According to a Google search: "Petit" means "little" and "four" means "oven" in French, which kind of describes the 1972-74 Honda CB350Four models. After Honda launched the CB750 in 1969, they followed with the 1971 CB500Four, then in late 1972, the CB350four editions. Just like the “Three Bears” one has porridge that is TOO HOT (and heavy), one was TOO COLD (underpowered) and one was just right, the CB500Four (later CB550Four). Honda was aiming to replace the buzzy CB/CL350 twins with a smooth four cylinder bike, but the added weight, complexity and low level of tuning left it as an also-ran in the 350cc bike category, during the 1970s. If only they had a little more fire in the oven!
I bought one new, in 1973, and enjoyed it as a lightweight transportation bike, but they were short-geared and always screaming at highway speeds. Acceleration was leisurely, front brakes were prone to squealing and gas mileage wasn’t all that great. Still, they were so much smoother than the twins and the styling of the 4:4 exhaust pipes and fuel tank shape gave it some definite eye-appeal. As time went on, it developed the common head gasket oil leaks, which required pulling the top end off of the engine, which can be easily done with the engine in the frame.
My friend, Marc, who was the subject of previous reports when I attempted to revive his ailing CB350 twin (and failed miserably), wisely heeded my advice to seek out one of the small Honda fours for his next vintage bike choice. He tracked down a clean, low-miles example that had been rehabbed in AZ and offered for sale on Craigslist. I called the seller and we discussed the work which he had done/not done and overall condition report. It seemed like a pretty good bike, so Marc went forward with the deal and brought back to San Diego, late last year.
I performed a tune-up and some carb work, and then he disappeared on it for a few months. A true bike enthusiast (as well as being a new dad, working two jobs and trying to have a life in between), Marc rode the bike to work every day, saving gas and being able to park the bike downtown easily, because San Diego has numerous free motorcycle parking spaces.
Late December, Marc checked in to say that he had taken the bike to Yosemite and back and was planning to go to Mexico for a trip, as well. He brought the bike by and I noticed that the drive chain was hanging low, due to lack of maintenance and LOTS of miles. I think he had run about 3,000 miles since my full service and things were getting a little ragged, here and there. He asked when I would have time to work on the bike, which finally wound up being in the beginning of Feb. The bike was trucked over to me with a grocery list of wants/needs: Head gasket leak, new drive chain, new rear tire and replace a previously noted front brake caliper piston (pitted at seal ring surface).
I had an OEM top and bottom end gasket set in stock for the bike, but ordered a new rear tire, tube, o-ring chain and aftermarket stainless steel caliper piston, then went to work on tearing down the engine. Having worked on 350-400Fs many times, the drill is always the same. Flip up the seat, remove the tool tray, air filter lid, air filter, then pry out the connecting tube to the air plenum chamber, which connects to the carburetor inlets. It is a tight fit to get the air box off the carbs and out the side of the frame, but no big problems. With carb cables off and carb rack removed, you can start to access all the cylinder head hardware and begin tearing things apart. The engine only had about 8,000 miles on it, so everything was as-built from the factory. Despite an overly extended oil change schedule, the engine parts all looked pretty pristine and shiny inside.
One thing that is really mandatory when removing the heads of these bikes is to replace the valve stem seals. The originals are now 40 years old and the rubber has long-since hardened into plastic consistency, allowing oil to drool down the valve stems and be drawn into the engine, which shows up in increased oil consumption and some engine smoke at start-up.
Taking your time, the process is straight-forward, removing all the valves which received a good wire brush treatment, then the new seals installed before final assembly. The gaskets all came off cleanly and scrubbing time was minimal, overall. After the head was renewed and gasket surfaces cleaned off, it was time to replace the seals around the oil orifices, dowel pins and the tachometer drive seal, before dropping the cylinder head back down over the studs. Because the engine is so tiny, you have to be careful to wiggle the camchain on the sprocket at just the right spot, then final check the cam timing before snugging down the two sprocket bolts.
