Topic: CB350F idle question

[Scott S]

 Long post ahead, please stick with me!
I recently helped a friend get her first vintage bike back on the road; a 1974 CB350F.

 We bought the bike as a "runner" from a guy that says he ultrasonically cleaned the carbs and he used vinegar in the gas tank to remove rust. I'm not sure I believe the ultrasonic stuff and the gas tank looked like crap.
 We pulled the carbs and cleaned all orifices and jets with carb cleaner and compressed air. Used EvapoRust on the tank and rebuilt the petcock. The tank sat for a bit with Marvel Mystery oil in it and flash rusted pretty bad, so I used a POR 15 kit and lined the tank. I'm telling you all this to let you know that I feel confident the fuel system is now clean.

 After starting the bike up again, it was pissing fuel from the vent tubes. Pulled the carbs and found a dirty/bad needle valve. Soaked the needle valves over night with SeaFoam, sprayed through the carbs again, lightly sanded all float hing pins, etc. Replaced bad needle valve. Hooked them up to my auxiliary tank and they seemed to work fine.

 We've done a 3K mile tune up: valves, cam chain, timing, new Pamco, trimmed plug wires, new caps, new plugs, oil, filter, brand new battery, bench synced carbs, etc., etc.

 The bike will sometimes get a high idle, often after getting a little heat in it. "Vacuum Leak!" you say! Well, I've searched til I'm blue in the face and can't find one. Carb cleaner sprayed around carbs reveals nothing. Manifold boots are good, clamps are tight, etc.
 I've also checked the other usual stuff; throttle cable not binding, choke operating properly, timing is dead on, you name it.

 The bike is stock except for a MAC 4-1 (and they usually don't require re-jetting, plus it's sorta intermittent. Improper jetting wouldn't come and go, would it?) Bike pulls fine through the gears and up top. It just gets a hanging or high idle sometimes.

 Here's the head scratcher for me: The 350F says idle mixture screws should be 7/8 turn, + or - 1/8 turn. This bike likes them best nearly closed.....just somewhere between 1/8 and 1/4 turn. I can get it to idle better that way and it seems to stave off the high idle deal.
 The only other unknown is float height. I couldn't find specific heights for the 350F, so I used the 400F height of 22mm. I didn't change ANY of the float heights, just checked them. They were either dead on or so close that I didn't want to mess with them.

 Lastly, when I bench synced the carbs, I marked the idle adjustment screw with a Sharpie and opened the slides enough to use a drill bit as a feeler gauge. After syncing the carbs, I turned the screw back the same amount. I can reach under the tank with a long screw driver and push down on the throttle linkage (where the cables attach) and make the idle drop. Again....it's NOT the cable that I can tell. I can see the proper amount of free play on the cable under there before moving the linkage.
 Theory number 2: Is it possible that when I bench synced the carbs, I have the slides in such a position that even when "resting" they are open too much? I can back the idle adjustment screw all the way out when it's idling high. Would that explain why I have to go so far in on the IMS screws?
 

[Scott S]

 EDIT: In the first post, I used the term "hanging idle". That's most often used to describe an idle that's slow to drop back to normal. That's NOT what is happening here. It just goes high and stays there.

 I also cleaned, checked and lubed the advance mechanism. It snaps back nicely.

[MoMo]

Question-did you remove the emulsifier tubes when you cleaned the carbs? ....Larry

[Deltarider]

They were either dead on or so close that I didn't want to mess with them.

Wise.

Theory number 2: Is it possible that when I bench synced the carbs, I have the slides in such a position that even when "resting" they are open too much? I can back the idle adjustment screw all the way out when it's idling high. Would that explain why I have to go so far in on the IMS screws?

This is possible. Some here had this condition. A detail: the CB350 Four does not have IMS screws but air screws.

[Scott S]

 Larry, no...I'm embarrassed to admit that I did not. I knew I probably should, but I didn't have new O-rings for the main jets on hand and didn't want to disturb them.
 But that would mostly cause a rich condition or problems in the mid range, right?
 
 FYI, all the jets are genuine Keihin, by the way.

[Scott S]

They were either dead on or so close that I didn't want to mess with them.

Wise.

Theory number 2: Is it possible that when I bench synced the carbs, I have the slides in such a position that even when "resting" they are open too much? I can back the idle adjustment screw all the way out when it's idling high. Would that explain why I have to go so far in on the IMS screws?

This is possible. Some here had this condition. A detail: the CB350 Four does not have IMS screws but air screws.

 That's what I was talking about. Just used the wrong term. I knew they are on the air box side and therefore meter air, not fuel.

[flybox1]

Do the clear tube test for fuel level in the bowls.    and be sure to clean the emulsion tubes.
I'd bet you a six'er that you're too high, and that about 24mm is the float height you'll need.
I had a LONG discussion with Hondaman when I first had my 350F, and it was only after being urged to do the clear tube test that my carb leak, high idle, and off idle throttle issues went away. 
21mm, like the shop manual and clymer say, is too high.
For my bike, 24mm got the fuel level at 3-4mm below bowl gasket seam.

[flybox1]

I just re-read your first post. what i said above still stands, but also....youre lean!
you have a mac 4-1.  its not stock, so you cant assume A/F mixture doesnt need attention.
you'll need to go up one size in your pilot jets(38) , and up to 78/80s on your mains.

