Topic: CB350F idle races out of control!!!

[350Mike]

Hey SOHC4,

I'm a new member and new rebuilder of a 350F.  Rebuilt the carbs, timed, new plugs, etc.  Bike fired up beautifully, but I'm facing a problem when I go to adjust the idle speed.  After starting, the bike idles okay (a bit high or low) but when I go to adjust it toward 1200 rpm the idle starts racing through the roof, and all I can do to stop it is to kill the bike. 

I set the air screw 1-1/2 turns out on all carbs.
Thought maybe I had some issues in throttle cables, so I took them off.
When I adjust the throttle stop screw it does decrease or increase idle, but it does not last long and the idle eventually races through the roof again.

Could it be a vacuum problem caused by my throttle valves?
Also, there are three rubber lines to the carbs.  The large one is the fuel line, but are the other two vaccum lines or just breather lines? (I don't have the correct pet cock)  Could this also be the cause if they are not connected to the pet cock (if they are even supposed to)?

Thanks for the help!

[NGL_BrSH]

cracked intake boots maybe letting in air when the vacuum rises splitting the boots?

I don't know your bike at all but i'll give it a guess that the second tube was to supply fuel as well and when you raise the idle it can't replenish the bowl fast enough leaning out the mixture?

sounded good right

the gurus will def help more than me.. but I thought i'd throw a couple ideas at it.

[350Mike]

Thanks.   

I'll check the boots again but they appeared to be in good shape.  Since I don't know bikes very well I'm probably way off, but I don't understand how the idle can increase so drastically if the throttle valves don't move up?

[jhasewhite]

The fuel petcock only needs one vacuum line from the engine side of the carbs. You can plug the other one with a vacuum plug cap(available from most auto parts stores.) Your engine is definitely  getting too much fresh air. Also check the tightness of your hose clamps - if they are loose they will suck in air and make your engine do crazy things.

[kslrr]

The three lines from the carbs are: fuel and 2 breathers.  The breathers are the small lines and these exit to the rear of the engine.  There are no vacuum lines on this engine.

[350Mike]

So that explains why all schematics for the petcock only seem to show 1 hose connection.

[hoodellyhoo]

I had sort of a similar problem. One of the synchronization adjusters on one carb had vibrated lose and was waaaaay different from the other three. This gave me an erratic and sometimes racing idle, but it also ran really rough and you have not said that yours runs rough so I am doubting that this is your problem.

[bender01]

I would start by cleaning the carbs again. Sounds like something sticking. If you just got it going from an extended rest theyre might be some junk that came from the fuel tank or something. Floats sticking? Slides? Just a thought. Oh and heat the air box with a heater or a dryer and the carbs go on alot easier. I know its a tight space.

[350Mike]

Thanks bender.  I'll post the results and let you know what I find.

[tonycb650]

Is your choke on?

[350Mike]

Hey guys,

Felt like I tried nearly everything today and still no luck.

Pulled off the carbs, took them apart inspected and cleaned everything including jets.  Everything looked fine.  Since I can't sync the carbs until I get the idle tamed down, I made all throttle valve spacing look the same by eyeing them and adjusting the vaccum screws (no major adjustments on these).  I could find no holes in the intake boots.  Still running the bike without throttle cables connected, so I can eleminate any cable adjustment as the issue.

So, when I fire the bike up it's still idling either way too high or barely enough to keep running.  I manually rev the engine, and when I release the throttle the idle stays high, usually around 3.5k rpm (By the way, the throttle fully returns when I relese it.  The spring is in good shape.  Nothing sticking.).  I try to adjust the idle down wth the idle screw and I can make it drop steadily until it gets to about 3K rpm.  Once it drops just below 3k rpm it falls way off and dies.

What the heck?  I don't even know what to try next!  Ahhhh!

[bender01]

the slides should only be open just a crack when they go back on the bike or youll run out off adjustment. Im sure you looked up bench syncing by now?  Maybe you need to do the timing again?

[350Mike]

bender,

I don't have any reference to bench syncing.  The only reference I have is a Clymer and that only gives me the manometer method, or homemade manometer method, but those require the bike to be idiling consistently.  Do you have a any reference to a bench method?

Thanks!

[bender01]

http://forums.sohc4.net/index.php?topic=5410.0
 I use a wd-40 straw instead of drill bit. Its under FAQ. Theres lots of help there or search bench sync at the top of this page.

