12/02/2007
The Honda CB750. For many, the name alone conjures images of a golden era of motorcycling, a machine that single-handedly redefined expectations and sent shockwaves through the established motorcycle manufacturing world. Announced in late 1959, and hitting showrooms just a year after its design concept, the CB750 was a revelation. It resembled a Grand Prix road racer, boasting four carburetors, a single overhead camshaft, four exhaust pipes, and, revolutionary for its time, an effective disc brake. This wasn't just another bike; it was a statement, a bold declaration of Japan's full arrival into the global motorcycle brotherhood. While its design was eventually superseded in 1978 by the 16-valve DOHC 750, make no mistake: the original SOHC CB750 engine is not out of date, merely out of fashion. Its deserved reputation for long life and reliability, often lasting 80 to 100,000 miles with proper care, speaks volumes. But what truly makes a good CB750, especially today, decades after its prime? It's about understanding its inherent strengths and applying thoughtful, proven modifications to enhance its performance and longevity, bringing this classic up to a standard that rivals modern machines in feel and capability.
The Enduring Appeal of the SOHC CB750
The Honda CB750 delivered a cold shock to complacent management on both sides of the Atlantic. While Harley-Davidson had a DOHC four-cylinder in development, they doubted its market appeal. English manufacturers, bogged down in management turmoil, stalled design progress, leading to their eventual decline. BMW even contemplated abandoning motorcycles entirely. Against this backdrop, Honda's lightning-fast development of the CB750, a feat of design expertise and sheer will, cemented its place in history. After initial teething troubles with chain breakages and carb sticking, the CB750 settled into a robust nine-year production run, with yearly updates reflecting changing tastes and regulatory requirements. Power increased from an initial 54 horsepower to 64 in later sport models. The core engine, however, remained a paragon of durability. This enduring quality is precisely why the SOHC CB750 remains a fantastic platform for enhancement. The goal isn't to turn it into a modern superbike, but to amplify its inherent virtues: increasing peak power, significantly boosting mid-range power, and doing so with minimal expense and absolutely no loss of the legendary Honda reliability.
Engine Enhancements: Unlocking Greater Potential
While engine technology may have advanced slowly in the 1970s, the principles of improving an engine's output and efficiency remain timeless. For the SOHC CB750, the most effective, easiest, and cheapest way to elevate performance is to increase displacement and improve its breathing. These modifications, proven over thousands of bikes, genuinely bring the Honda 750 up to date while maintaining its original dependability.
Increased Displacement: The 836cc Bore Job
A simple bore job can increase the 750's displacement to 836cc, adding 110cc over stock. When coupled with improved breathing, this can reliably deliver around 75 horsepower from an otherwise unmodified engine. Kits, historically featuring cast pistons manufactured by Honda's original piston maker and rings from the Honda 350cc twin, offered excellent reliability. It's crucial to acknowledge the Honda's weakest link: its connecting rods. If the red-line is observed and peak power is kept to 75 horsepower or less, the stock rods are usually fine. Exceeding this will necessitate splitting the cases for expensive heat-treated or aftermarket rods.
Camshaft & Valve Gear: Optimising Power Delivery
Adding a high-lift, short-duration (torque) camshaft significantly improves the engine's power potential, leveraging the extra displacement while lowering cranking pressure to better accommodate modern petrol. While not strictly essential, a good cam enhances the broad power-band of an enlarged engine. Pairing this with appropriate valve springs and stock retainers ensures valve train control. Honda's original valves and valve guides are of excellent quality and rarely need replacing for street use, even after significant mileage, unless there's evidence of burning oil or excessive wear. Regular inspection for contact width and proper cleaning and lapping are usually sufficient. The key is to find a mechanic who truly understands how to perform a proper valve job, as the hardened seats of most CB750s require grinding, not cutting, a delicate process demanding high skill.
Breathing Easy: Air Intake and Exhaust Systems
An engine needs to breathe to make power. While the Honda's airbox and 28mm Keihin carburetors are surprisingly capable, the stock paper air filter is often too restrictive. A high-flow cloth filter, such as a K&N type, is a simple yet effective upgrade. Exhaust systems for the 750 changed considerably over its production run, with earlier versions being louder and less restrictive, thus better for power. Later designs became quieter, heavier, and more restrictive. Aftermarket four-into-one pipes offer significant advantages: they are lighter, more durable, and typically produce more power than stock items. They also improve ground clearance. Jetting for these modifications, particularly with a high-flow air filter and aftermarket exhaust, is relatively straightforward, involving minor adjustments to pilot and main jets and careful carb synchronisation.
