Topic: 1975 CB550 carb lean problem

[dilbone]

Ok guys, I'm new here to this bike and forum, but I've been searching all over the place to find info for my problem.  First off, here a couple of pics of the bike in question...before and after shots...



I picked this bike up on the cheap since it had been sitting for at least 5 years and collecting rust and dust.  First thing I took the carbs off and doused everything I could with carb cleaner.  I put the carbs back on and with a new battery and fresh gas the bike fired right up.

Now that I've been messing around with it for a week or so(and put an entire bottle of SeaFoam through it) I'm seeing that this bike only likes to run at 1/2 choke.
I turned the air screws in a bit to help and I think it did to a degree.
Now if I'm in the first 1/4 throttle it will run pretty well without choke, but once I get beyond that I've got to go back to between 1/3 and 1/2 choke to get any acceleration.

I just talked to the previous owner and asked him about the choke issue and he said, "you know, I think I always did have to run it with a little choke."

What is my next step(I assume you're going to tell me to take them back off and take them completely apart)?  My problem is that as much automotive repair as I've done, I've not done any carb work, either automotive or motorcycle.  Is there a good resource somewhere that can tell me what is what, what to do, and what not to do as I proceed?  I've seen some of the great info here on the site, but I'm looking for something with pics too...

Thanks in advance for any help on what is probably a common question.

[Bill Vaughan]

If you haven't done so already, take a look at the sticky entitled "download manuals" located in the top of this forum.  The service and parts manuals for your bike are located there.

You'll also want to read through the article at this link: http://data.sohc4.net/KeihinRebuild.pdf  It provides the overhaul procedures with lots of pictures for a set of 750 carbs, but is closely related to the 550.

[750goes]

I think you will find that the pilot & main jets will need to be cleaned  - this can be done with the carbs on the bike, but I would take them off to do a proper clean.. and I think you will find the emulsion tubes will need a clean as well....

You do not have to separate the carbs but just do one at a time - leave the rack together...

I know its a pain but it will help the running of the bike...and it's not hard to do, only a bit of time to do it, probably two hours tops..... not including dismounting/mounting...

 

[1timduke]

I see the airbox plenum in the shots, the bike looks really nice!   Is the rest of the box complete, i.e a proper filter with the lid (tool tray) fitted?   This can affect mixture, if everything isn't there or you have leaky rubber boots somewhere, it can run lean.  

+1 750goes, jets and tubes definitely need cleaning.

-Tim

[TwoTired]

If you need choke to keep it running, it usually means the slow/idle/pilots jets are plugged and not feeding fuel. Applying choke forces more fuel through the throttle valve circuit than would normally occur.  Check the head pipe temps, as a cool(er) one usually corresponds to the carb or carbs that have a problem.  (Assuming you can make the engine idle at 1100 RPM-ish.)

You can clean slow jets with the carbs on the bike if you are skilled enough.  You must be able to see light through the 0.016 inch hole withing the removed jet tube.

Remember, these bikes need a complete tuneup at 3000 mile intervals.  Deal with that checklist before attacking performance issues.  The bike just may have been cheap because the tuneup checklist hasn't been attended to.

Cheers,

[dilbone]

I see the airbox plenum in the shots, the bike looks really nice!   Is the rest of the box complete, i.e a proper filter with the lid (tool tray) fitted?   This can affect mixture, if everything isn't there or you have leaky rubber boots somewhere, it can run lean.  

+1 750goes, jets and tubes definitely need cleaning.

-Tim

Tim,
Yes it is complete, The only problem is that the breather tube isn't attached very well at the bottom of the box.  It looked like the inlet elbow had been chewed up pretty well where the tube actually slides on.  It looks like it had been split/cracked so that it isn't a tight fit into the tube at all(it's basically just lying there in contact with the elbow and that's about it).  I'm not sure how that elbow is mounted into the box and if that can be replaced, or if I can rig something up that will fit tighter.

[dilbone]

If you haven't done so already, take a look at the sticky entitled "download manuals" located in the top of this forum.  The service and parts manuals for your bike are located there.

You'll also want to read through the article at this link: http://data.sohc4.net/KeihinRebuild.pdf  It provides the overhaul procedures with lots of pictures for a set of 750 carbs, but is closely related to the 550.

Bill, thanks for the link and info...very helpfull already.

Looks like I'll be taking them off again for a much more thorough cleaning

[dilbone]

Remember, these bikes need a complete tuneup at 3000 mile intervals.  Deal with that checklist before attacking performance issues.  The bike just may have been cheap because the tuneup checklist hasn't been attended to.

