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Discussion Starter · #1 ·
The left cylinder has been giving me trouble. This is a craiglist buy from a group of old fellas who just restore bikes to a running state and sell 'em for fun, so it wasn't spectacular but it certainly felt fine on the test ride, and some short rides after.

But having only changed some suspension and working on a seat pan (for a short rider) I'm now not getting a strong engine like I was. The bike did sit in the cold from January when it was bought, and I didn't make the proper steps to winterize. I haven't got a clue how old the various gaskets are, but I was made aware that the carbs were off and cleaned (the guy said it took a few days in the ultra-sonic cleaner to get 'em good.)

When I started tinkering, I found the left cylinder wasn't firing unless the throttle was opened, so I figured it was too lean at idle on that side. I've since been trying to adjust it and having no luck. Recently, white smoke started appearing on the left exhaust which I believe is tell-tale signage of an oil leak in the cylinder. I also pulled the spark plug and found it wet and smelling of gas. It's also holding high rpms after I open the throttle and let it fall back, and generally it continues to rise until I shut the engine off.

Tear down the top end? Rebuild the carbs? I'm bummed because my girlfriend is at this moment on day 1 of her MFC and doesn't have a ride when she completes it tomorrow. :(
 

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Unless you put ethanol free in it in January 5 months is long enough for gas to break down and probably clog something.

Your conclusion of it being too lean doesn't make sense to me. If it's too lean adding more air by opening the throttle should make it worse or just kill the engine (if you choke it and it improves then it's too lean). More throttle shows too rich, because when you add air (throttle), the mixture is sufficient again for combustion and it fires. The gas soaked plug also supports too rich, as well as white smoke (unburnt excess fuel).

The holding high rpms sounds like a vacuum leak somewhere as well but after checking your carbs out reset all your carb adjustments and see if the symptom persists (not sure how on a cb350)

Also oil burns blue, your top end is fine. Someone who knows cb350 carbs will tell you what to look for in addition to drying out that spark plug
 

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Discussion Starter · #3 ·
Thanks for the reply - that info makes me feel better overall.

We've been running it on and off the last few months, just not riding it anywhere. The bike had some gas in it when we bought it, and put fresh fuel in as necessary and a little bit of seafoam to help things along.

I've been trying to follow some guides regarding "synchronizing" the carbs on the bike just using exhaust pressure and my hand (not ideal, I know) and that left cylinder is pretty much always under performing the right. I lower the idle, run the fuel screw in almost all the way, and it doesn't seem to change. It definitely benefits from the choke being opened, sometimes even causing the RPMs to take off and not slow down.

Is there a better way to check from vacuum leaks other than WD40? I sprayed the top of the intake boot at both ends (engine and carb end) and saw felt/heard no change in the RPMs.
 

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Did you turn off the petcock before you parked it for the winter? If not you likely have gas in your oil in the crank case now.
Check your oil and if it smells of gas, change it ASAP.


ALWAYS turn off the petcock when not riding.

I agree that the "holding high RPMs" is a classic sign of a vacuum leak.
 

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Well that sounds less likely to have clogged just by old gas. How is your tank inside? If it's rusty it could be clogging the carbs that way, if google is your friend because there are many ways to do it and you'll find one you like/have supplies to do. I used metal rescue and it worked well, just be prepared for flash rusting (for all methods really).

I did the wd40 way and it's a very slight increase (and not exactly nice having your bike rev at 3000 just to test).

People recommend carb cleaner, starting fluid or an unlit propane torch (use the stream of gas). Carb cleaner on rubber is a bit sketchy, both are very flammable so be careful

This guy's website is really helpful as well http://www.dansmc.com/mc_repaircourse.htm
 

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Discussion Starter · #6 ·
Did you turn off the petcock before you parked it for the winter? If not you likely have gas in your oil in the crank case now.
Check your oil and if it smells of gas, change it ASAP.


ALWAYS turn off the petcock when not riding.

I agree that the "holding high RPMs" is a classic sign of a vacuum leak.
Curious - how does that come about? I think it's due for an oil change anyway, and plan on doing it but will do so sooner rather than later.
 

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The carbs on these old bikes are notorious for leaking fuel into the cylinders which will then drain into the crank case and dilute your oil. Do not start the engine until you're sure the oil hasn't been contaminated.
 

