Topic: 74 550K0 running really lean? [Stock airbox + MAC 4-1]

[TwoTired]

Put them on with the stock airbox, and started it up. Seems like it's still running lean, as it still won't run without choke. I haven't messed with the a/f screws yet though. I also noticed when I was putting the carbs back together, the idle screw seemed that it needed to be screwed almost all the way in (fully compressing the spring) to make contact with the slide arm (?).. I don't remember it being that far out, did I mess something up?

Common mistake when using the drill bit method.  You probably adjusted all the slide adjusters, instead of picking one as master, and adjusting the other three to match it.  Now all the slides are skewed, and that is why the idle knob position changed.  Some have skewed all the adjusters so that the idle knob is ineffective and the slides can't close fully.
Anyway, that is why I have never recommend the drill bit method.  I use the light in the bore method.

I still can't figure out why it's running lean though

Do check the idle air bleeds, but that is unlikely THE problem.  I'd guess you have overlooked something in the pilot circuit.  Or, you have some leaks between the carbs and the engine intake valves.

Have you checked the vacuum balance port screws?  If they are missing...

[Dimitri13]

So.. The slides should be fully closed at the front end? That would.. probably explain some leanness.

What exactly are the idle air bleeds? Not sure I've heard of those.

Those screws are still there thankfully.

[TwoTired]

So.. The slides should be fully closed at the front end? That would.. probably explain some leanness.

The slides should be capable of fully closing.  But, the slide opening is directly related to engine RPM.  I can't imagine how that could effect mixture RATIOS. which are determined by fuel metering devices in the carb.

What exactly are the idle air bleeds? Not sure I've heard of those.

They are on the side of the carbs.  Hopefully, those are still installed, too.

[Dimitri13]

Oh, those. I just call them air/fuel screws >>;

As for the slides affecting ratios, I thought that on the inlet side (towards the airbox), the amount of air going through there affected the ratio, as well as the amount of air flowing throught the front side (towards the cylinders). Or at least that's what I thought I read.

By fuel metering devices, you're referring to the jets/needle, correct?

And again, the floats are all at stock height, and fuel is coming out of the .. I guess they're called "Bulk Hose 3.5x3000;" the ones coming from the upper part of the carbs, at the end of the 'fuel flow.' Not the float overflow tubes coming out of the bottom of the floats. Unlike before though, it's not pouring out at the same rate as the fuel flows, but it's dripping substantially. It's enough to make a 6" circle plus runoff after the fuel is turned off, with the bike sitting still. Since I made sure I cleaned all the passages and confirmed that all the circuits are clear, I can only think that it's because the floats, at stock height, are too high, because either the tangs are worn out, or the floats somehow wore out.

[Dimitri13]

So after riding it for two days, I've come to the definite conclusion that it's slightly lean when cold, and slightly when warm, WITH full choke. WITHOUT choke, it's too lean both cold and warm, and won't run.

I guess I'm pulling the carbs again to try diagnosing the fuel leak and verifying that the float gaskets are seated properly (fairly certain I pinched one inside the bowl, though :/

I'll also be replacing the float bowl screws, as 12 of the heads are stripped. According to Z1, the screws are MM4X12X25. Could I just go down to the local home improvement store and pick some up, or are they special in any way, where I'd want to get them from a specialty store/site? Also, the drain screw isn't sold separately from NOS sites (just with a float bowl set). Anyone know the lengthXthreadsXpitch on it, as well as the little gasket?

[TwoTired]

Just to cover all bases...  Are you aware that the choke lever is pressed up for choke on and down for choke off?

[Dimitri13]

Er uh... Oh?

So you're saying this is choke OFF?


(grabbed the image from another sohc4 member)

What does it mean when it says "OPEN" then?

[Deltarider]

The carbs in the picture have the chokes OFF. This the normal position when riding. For cold start raise the lever in horizontal position. With this type of carbs soon the chokes - even before riding away - can be put off. That is... when everything works normal.

[TwoTired]

Er uh... Oh?

So you're saying this is choke OFF?

Yes.

What does it mean when it says "OPEN" then?

The lever controls a butterfly plate at the entrance of each carb.  The plates "choke" the air supply, causing fuel mixture enrichment.

Therefore, choke open means choke off.

Problem all fixed now?

[Dimitri13]

Well, that makes sense now that I think about it. With the choke 'closed' the plates in there are closed, and the small little inlet plates open up from the air pressure (correct?)... I'm thoroughly embarrassed It's not that I'm not familiar with choke (my ninja 250 had a choke), I've just never needed to use one (Hawaii) or even took the carbs off of that bike.

I got my manometer in today and tried to sync it. Turns out I have compression leak in cyl #2 and #4. Or at least that's what it seems like, I haven't done a compression test yet, but I'll be doing it tomorrow. The pressure for the carbs should be between 14-15, correct? (Because the.. optimal psi is supposed to be like 14.7psi? Or so I've read.. I think)

Thanks for your help everyone, especially TT.

[Rgconner]

There is no "number" for the vacuum, so long as they are all the same across the board.

Do not set the choke while using the manometer, it will damage it.

[Dimitri13]

In my service manual it said it should be somewhere between.. 13-17 cm^3 iirc? Choke wasn't set.