Cooling & Clutch: Ensuring Longevity Under Load
A highly stressed, large-displacement Honda four can generate oil temperatures exceeding 275 degrees Fahrenheit, especially during hard riding or heavily loaded touring in hot conditions. Such extreme heat can cause gaskets, particularly the head gasket, to become brittle and leak, and rubber cam-chain rollers to harden and crumble, potentially leading to catastrophic engine failure. Installing an oil cooler is highly recommended, regardless of power increases, to protect engine components and allow for extended oil change intervals. For riders who engage in drag racing or simply desire a better clutch feel, a set of upgraded friction plates can significantly improve the clutch's performance and durability, even for the gentlest of riders.
Transforming the Chassis: Handling and Stability Upgrades
While engine technology saw gradual progress, the 1970s brought major advancements in chassis technology, with stiffer frames, cast wheels, and remarkably well-controlled suspensions. How does the SOHC Honda CB750 stack up? Surprisingly well. Its frame is stiff enough for the best street tyres available. However, critical areas benefit immensely from modern upgrades.
Steering-Head Bearings: Precision and Stability
All SOHC Honda 750s were factory-equipped with loose ball-type steering bearings, which are cheap and inadequate for the bike's weight. These wear out quickly, causing jerky steering and contributing to high-speed instability. Replacing them with tapered roller bearings provides the most significant improvement for money spent. These bearings prevent race-denting, ensuring the forks rotate freely without play, dramatically enhancing the bike's handling and giving a noticeable feeling of stability and security. Regular adjustment after initial use is key to maintaining optimal performance.
Swing-Arm Bushings: Firming Up the Rear
Honda's 750 swing-arm design was commendable for its stiffness, even suitable for road racing before the advent of slick tyres. However, fitting plastic bushings was a cost-cutting measure. These plastic bushings wear out quickly, introducing unwanted play. Installing silicone-bronze bushings is a highly recommended upgrade. This modification, coupled with proper installation and lubrication, ensures the swing-arm operates without binding but also without any side play, resulting in a more solid and stable rear end. This improved stability is immediately noticeable and contributes significantly to the bike's overall handling.
Rear Dampers: Tailoring Your Ride
Almost any quality aftermarket rear dampers will outperform the originals. For touring riders, air or air-assisted dampers offer adjustability for varying loads. For sport riding with occasional light touring, conventional dampers with suitable springs offer better handling for less money. Various satisfactory damper assemblies are available for the Honda's specific lower shock mount. Selecting the correct spring rates based on bike weight and intended use (e.g., solo sport riding vs. two-up touring) is crucial for optimising comfort and handling. A well-chosen set of dampers, combined with the upgraded swing-arm bushings, will make your Honda significantly more stable, comfortable, and responsive.
Front Forks: Reducing Stiction, Enhancing Control
Honda did a decent job with the 750's fork damping values, but the primary limitations are engagement length and stiction. While engagement length is fixed, stiction can be drastically reduced. Early CB750 forks (pre-frame No. 2093730) with 35x47mm seals can still be improved by replacing seals with 'Leak-Proof' types and using quality 5W fork oil. For the best performance, especially for touring or spirited riding, converting to air-assisted forks is highly beneficial. This provides superior ride quality, more secure braking, and adjustable handling. This involves selecting appropriate springs, carefully setting spring preload, and increasing oil volume to raise the fork's compression ratio, leading to a useful rise in air pressure during braking or over bumps. Using seals designed for air fork use, even if slightly oversized, can further reduce friction. Proper fork alignment after any disassembly is paramount to prevent binding and ensure smooth operation.
| Component | Original (SOHC CB750) | Recommended Upgrade | Benefit |
|---|---|---|---|
| Steering Bearings | Loose Ball Bearings | Tapered Roller Bearings | Eliminates play, enhances high-speed stability, smoother steering |
| Swing-Arm Bushings | Plastic Bushings | Silicone-Bronze Bushings | Reduces play, improves rear-end stability and feel |
| Rear Dampers | Basic, often inadequate | Adjustable Air-Assist or High-Quality Conventional | Improved comfort, better load carrying, superior handling |
| Front Forks | Good damping, high stiction | Air-Assist Conversion / Low-Friction Seals & Optimal Oil Volume | Smoother action, better braking, adjustable ride characteristics |
Refining the Ride: Ignition and Tyre Choices
Beyond the core mechanicals, two critical areas significantly impact ride quality, safety, and overall performance: ignition and tyres.
