Upgrading the performance of an early 80s cb750 using the camshafts from a cb900 is not only cost efficient, but doable for the shade tree DIYer thanks to the existence of forum experts and the wealth of searchable knowledge from others who have asked the same questions before. I found a set of cb900 cams on ebay for small change last year (under $30 shipped). There are lots of folks parting old motorcycles and DOHC cb750s and cb900s sold well enough to support bike dismantling to keep the existing bikes on the road for years to come. Since there are so few people actually riding these classic bikes, there's not alot of competition for many of the parts. With the exception of carburetors, most parts can be acquired for a reasonable amount on ebay. So what will 900 cams do for a 750? From the seat of the pants, there is expected to be a very noticeable gain in midrange performance with most owners exclaiming the upgrade to be well worth the effort even though some of them experienced problems with removing or installing. One poster suggested +10hp @ 10k RPM and +3 ft/lb torque @ 7k RPM using engine peformance software to calculate the improvement, although it has been thought to be optimistic for a bike that rolled out of the dealership 30 years ago with 72hp @ 9k RPM.
I needed to replace the 30 year old valve cover gasket and grommets, but had some cash flow issues this winter so decided to re-use the valve cover gasket one more time since it was still fairly soft and pliable. Instead of RTV silicone, I used grease on the 4 small corners of the VC gasket near the ends of the cams. Since I couldn't afford a new valve cover gasket, I decided to try only buying new bolt seals in the hopes of getting the valve cover tight enough to not leak. The bolt seals are responsible for the clamping pressure to keep the valve cover tight.
I did not have a small torque wrench to torque down the camshaft holders and though the recommended torque of 9 ft lbs is not alot, I was not confident of getting properly tightened bolts by manually torqueing them down using only my small 1/4" socket. The 2 long bolts holding down the oil tube seemed to need more attention even though all the other bolts felt like they were snug. I needed to find a torque wrench and my big one for the wheels on my car doesn't even go down to 10 ft/lbs. I picked up a 3/8" torque wrench from harbor freight and proceeded to overtighten all the camshaft holders down when I realized the torque wrench wasn't clicking. I decided to check the wrench on the lugs nuts on my car and no clicking at 9 ft lbs even though the car was moving with my weight. I ended up returning the faulting torque wrench to Harbor freight and exchanging it for a 1/4" torque wrench. Then I loosened each bolt and re-torqued as insurance against future failures.
The factory service manual covers the procedure of cam removal and install very well. The big problem with the procedure is the potential to break the brittle hardened steel bolts that hold the cam caps down. The bolts are torqued down to 9ft/lbs and definitely should not exceed 11 ft/lbs while gradually tightening the cam caps in a criss-cross pattern to ensure that the cams are tightened down evenly. Pay attention to what you disassemble and keep everything sorted instead of one big pile of parts. I kept my parts grouped by camshaft holder.
Make sure to stuff a towel down where the cam chain connects with the crank. There is a real potential of losing a bolt or nut down in the recess.
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Honda CBR Coils are a cost effective improvement for the weak ignition system that came standard on the DOHC Hondas. Used coils are plug and play and reportedly throw a much more intense spark than the original units. The advantage is improved combustion and driveability though there are no expected horsepower gains.
The CBR Coils are shorter than the OEM Coils so you will need to either zip-tie or drill a hole for one of the mounting spots on the frame.
Very nice. How is it doing after all these years?
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