The bike didn’t stay intact for very long. I put it up on the workbench and proceeded to dismantle the bike’s exhaust and carburetors to allow the disassembly of the top end to find out what the source of the oil leak was and rather uneven running at idle. Visually, the tach drive seal had been leaking for a long time and drooling oil down the front of the engine.
As the cylinder head was removed, it became apparent that the head gasket had blown out between the number 3 and 4 cylinders. This is highly unusual in my experience, especially having raced 350 and 400F models under severe conditions. Here was a stone-stock model with a burned fire ring and adjacent gasket material. The combustion chamber was carbon-ed up, as were the valves with burned oil, but the cylinder bore looked completely fine and just like the other three.
The cylinder head had a depression and signs of some erosion from the blown head gasket area. I measured the depression to about .004” and sought out a machine shop that could shave off about .006” to flatten out the head and erase the low spot between cylinders.
I took it to a local automotive machine shop but they were unable to jig the head up in their machine, so they walked it down the street to another machine shop to see if they could do the deed. I suppose that you might be able to skim-coat some JB weld down in the valley and coat the gasket with some sealer to ensure a good connection, but that would be a last-resort effort. Certainly, another cylinder head would be available on eBay, but you never know what you will get in a used part like this.
Trials and Tribulations….
Well, the second machine shop did a beautiful job of skimming off just a bit of the uneven cylinder head surface, so all I had to do was to remove the rest of the valves, de-carbon them, change the valve stem seals and put it all back together again. Reassembly is pretty straightforward on these engines, which are easy to work on in the frame. After I got the top end back together and the valves adjusted, I checked the compression readings which were all about 150 psi. I left the pistons/rings alone as the bores looked pretty clean, but it does have 17k miles on it, so we’ll see how it runs and if it wants to use some oil. The valve stem seals were definitely cooked, so the new ones will reduce oil burning quite a bit.
The carburetors were split apart enough to replace the o-rings on the fuel fittings that feed gasoline across the four carbs from a central point. I had to drive out the shaft that had a couple of keys in both ends to engage the throttle arms. In the process, one of the copper bushings got damaged and they are not a separate part from the base plate. I went on eBay and found a copper bushing that was 3/8” x 1/2” by 3/4” which is actually the size of the OD and length. The ID was a little bit tight so I drilled out the inside with a drill bit of a close size and it all went back together again. The float bowls and jets were all ultrasound cleaned and the o-rings were replaced from my handy K&L Keihin carb repair kit box.
Again, the float settings for the carburetors were off more than a bit. The last set for my friend's CB350F was much worse, though.
The next challenge was to install new aftermarket intake manifold rubbers. There are two different part numbers used on 1-4 and 2-3, but they are slightly different shaped and I spent a half hour trying to get the carbs pushed into place, thinking that the little part numbers indicated on the manifold faces were all to go down. Finally, I had to remove them from the head, push them onto the carb spigots, and turn them until the spacing matched the intake port dimensions. In the end, the two left sides were installed with the lettering down and the two right sides were installed with the letters up! The carbs slipped into the manifolds securely and the next job was to install those shiny new 4:4 mufflers, which were worth more than the bike.
I had installed a set of these for a friend, when they were available about 5 years ago, and recall having some problems getting them to fit up to the brackets properly. I started with the left side 1-2 pipes, which are bolted together at the rear along with the interconnection gasket. When I tried to get the flanges and collars to fit up into the cylinder head, they wouldn’t go deep enough to get some threads for the 6mm nuts to tighten them up. I did finally get the #2 pipe into the head with the stock collars, but the #1 was not having it. I dug through a box of misc flanges and collars and came up with 4 that were the right OD but about 1/4” shorter in height. These eventually allow the #1 pipe to be fitted into the cylinder head, along with a fresh exhaust pipe gasket.
