Airbox and stumble

[Ed_Ramberger]

First let me thank everyone for all their help.  This has been a great group to learn about this bike from!

The bike had a slow speed miss - I removed and cleaned the fungus out of the carbs, and the bike responded beautifully.  The only thing I am struggling with, is if I whack the throttle, there is a stumble until  the motor  seems to catch up.  This stumble is absolutely non-existant under normal all around driving.  If I downshift and go WOT, though - stumble.  THe only thing I have yet to do is sync the carbs, but this is not a sync issue - it's a miss/stumble.

The airbox has a vacuum actuator.  Is this the "update" I see mentioned?  Also, the air cleaner does not appear stock.  It has yellow foam around white foam.  Is this a re-useable filter?  

This is a great bike, I feel really good that she got this one as her  first bike.  I think that it will last her a long time.  

As always, thank you for all the help on this ride!

Ed

[Rick G]

Hi Ed, Yes, that is a reuseable filter, (i have a couple of them) just wash it in soapy water and when dry LIGHTLY oil it.  The stumble can be remidied by epoxying a nickle to the top of the damper door on the air box inlet. (I have two)  this will reduce the air flow at the 4000 to 5000 stumble point . The problem is that the air entering the carb ( which is largish for a 550) is moving too slowly to mix the fuel properly  when the door is partialy closed the air is speeded up and mixes the fuel properly. I'm paraphrasing this from the article I read it in. all the current  1000 and larger double overhead cam four valve v twins on the market now  have fuel injection which eliminates the problem.  IE: Aprilla, Voxan, V-rod et all. dont forget to sync your carbs , it will run smoother and better for it.

[jasonm.]

I eliminated the "stumble" by setting the low speed screws on mine. I used a digital tach. And just tuned for highest idle. I have found that 3.5 to 4.5 turns out is the range. Try four out and check. But the carbs must be sync'd!

[Ed_Ramberger]

Question regarding tuning for highest rpm - that goes against everything I ever did regarding setting carbs with the lean drop procedure.  I have always fattened up/leaned out until rpm/s no longer change at ecah extreme, and then set in the middle.  

Sounds to me like you went to one extreme and left it there.  the only thing I can think of is that your base mix is so extreme it is affecting your transfer from slow jet to main by overlapping.  

[Rick G]

Hi Ed, when I worked as a wrench, that was the way we set all carbs ie: set idle as low as it will maintain tick over , then set low speed screws for fastest idle ,then set idle to factory specs. Of course we did this with no tach on some  bikes in the sixties and seventies as the didn't come with one. Jasons idea of using a tach is great, but I do mine by ear and it works fine.

[Lucky]

my preferance is adjust the pilot air screws for middle range, smoothest idle, then 1/4 to 1/2 richer to keep the valves cool.

Jason's range seems to be about average, +/- 1 turn.  for whatever reason, these carbs don't all react the same, probably due to varnish and age, so tune for your best performance.

--Lucky

[ricks_83]

if you are having problems with tuning you might need to pick up the weber tuning manuel it is a pretty good copy of this carb you will have to convert what there standards are to the mikunis you can make all the changes with out changing carbs the 82 has more off the bottom than the 83s if you need help i would be glade to help what part of the country are you from and what year and what changes have been made to bike

[jasonm.]

The '82 carbs have richer pilot gas jets. Which if the screws are turned out too far will make the bike run poorly. My '83 it's almost impossible to do that. The L.S. jets on the '83 are smaller. Screws would fall out before you could get as rich as an '82. There are really only 2 gas jets in these bikes. Low and High. There is an overlap.  And the flapper has to do with this overlap also. The flapper is suppose to open as the mains come online. About 5000rpm or so. Remember these are TRUE downdraft carbs. With no moving parts except the butterflys. Vacuum is everything here.

[Paul_Jungnitsch]

Hi everybody. Gunson sells a clear sparkplug, which is useful to see the flame color in the cylinder. My '82 had an annoying stumble when I got it that turned out to be the carbs set waaay too lean at idle. With the clear plug it is easy to see exactly what is going on.