-check that all main jets (#75) have good o-rings on them, and the butterfly clips to keep them seated.
my WOT was lean, so i went to #78 mains - 4->2 jardines was the only mod, you'll need to go 78/80's

-twist out the pilot jets(#35) and check the tiny holes.  with your 35's in there, you're lean at idle, which is why your bike likes the air screws turned in to decrease the air at idle.  i bet you'd be really close with #38 pilots.

-remove the air screws and blow carb cleaner/air thru them.  Air screws at 7/8 turn
dont start it without a new D8EA plugs.  you'll want a good sample of idle deposits to clue you in to mixture here, but as i stated, your lean...they'll probably come out white.

[pamcopete]

scott,

I have had some luck as well with bench syncing the carbs on my CB750C, but it doesn't hurt to double check the sync with the gauges. What you have sounds like one of the carbs is just on the edge of working or not working and when it decides to work, the revs go up. The other way to check for this possibility is to spray some water or Windex on the exhaust pipes after the engine has warmed up at idle speed to see if all four pipes are hot. If one is cold, that's the carb that will "wake up" suddenly and drive the revs high.

It also wouldn't hurt to check the advancer springs to see that they are not loose.

[Scott S]

  Thanks all for the suggestions. I think flybox1 was on to something, plus I found a couple of things when I pulled the carbs.

 I set the floats to 24mm, per his suggestion. I propped the carbs up so that the floats *juuuuust* touched the needle valves without compressing them (I don't have the stuff to do the clear tube method).


  I also took this time to pull the emulsion tubes (all nice and clean already), check the mains and the pilot jets.
 The floats looked good when tilted like that, but when I turned the carbs upside down, I noticed this one float....



 First thing I did was put it in the sink and make sure the float was good. It was, so I checked the needle valve. It seemed OK, but I took one from my parts box, replaced it and reset the float on that carb. Bingo!



 I also took this opportunity to bench sync them again, this time using a smaller bit. This enabled me to actually have some "travel" on the idle adjustment screw. I set the air screws to 7/8 turns and put the carbs back on the bike.
 That got it! I have a nice idle at about 1,100 rpm. No more off idle stumble. Bike runs the best it has since we got it and started working on it.

 And to just address a few of the other suggestions: Advancer springs are good and it snaps back nicely. The Pamco timing is dead perfect (Thanks, Pete!). New plugs during the freshen up.....less than 30 miles on them.
 I REALLY don't think I need to re-jet now. It performs well, but I'll certainly be keeping an eye on it and the plugs as we put some miles on it. Plugs look great so far.
 I ended up right at 1 turn out on the air screws with maybe the thickness of the slot one way or the other on a couple of them.

 Thanks again guys! And since everybody likes pics, here are a few of the bike in question:

[MoMo]

Nice looking bike and good work on the repair...Larry

[HondanutRider]

At the start of this post you claimed the bike was a 1974 CB350F.  From your last pictures it would appear not to be, but rather a 1972/73 model.  The paint and side emblem are the wrong colours for the 1974.  (Not that here would be any differences in your carb tuning.)

[Scott S]

 I agree  that the color seems wrong, but the VIN indicates 1974. Sort of a head scratcher and a topic of discussion here, too!

[flybox1]

Nice work. 

[HondanutRider]

I agree  that the color seems wrong, but the VIN indicates 1974. Sort of a head scratcher and a topic of discussion here, too!

The VIN sticker on the headstock should include a date of manufacture, given as month and year.  The 1974 models may have started manufacture late in 1973.  The build date of mine is 5/74 and is definitely a 1974 model. 

The frame serial numbers for 1974 begin with "CB350F 200..."  I believe the 72/73 begin with "CB350F 100...".  The engine numbers are similar (but not the same/matching at the end of the numbers) except that they have an "E" after the "CB350F".

[Scott S]

 Build date of 12/73.

[Scott S]

 OK, we have about 100 miles on the bike since the above "repairs". When we got home from a 75 mile ride today, the idle was a little funky again. Not as bad as before, though.
 I started smelling gas in the garage again and the overflow tube on #1 is damp at the end. That's the carb that had the sinking needle valve.

 I've had issues with two of the needle/seat valves now, one of them twice. I've "repaired" it with used parts from my CB500/550 parts bin. I think it's time for NEW needle valves and/or seats.
 I know the Keyster kits aren't that great. What do you recommend for needle valves? And should I use the stock Keihin seats? Or get all new needles AND seats?

[HondanutRider]

If by "needle valve" you are referring to the float valves, then replacement ones are available separate from a complete carb kit and should work fine as long as the float heights are set correctly.  I replaced mine a number of years ago to cure a leaking/overflow condition, as the original seats were worn, although I understand you can hone the worn ones as long as the seats aren't rubber.

Regarding the other brass parts (ie. jets and throttle needle), they should clean-up without replacement, as long as you don't damage their orifices or scour the profile of the throttle valve.  One thing that will undoubtedly require replacement though are the O-rings.  There was someone on this forum who sold a correct complete set.

[flybox1]

these are keihin replicas, not keyster.
nothing but good things to say about the folks at 4INTO1 
http://4into1.com/replica-keihin-float-needle-valve-assembly-16011-329-004/

[Scott S]

 Flybox1, you ROCK! I'm ordering those tonight!

 And, yes...all I need are the float valves. All other parts are fine.

[Bodi]

Those are probably the same as Keyster sells, from the same machines in the same factory.
For peace of mind (and after much grief with aftermarket carb parts) I use OEM Keihin parts if available. They cost a lot more, but re/re time on a carb bank is not fun.