Topic: CB550 -- top speed and jetting

[nilsey]

Hey everyone...

Recently i went from pods and a 110 main jet to a stock airbox configuration and 100 main jets. I felt like performance improved overall.

with the pods i was struggling to keep it around 80 at WOT. maybe hit 85 with enough patience. when i went to stock airobx it seemed to hit 80 more easily and get to 85, but no fastaer than that.

it seems to me lik this bike should be able to exceed 85, so on a friends recommendation i went to 110 main jets. he said that the limiting fatcor to attaining higher speed at WOT was liklely the main jets.

Well i got it together tonight, there was too much traffic to really play with speed but i feel lik eteh performance is not as good as with teh 100 main jets.

i guess if the performance got worse that would mean i am running too rich now right?

anyways, whats my next step? can someone give me a tip on improving the top speed with the stock airbox? i should also mention that the bike has an aftermarket exhaust (4 into 2) of unknown brand, age, or specification.

Thanks for any advice.

[Rgconner]

Totally stock, 85 is easily doable on my CB550, not at WOT. 100 is doable, but unnerving because of the decrepit suspension, but the engine pulled all the way there.

So I would suggest putting everything back to stock and get it to run right, then you can F with it to your hearts content, knowing stock will run well.

[Doggie]

At 100 you can be one of the "Ton-up Boys"
                                                                 Doggie

[bollingball]

I have a 750 TT and Dave500 will help. They and a few more are the go to guys for these bikes. But my first thought was if the jets were changed for the pods maybe the needles were also and the floats. You may need to go completely though them for all changes that may have been done for the pods. The next time you can get to WOT do a plug chop that I would do first.

[Rgconner]

At 100 you can be one of the "Ton-up Boys"
                                                                 Doggie

Until I get better suspension going.... I will pass.

My point is, if the performance mods are getting you less performance, put everything back to stock and make it perform as best as stock can.

If something is fundamentally wrong with the bike, such as a worn cam, no amount of tuning is going to help.

[Doggie]

Just a dream of mine. Working on getting mine past 4th gear..lol
                                    Doggie

[Danno]

what air filter do you have if you have an after market foam filter you will need 40 slow jets  these carbs are cumulative so if the slow jet is too small it will cap top speed
also how loose is the exhaust? you could also need 105 main jets if it is looser than stock but not enough for the 110 jets
I have had my 74 550 for 27 years it runs 110 mains and 40 slows had the same problem you have till i put the 40 slows in now it goes well past 100 the top speed for any of the years stock after 74 was 105 mph the 74 will go 125 the main difference is the epa stuff they did to the bike following 74 could not tell you what all they did as I have the 74
also every time you change any of the jets you will need to adjust the air screws which also have a lot to do with the bike running lean at top end you would not think so as they are supposed to only be for idle but as I said the carbs are cumulative so one thing out of place and it effects the whole thing which usually caps top end

[dave500]

what carbs are on it?the type one or the PD?before you touch the carbs make sure your ignition is absolutley spot on or youll chase your tail everytime fiddling with the carbs,be sure the advancer is free to move,dont just asume or think its ok go and recheck it,my 550 with four to one and stock airbox and jetting in type one carbs im sure will do the ton,80 miles per hour easy,go over the ignition first,use a dwell meter and timing light for this,if you have a compression guage or can borrow one just check it to get it out of your mind,when was it last vacuum synced?just out of interest i actually dropped from a 40 to a 38 slow jet and kept the 100 main,the needle is clipped in the middle slot,my air screws are the solid type with no hole,the ones with the hole you can only enrich to a certain point,,like a smog limiter idle mixture screw on a car,dont copy my jet settings your results may vary.

[bwaller]

A little back ground info maybe. Is this a new to you bike. Is there 100,000 miles on the engine? As Dave mentioned start from the beginning, do a complete tune-up, including plugs & air filter and focus on jetting once that is done. Start with as good a package as possible.

Check around here for a step by step approach to jetting. It's not difficult but your engine may be different from all others, so don't change anything until you do some reading homework. Take a logical approach from the beginning with a plug chop and fresh plugs.

Only then can folks here help pinpoint solutions if you need it.  Good luck.

[nilsey]

some more background  as requested:

the bike has 35K miles on it. i replaced the points ingnition with a Dyna-S electronic ignition and timed statically. compression is equal across all the cylinders. the filter is a UNI foam filter, new and newly oiled about a month ago.

gonna have to find a stretch of road to do a plug chop -- not easy here in the city of chicago.... also going to

now for a really dumb question - which is the slow jet? is that the long jet which is in teh top part of the carb, not the float bowl end?

