XJ550 Seca Carb Rebuild....One Last Issue

Ok...for those who may not remember, I rebuilt a set of carbs for a friends XJ550 Seca. Deluxe rebuild kit came from Len...went to church on the rebuild (complete disassembly, all metal parts soaked in carb cleaner for at least 8 hours) , no issues with slides (passed the "clunk" test), diaphragms had no pinholes, dry sync'd, wet set, running sync, valves in spec...basically bike starts and runs flawlessly up to about 5k, at which point it'll begin to stumble...it'll pull to redline, but it just doesn't "like" being pushed past 5000 rpm. I have used both propane and carb spray and cannot locate any vacuum leaks...stock air box w/new paper filter.
I no longer own a ColourTune and set the bleed screws by ear (that shouldn't matter...should it, since the stumble is in the higher RPM's?).
I also replaced the fuel line and vacuum line to the petcock (at this point I removed the inline filter just to verify no air bubbles).
Have yet to rebuild the petcock itself as it seems to operate fine and does not leak...it sure feels like a fuel delivery issue....could it also be a float issue? I know the Mikuni's need to be spot on, but I'm pretty sure I had them set bang-dead on! What else could cause such a stumble?

 

gas cap vent cleaned? think of the fuel system as starting from cap and ending at manifold boots.
test run with petcock set to prime? should tell you if petcock is a problem.
internal tank filter clean.
full tank of gas.

your stumble sounds like what happens when I have to switch to reserve

 

Good Call!
Have not checked the gas cap vent! Also all test performed with petcock set in the "Run" position. Will check those right away.
Could it be a float issue? Or, could it have had anything to do with the fact I didn't use a ColourTune?

 

Did you make sure the carb slide diaphragms were seated properly while putting the carb hats on. One or two could have slipped during reassembly.
Were the main jets replaced during the carb rebuild and are they the proper size?
If your slides don't raise properly you will never get passed 5000 rpm.

 

if floats are with in the spec -2 mm from flange +_1mm it should be good I will say that bench wet set does not mean proper setting on the bike when carbs are installed.
before i got a color tune i set carbs between 2-5/8 turns and 2-3/4 turns by ear bike ran well.
to check vent you can ride with gas cap unlocked.

as mlew stated the diaphrams are the next thing to check.

do you have any Idea on compression values ?
ignition system specs
spark plug gap/age?
was cam chain tensioner set to spec? easy to set if carbs are off a little tight under the carbs.

 

[QUOTE=".
to check vent you can ride with gas cap unlocked.[/QUOTE]

Not really a brilliant idea unless you have another gas cap handy. I would not want to ride without a gas cap locked in place. Way too easy to spill get gas all over yourself.
It does not take long to remove a gas cap and clean the vent hole and check ball. Check it before you ride.

 

Possible ignition issue? HIgh RPM misfires could be from bad plugs/plug caps, coil grounds, or even a coil...

 

New plugs and gap set by me...
Did not check compression (other than thumb over hole while cranking), and no I did not check cam chain tension

 

Did check slides before putting carbs on bike...they were fine. Had to replace mains as they were too buggered up...used OEM Mikuni...pretty sure they were #35 (was whatever the stock jet was)

 

I had this symptom and found one of the main jets had worked loose (or more likley I didn't tighten fully) and was lying in the float bowl when I took the carbs off again..

 

I hope that's not the case....I'd really rather not take the carbs off again!

 

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this is the accepted method here, not my Idea , but comes from people more in the knowledgeable than me.

 

Too low of a fuel level in the bowl (meaning the numberical reading will be too high when checked) will also cause the condition your describe....check it using the "clear-tube" method as outlined in the service manual.

 

Haha. I HAD same problem my 750 just did not want to go past 5000rpm. Pulled carbs # 3 pilot jet was laying in bottom of the bowl...this answered why the plug was black sooted up. You might want to pull carbs and check if any are loose or fell out.

 

First I have seen that suggestion. Its still not a good idea. Very unsafe. Better to just test the cap if you suspect it is clogged before you ride.

 

Check the color of your spark plugs. One or two that are a very different color that the others will indicate a carb issue. Remember the plugs should be a light tan color. An extreme lean or rich mixture in one or two carbs will make any XJ engine struggle to achieve high rpms.

I really think you will have to remove the carbs again to get the issue fixed. Something in them is not 100% right. From what you have told us , I would check the carb slides again. Push them up and let the spring push it back down, all 4 of them should feel about the same and move at the same speed( close to the same). If they don't that will indicated one with a out of place spring or diaphragm.
Out of place air jets and float levels will also cause your problem.

 

1. We aren't riding around for hours to test, just a few minutes.
2. Ride until the problem occurs, pull over, open gas cap, pull back onto the road for a few hundred feet. Does problem go away?
3. Riding a motorcycle is dangerous.

Plugs aren't necessarily going to be a light tan color, even when fuel mixtures are dialed in perfectly. I've seen anything from the classic tan, to a light grey, depending on the characteristics of the local fuel.


My advice: start with all of the easy things (gas cap, checking float levels (wet), plug and plug wire condition, air filter condition, petcock flow rate) before taking the carbs back off.

 

Still not a good idea to tell someone to ride with a loose gas cap. Even a few hundred feet can get you in trouble. Better to open it to vent the tank then put it right back on.
Yes plugs can be a number of colors. Tan or close to that is normal, we don't want black or white on them. That's all I was looking for.
We know the obvious , quit being so quick to point them out

 

I'll keep pointing out the obious things, because often they get overlooked, and they are easy to check before tackling the more time-consuming things.
Troubleshooting demands methodical thoroughness, and doesn't care about anyone's ego.

 

I agree , so do I. Recommending someone ride with a loose gas cap is one of them. Check it before you go ride, its a quick simple thing to fix.