Bi-polar CB750F2 undergoes a polarity shift…

Jeff K. Barberie

Sometimes the universe serves up some interesting scenarios to consider as a way to make change and move forward in life. Such was the case when the RC Engineering-equipped CB750F2 failed to gain a single bid over $2500 recently. I decided to just sit on the situation for awhile and see if any interested parties might suddenly show up with an offer to buy or perhaps with something interesting to trade instead.

Not long after the auction closed (after having spent a couple of hundred dollars on eBay fees for no sales), I was contacted by a gentleman from Northern California who had read the end of my auction where I mentioned a possible trade for the bike. What I had in mind, when I wrote the text was some kind of trade and cash, but this offer was for a fully-restored 1971 SL350K1 Motosport 350, basically straight across. Given the lack of enthusiasm from the auctions to date, it seemed like there was a better possibility of regaining some of my investment money by swapping for the 350 and selling it afterwards. The offer came with some nice photos of the bike with a winning show trophy and a more detailed shot of the engine section, which was highly detailed. I have always admired people’s efforts to really detail and polish engine cases and go all out in bringing the bike back to near factory original condition. The CB750 was dormant in a garage for some 20+ years, which takes a toll on the bike’s shiny surfaces, especially chrome and polished aluminum. I barely took time to wipe it down with some polish and cleaners here and there, as my focus was making something useful out of the bored and stroked 1160cc CB750 motor, so it could be useful for someone who had a passion for this model.

After a few e-mails back and forth, we struck a deal to do the swap, but the remaining barrier was that he was in Monterey, CA and I was about 450 miles south of there. Fortunately, he and his wife were about to take some vacation time and come down the coast to Morro Bay for a few days. At first we thought about meeting half-way, but I didn’t have ready access to a reliable truck, so fortunately he and his wife decided to extend the adventure all the way down to San Diego and bring the bike down in a closed trailer. That was a great relief for me, as dragging that 500+ lb bike in and out of a pickup truck bed has little appeal to me, these days. It has only been about 90 days since I received a knee replacement, so I am trying to be a little bit gentle with it in most situations.

Happily, they arrived mid-week, but having gotten caught up in the southbound 805 afternoon traffic, the last thirty miles took over an hour to achieve. After they checked into their local hotel room, they drove the truck and trailer to my place, ready to make the swap. The CB750 got the first introduction with a quick start-up and warm-up while it spit and sputtered, as it came to life. Despite the stock cam, the very large cylinder bores and ported head creates some serious air flow through the carburetors which are not calibrated or designed for those kinds of airflow velocities. Enlarging the idle jets with jet reamers seems to have helped the situation somewhat, but it takes a good ten minutes of run-time in order to get the idle speed to settle down and the engine to smooth out further.

The combination of the big piston weights acting upon the stock crankshaft, which was balanced for much smaller slugs, generates a lot of primary chain noise when the engine is cold. It quiets considerably when the engine warms up, but there is quite a mass of aluminum and 4 quarts of oil to bring up to operating temperature. My local test rides included a quick blast to 90 mph going uphill with about 6k rpms showing on the tachometer and seemingly lots of power left in reserve. Quite the thrill ride, but once was enough and I let it rest until the NoCal rescue crew arrived.

I had advised that this engine could have a short fuse in it, given the amount of modifications done and the fact that the cylinders were filled with RTV sealer to keep the oil from leaking past the giant cylinder sleeves machined into the block. This blocks any air flow from reaching between the end pairs of cylinders, but the retention of the oil cooler should help to keep the engine temperatures down into the realm of reason, at least for awhile. It never gave indications of wanting to overheat and the spark plugs generally came out looking right for the jetting setup.

We wheeled out the SL350, which is titled as a 1972 model, but the build sticker on the frame says 10/70 and the bike’s attributes are all SL350K1. The SL350 came out in 1969 and split the year into 1970, before the K1s were released. While the K0 versions were basically reframed CL350s, complete with their CV carbs and electric starters, the K1s were reworked to use smaller 24mm slide carbs and were a kick-start only machine. Losing the starter motor was an immediate reduction of some 10lbs of machinery and the small-bore carburetors had better low-end performance. Also increasing the low end power output was a new cylinder head with high velocity small ports and a matching camshaft that all helped to rein in the 10,000 rpm CB/CL power peak down to where it would be useful out on the trails. The K1 redline is only 9,000 rpm, showing its desire to make power down low. Despite all the changes to the chassis and engine, the bikes were never really a serious threat to DT-2 Yamahas for desert and off-road use. However, a highly modified SL350 did win the Baja 1000 in 1968, so in the right hands and with the right setup, they can get the job done. http://www.ravenwestguitar.com/1968baja1000.html

The bike gleamed as it was removed from the trailer. It has a very high-quality paint job, lots of polished alloy parts, fresh tires and correct OEM mufflers, which have been rare as hen’s teeth for the past 20 years. The left side muffler has a repair patch on the side from where the sidestand gets knocked into the muffler, but the right side muffler is a nearly virgin item. Installing a featherweight ni-cad battery allowed sparks to flow to the ignition and lighting. With a few prods the engine came to life and sounded healthy. After a quick warm-up, it had a trip around the block test ride and all seemed to be in order except for a bit of a tappet noise on the left side. The bike has been ridden very little since it was built, owing to the fact that the owner had nine motorcycles at the time. I felt confident that the little noises and running glitches would be solved easily, so we shook hands and completed the deal.

The next day, the CB750’s compass was pointing NORTH, just opposite of the SL350s new arrival in San Diego county. Hopefully the change of scene will benefit both the bikes and new owners. So far, so good, on this end of the pole, anyway. A thirty-minute tuning session, including a needed valve and timing adjustment, brought the engine’s tapping noises to a halt, allowing the 180 degree twin to sit and idle happily and quietly as if it were just pulled from the factory crate. Even with 18k miles showing on the odometer, the engine only required a hone and re-ring job on the standard bores, showing that these engines can be long-lived if treated with some level of respect and regular oil changes. There was a specific Honda tech bulletin regarding “backfire” conditions on the SL350K1s and this one has exhibited a bit of that at idle. I did find the needle clips in the #2 needle slots, rather than the suggested #4 positions. It idles well, but the transition from idle to part throttle has a little hesitation issue.

My previous dealings with CB/CL350s were less than rewarding or even the slightest bit of success in one case. The bikes are raced extensively in AHRMA vintage races and have been made to make double the stock horsepower ratings and do it successfully.

http://www.thevintagemonkey.com/#!The Vintage Monkey Motorcycle Racing/zoom/c1xi5/i4iho

http://www.thevintagemonkey.com/#!The Vintage Monkey Motorcycle Racing/zoom/c1xi5/i3i1s

Having fine-tuned this one, knowing that it has been gone through completely and is running perfectly now is a comfort to my soul, these days. Due to the kick-starter only feature of this bike, it can’t remain in my custody for very long as a useful daily driver. My “equity” in the CB750 disaster (financial anyway) is in the 350, so I hope to cash out soon, get back to just one or maybe two bikes and call it good for now.

The CB750 and previous 350 twins have had a polarizing effect on my life in various ways. Now that the poles have shifted, with the 750 going north and the 350 coming south, perhaps the transition will bring about some positive forward motion in my life and finances this year. The bikes are polar opposites, but in the end, the solutions to two people’s problems have been solved by the shift.

Bill “MrHonda” Silver

Bi-polar CB750F2 undergoes a polarity shift…