Before the top rocker arm cover is installed, you MUST back off all of the valve clearance adjuster screws, allowing the rocker arms to be positioned in a full UP position before the cover is lowered down on top of the cam and valve stems. If a rocker arm adjustment screw snags the edge of the valve stem on the way down, it can bend the affected valve, causing immediate compression loss and more problems as you try to remove the bent valve from the guide without damaging the guide, as well. TAKE TIME to do it right and you will have no regrets, later on.
While the carbs were off, I rechecked the needle positions, which I had altered before in hopes of curing the little off-idle, part-throttle hesitation which is common with these bikes. You have 20mm carbs feeding 75cc cylinders, which is a little large for the mild tuning of these engines. The rush of air into the ports seems to cause a stumble, which I have always thought was a lean condition, due to EPA emission regulations. When I pulled the spark plugs, before engine tear-down, I discovered that all of them were carboned-up indicating a rich condition at low and medium speeds. The jetting is all OEM stock at #35/75 idle/main jets with correct float levels set at 21mm. The only “non-stock” item on the bike is a set of Japanese-made 4:2 muffler systems, unlike any that I have seen previously. Normally, the old mufflers of this style use the stock header pipes, which are sawed off at the old muffler joints and the new 2:1 muffler slides over a pair of pipes on each side. This system included new header pipes with a little kick-up at the back, so the mufflers were angled up for more ground clearance and a sleeker look.
The exhaust system, however, had some design flaws which caused the pipes and mufflers to be in a bind, when installed. I had to re-drill a new mount plate hole on one side and do some other fancy work to get the opposite to fit without having to force things together. The original installation caused the #3 header pipe flange to be angled out, causing an exhaust leak at the port. Once the system was manicured and properly fitted, the noise level was decreased and the engine tuning was easier. Perhaps something in this exhaust system was causing the carbs to run rich, so I dropped the needles one more notch, which put the clip in the #2 slot (out of 5 notches). Leaning the carburetion out seemed counter-intuitive, as almost all of my tuning efforts lately were to richen the fuel delivery up to compensate for the alcohol in the gasoline. Once the bike was back up and running, I notice that the throttle response seemed to be improved and only a few times was there a slight sign of a stumble at part throttle settings, when accelerating. It WORKED!
Another hour or so was devoted to pulling the rear wheel, changing the tire/tube and installing a new o-ring chain, which I thought might be more serviceable for the kind of work it has to do on a daily basis. The final task was to replace the caliper piston with a new one. While replacement went pretty smoothly, I noticed that the caliper bracket, which pivots on a bracket on the inside of the fork, was basically seized up! Getting to the hardware for removal, required taking the whole front wheel off, then the fender bolts on the left side. The bracket was taken to a vise, where I sprayed some penetrating oil down the shaft/bushing interface, then wiggled it back and forth until it came free. I wire-wheeled the shaft, greased up the parts and reinstalled it all again. Time for a test-drive.
These bikes are a little cold-blooded and the idle speed varies, depending upon the temperature of the engine oil. With fresh 10-40 Honda motorcycle oil and a new filter, you could almost feel the churning inside, as the motor woke up for service. Out on the road, the engine was singing along, making its best power above 6,000 rpms. Out on the freeway, where everyone is going 75mph, the engine is spinning past 7,000 rpms, heading towards the redline a few thousand revs later. It REALLY needs another gear in the transmission, which eventually did show up when the 350 motor was reconfigured into a CB400F Super Sport. Talk about making a silk purse from a sow’s ear; that is exactly what happened with that upgrade.
So, with the chores all completed, the bike went back to Marc with a smoother power delivery, no head gasket oil leaks, a new drive chain and even the brakes quieted down, either due to the new billet stainless steel piston or freeing up the caliper bracket (or both).
I spent the better part of 2.5 days crawling around on my hands and knees, getting this bike dialed back in again, but it was worth it in the end. Just don’t tell that to my ankles and knees!
Bill “MrHonda” Silver