Then the problems really began as the rear mount which incorporates the rear footpeg was about a 1/2” off center from where the bolt goes through the footpeg and the exhaust bracket then to the frame mount. I tried to adjust the stud holes that attach the two pipes together but it wasn’t going well. Finally, I loosened the flanges a bit and pulled the rear bracket up so the bolt would go through. Not only was it off in height, it was about a 1/4” outwards leaving a gap to fill in. Eventually, I was able to bolt everything up, but the pipes were in a bit of a bind. I think that they missed the mark on bending the header pipes correctly which caused the misalignment at the rear.
When I looked at the right side pair, before installation, I compared the flanges of the old and new pipes and discovered that the old pipe flange was welded flush with the pipe, leaving a nice 90-degree corner for the collars to push up against them to hold into the cylinder head port. The new ones had thicker flanges that were welded in where the flanges would normally rest, causing them to appear to be too short to install. I used one set of the shorter collars on one pipe and wound up cutting down the stock set about an eighth of an inch to allow for the flange problem.
Again, when the headers were bolted into the exhaust ports, the rear bracket was a little low and about a half inch outwards away from the frame mount. I installed a set of thick washers to take up the space and sourced a longer bolt to finish up the installation. I spent more than 4 hours just trying to fit up the exhaust system and even though they are nice and new and shiny, they really don’t fit the bike properly.
The mufflers had little stickers that were from QC and said “checked” but I am not sure that whatever they checked was accurate. Anyway, they are on the bike. I did start it up on a remote bottle to see how it would run now but there were some lingering carb problems that needed to be addressed before a good test ride is undertaken.
I removed the carbs again and switched the connecting shaft around 180 degrees to get the keyways and shaft machining oriented properly. The damaged bushing left a little bit of extra slop in the system, but they worked much better this time. However, when I put the gauges on them to sync all four, the #3 slide adjustment wouldn’t sync with the other three. I carefully dismantled the #3 carb top and extracted the slide to check for any irregularities, then reset everything once again. Finally, they were all reading the same and the idle smoothed out considerably. If the #3 slide was always higher than the other three, previously, that might explain the overheating of that cylinder and the head gasket failure.
Two things led me to think that the brake shoes and points were all original to the bike at 17k miles. The rear brake rod needed all the adjustment used up to get the rear brake to function properly, which generally means that the shoes are all worn down. For riders who have some fear about heavy use of front brakes, often wear out the rear brakes prematurely because that is their main source of stopping the bike. Imagine my surprise when I removed the brake panel and found the shoes to be at close to 4mm, which is apparently the stock thickness. I had purchased a set of Vesrah aftermarket brake shoes from an eBay seller and they came out of the box at 4mm! So, once installed on the bike, the rod adjustment nut is still mostly threaded all the way up. There is a pedal adjustment to lower the standard height setting, so that will affect the adjustment nut setting, but normally it should be way at the end of the rod when everything is new. Perhaps, the drum is worn out, which seems unlikely.
In adjusting the ignition timing, I had to rotate the backing plate all the way in the adjustment slots in order to retard the ignition timing at idle and it was still going a bit past the full advance marks. Looking at the point gaps, they were down around .008” instead of the normal .012-.016” range, which indicates that the rubbing blocks were all worn down from 17k miles of operation. Normally, the points should be changed about 12k miles, so they are undoubtedly the originals from 1973. I ordered one of the inexpensive Chinese-made electronic ignition plates and that should stabilize the ignition timing.
Apart from replacing the tires and drive chain, all it needs is a really good detail and polish to bring it back to its former glory. I think that the previous, now deceased owner would approve of my handling of his precious CB350 Four.
Bill Silver aka MrHonda
The carbs had to come off again as the linkage bushings were allowing too much play. I bought a used bracket plate from an eBay seller and also re-jetted the mains to #78, up from the stock #75 sizes. This, coupled with the newly installed e-ignition allowed the bike to run smoother with a bit more pep. The carb synch went well and all four are staying equal with each other.
I wound up replacing the fork seals, as they were the originals and weeping a bit. I put the bike into my name and registered it. It gets ridden a few times a week, but the suspension is still rather rough for me. My Sunday ride is a 1991 NT650 Hawk GT and I really enjoy that bike. So, the 350F will go on the market and hopefully to a good home where it can be appreciated for its glory after 50 years.