Topic: '72 CB350F Choke Symptom

[Evan]

I recently bought a 1972 CB350F.  With the choke off it won't start.  It will run fine all day with the choke on, but if I close it, even after it is warm, it will sputter and and die. 
I seem to be topping out at 60 MPH and getting around 32 miles per gallon.  Should I be happy with that from my bike?
From what I've read here it sounds like I should clean the carbs.  Any other suggestions?
I am considering buying a carb rebuild kit, but I have read a few threads that warn against cheap kits and I might be better off cleaning the originals.  Any thoughts?

[hoodellyhoo]

Cleaning the carbs is pretty much a requirement on these old bikes unless you know it has been stored or maintained properly. When you do, make sure you check that all the tiny jet holes and passageways in the carb body are clear.

As far as cheap carb kits, I bought some and just ended up using all the gaskets/o-rings in them. Unless your float valves don't work or your needles are badly worn you should be fine with the original honda parts. If you want, I think you can still get carb gasket sets from honda.

Once that's all done, make sure you do a full tune-up.

On questions such as this it is also helpful to know what your intake and exhaust setup is

[CrashBar]

You will want to pull and clean the carbs.   You should get above 80 without a problem (except maybe the cops) and get mid-40s MPG.   I used the aftermarket carb kits and had good luck.  I didn't break the 4 carbs apart, just pulled the float bowls, floats and jets and blasted with Carb cleaner.   Setting the proper float height when you reassemble is critical as well.

It's more of a job getting them off and back on than it is to clean/rebuild them. I found it easier to pull the rear fender, and then the airbox, and then the carbs.  It is still a tight job.   Good luck

[Evan]

Hoodellyhoo,
I have the stock intake (just replaced the air filter), and close to stock exhaust except two into one pipes.  What exactly would you consider to be a full tune up?  Float level adjustment, idle mixture, idle speed, synchronization and throttle linkage? (that from my Clymer manual)

Crashbar,
Would you say that if I pull the carbs out as an assembly, open them up and blast them with carb cleaner, I should not have to mess around with synchronization?

I have no experience as a mechanic and do not want to get in over my head, although I am eager to learn.

[flybox1]

do you have 4 into 2, or 4 into 1 exhaust?

Yes, low 30's mileage is poor, and you should easily run faster than 60.

Step 1. pull and clean your carbs.  ensure the float valve, idle and main jets are clean and clear.  set your float heights to 22-24mm.
I had no experience with carbs before my 350F. you wont get in over your head.  trust me.
you dont need to separate the bank of carbs, just remove the bowls, and carefully lay each piece out on a towel after you clean them with carb cleaner.  do one at a time.
and you really should bench sync your slides while you have the carbs off.  get them all close.

Step 2. as mentioned above, do a full tune up.
Points/condensers/timing/valve adjust/oil and filter/air filter, and vac sync your carbs.

report back to us 

[flybox1]

BTW, for these carbs, choke ON is in the UP position.  OFF is DOWN.   
start your bike with it in the UP position, and slide it down as it warms up.
down all the way when warm.

[hoodellyhoo]

Another thing to do is set the cam chain tension. The cam chain tensioner is really the weak point of the 350/400 fours. The procedure in the manual no longer works because the spring in the system is so old. If you do a search here you will find many posts about how to do it properly. One of these days I'm going to make a youtube video about it.

I wouldn't worry about getting in over your head. We are a very helpful bunch on this site and are willing to help people though anything. 

[Evan]

OK great.
Actually the fact that the choke is on in the up position is very helpful.  That means that the choke is working fine. (The arrow engraved on the choke lever is a little misleading).
So I will attempt the full tune up as suggested and check back in with more questions.
Thanks so much everyone.

[Evan]

Oh, I have four into two exhaust.

[camelman]

I had trouble getting my 350F to run consistently.  I had to take the carbs apart twice to clean them out because junk from the tank was getting through the cheap fuel filter I put on.  So, I highly recommend putting in a good fuel filter before trying to run the bike.  You will also need the entire air filter assembly in place with the plastic cap over the top of the air box.

