the book calls for, I think D7EA- My riding environment is cold now, 30-45 degrees, should I try a colder or hotter plug? 

[TwoTired]

my air cleaner is one thats been sitting in the bike during a 10-15 year sit.  It looked clean but I'm beginning to doubt its cleanliness, I'm going to try a new one and see if it helps.  My plugs are what the book calls for, I think D7EA- My riding environment is cold now, 30-45 degrees, should I try a colder or hotter plug? 

It's amazing how these old things respond to a complete a thorough tune up.

Honda's stated maintenance schedule is to clean the air filter every 6 mos. or 3000 miles (whichever comes first) and replace the air filter every 12 mos. or yearly.  So, you've already got 10-15 times the normal expected life of your current filter.  (cheapskate  )
Really, the paper filters break down over time with moisture from the air, lose their "fluff" and can become more restrictive.  New ones should come in a plastic bag if it's been on the shelf for very long.

I'm currently using the Uni NU-4055 foam air filter which I clean and reuse about once a year (usually).  (too many bikes-not enough miles. )

I've never needed any plugs other than the NGK D7EA heat range on the 550.  I tried D8EA a couple times.  Definitely not for cold weather use or normal street driving. Bleah 

Cheers,

[HawaiiMike]

Hey guys,

It's been a few days since I posted and just took a look at your answers.  Thanks so much for your help!  When I find some spare time I'll pull the carbs and move the needles up a notch to see if it will get the bike to run better.

I don't think my CB550 has the fast idle connected to the choke.  I'll let you know how it goes.

Aloha, Mike

[MoTo-BunnY]

  Just thought I would point this .pdf out on Keihin carb rebuilding on my business site, that I scanned awhile back:

Keihin Carb Rebuild - Bike Bike - JULY 1974

Hope it helps!

[patpollin]

Two-Tired could you tell a little more about the uni nu-4055 filter?  I see them for sale on a couple of different websites but it doesn't show a picture or say anything about them-do they have the same airflow rates (rejet required?),  are they the  kind needing oil?  are they shaped similar to the stock paper one, so they fit in the same spot as the stock one?  ah and do they last longer?  Thanks  for the info!  Pat

[TwoTired]

Sure.
The Uni NU-4055 is the same form factor as the stock paper filter, and fits into the stock air box in a like manner.  I did not need re-jetting on any 550 with the early Keihin carbs.  I can't remember if my 78 has one.  Probably.  But, the 77 and 78 models would be the most sensitive to filtration changes. Might have to re-adjust the IMS on that one.
There is some slight leaning of the mixture due to it having less restriction than the paper at mid to idle RPMs.  They likely do not flow quite as well as a brand new paper filter near red line operation, due to the nature of foam filtration.  But, it wasn't a noticeable power change as I recall.  I haven't ever changed any carb settings to compensate for the air restriction difference.    The fuel mileage improved some and they don't seem to get nearly as restrictive as the paper ones do when they get older. So the mileage improvement is more consistent during time between servicing.  They do have to be oiled, and I recommend the oil and cleaner aerosols Uni sells for that purpose.  The first Uni filter that I bought lasted 15 years before I noticed the foam begin to deteriorate.  I still clean and re-oil them at least once a year.   They should probably be cleaned at the regular Honda maintenance schedule (6 mos./3000 miles).
I would still prefer the paper type if I operated in very dusty environments, as I feel it will trap more particles than the Foam type.  But, I've got 42000 miles (so far) on the 74 using the Uni filter in the San Jose, Ca area. 

I think the stock Honda paper filter is around $50.  The Uni is about $25-30 and even if you add the oil and cleaner aerosols, it is more economical by comparison after the first year or two of use.

Cheers,