Cheers,

Thanks for the input, the bike was cheap because 1 it was a mess and wasn't running and 2 I had done some lawn mower repair for the owner... for $45 in parts for the mower and $100 out of my pocket I got the bike... not a bad deal considering it actually runs very well even considering the lean condition right now.

I'll start on the checklist and see what happens, at this point for what I have in it I've been very pleased.  Fixing the lean issue will make this bike a real keeper.

[dilbone]

Ok guys,

I took off the carbs today and cleaned and blew out all the jets, passage ways, emulsion tubes etc...
I was also able to get the breather tube hooked up and secure to the air box again hoping that between the carb cleaning and fixing the hole in the air box that I'd notice a substantial improvement.

I put it back together and just buttoned it up.  There was little if any change in it's condition.

I can get up to about 50mph with the choke open, to go any faster I've got to choke it to about 1/3 to 1/2 closed.

Not sure what else to do... any ideas...I may just have to have it looked at...by a professional

[750goes]

Next step I would take would be to check the mechanical side of things.

Are points set correctly, is timing correct. I would definitely check the advancer mechanism, as it may not be really advancing the timing when required.. simple enough job as well.

Are all the plugs, leads, coils been tested or inspected??

Valves adjusted ??

keep at it  - 

[1timduke]

If it doesn't get above 50mph, was there any indication of lean running in other throttle positions in the lower gears?   If you are throttling up before you shift, but shifting early to compensate for a poor mixture, you may have the wrong needle position or main jet fitted (or both).

You may also do a plug chop test to read the plugs in a certain throttle range.   You basically ride the bike at a certain throttle position (may help to mark your throttle w/ tape or something), then pull in the clutch and kill the motor.   Coast safely to a spot where you can pull the plugs that are easy to get too and see what they tell you. 

You could also have possibly some leaks around the rubber boots, Higher RPMs require more air to maintain the balance of fuel and air, if you've got leaky boots on the carbs, you may be sucking too much (unfiltered) air.    Easy test is to spray WD40 on the boots at idle.   The WD will temporarily seal the boot a little better, if RPM improves, you may have a winner.


WOW! $145 for the bike!   Now that's a bargain, running or not.

-Tim

[dilbone]

If it doesn't get above 50mph, was there any indication of lean running in other throttle positions in the lower gears?   If you are throttling up before you shift, but shifting early to compensate for a poor mixture, you may have the wrong needle position or main jet fitted (or both).

If I throttle up in any gear it starts to bogg, but that gets worse as speed increases.  How do I change the needle position?  Is that via the little 10mm bolt in the top of the carb?  There appears to be a little clip on it holding the head of the bold from turning, does that have to be taken off?

You could also have possibly some leaks around the rubber boots, Higher RPMs require more air to maintain the balance of fuel and air, if you've got leaky boots on the carbs, you may be sucking too much (unfiltered) air.    Easy test is to spray WD40 on the boots at idle.   The WD will temporarily seal the boot a little better, if RPM improves, you may have a winner.

I'll try that and see what happens.  The thing is that it idles well regardless of choke position, when idling and popping the throttle I've got good response, and at 1/8 to 1/4 throttle you'd never know there was an issue... then you'd think you're on a different bike once you get up to 50mph or so as it just can't handle it without choke.
I did notice that the breather filter is missing when I took out the air box to fix the breather tube.  That certainly couldn't cause this drastic an effect could it?  I wouldn't think it'd be pulling that much air up from the breather?

I've also tried spraying starting fluid at a few places I was suspect of for vacuum leaks and found nothing, I'm not sure the WD 40 will help because it idles very smooth regardless of what I do to it.

WOW! $145 for the bike!   Now that's a bargain, running or not.

-Tim

I thought so too, now if I can just get it running right...

[dilbone]

I just checked/cleaned/re-gapped the points, checked/re-set the timing with my timing light...timing advance is definitely kicking in.  Everything looks/sounds good there.  I sprayed starting fluid at all the boots and any other places I could think I might have a vacuum leak and listened for slight idle changes...nothing...

I even took some T-shirt fabric and made a multiple layered make-shift "filter" to place down in the breather area hoping maybe I was sucking too much air from down there.  It may have helped just a hair, or it might just be wishfull thinking.

I do think it's running better in all other phases.  It's idling smoother, take off is smoother, low acceleration is smooth but I just can't get past the bogging when I go past 1/3 throttle without at least a little choke.