Make sure you clean the emulsifier tube.  It is below the main jet (the one in the center of the carb).  It may be hard to get out, but just work at it with something that won't damage it.  The tube has holes in it that need to be clean enough to see through.  You should also clean out the hole that the tube slides into.  Bench syncing is critical.  If you do not get the carbs close, then the engine will run very badly.  Make sure you sync the throttle plates and the choke plates both.

If you pull the carbs apart after running, then the float bowl gaskets may stretch a bit.  I had this problem and trimmed a section out so they would fit in the groove.  I then used blue RTV from the car shop to seal around where I cut the gasket.  It hasn't leaked on me yet.

For removal, just unhook the throttle cables, unscrew the clamp rings (including the one to the airbox), pull the oval tube that transitions from the airbox to the plenum, and then remove the plenum.  Once the plenum is out of the way, there is plenty of room for the carbs to slide out the right side of the bike.

Camelman

[fmctm1sw]

One of the problems I have with my choke is that if I adjust them to where the butterflies are fully closed at full choke, they don't quite open fully.  Adjusted for full open and 3 and 4 don't quite close the whole way.  I opted for full open and it may be why the bike is hard to start when it gets colder.  Just thought I'd throw that out there, I'll have to loo at it next time I have the carbs off...

[flybox1]

Camelman
+1 on the emulsifier tubes/choke plates.

Evan,
if the holes on the sides of the emulsifier tubes are clogged, soak it in carb cleaner, then use a tiny copper wire to clean out the holes.  the choke plates have adjustment screws/linkages.  adjust them so that all plates fully close, and move up and down in unison.

your bowl gaskets might expand when they come in contact with gas.  let them air dry, and they should return to normal.  they sometimes can be a pain to get seated.

you will hear a lot of +/- on an inline fuel filter.  many here swear by them.
my thought is if you can ensure your tank is clean of all rust/debris, you rarely will have to clean your carbs.  an inline fuel filter doesnt really fix the problem of a dirty tank, and it can cause fuel flow issues down the road.

I have a 4 into 2 exhaust and went to #78 mains to get higher rpm/speed.  You might not need to do this once your tune-up stuff is done. #78's 80's and 85 mains are available. although i do not ride at high speeds/rpms on this bike, 70+mph is easily reached when needed, and average 40+ mpg.

enjoy working on your 350F!

[Evan]

Update #1
I was able to remove the carb assembly without too much trouble.  I have the main and slow jets off as well as the float valves and am in the process of soaking them in carb cleaner.  I just read through all of your responses again and see now where the emusifier tubes should be located.  I will get those out and clean them too.
Thanks everyone for your posts.  I have found them to be invaluable so far.

[HondanutRider]

Sounds like you are past this stage now and I don't know if this has ever been mentioned before, but I have found removal of the main jet and the float valve often to be  a real PITA.  The problem has been that on these carbs they aren't screwed-in but rather sealed with an o-ring that often gets gummed and stuck with age.  Trying to extract a "frozen-in" jet or valve with pliers leads to a badly distorted piece, although the actual working part should still be OK.  I have found if I first tap it a bit (with a drift) on the top and sides, it will come out easier, as the tapping seems to break loose the frozen parts.  Just don't whack it so hard as to distort the valve or jet, as they are brass material and can be damaged that way.

[Evan]

Update #2
I got all the jets out and all cleaned up.  I adjusted the floats to 21MM (according to my Clymer book).  When I put everything back together and turned the petcock valve on, gas started pouring out of the float bowl overflow tubes.  (Leads me to believe that I have the floats adjusted too low.)
I started it up anyway and after it was run for a while the gas leak was down to a drip.
I noticed a few things after my test run.
1.  On first start up, having the choke on/up (valves closed) didn't seem to help starting up, only when I opened them up did I get her to start.  Not sure if this was a choke problem or related to the first start after draining all the gas out of the system.
2.  My top speed seems to have increased to 70 which is good.
3.  On the deceleration I am getting small backfires.
Any advice?

[camelman]

Sounds like you might have missed some passages in your carb.  There is more to clean out than just the stuff inside the bowl.  I should have listed this out for you before, so here goes:

1) remove bowl and jets
2) spray carb cleaner through each hole in bowl area
3) spray carb cleaner into each opening in the front of the carb (there are holes in the venturi - where the air flows)
4) spray carb cleaner into the fuel line until you see some flow into the bowl location
5) repeat for each carb body individually
6) WEAR GLASSES/GOGGLES when you do this
7) Bench sync the butterfly valves, or it will run like poop

The choke really isn't needed much on these bikes.  They are pretty warm blooded in the stock configuration.  You will need a little choke, but probably not full choke.  When the carbs are clean and adjusted correctly, it will need a bit more choke.