P.S. - I am sooo sick of taking teh carbs off with the airbox setup, it was so much easier with pods... ha ha.

[bwaller]

I won't harp further after this. If the compression is good and even then you adjusted valve clearances? If the carbs are meticulously clean have you adjusted fuel/float levels? Yes the pilot/slow jet is the long one threaded into the carb body.

[TwoTired]

What exhaust is on it?

Are all the head pipes hot, both at idle and with the throttle opened a bit?

Have any of the jets been drilled to a larger size?

Have the mains emulsion tubes been cleaned?

The pilot/slow/idle jet S/B #38, screws in next to the main carb post casting where the main and emulsion tube lives.

Remove the bolts between filter box and plenum box so the plenum box will drop down in the frame.  Also, take the bolts off the filter box so it will push back.  Makes it easier to pull install carbs.

The jet needle is attached to the slide.  Also called the Throttle Valve, it moves with the twist grip.

You should have no trouble getting to 90 MPH in 13.9 seconds.  Some will do 100 indicated, kind of depends on speedometer accuracy, tire sizes, etc.  Big tires slow the bike down some.  Stock top speed was measured at 98.77 MPH actual @8072 RPM
Assuming you have stock size sprockets. 17 F / 37R

Cheers,

[bollingball]

You can do a lot of the setup of the carbs on the work bench. Take a few minutes to make a jig like this.

[crazypj]

Simple test.
 Run it to redline in 3rd gear
Redline in 4th gear?
 If it won't pull to redline in lower gears, it ain't ever going to get there in top

[BobbyR]

Speaking of gears, are you using the stock gearing.

[bigfish_Oh]

How big are you guys?  up to 100mph it may not matter much on the weight, but I can only hide my mass so much out of the wind !!  I guess I'll quit tuning when I get to 100 ?

[nilsey]

What exhaust is on it?

as mentioned, unmarked 4 into 2, age, brand and provenance unknown.

Are all the head pipes hot, both at idle and with the throttle opened a bit?

i'll check that out, but i think the answer is yes. i guess i should get one f those point-and-measure thermometers....

Have any of the jets been drilled to a larger size?

no.

Have the mains emulsion tubes been cleaned?

i bought carb rebuild kits and replaced the mains and pilot jets, and airscrews, and the float bowl valve but did not mess with the long needle jets because i wasn't sure how to replace those. which are the "mains emulsion tubes?"

The pilot/slow/idle jet S/B #38, screws in next to the main carb post casting where the main and emulsion tube lives.

got it, thanks, that was replaced with the carb rebuild kit, i assume its the stock #38.

Remove the bolts between filter box and plenum box so the plenum box will drop down in the frame.  Also, take the bolts off the filter box so it will push back.  Makes it easier to pull install carbs.

i've been pulling teh whole airbox and battery holder to get teh airbox in place, which seems to make things easier, but i'll try your method, if it works for me that would be much less hassle.

The jet needle is attached to the slide.  Also called the Throttle Valve, it moves with the twist grip.

how is the jet needle replaced and/or adjusted? the manuals i've looked at are very confusing on this point.

You should have no trouble getting to 90 MPH in 13.9 seconds.  Some will do 100 indicated, kind of depends on speedometer accuracy, tire sizes, etc.  Big tires slow the bike down some.  Stock top speed was measured at 98.77 MPH actual @8072 RPM
Assuming you have stock size sprockets. 17 F / 37R

i have the stock sprocket sizes.

i feel like i should go back to teh 100 main jet size and adjust the jet needle. This afternoon i performed a plug chop, pulling the right hand plug and it looked good -- not fouled, a little on the lean side, if anything.

i'm no carb tuner. the thing was running great from 0 to 80mph woth the stock #100 main jets, its running about the same now, maybe a little slower with #110s. i juts need that extra power on the top end it should have. its not fun trying to keep up with traffic on teh expressway when you want to make a quick pass and everyone's at 80mph, i've got nowhere to go.....

[BobbyR]

I hate to say this. If you want to run at 80 mph in expressway traffic and have some reserve passing power. I think you are in CB750 territory.

[TwoTired]

What exhaust is on it?

as mentioned, unmarked 4 into 2, age, brand and provenance unknown.

Sorry, I missed that. The 4 to 2 that I had for 10 years didn't need any carb rejetting with the stock air box/filter (74 CB550).

Have the mains emulsion tubes been cleaned?

i bought carb rebuild kits and replaced the mains and pilot jets, and airscrews, and the float bowl valve but did not mess with the long needle jets because i wasn't sure how to replace those. which are the "mains emulsion tubes?"

