I bought my first bike, a '71 CB350, a couple weeks back. It ran well for the first 100 miles. I changed the oil and on the way home it began running poorly, coughing when I poured on the throttle. The oil change could be a coincidence, but then again, maybe not.
Did some research and rebuilt the carbs, hoping that would take care of it - but the bike is running just as poorly as it had before. I figure that it is running too rich because 1) the spark plugs are black even after a couple minutes idle 2) exhaust note is very "poppy" when running, and 3) choke air intake needs to be left wide open, else the bike dies immediately.
What is the solution? Do I need to adjust the floats? (But this doesn't seem to explain the sudden onset of the issue, as both floats are in good working order, with no leaks). Can anyone help?
Subject: Re: Rebuilt carbs but still running poorly
I've decided to work through the ignition/valves per Clymer 1) cam chain tensioner 2) valve clearance 3) breaker points 4) ignition timing. If that doesn't work, I might just throw a tantrum, and then start messing with the float adjustment - although when I rebuilt the carbs the measurements were as spot-on as my standard ruler could measure.
Does anyone have any thoughts before I spend my afternoon doing something that may or may not be a waste of time? Help? Please?!
Subject: Re: Rebuilt carbs but still running poorly
Hi Nathan,
Did you check the throttle slides? The diaphragms could be torn or have hardened from age. One way to check them is to lift the slide up through the throat of the carb, it should come back down all by itself when you let go of it. If it doesn't the diaphragm has probably hardened and is bad and will cause the slide to stick open. You will have to check the diaphragms visually for tears or cracks, these can keep the slides from opening properly. Also did you check the air cleaners? I have seen these cause poor running conditions when slightly dirty.
Good luck !
Tom
Subject: Re: Rebuilt carbs but still running poorly
Hi Tom,
Thanks for your advice. The rubber boots were in good order when I rebuilt the carbs last week, but I'll pull them again and recheck. Yesterday I set the floats really high (27mm on the right and then 28mm on the left) and it still runs very poorly - even worse than before - but at least now the spark plug tips aren't solid black after a few minutes running and are instead mostly tan. Now I'm wondering, though, if the engine is starved of fuel instead of getting too much.
I'll post an update after I get through the electrical system and check the rubber bits, but any further advice or ideas would be most appreciated.
Subject: Re: Rebuilt carbs but still running poorly
Nathan,
The factory specifications for the float height is 19 /- .5mm. If you had to set them to a higher number and it ran better it is probably running to rich. Another thing you could check if you haven't already is the battery condition. If the battery is low or in poor condition it could cause it to act like it is running rich.
Subject: Re: Rebuilt carbs but still running poorly
you might want to try the spark plug caps,,they should have 5kohms resistance ,,if you dont have a multimeter just buy new caps they are about five bucks each ,when they get old the resistance value can go up too high and cause missfiring and stuttering intermittently ,some times only when hot.the spark may look good but under compression it can go all weak in these conditions,go right over the contact points aswell ,the points may be fine but the condensors might be soggy if they are old ,,these are cheap aswell ,some auto ones can be used,,getting the ignition tip top first is easier than pulling carbs down over and over!! the float height isnt real important so long as it isnt really out..good luck.
Subject: Re: Rebuilt carbs but still running poorly
This is one of those things where you just need to comb through the ignition and carbs methodically and set everything just right. I'd start by checking all connections from plugs to coils to points and make sure they're all tight and clean. Then retension the cam chain. Then set point gap and spark timing. After all that is perfect, turn to the carbs.
Speaking of float height, I thought I'd clarify. Different carb models had different float heights. The model number is stamped into the side of each carb.