Fuel ran out of the overflow tubes because the float valves don't seal well on these bikes if they get dry.  When you put the carbs on (or if you ever run out of gas) you need to tap lightly on the side of the carbs once fuel starts to flow into them.  This unsticks the floats and lets them seal.  My bike does the same thing with freshly rebuilt carbs.
You mentioned the carbs are still dribbling a bit of fuel. That could be from high float levels, or it could be from grit in the fuel that got stuck at the float needle seal.  Make sure to clean that area when you pull them apart.

I highly recommend pulling the petcock apart for a good cleaning.  A lot of junk collects in there.  You don't want that crap getting into your carbs.

Good luck.  I imagine it will run well after this final cleaning.

Camelman

[Evan]

Thanks camelman.
I will get back in there.

[flybox1]

Evan,
i concur with camelman. clean them again and pay particular attention to the float valves and overflow tube stems in the bowls.  any have cracks.  fill the bowls with water and check to see if it drips out the overflow tubes.

Clymer book is off on float height.  I went with 24mm on a suggestion from a seasoned vet here.
I've had no issues since and good fuel height(see test below).

what is the setting of your idle screw on startup?

when you bench sync your carb slides (not butterflys), you should first, before any adjustment, adjust your idle screw to touching +1/2 turn.  then bench sync your slides with a 1/8" drill bit.

once you reset your float height and reinstall your carbs, use the method below to check if float height is accurate.

First. take out your bow drain screws so your bowls drain, then head down to your local hardware store and get a 2' section of 1/8" clear tubing. $1
cut it in half.
get a candle, or hold the tubing over a burner to heat it up.  youre gonna want to heat stretch the tubing in the middle so it thins out.  cut the tubing at the smallest point, and screw the small end into each of your 4 bowl drain screws.  bend the tubing up so it sits next to the bowl gasket seam and secure it there.
when you open the petcock the bowls will fill up and hopefully wont overflow this time.
the level of gas in the tube should be 4mm below the gasket seam.

[camelman]

Oops, good catch.  How un-SOHC4 of me to mention butterflies instead of slides.   

Camelman

[Evan]

Update #3
I got the carbs out again and blasted everything as suggested again.  Adjusted the floats to 24MM and the air screws out 3/4 of a turn.
The test run was sucessful.  The idle and low end are running very well now.  I am still around 70 MPH top speed.
This weekend I will finish the rest of the tune up and check back in.
Thanks everyone.  Very helpful.  I am having a great time with it too!

[fmctm1sw]

Adjusted the floats to 24MM and the air screws out 3/4 of a turn.

Thought it was 21mm?

[camelman]

Evan,

It sounds like there might be other issues.  I can get my stock 350F over 90MPH pretty easily.  Have you checked your ignition system?  I think your timing advance unit is not operating.  I recommend pulling off the points cover and fiddling with it to see if it moves freely.  Be careful not to force anything though.

Camelman

[Evan]

Camelman,
I did get new points and condensers, so my next project is to get those replaced and do the timing.

fmctm1sw,
I tried 21MM and it was definately too low, feul spilling out of the float bowl relief tubes.  I went back to 24MM on some other advice and think that is where I need to be.

[camelman]

Evan,

The points and condensers will make a difference if the current ones are bad.  The advance is a separate part within there though, and it makes a huge difference in the higher RPM.  You'll see it when you get to your points.  Follow the directions when setting your point gaps and dwell too.  You can lose a lot of power to poorly set up point gaps/dwell.

Camelman

[Evan]

Last weekend, when I removed the cover to the points and condensors, I saw that there is an electronic ignition kit installed (Marakell or something?). 
I have read a few strings on systems such as the Dyna S, and it seems to be OK if not better to use the electronic ignitions.  I did already buy new points and condensors, so I guess the question is:
1.  If I keep the electronic ignition system, do I have to set the timing again?
2.  Would I be better off removing the electronic ignition syatem and going back to points and condensors and finish my tune up?