Hi all,
Long time stalker, first time poster. I've been working on a CB350f that had been running rich throughout the rpm range and I finally figured out what it was. I'll give a history of the bike, what I had done, the symptoms and then what FINALLY fixed it. I hope this helps someone else who has these problems.
Bought a cb350f basket case. Motor was locked up and the valve seats were worn. Got a used set of cylinders with much less pitting, honed them and put new rings on the pistons. Cut new valve seats and lapped the valves. Installed a new electronic ignition.
Engine went together smoothly. Cam timing lined up perfectly and the tensioner snug everything into place. I cleaned and rebuilt the carbs with brand new stock sized jets, emulsion tubes and needles. I am using the stock airbox and a 4 into 2 exhaust.
The bike tried to start, but just wouldn't run right, it was only firing on 2 cylinders. After some testing, it turned out that the brand new ignition was only firing on 1 and 4, not 2 and 3. Got a replacement ignition and the bike was now running on all cylinders but running terribly.
This gets us to most frustrating part of getting this bike running.
The symptoms were a high idle 3k to 4k rpm, it would die below that, blue smoke out of the exhaust, would not rev well below 5k and was fouling plugs almost immediately. The idle mixture screws had almost no affect on how the bike ran at idle, I'd get to almost 2 full turns out before the bike would run right.
I focused first on getting it to idle right.
Everything pointed to it running very rich, but there's very little info about that so everything I read said it was a lean hang for the bad idle. I replaced the intake boots with brand new ones and new o rings. Same symptoms.
I replaced the pilot jets with one size up and it ran even worse. I couldn't even get close to 7/8s turn with the idle mix screws, it was still 2 or more turns to get it to run close to right. I should have known better from how far out the idle mixture screws needed to be that it wasn't a lean hang, but I wanted to eliminate every possibility. So I put the stock ones back in.
The plugs were all sooty, covered in carbon. There had to be a reason for it. I read a post about valve guides being worn out being a possibility for running rich and fouling plugs, so I took the head off again, got new honda guides for intake and exhaust. I measured the valves and reamed the guides to be .0008 bigger. Installed the head and there was slightly less smoke but it still ran incredibly rich at idle.
I then took the carbs back off, adjusted the floats to be lower than stock by a few mm in case fuel was getting into the engine, but not hitting the over flow. No change.
At this point, I put a bung on the exhaust and hooked up an O2 sensor.
I took the needles and lowered them to the the top notch and took the bike for a ride with the O2 sensor on. With the help of the sensor, I could get the bike to idle around 11 to 12:1, but when you twist the throttle, it would bottom out the sensor, reading flat 10:1 because that's as low as it would read. All the way up to 7k or so before it would lean out to 12:1 with at least 3/4 throttle.
I had rebuilt the carbs to spec, bench synced them myself, but maybe the slides were backwards? Why else would it be dumping fuel at such a low rpm? I took the carbs off and cleaned them while I was in there.
THIS IS THE FIX I FOUND. When cleaning, the emulsion tube simply fell out of the carb I was working on. I got out my micrometer and measured where the brand new tube seats into the carb, it was .192 thousandths. I got out the original honda one, it was .195. So I swapped out the new ones for the old and they all went into the carbs snug, no falling out.
I put the carbs back on and the bike ran like a top. Idles at 13:1, revs through the whole range between 12:1 and 13.5:1. The idle screws are now very close to 7/8 turn out. Moving the needles and adjusting the mixture screws have an actual affect on the bike.
Lesson is, check the new parts against the old ones. Most of the new parts from the kit I used have been swapped out for the original stuff. I should have known better when the bowl gaskets didn't even come close to working, but it was a reputable store so I was hoping it was just a fluke.
Hope this helps someone that may have the same problem!