Ignition System: A Stronger Spark
The Honda 750, like many motorcycles of its era, had a weakness in its ignition system. Signs of a poor spark include slow warm-up, short spark plug life, and extreme sensitivity to carburetion changes. While sheer voltage isn't everything, the total energy delivered across the spark gap is. Aftermarket electronic ignitions can dramatically increase the spark's energy by delivering current to the stock coils for a longer period, allowing them to store and deliver more energy, especially at high RPMs. Such an upgrade can lead to quicker warm-ups and improved engine responsiveness, making it a highly reliable and low-maintenance improvement once installed.
Tyres: Grip, Comfort, and Longevity
Modern tyre development has brought a magnificent selection of treads to our bikes, offering choices for all-out traction, longest possible life, or balanced performance. The right tyres can transform a bike's feel. For a balance of ride comfort, long life, and broad compatibility, certain front and rear sets offer excellent all-around performance. For riders prioritising wet-weather performance and maximum traction, specific series of tyres are renowned for their grip, even if they don't offer the longest lifespan. Conversely, certain hard-rubber, stiff-casing tyres provide surprising traction and exceptional longevity. For ultimate traction and feel, some club-level road racers favour specific models, though these often come at the expense of wear life. The key is to choose tyres that match your riding style and priorities, knowing that today's options far exceed those of the 750's original era.
Why Invest in a Classic CB750 Today?
The original Honda CB750 engine is renowned for its durability. When left stock and reasonably well cared for, it will last 80 to 100,000 miles with no major surgery. It can withstand overheating, drag-racing, and almost any abuse except over-revving and too-infrequent oil changes. The modifications discussed here aim to build upon this inherent strength, offering even more reliability while significantly boosting performance. The Honda that results from these thoughtful modifications is a far superior machine than its stock form. The engine will deliver more power, especially in the mid-RPM range, with better response and enhanced reliability. Even more impressive are the suspension changes. While Honda eventually updated some suspension parts late in the 750's production life, they never offered truly high-quality rear dampers. The addition of modern air-fork technology, coupled with high-quality aftermarket dampers and improved steering and swing-arm bearings, brings the CB750's chassis performance remarkably close to many contemporary bikes.
In an age of economic pressures and a growing desire to hold onto well-made possessions, updating a Honda CB750 makes immense sense. It is, without doubt, one of the best motorcycles ever produced. By applying these proven modifications, you're not just maintaining a classic; you're elevating it, ensuring these fine machines continue to grace our roads for decades to come.
Frequently Asked Questions
Q: Is the Honda CB750 still a good motorcycle to own today?
A: Absolutely. The Honda CB750 is celebrated for its robust engine and timeless design. With thoughtful maintenance and strategic upgrades, a CB750 can offer a highly enjoyable and reliable riding experience that rivals many newer bikes, particularly in terms of character and historical significance.
Q: What are the most crucial upgrades for an SOHC CB750?
A: The most impactful upgrades focus on chassis stability and engine breathing. Replacing the stock steering-head and swing-arm bushings with modern tapered bearings and bronze bushings, respectively, dramatically improves handling. For the engine, a high-flow air filter, a good aftermarket 4-into-1 exhaust, and potentially an increased displacement (e.g., 836cc kit) coupled with an appropriate camshaft significantly boost power and responsiveness.
Q: Can I use modern parts on my classic CB750?
A: While many specific parts from the 1981 article are no longer available, the principles of the upgrades remain valid. Modern equivalents for components like tapered steering bearings, bronze swing-arm bushings, and high-quality aftermarket dampers are readily available and designed to fit the CB750, allowing for significant performance enhancements.
Q: How much power can a modified SOHC CB750 reliably make?
A: With an 836cc bore job and improved breathing (air filter, exhaust, cam), an SOHC CB750 can reliably achieve around 75 horsepower. Exceeding this figure typically requires more extensive and costly modifications to the connecting rods and other internal components, which can compromise the engine's legendary reliability if not done correctly.
Q: Why is an oil cooler recommended for a CB750?
A: While Honda initially stated it wasn't necessary, an oil cooler is highly recommended, especially for bikes ridden hard, in hot weather, or heavily loaded (e.g., touring). High oil temperatures (over 275°F) can degrade oil, harden rubber components like cam-chain rollers, and make gaskets brittle, potentially leading to leaks or premature engine wear. An oil cooler significantly extends engine life and allows for safer, longer oil change intervals.
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