[350Mike]

I'll pull the carbs off again and check the gaps in the slides.  You say the throttle slides are only open a crack when you fully release?  Just out of curiousity, can you give me a rough idea of how much the vaccume screw heads are sticking out from the lock nuts on your carbs?

[martino1972]

when you cleaned your carb's.,did you take the idle jets out nad confirmed that all the orfices where clean and could be sprayed trough with brake cleaner or carb cleaner??
many of us here had to take the carbs apart a few times to get it cleaned right...
it really sounds like your idle jet/orfices are plugged,below 3000 rpm dropping dead etc..

to bench sync. the carbs,take the top lids of the carbs,3 of em will have a adjusting bolt/nut and 1 wont have that..
start with the one that doesnt have it,turn up idle screw and insert a wd-40 straw or small drill bit under the carb slide,then turn out idle screw till straw can just slide in/out....
then with the top adjusting screws set the other 3 carbs with the straw till you get the same result..
this will get it close enough to be able to idle decent..
then reinstall carbs ,start engine and turn down idle screw.....

if carbs are indeed cleaned perfectly,and bench sync is set right,and intake boots are installed correctly,and boots dont leak or cracked the bike should idle fine..

when bike is running,did you try and spray wd-40 on the intake boots??? if there is a air leak,it will show up by rpm changing when sprayed with wd 40..

good luck..

[bender01]

Yes only open a crack when released or at idle. That way you will have plenty of adjustment on the idle screw. You have 4 adjusters  not 3 like the previous post and cleaning the carbs 3 times so far has been true for me. Youll go nuts guessing by how far to adjust the vacuum by the height of the screws.  Pull em and go over them again.
 Lets hear when its running fine.

[350Mike]

Man, the idle is still all over the place.  Pulled the carbs and followed the bench sync instructions.  The intake boots look like they are in great shape, no cracks or holes.  Cleaned all jets.

When I start the bike now I have to hold the throttle even for it to stay going while I adjust the idle screw.  By the time I get the idle screw to hold a basic idle one of two things is hapening:

1.) If I pull the throttle the bikes revs, and then usually sticks close to the rev rpm (usually drops a bit but its still high, around 3000 rmps);

OR

2.) Barely hanging on to a basic idle, the bike will suddenly race quickly up to about 4,000 - 5,000 rpms almost as if it were running out of fuel. 

We checked the fuel again and everything checks out.  The floats have been set and tested so I know the bowls are holding fuel.  Tried all combos of air screw from lean/super lean to rich/super rich.

Do you have anything left for me guys?  I understand how if the carbs are getting extra air the idle would be high, but I don't get how it can race when the throttle valves are not moving at all?

[martino1972]

did you try to spray wd-40 on the intake boots(between carbs and cylinderhead) with the bike running,to see if the rpm's change ??

[350Mike]

No, but now I see why you mentioned it earlier.  Possibly cracked or missing O Rings between the boots and head?

[martino1972]

well,it will tell you if you got an airleak there,cause it strongly sounds like an airleak...

[birdmannn101]

Hi Mike,

I have the very same problem.  Been restoring this 350F from a parts bike.  I bench synced the carbs to 2mm with a hex wrench after turning the idle screw in enough to open the first slide, then adjusted the other 3 slides to the same 2mm setting, then turned the idle screw back down to "0" but didn't look to see if all the slides were closed at that time and not leaking air.  It idled high as soon as I advanced the throttle and didn't come back down.  So, tomorrow I will turn each carb down 1 full turn to try and close the 4 carbs down to make sure that the carbs aren't leaking air while I wait for my neighbor to come buy with the guages to set them to 16-24 cmHg.

Dan

[Kev Nemo]

well,it will tell you if you got an airleak there,cause it strongly sounds like an airleak...

I second that. I had this problem and went through the leak check-those inside carb boots can be a pain. Check them for seating

[350Mike]

Hey guys,

Finally got to work this weekend.  Pulled the carbs off again and then took off the insulators (Had to extract half the screws becuase they were stripped!).  No cracks, rubber looks to be in great shape, but the O rings between the head and the insulators are dead flat and hard as a rock.  Also, looks like someone tried some liqued gasket at some point.  I'm thinking this is where the air is getting in.  Any similar experiences with these O Rings?

Thanks again guys.

Mike 

[Trevor from Warragul]

Another thing to check is the choke mechanism.  On the CB350F the choke also opens the throttle.  If it is adjusted poorly the choke will appear to be off but will still be lifting the slides.