Topic: '78 550 Carburation Issues

[Brian G]

Hello All -

I apologize in advance for asking a question that has no doubt been answered previously. It's not easy to phrase a Search Term which returns less than a couple of hundred responses. Maybe its just easier to ask again 

1978 CB550K4 - which I acquired a couple months ago - and with what I believe to be a Mac 4 into 1 exhaust. The P.O. says he made no jetting changes whatsoever when he replaced the standard exhaust, so I'm expecting her to run a bit lean at W.O.T. Sure enough, shifting into top gear at 7K rpm, she seems to bog a little; closing the throttle 1/4 of a turn improves the response slightly - which also says to me she's running lean.

Would someone be so kind as to let me know what the standard main jet sizes were on the '78 CB550K4, and what size might be suggested in order to actually get to the redline in top gear?

Sure do like this bike, and would love to get her running right...

Thanks,
Brian

[gene03079]

The book lists it as a 90.

I have a a 4 into 1 on my 78 and after looking here I was able to dial in my carbs very easy. The FAQ section has a whole bunch of info on setting up the carbs.

[Brian G]

Thanks for that. As you say, there's such a wealth of information in the FAQ's (and elsewhere on the site) that sometimes it's difficult to locate exactly what you're looking for. Might I ask what size main jets you 'dialed in' for your 4 into 1 exhaust? I'd rather spend my money on jets which will be close to doing the job rather than using the 'scattergun' approach!

Thanks and regards,

[ieism]

I don't think you have to rejet for a Mac 4-1. Many people here run one without rejetting. If you just got the bike, i would start with a normal tune-up:
Oil, oilfilter, airfilter, plugs, valves, timing, carbsync.

Then put some carbcleaner in the tank (i think you call it seafoam in the US) and run it all day at highway speeds. I'm pretty sure the bog will clear by itself. If not you need to clean the carbs, and they are a b!tch to take off.

[gene03079]

I still run he stock jets in mine. I may be a bit lean wide open though the plugs look fine. But am changing the exhaust back to a 4 into 4 so I never got into re jetting.

Make sure you carbs are real clean. I did mine twice the on the third try i saw what real clean is.

[Brian G]

O.K., thanks, guys. I had planned on doing a normal tune-up; i.e: carb cleaning and synch, valve clearances etc., this winter anyway so this just confirms it (my darling wife is getting me a decent timing light for Christmas  ).  I'm a bit surprised that no re-jetting is needed when changing from a 4-4 to 4-1 though - if I were to change the exhaust on my R3 to a 3-1 collector from the stock 3-2, Conventional Wisdom says that the Amals would definitely need an upward move for the mains due to the decreased back-pressure in the exhaust. After all these years, I'm still learning new stuff every day... And I wish I could find a stock 4-4 system; not only would it fix my mixture problems, but it would look far better as well! You wouldn't happen to have a supplier in mind, would you Gene?

Thanks again,
Brian

[Tattoo]

I just bought a stock or close to stock 74 550k that back in the 80s it had a 4 into 1 exhaust, 105 jets installed from the stock 100s. It has had 200 miles since the change over because it ran like crap. After changing the jets back and cleaning the cabs and the emulsion tubs as they were full of junk it runs great now. So I wouldn't change the jets unless you want to pull the cabs off again for #$%*s and giggles, Tattoo

[ieism]

I rarely see a 550 with original pipes here, and I do not believe everybody rejets. My 77 550K came with 4into1 pipe and stock jets it ran fine.

[Brian G]

Don't you just love the Internet? A couple of hours ago I posted a question from eastern Canada and since then I've received comments and advice from the Southern States and North Eastern U.S of A, and from half way around the world in Amsterdam. If I can't get this 550 running properly, it won't be because of any lack of support or advice from other SOHC aficionados Ride On, Brothers!!

[edbikerii]

I had basically the same problem as you.  I have a 77 CB550K which has essentially the same lean-burn PD46 carbs as the 78.

Original pipes were rotted out, so I replaced them with a MAC 4-1 many years ago.  I also had a UNI foam filter in the airbox, which may have leaned it out even more.

In any event, if you want a THOROUGH discussion of the carbs on your bike (and mine), the MAC exaust, running temps and possibly even pod filters, have a gander at the thread referenced in my signature below.

I was definitely running lean and hot with my setup (MAC 4-1, airbox w/uni filter, stock jetting), which is similar to yours.  I felt that it was just easier to re-jet for pods than to re-jet for the stock airbox/filter with MAC 4-1.  Anyway, read that thread and you'll learn a whole lot.

[Brian G]

Thanks, Ed - You actually mentioned a symptom I hadn't noted in my original posting - i.e. running relatively hot. Compared to the temperature of my R3 after an equivalent run, the 550 was quite a bit hotter (based on a decidedly subjective test, i.e. hand on the fins). That's quite an informative thread to which you directed me, but unfortunately its too late at night right now for me to give it the consideration it deserves, so I'll follow up on it tomorrow. Looks like it may have the answer, though...

Thanks,
Brian

[bryanj]

From one Bryan to another there aint no such animal as a K4 550, Its just a 78 550. Standard exhausts have long been deleted and the earlier "trumpet" type that are available dont fit.

The 77/78 Carbs have the pressed in pilot jet, which cruds up easily and it can be got out carefully and cleaned (many posts on that) whilst the mains are screw in type like the 750's

[Brian G]

From one Bryan to another there aint no such animal as a K4 550

Well, that was a surprise! I just assumed that as the '77s were K3's....... Signature altered accordingly!
I wonder why Honda changed the progression?

Thanks for the info on the pilot and main jets. They're forecasting snow for the weekend, so they'll be on the workbench soon.

Regards,

[mmtsquid]

Before you yank the carbs off the bike, try this - just remove the bowls from the bottom, and you can get to the jets that way.
If its something as simple as a clogged jet, you can remove them from the bottom without removing the carb bank.

Personally, I have to be absolutely positive that removing the whole carb bank is necessary before I'll pull 'em - they suck that much!

[Tattoo]

Before you yank the carbs off the bike, try this - just remove the bowls from the bottom, and you can get to the jets that way.
If its something as simple as a clogged jet, you can remove them from the bottom without removing the carb bank.

Personally, I have to be absolutely positive that removing the whole carb bank is necessary before I'll pull 'em - they suck that much!

I've had mine off twice in the last month and I didn't find it that hard to remove them. I find it harder to get the two inside bowls off with them on the bike but I guess it's my big hands that get in the way. Tattoo