Previous stories have alluded to my rather unwise purchase of the “monster” 1978 CB750 Super Sport and my attempts to revive it and make it street-worthy. After installing the stock electric starter system back into the “kick start only” engine cases, it became apparent that the stock starting system was overwhelmed by the demands of the 1160cc, bored and stroked SOHC motor. The starter clutch system spun loose from the end of the crankshaft on one occasion. As the engine internals were really never verified by the chain of owners since the bike was built in the 1980s, it seemed the best course of action was to dismantle the engine for inspection and verification of what work was performed on this machine.
The original stock “emission control” carburetors had been replaced by a set of the earlier K1-K6 28mm carb set. The information that came with the bike stated that the carburetors were “bored” to some oversize figure, but inspection of the carb bodies revealed the normal double-oval throat bores. The main jet sizes were #130, up from stock #110, but little more could be seen in the way of performance mods. The carbs had been mounted up on ancient intake manifolds, aided by liberal doses of orange colored RTV sealant. New manifolds were purchased and installed during the early revival attempts and eventually the engine fueling tasks seemed to be under control.
With carbs and the original RC Engineering 4:1 exhaust system removed, work began to remove the cylinder head from the engine, while it was still installed on the chassis. Normally, the engine must be removed from the frame to service the top end, but the previous owner had cut away the two side brace tubes to allow engine work to be performed in the chassis. The engine assembly weighs in at around 200 lbs, so becomes a 2-person chore to R&R the powerplant. With the frame tubes out of the way, access to all the fasteners holding the cylinder head cover was greatly enhanced. The top cylinder head cover had been welded and modified for additional fasteners to help anchor it in place, a common modification when these engines are highly modified. Once the top cylinder head cover was removed, the heavy-duty valvetrain components were revealed. Alloy valve spring retainers capped off some hefty valve springs which were closing stainless steel valve stems. The camshaft lobes were quite aggressive in profile and according to the cam spec sheet which came with the bike, the cam was a nearly full-race grind. The camshaft sprocket was slotted to allow liberal amounts of advance/retard settings of the camshaft’s lift and duration specifications. All of the engine’s internals were bright and shiny with little or no wear evident, so far. Removal of the camchain tensioner and the camshaft sprocket bolts allowed the camchain to be removed from the mounting shoulders. The camshaft is held down with two bolt-on camshaft bearing holders, which are line-bored and matched at the factory. With the cambearing caps and hardware removed, the camshaft slipped out from beneath the endless camchain and set aside for inspection.
Once the cambearings are removed, access is gained to all of the cylinder head hold-down nuts. After fishing them out from recesses, some beneath large rubber flat plugs, the cylinder head was then lifted off the cylinder block. A copper head gasket was secured to the head with some adhesive so came off with the cylinder head assembly. Minimal carbon buildup was seen on the combustion chambers and valve heads, again indicating little actual use in the as-built condition. It is hard to get a lot of riding time in when your engine won’t reliably start up due to the massive 50% engine displacement increase. Inspection revealed that the valve head sizes were normal CB750 dimensions, smaller than the normal “F2” canted-valve head types. Apparently, there may have been some kind of interference issues with the larger valves, being operated by a full-race camshaft that precluded use of the F2 head on this engine build. So, the cylinder head, which was ported on both sides, was a regular production CB750K item, instead of a modified CB750F2 part. More surprises were to be found as the engine teardown progressed, later on.
The cylinder bores were quite close together, indicating the installation of the largest big-bore piston kit that can be installed in this series of engine. The CB750 has a 61mm standard bore. 836cc “big bore” kits are 65mm. These bores measured about 72mm. Once the cylinder block was raised up off of the pistons, the shiny, highly-machined pistons were revealed, along with beautiful aluminum wrist pin “buttons” to hold the piston pins in place, instead of wire wrist pin clips. At the same time, the glimpses of the famous RC Engineering “Golden Rods” were revealed. These enormous rods were made from treated aluminum and anodized in a yellow-gold color. Light, but strong, they were used in all of the RC Engineering racing engines. The twist, in this case, is that the rods are shortened to allow for the reported 1/4” stroker modification of the crankshaft throws. In after-the-fact research, it was discovered that the stroker crankshaft setup was never recommended for street use by Russ Collins, himself.