Often overlooked during cleaning.  They live behind the main jet in the "tower" of the carb body.  They are pressed out from the slide needle end if they are stuck in there.  They aerate or emulsify the fuel before it is fed to the carb throat, which aid fuel flow and mixture atomization.

how is the jet needle replaced and/or adjusted? the manuals i've looked at are very confusing on this point.

The needle is in the slide and captured by a plate with two cross head screws holding it down.  It is accessed from the top of the carb and the screwdriver must have a slender shaft to fit in there.  First take of the top cover, and remove the hex bolt on the shaft.  There is another hex head over the slide which must be removed, as it captivates the ball link.  That ball link is pushed out and you should be able to remove the slide assembly see the two cross head screws that hold the linkage to the slide.  With the linkage plate removed from the slide, the needle will come out the top of the slide.
With all these parts disturbed, a vacuum sync is required after reassembly.

i feel like i should go back to teh 100 main jet size and adjust the jet needle. This afternoon i performed a plug chop, pulling the right hand plug and it looked good -- not fouled, a little on the lean side, if anything.

I don't see why the stock jet sizes and needle settings wouldn't work for you.  But, certainly the carbs need to be clean.  And, if you can't be certain the slide needles are in the stock position (4th groove from the top), you'll have to go in and look for yourself.

I hate to say this. If you want to run at 80 mph in expressway traffic and have some reserve passing power. I think you are in CB750 territory.

Not so, unless you limit to throttle twist only.  For the Cb550, if you want to pass at 80 MPH, just downshift a gear or two to get the RPM up.  It'll scoot then. (80 to 90MPH in 4.4 seconds.)

Cheers,

[Scott S]

what air filter do you have if you have an after market foam filter you will need 40 slow jets  these carbs are cumulative so if the slow jet is too small it will cap top speed

I have had my 74 550 for 27 years it runs 110 mains and 40 slows had the same problem you have till i put the 40 slows in now it goes well past 100

 At the speeds he's talking about, he should be on the needle and the mains. I don't see how the slow jet would affect him there, even if there is a "cumulative effect".

[nilsey]

so lets assume that i go back to the #100 main jets and find that teh needle jet is in teh correct position. Whats my next move to try to gain some power here at teh high end?

[crazypj]

so lets assume that i go back to the #100 main jets and find that teh needle jet is in teh correct position. Whats my next move to try to gain some power here at teh high end?

Turn the throttle further
 Change down, open up.
 I have well over 100,000 on my 550, except at very top end, (100mph+) never had any issues overtaking, plus, it's a much lighter bike than 750

[TwoTired]

so lets assume that i go back to the #100 main jets and find that teh needle jet is in teh correct position. Whats my next move to try to gain some power here at teh high end?

When you are looking at the slide needle, write down the number stamped on it.  Some have reported problems with non-Honda needles.

Do check that the choke plates open fully as well as close fully.

There is also a slide up-stop that could have been tampered with, which could limit wide open throttle position.

Dearth of real info/status about the problem bike here.
Need to find out if all the spark plug deposits are same.  If not, correct that.
Then another run with clean plugs and preferably a plug chop at the problem speed, to tell us what the combustion conditions are when the bike loses power.

It's fun to keep guessing, I suppose. 

Cheers,

[nilsey]

so lets assume that i go back to the #100 main jets and find that teh needle jet is in teh correct position. Whats my next move to try to gain some power here at teh high end?

When you are looking at the slide needle, write down the number stamped on it.  Some have reported problems with non-Honda needles.

Do check that the choke plates open fully as well as close fully.

There is also a slide up-stop that could have been tampered with, which could limit wide open throttle position.

Dearth of real info/status about the problem bike here.
Need to find out if all the spark plug deposits are same.  If not, correct that.
Then another run with clean plugs and preferably a plug chop at the problem speed, to tell us what the combustion conditions are when the bike loses power.

It's fun to keep guessing, I suppose. 

Cheers,

you're quite right, TwoTired. i'm very inexperienced at carb tuning, thats why i think my best bet is to get as close to stock as possible. Lemme do that, do a valve adjust, timing check, and carb sync, and get back to you guys.

Thanks for your help.

[nilsey]

so... i did a valve adjustment, timing chain adjust, and checked teh timing (statically, dyna-s elec ignition).

to recap: UNI foam filter >stock airbox > #38 pilot/#110 mains (two sizes up from the #100 stock) > 4 into 2 exhaust.

while at top speed i slowly closed the choke... no difference in power until choke was pretty much closed... then big loss of power.

result: top speed 88mph (seemed a little faster after the valve job).

whats my next move? adjust the needle clip to richen the mix? increase main jet again?

oh yeah, i should probably sync the carbs again before i do anything else.