[ekpent]

Check your petcock for fuel flow,make sure its not clogged or running slow,can't keep up with the rpm's, and your cap vent is clean and fuel line running down for the gravity feed.Seems to be starving for fuel,maybe turn off gas right after a hard run,drain bowls and see how much is in them before it can catch-up from sitting.

[1timduke]

Check your petcock for fuel flow,make sure its not clogged or running slow,can't keep up with the rpm's, and your cap vent is clean and fuel line running down for the gravity feed.

+1

The needle position is indeed changed through the top portion of the carb.   You'll have to remove the bolt (just bend the washer tab, and possibly the other bolt that is in there as well (I believe it's a 7mm).   Then remove the knuckle that connects the slide to the arm thingy (I'm using very untechnical terms here).   Careful, there is a little dark looking cylinder thingy and a spring inside.   Then, remove the two very tiny phillips head screws at the bottom of the chamber, rotate and viola! it should come out.   Then check the needle position, raising the needle (putting the clip closer to the sharp end) richens the mixture, lowering the needle (putting the clip closer to the top, blunt end of the needle) leans out the mixture.   Do this in a really clean space, these parts are tiny.   You shouldn't have to force anything, all the parts should come out fairly easily.   I believe this is how I did it, although I may have left something out in the procedure.

The breather filter shouldn't affect it too much, it's more of an environmental thing, recycling case vapors to be recombusted.

Before you tear down the carbs again, run the bike in it's 'bog' stage, kill the motor and check the #1 and #4 plugs, if they're whitish, it's running lean, but if it's a tannish brown, you may be looking at something other than a carb issue.

-Tim

[dilbone]

Thanks again guys for all the advice, I'm learning an awful lot here.

The fuel checks out, no problems there.

I'll do a plug check and see what I get.

Thanks for the info on the needles, that'll probably be next on the list.

Also, I was just out riding around and it looks like I can place 2 layers of T-shirt fabric over the airbox opening at the screen under the seat and run it with choke open with much better acceleration.

Is that a safer way to run it until I can figure this out than going 1/3 to 1/2 closed on the choke?
I figured if I could cut down the air before it gets to the carbs it's better than choking it and having the venturi effect inside the carb pull more fuel...
Any confirmation on that theory?

[TwoTired]

If the breather filter and its housing are missing, it allows oil vapors to saturate/clog/destroy paper air filters.

You do need to read spark plugs in order to verify a lean condition.
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

[dilbone]

housing is there, filter is gone,  Cotton fabric has been placed there for the time being

Thanks for the spark plug link, I'm going to go check the plugs now.

[dilbone]

checked the plugs, definitely whitish looking...

pulled the first needle...I can only bring it up one more slot...looks like it may have already been raised for a lean problem.  Can moving it up only one slot make a big difference in the flow?

[TwoTired]

Yeah, I guess.  Did you write down the needle number stamped on it?

[dilbone]

272304 is the number on the needle,

It has 5 slots and so I moved the clip down to the bottom slot.  Tomorrow I'll go ahead and take off the carbs again and get the needles moved up to the bottom slot and hope it makes a difference.

[Bill Vaughan]

For what it's worth...

Regarding the needle clip position:  If they're the original carbs, they should be an 022A set-up (check the stamping located on the top side of the flange).  Assuming you have the stock/correct needles, the standard clip position is #4 (4th groove down from the top).

[dilbone]

For what it's worth...

Regarding the needle clip position:  If they're the original carbs, they should be an 022A set-up (check the stamping located on the top side of the flange).  Assuming you have the stock/correct needles, the standard clip position is #4 (4th groove down from the top).

Yes they're the 022A stock carbs.  That's good to know, I'm just curious as to why this thing seems to have always run lean according to the previous owner.  It's been in they're family since it was new so I know it's been well taken care of.

Will changing the clip to #5 make a big difference?

[Bill Vaughan]

Based on my limited experience is does make a difference, but not necessarily for the better.  I always found that my stock Hondas ran best at the factory clip position.  If I had a problem, a clip adjustment might improve the situation somewhat, but the real cause of it was elsewhere

[cycleman]

Just remember that if you change one thing it will affect something else. Everything has to be taken together when you are dealing with carbs. The needle comes into play from about 1/4 throttle & up. You could try going up one main jet size to see if that helps, rather than moving the clip position. Bad carb boots can cause your type of problem.

If you are running with no air filter or the stock air filter housing loose, it will affect the running of the bike and make it run lean.  Also other than stock exhaust will affect the driveability.

Engines are much more senstive on the intake side than the exhaust side.

Check for vaccum leaks in & around the carbs.  Use an unlit propane bottle & pass it around the carbs.  If idle speeds up you've found your leak.