A few days prior to going in for knee surgery, I finished removing the rest of the engine from the chassis, using a floor jack and then transferring the lower end to a rolling cart, which was wheeled into the shop area. After three weeks of recovery time, it seemed to be time to finish pulling the rest of the engine apart, which went along fairly smoothly. Once the clutch was removed on the right side, the charging system components and shift selector parts removed, the case bolts were loosened and set aside. The engine cases parted easily, revealing shiny engine components inside, including obviously modified crankshaft connecting rod journals. I fished off the camshaft and primary chains from the crankshaft’s central sprockets, then checked the crankshaft journals for wear and signs of issues around the modified crankshaft throws. Everything looked nice and clean, with little debris or dirt inside the crankcases or the oil pump screen. It was a happy engine that was just built a little too big for its own good… at least for the street.
The final drive setup was massive and obviously setup for drag racing, including a fabricated swing arm with three axle and shock absorber locations, plus monstrous chain adjusters and a duplex final drive chain and sprocket combo. As I was working towards returning the bike closer to stock configuration, the swing arm and sprocket combo had to go. It took three different eBay auctions to come up with most of the stock swing arm components and axle spacers. The swing arm came off easily, but when the stock axle was fed back into the rear wheel, it was evident that the axle spacers had been modified. Good used spacers were procured, but there was still a gap at the sprocket side of the rear wheel. Without checking the dimensions, I had imagined that the rear axle setup was thoroughly modified to use a larger rear axle. The axle, itself, was fabricated and threaded from bar stock and a large nut welded one end. I misread the bearing numbers on the rear wheel and convinced myself that some kind of major engineering work had gone into the rear wheel, using larger ID bearings, then boring out the rear caliper mount hole and using special spacers to make it all come together.
It turned out that the axle OD was the same size as the OEM Honda axle, so all of my concerns proved to be false. The remaining gap at the axle spacers proved to be caused by a narrowed caliper holder at the axle hole, instead of some mysterious combination of spacers, themselves. I had ordered a $5 caliper bracket along with some other missing parts for the rear fender, so using it to compare against the one on the bike I finally solved the mystery. It may have been that the racing swing arm was built to regular K series specifications, which use a drum brake on the rear wheel and different axle spacers. Narrowing the brake caliper bracket bosses apparently became necessary to get the rear chain alignment setup correctly.
Other oddities were a centerstand which had been cut down to the point where it wasn’t stabilizing the bike when placed on the stand, itself. A replacement centerstand was purchased and installed to help stabilize the bike, which has been parked on a sloping cement driveway. In the beginning, the bike had NO brakes at all, so it wanted to keep sliding down the driveway until I put a big automotive wheel chock behind the front wheel. Once the brakes were all rebuilt, the bike could be moved with more control and confidence.
The final disposition of this bike can be in one of several forms.
1. Sell the rolling chassis, as-is, with title. New owner can plug and play a whole stock engine into the chassis, which only has 13k miles on it and the bike gets a new life, but with a different engine.
2. Find a complete engine to install myself and sell it as a more or less complete stock machine.
3. Sell off the RC Engineering parts and replace them with OEM stock parts, thus keeping the engine serial numbers as a match for the title.
4. Part out the whole machine.
So, faced with a 1978 CB750F2(3) engine carrying a K-series cylinder head and carbs, putting it back close to the original pumped-up OEM specs may be an expensive and daunting task. I missed a couple of opportunities to purchase complete F2 motors for as little as $500, but when I was ready to make that move, the engines were no longer available.
Hopefully, with the sale of the sought-after RC Engineering parts, the funds can help facilitate the engine rebuild of the stock cases. The top engine case has been heavily bored out to fit the big bore pistons, but the majority of the rest of the engine should go back together again, but you just never know when you are trying to de-engineer some of Russ Collins’ high performance racing modifications.
The first step in that “return to stock” direction came on Christmas Day, when an eBay message came into my Inbox stating that the rear swing arm had sold for almost $400! It’s a start…