Topic: Cb350f Carb fun, and low speed/no fire on one cylinder

[davebaker]

So, finally registered, inspected, and all that, I decided to take the bike for its first real spin down to fill the tank. No odd noises that I could find, but cruising around 50mph, I felt an slight surge. I'm not sure, but I think I read somewhere that this is referred to as "hunting"? Then, when I found a nice stretch of highway, she would cap out at full throttle around 60 to 65. Conferred with my HD driving father, and he feels it definitely should go faster. There's no airbox on the bike, so maybe that relates.

Replaced the seals on the carb drain plugs (one seal was missing entirely), and cleaned them. Two were pretty clean, and the center two were really clogged. Cleaned and replaced, then fired the bike up. The center exhaust pipe on the side of the horn felt cool. I thought maybe I hadn't waited long enough for the pipes to warm, but slightly burned myself touching another pipe, so I think a piston might not be firing.

A local buddy said it sounded like a bad carb, so I went and bought some chemtool and came home in the failing daylight to begin to remove the carbs. Got the carbs out, and removed the bowls on the center carbs, since they seemed their drain plugs seemed to be the dirtiest and might relate to the piston problem.  The bowls were extremely clean! Hardly any gunk. The rings look new.

While scratching my head at this (since I was certain they were going to be filthy), the o-ring on one of the bowls came off, and I found that it appeared to be held in place by a clear jelly material. When I tried to fit the ring back, it doesn't appear to fit to the groove. There's about a quarter inch too much ring!

Now I'm new to bikes, but I would think the rings should fit rather snug in the groove. Conferred with the father, and he agrees, but I thought I would come and give you good folks an update on my bike and listen to your thoughts.

Do o-rings run slightly large, and I need this clear gel to hold the thing in place? Should I buy a set of rebuild kits for the carbs and strip them down completely, despite the clean look of the bowls?



Oh, and Happy New YEar guys!

-D

[Gordon]

If you search this forum you will find literally dozens of topics with many helpful answers regarding this same topic, but here's a synopsis:

A cold pipe at idle typically means a clogged idle jet or idle circuit in the corresponding carb.  There are no shortcuts, the carbs and all jets need to be thoroughly cleaned, soaked, blown out with compressed air, and generally poked and prodded to ensure all of the tiniest of holes and passages are completely clear.  The clear jelly-like stuff used to hold the bowl o-rings in place may be the culprit, or they could just be dirty from varnish or other crud. 

The air box and filter need to be installed before making any running engine diagnosis as the bike will run lean without them in place.  This could easily be the problem with your lack of top speed. 

Most aftermarket o-rings will swell when soaked with fuel.  Dry them out and try again.  There was a recent thread on various ways to hold them in place while installing the bowls, but the concensus was pretty much whatever you use, use as little as possible because it will dissolve in the fuel and end up re-clogging your idle jets.

And check your tank, fuel lines, and filter to make sure nothing in them is gettin into the carbs and causing problems. 

[davebaker]

Thanks Gordon. I had been looking in Faq on the carbs, which is how I got them off the bike, but for whatever reason hadn't thought to doa search on the cold pipes.  I think I remember something about the adhesive for the carbs, but maybe I'm just imagining things. Thanks for the quick rundown.

-D

[Trevor from Warragul]

Sometimes a cold pipe is caused by no fuel is getting through - could be as simple as a bit of crud blocking the float valve or a sticking float.  With the fuel tap turned to "on", undo the carbie drain screws one at a time - if no fuel is getting through then one bowl will be empty.  A simple and often effective solution is to get a long, thin piece of wood, hold it against the offending carbie, and give it a few taps with a hammer - voila, fuel starts to piss out of the bowl!  I have a 350/4, and this has worked well for me in the past.

[davebaker]

morini,

Just thinking here, but shouldn't it be sending gas out someplace though if it were blocked? Like the overflow tube, or into the valves, or something?

I've still got the carbs off, and I think I'll go ahead and thoroughly clean 'em. I don't want to, everyone round here seeming to think it's a huge pain in the rear, but I think I should go ahead.

[750goes]

if you really want to be confident about the bike performing and running better, then you need to go the whole nine yards and clean those carbies... at least then you know thats one job done, and you learn a lot in the process.

[Gordon]

morini,

Just thinking here, but shouldn't it be sending gas out someplace though if it were blocked? Like the overflow tube, or into the valves, or something?

I've still got the carbs off, and I think I'll go ahead and thoroughly clean 'em. I don't want to, everyone round here seeming to think it's a huge pain in the rear, but I think I should go ahead.

If the float valve is stuck closed, there would be no fuel to go out of the overflow tube.  The bowl would be dry. 

The main thing that's really a pain in the ass about cleaning the carbs is not doing it right the first time.  (or second, or third, etc...)  It's the repeated removal and re-installing of the carbs that becomes the biggest problem. 

[schneider419]

hey y'all i've got a 72 350f, and had the carbs off a go-zillion times. (a lot) i found a easier, faster way to take them off. the problem is, when you get all the carbs, and airbox loosened up, there is no room to get it all out of the way to get the carbs off. what i did was take the connector out of the aircleaner box that goes in between the plenum and air cleaner box. take the rubber seal off of the connector, put the seal back in the air cleaner box. then use some honda shine or other lub., and put some on the outside of the connector. push it through the seal and into the plenum. put the air cleaner back in the box. when you put the spring back in that holds the air cleaner in, it will push the air cleaner into the seal and seal the whole assy. together. next time you pull off the carb rack, you just pull the connector tube back and there is lots of room to get the carbs out. it saves time, and you don't have to make up new swear words to get the job done. i'm a small guy and i fell in love with the 350f. lots of parts on line and they look sooo cool.
                                                                                mike

[Gordon]

Any update on this one?  Just checking.

[davebaker]

Cleaned out the gunk and rebuilt the carbs. Put the Carbs back on, and she starts and purrs along. Allow me a moment to cheer.  Checked the pipes, and they all warmed.

I've ordered air filter/pods to correct the airbox issue. I may still buy the stock airbox later, but I went with the shiny crome reusables this time around. They're being shipped UPS ground, so should have them in a couple days.

I've begun replacing the spark plugs. The two I've pulled were sooty black. The other two have a 3/4 radius nut on them, and I've still to find the socket so I can pull them.

I'm replacing the oil and filter tomorrow/later today while I wait for the pods to arrive. 

[Gordon]

Glad to hear you got them cleaned out well!  

With the pod filters, chances are you'll still have the lean running condition since they breathe much better than the stock air box/filter.  Give it a try, though, and pull the plugs to see what they look like.  Then compare to how it runs with the stock setup.

[750goes]

Excellent result, makes you feel all warm inside... you have passed your initial carbies sohc'ers ritual...

Anything mechanical with fuel passing throught it, irrecpective of age and condition will need some sort of maintenance at some time, but without being fanatical about polishing the idle screw everyday, most things on this age and style of bike is pretty basic and don't ivolve a huge amount of brain power...thats why I got  one anyway...you just have to use logic...
now where did I put that logic

well done.....