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Discussion Starter · #8 ·
https://imgur.com/a/eaBrfBa

Don't you have those moments where things are bad and you can't figure out why, but then you wait a day and come back with a calm, more clear head and reset things and it all just works better? Yeah, that's today. But there's still too much fuel in the left cylinder. I've WD40'd the intake boots on both carbs with no change, have the right cylinder fuel screw to 1/2 turn out, and the left fuel screw to full-in, and I'm still getting tons of fuel in the cylinder, coming out the exhaust both as a liquid and burning into white smoke.

EDIT - also, is that hole in the muffler meant to have a drainage hose on it or something? It doesn't have much room to attach anything, but it's where a lot of fuel/carbon is dripping out.

I'll be checking the oil once the bike cools down and probably just flushing it anyway to be on the safe side. Perhaps maybe it needs a valve adjustment?

Thanks for all your replies.
 

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Sounds like the float is adjusted wrong, there is garbage causing it to stay open or the rubber tip is messed up. New fuel will eat the old rubber used in these carbs. Pull the float bowl and have a look.
 

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First off terminology: You "close" the choke to start a cold engine. The choke is "open" normally when running a warmed up engine. Not sure what you were saying a few posts back when you said it ran better as you "opened" the choke.

The drains on the bottom of the mufflers don't need hoses. They just drain water which is a byproduct of burning gasoline. As the bike warms up (and the mufflers warm up) the water should exit the mufflers as vapor.
 

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Discussion Starter · #11 · (Edited)
Sounds like the float is adjusted wrong, there is garbage causing it to stay open or the rubber tip is messed up. New fuel will eat the old rubber used in these carbs. Pull the float bowl and have a look.
I took the carb bowls off and found the left carb floats to be floating higher than the right carb, so I bent the tang to allow the float need to shut the gas flow sooner - maybe that will help. I know everyone ****s on the Clymer manuals around here (and I'm not a fan, too, though this is the first bike it's been so bad) but it has the float height at 26mm. Anyone have better specifications and/or the best way to measure while the carbs are still on the bike?

Also, neither float needle had any rubber in them whatsoever - perhaps it was sticking inside the float needle intake?

First off terminology: You "close" the choke to start a cold engine. The choke is "open" normally when running a warmed up engine. Not sure what you were saying a few posts back when you said it ran better as you "opened" the choke.
Okay, so maybe I have the choke in the wrong position? When it's down, it clearly says to "Open -->" which is moving it forward and up. What's the running position vs starting position?
 

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+1 on checking for gas in the oil.

When I got my bike it was blowing a lot of white and/or blue smoke and the oil was about 1 quart overfilled. It did not smell like gas to me but I changed the oil and the smoking stopped immediately, I think my oil was thinned out with gas from an open petcock from the PO.
 

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Discussion Starter · #13 ·
+1 on checking for gas in the oil.

When I got my bike it was blowing a lot of white and/or blue smoke and the oil was about 1 quart overfilled. It did not smell like gas to me but I changed the oil and the smoking stopped immediately, I think my oil was thinned out with gas from an open petcock from the PO.
That's my next step. Should I clean up the filter and such this time around, or can that be done while there's oil in the crankcase? If I have to order new gaskets I wanna do it right away.
 

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That SHOULD have been your first step IMO. Oil that has been contaminated with gas will not be very good for engine lubrication and this can cause a lot of damage (sometimes severe even) to your engine if you start it.
 
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Okay, so maybe I have the choke in the wrong position? When it's down, it clearly says to "Open -->" which is moving it forward and up. What's the running position vs starting position?


Actually when the choke lever is pointing down the butterfly is open which means the choke is open. When the choke lever is up in the horizontal position it means the butterfly is closed and choke is closed.

 

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Discussion Starter · #16 ·
Actually when the choke lever is pointing down the butterfly is open which means the choke is open. When the choke lever is up in the horizontal position it means the butterfly is closed and choke is closed.
That clears it up, thank you!
 

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Discussion Starter · #17 ·
That SHOULD have been your first step IMO. Oil that has been contaminated with gas will not be very good for engine lubrication and this can cause a lot of damage (sometimes severe even) to your engine if you start it.
Had I known it to be an issue I would have made it my priority. As it stands now, the gas tank is off with the exhaust, so there's no running it at all until the oil is changed.
 

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One thing about setting Float Heights.

The TIP of the Float Needle is SRPING LOADED.
That is why you MUST hold the carb at an angle until the TANG just touches the tip.

Holding it full upside down the weight of the float will compress the tip and cause an incorrect setting.
 
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