Topic: CB350f carb tuning

[mnkjoe]

So i was synching the carbs yesterday but i am stumped . the number 4 carb is requiring a drastically different setting than the others  others. what would cause low vacuum on one cylinder? It seems like its not burning properly either. the cylinder is hot though. could the valves be out of adjustment?

thanks

[flybox1]

verified there are no vacuum leaks?
how well did you bench sync them?

[mnkjoe]

I checked for leaks with starter fluid, no leaks found. And to my understanding bench syncing is an OK alternative to using a sync gauge which i was using.

[flybox1]

Bench sync is only useful to get the slides relatively close to eachother.  good for getting a bike started.
A vacuum sync is needed to get the carbs all pulling the same vacuum, and is required maintenance for a properly tuned and running bike,
bench sync is not a substitute for a vacuum sync.

your results indicate #4 slide height is off compared to the others.
To make this easier....reset the vacuum sync screws to where all are showing the same threads above the set screw, and then bench sync them all.
once this is done, there should be very little adjustment needed, as far as screw turns go, to get them adequately vacuum sync'd to each other.

[mnkjoe]

Well 3 gauges are synced but #4 is requiring a far greater adjustment. It seems to be spitting out unburned gas as well a bit. what is this idicative of?

[mnkjoe]

COuld this have to do with an out of adjustment valve?

[flybox1]

Well 3 gauges are synced but #4 is requiring a far greater adjustment.

I just explained why and what to do about it above 

It seems to be spitting out unburned gas as well a bit. what is this idicative of?

highly unlikely.
it IS likely that youve had the bike idling off and on, and not really getting it up to operating temps to burn off any condensate in your pipes. . .

[mnkjoe]

I do appreciate the Help. But can you help me understand why 1 carbs vacuum is so different from the others who are all synced? I will follow the procedure though.

[harisuluv]

I really don't understand what you are saying.

You say during a vacuum sync #4 requires far greater adjustment.  What does this mean?  If they are all the same vacuum then they are all equal.  It doesn't matter where you started from, that is the purpose of the vacuum sync to get them all equal.

Are you saying perhaps that there are way more threads on the #4 adjuster than the others?  Cause otherwise I don't understand what  you're saying.

[carnivorous chicken]

I do appreciate the Help. But can you help me understand why 1 carbs vacuum is so different from the others who are all synced? I will follow the procedure though.

Let me take a stab, seems to be some miscommunication going on. Using your manometer, can't you adjust the #4 carb to be equal to the others? If so, do that, and don't worry about why it was off (unless you've got other issues). bench syncing is eyeballing, and vacuum syncing is dialing it in, so it might've just been off after the eyeballing.

[mnkjoe]

OK well i got them equal as far as sync gauge numbers for the most part. My issue is that carbs 1,2 and 3 have nearly the same adjustment and thread exposure. Carb 4 is requiring  a far greater adjustment which is causing me concern. the threads exposed are about half of all the other carbs.  meaning i have to screw it in much further. Also after using a temp gun the other cylinders are hotter than the #4 carb. #4 is about 20 degrees cooler. 

[flybox1]

if they are all pulling the same vacuum, thread count above the set screw is irrelevant.

if you have a cyl running colder, you need to focus on mixture.

[harisuluv]

Ok that's what I thought.  That is not uncommon.  If you look at the arms (which are spot welded) they aren't always at the exact same angle.  That combined with a slight bend in it can cause what you're describing.  Even just taking them off the backplate and putting them back on will cause a variation than previously due to slight changes in angle tightened down on the backplate.

Basically, the adjustments are there to make the vacuum the same.  Unless it is very extreme (which it doesn't sound like) what you're describing is perfectly normal.  Remember you can also unscrew and adjust the two screws on the main throttle shaft which will allow you to move the two arms on either side left or right a smidge to center them.

[mnkjoe]

ok thank you both of you! What do ytou think i should do to start fresh from square 1 and make sure everything is good ?

[carnivorous chicken]

if they are all pulling the same vacuum, thread count above the set screw is irrelevant.

if you have a cyl running colder, you need to focus on mixture.

Or on the possibility of a bad or clogged jet, or float height.

[harisuluv]

ok thank you both of you! What do ytou think i should do to start fresh from square 1 and make sure everything is good ?

I wrote this up in another thread about bench syncing:

"Really the fool proof way (almost) to do this is (and I've been meaning to make a video of it for the longest time):  We'll use a 1/8" drill bit for the example tool from airbox side.  Get all the slides off "the deck" that way nothing will bind and throw everything else off.  Designate #2 the master.  Back off the idle screw so it's not making contact.  Now get the nut on #2 to where it has about the normal amount of threads showing like you want on your picture on the right (3-4 threads, 4 is good).  Now here's where the foolproof part comes in:  You have only set where you know the threads at the top are a perfectly acceptable honda factory level.  None of the other slides are BELOW the level of #2 (which would mess this up).  Now, adjust #2 with the IDLE SET SCREW with your drill bit (just close).  Verify range of motion IN and OUT with the idle adjust screw (should be making contact with the stop when turning in and out for providing a reasonable range of adjustment).  Next, fine adjust #2 with your drill bit (or whatever).  Now you are going to have for sure the sync set IN RELATION to the idle screw.  In other words, you're not going to get them all synced to each other only to find when you adjust the idle screw it's not even touching and you have no adjustment!  Now all you have to do is set the rest to the same level as #2 (adjust each with the individual adjuster, you don't need to really mess with the main idle adjust now).  Easy!"

[mnkjoe]

Thanks for the write up but it is a little confusing. Ill try again.

[HondaMan]

ok thank you both of you! What do ytou think i should do to start fresh from square 1 and make sure everything is good ?

Not until you have replace the old O-rings inside those castings that hold the carb hoses to the head. They are hard as plastic now, and leak vacuum. I see this on more than 80% of the 500/550/350F/400F bikes I get today.

[mnkjoe]

I tested for leask and everything checked out.