A referral from another customer, dating back a few years, lead the owner’s of a 25k mile CB350F to my door, owing to poor performance issues. Initial compression readings were in the 120 psi range, except for #3 cylinder which measured 80 psi! The bike was picked up from a local auction company and was in somewhat sad original condition. After purchase, the bike was disassembled and the chassis powdercoated. A tapered steering head bearing kit was installed and lots of work was done to brighten up the engine surfaces. The oil pan was leaking, the valve cover was leaking and the oil pump was leaking oil, and those issues were remedied before it came to me
Day 1
The first thing I did was to remove the spark plug cap and measure the resistance value. Typically, you want spark plug caps to read out less than 5k ohms. This one measured 11.5k ohms. High resistance spark plug caps are a leading cause of poor performance due to weak spark at the plugs. So, first thing on the list was a set of plug caps. Carb kits were installed, but apparently only the float valves and jets were replaced. My experience that in these days, stock #75 main jets are a bit too lean, especially when using today’s E10 alcohol gasoline. In the past I have installed #78 or #80 main jets with good results.
After the owner’s departed, I proceeded to remove the air box and the carburetor rack, plus the coil set from the frame. I was warned that the #4 cylinder spark plug threads were damaged and was shown a spark plug that had rounded off threads, but just way at the end in the first few threads. I have a thread chaser tool to help clean up the threads, so spent some time in working it down into the head to attempt to clean up the threads. The hole was pretty badly damaged, so may need an insert installed. The bike was dropped off around 7pm, so I concluded my work for the evening, but was noticing that the valve clearances seemed to be very tight, so the next step is to check and adjust the valves, then do another compression test, followed by a leakdown test to see where the low cylinder pressure is being lost.
Day 2
The compression readings went up to 150/150/150/120. The head was determined to be a write-off, so another one was purchased from an eBay seller. It supposedly had only 14k miles on it, but there was a considerable amount of carbon build-up on the back sides of the valves. Valves were all removed, cleaned and new stem seals added. The combustion chambers were somewhat carboned-up, so they were all cleaned out with a rotary wire brush. Once everything looked good, the head was reinstalled onto the cylinders with new gaskets and seals. After torquing the head down, the rocker arm cover was installed, valves adjusted and new spark plugs installed.
The carb “cleaning” was apparently from some months ago, so even though the engine fired up on full choke, it wouldn’t run off-idle properly. Off came the carbs again and this time all the idle jets were checked and all found to be plugged up. Not only that, the emulsion tubes were all plugged up with old fuel deposits, so the fueling was completely out of whack.With #78 jets installed and the emulstion tubes and idle jets cleaned out, the carbs began to work normally. I spent over 6 hours in one day, cleaning and installing all the bits and pieces.
Day 3
The bike came in with some older Ram-Flow 2into2 mufflers, which were trashed. The owner’s had rounded up a set of “new” OEM 333 code 4nto4 mufflers, but reported having difficulties with the #3 muffler which was twisted about 10 degrees from vertical and was about a 1/4” off at the inter-connectors that balance the exhaust chambers together. In addition when the two mufflers were installed on the bracket, the header pipe flanges were not parallel to each other.
Installing each muffler in the cylinder head highlighted the fact that the #3 muffler was incorrectly welded onto the header pipe at the factory. Carefully cutting the weld away from the header pipe and muffler, allowed the muffler to align up with the #4 pipe connection and mount up up the rear braket properly. The system was installed again, so that the header pipe could be marked for a re-weld.
I hauled the muffler down to a local automobile muffler shop who had reworked my Jaguar XK8 mufflers about 5 years ago. I was able to hold it just right for long enough to get a tack weld done. I rechecked the marks and gave him the green light to weld it all up again. The muffler was remounted on the bracket and everything lined up as originally designed.
The customer came by with the repainted fuel tank, which was installed and fuel turned onto the carbs. The bike fires up quickly and sounds like a 350 Four should sound with the original mufflers installed. There was, of course, more difficulties with the tank. First, there were remnants of rust and corrosion left inside and secondly the petcock lever required two hands to move from one position to the other.
After the tank was drained so the petcock could be serviced, it was noted that the 4-hole gasket was kind of out of index. The gasket was new, but something was causing excessive drag. The lever face was cleaned up and reassembled, but the problem remained. In the third round of inspection, a second wave washer came loose from the other! Apparently, the old gasket was leaking, so the additional washer was added in an attempt to seal the leak! Removal of the second washer resolved the petcock lever issue, at last.
The bike got a quick test ride locally, as it doesn’t have any license plate. It pulled up to redline in third gear and was running happily now. These bikes were never particularly fast, but are relatively comfortable as a beginner bike with a smooth motor.
Bill Silver
aka MrHonda