But I'm still having the problem with the fuel leaking from the upper overflow tubes.. Floats are at stock height and the seats/needles are new and cleaned twice. Dropping the float height to something like 27-30mm makes it not leak.

[TwoTired]

I don't know what you mean by "upper overflow tubes".
I would think maybe the vents. But, if the fuel level rose that high, then the carb throat and inlet runners would fill with fuel before the vents started leaking.
So, I need some clarification before making any suggestions.


I never paid much attention to the numbers scale on the vac gauges during carb synch, except that they be equal among all four cylinders.
It's not the carbs function to create the vacuum.  The engine does that.  You can only make the the carbs balance what the engine demands in the way of vacuum.  In other words, the carbs create no vacuum at all if the engine isn't running, or the pistons moving.

[Dimitri13]

Oops. Seems I messed up checking the tube's routing. Turns out it was from a float bowl. I've once again taken the carbs off and will be cleaning everything out some more.

[Dimitri13]

Well, turns out the brass overflow tube in the float bowl was cracked on the leaking carb. Sealed it with heat-wrap tubing, no more leaking (checked all the tubes with a vacuum gauge). There should be no more leakage from anyway now with floats at stock height (I also checked the buoyancy of each float and they're all the same).

I figured out what the problem is with not being able to sync my carbs:

In this picture, you can see that the slides are fully closed, but the arms that attach to the throttle bar (whatever it's called), are uneven.


In this picture, I made the arms even to each other (how they would be on the throttle bar), and one slide is fully closed, while the other is significantly more open.


Now that I've diagnosed the problem, I have NO idea how to remedy it. I've fully taken apart the carbs and reassembled them and ensured that everything is the way it's supposed to be, but that didn't help. This is the last hurdle in getting my bike running, so I'm very desperate to get past it. Hopefully someone can help me out.

[Tews19]

Glad to hear you were able to find the issue out. I am pretty sure someone will chime in who knows how to help. How is it living in Hawaii? You ever go to the military base?

[Rgconner]

If, and that is a big if, all the carbs were exactly the same, and all the cylinders were the same an drew the same vacuum, you could sync the carbs with a feeler gauge. All the slides would be at the same night and life would be good. We don't worry about the arms, only the slides.

But they arn't. So we set them so that the slides are all at the same as a rough guess and get the bike to run. Then we use the manometer to account for the diffences in vacuum across the cylinders by adjusting the opening of slides so they all draw the same vacuum, but the slides might not be all at the same height when they are balanced.

Follow the instructions found here and in other threads and stop trying to make it work the way you think it should.

Life wil be better.

[grcamna2]

I think that trying to match those arms...I'm just not sure if that is the best;those arms can be different, I think...as long as the slides are exactly the same height on all four by eye.I know you can then put it together and synchronize all 4 carbs w/ your gauges;I'm fairly sure if you use the #2 carb to start with & match them all to that one.I know other members will help you get to the next step from here. I hope all your float levels match.
                                                         You have a good weekend.

[dave500]

go over your ignition first,make sure its spot on,if you have points make sure the condensors are good,,if you dont know how old they are replace them,youll chase ghost carb issues forever with a not quite right ignition even though itll run "ok"check the plug cap resistance,you need 5kohm,use a strobe timing light to set the timing and a dwell meter,a bit like bench carb syncing,the light bulb and feeler guage gap gets you in the ball park.

[Dimitri13]

Thanks for the replies guys. The reason I'm concerned about the arms and everything being so uneven, is that when I was trying to sync the carbs, 2 and 4 both had very low vacuum, even with the slide adjustment screws ALL the way in. That's why I'm worried those being uneven will affect the carb's performance. I'll try syncing the carbs off the lowest vacuum carb (#4) when I get everything back together.

Oh, and on a completely unrelated subject:

The throttle return spring is extremely stiff. It can definitely pull the throttle closed fast, but riding for just half an hour will give me some bad forearm pump and wrist pain. It isn't just from the bar position (clubmans), because I had the same problem with the stock bars. Is there anything I can do to the current spring? Or are there replacements?

dave: Got ignition spot on, as well as the valves.

[dave500]

ok cool,those return springs were a bit heavy,most guys just use a little lighter one, your local bolt bar should have a selection of springs to compare with.

[Rgconner]

Stupid question: what rpms are you at when syncing?

3000 is the proper setting, use the idle set screw to crank them up. Spark advance should be engaged at that speed.

[Dimitri13]

Hm.. my Clymer manual says sync the carbs at idle speed, and they vacuum should be between 16 and 24 cm-hg (3-5psi I believe) (not worried about that too much).

I know in mrmax's carb sync vid he does his at 3k rpm, but my manual says different.

[TwoTired]

Hm.. my Clymer manual says sync the carbs at idle speed, and they vacuum should be between 16 and 24 cm-hg (3-5psi I believe) (not worried about that too much).

I know in mrmax's carb sync vid he does his at 3k rpm, but my manual says different.

...And the manual is correct.  Vacuum sync at idle. This is where it is the most sensitive and can balance most accurately.  @3000 RPM vacuum sync is almost inconsequential and largely a waste of time.

The slide position is the most important, moreso than the arm position.  The arm position can vary with these carbs along with the arm bolt hole tolerance (under the top cover).

[Dimitri13]

Thanks for the clarification TT.

I'll try syncing them up to the lowest vacuum carb (#4) and see how it goes.