Those of you who have been following the exploits of this column recall the acquisition of the 1976 Benelli Sei (750cc 6 cylinder) model which was unearthed after a twenty year snooze. When offered for sale, it would start up, but barely run above idle with attendant backfiring, misfiring and generally uneven running characteristics.
The first order of business was to check compression readings, which were all rock-solid at 180 psi. After that, the carburetor rack (3 on one bracket) was removed for inspection, cleaning and replacement of any damaged, contaminated or worn parts. The bike was showing 18K miles on the odometer and it had only gone through two previous owners before purchase. Carb inspection and subsequent performance issues were traced back to a sinking plastic float and a sticking enrichener valve on the middle carburetor. Despite the half dozen cylinders that needed feeding, the intake system consisted of three 24mm Dellorto carburetors, which are a bit on the small size for an engine of this displacement, especially when each carburetor is sharing a pair of cylinders via the 2:1 manifold system used from the factory. The bikes were rated at 71 horsepower vs. the 64 horsepower rating of the first generation CB750 Hondas. The Japanese brand was carrying a quad set of 28mm mixers, which should have given it the edge in power readings, but performance was pretty much the same for both machines.
When the carburetors were removed, the intake manifolds were checked and one was split nearly all the way around. Another was cracked to a smaller extent and the final one was about to open up as well. Because the engine is actually based upon a Honda CB500 Four, many of the parts are similar or exactly the same as the donor model. A set of three intake manifolds were ordered up and received quickly, however it was apparent that the Honda brand units were about ¼” longer than the OEM Benelli parts. This extra length probably would have caused difficulties if the stock airbox had been used, but the bike was reworked with K&N pod filters which just did fit onto the ends of the intake mouths.
Further inspection revealed flattened out intake manifold o-rings, so those were replaced in order to keep the intake tracts air-tight, which reduces backfiring and uneven engine performance. Still, after all this effort and the installation of new parts, the bike was still running poorly off-idle. With the intake system pretty well sorted, all that is left is the ignition system, which was a big can of worms from the outset. Benelli had to place three sets of ignition points on the base plate which was designed for two sets plus the pair of condensers on the original four cylinder design. The Italian solution was to cast recesses into the engine crankcase castings which allowed the three separate condensers to be screwed into the cases, right behind the ignition point base. Seems like a good solution to use the space available for the additional set of ignition components, however the points were of a rather inferior quality and the condensers were now bathed in hot, hot crankcase temperatures. While designed for high temperature environments, condensers would much rather be left to cooler conditions during operation. The stock ignition system was known for hot start problems, as the components cooked in their places, all covered with a nicely sealed chrome metal point cover.
The five-hundred dollar solution was to order a crankshaft mounted digital ignition system from Germany, which was designed for this engine series. While the ignition system found its way across the Atlantic and then across the US from NYC, work continued on the stock ignition system components. One set of points appeared to be partially seizing up on the pivot shaft and the point gaps/timing all seemed to be somewhat off specifications. Once the ignition timing was setup properly, the engine responded somewhat better, but not really the way they should be running with stock parts.
One of the ignition coils, originally supplied by Nippon Denso (same as the Honda items), apparently failed due to heat rising from atop the cylinder head. In all probability the CB750 coils would have worked as replacements, but the owner chose to order some large metal-jacketed coils that required creation of a mounting bracket and the extension of wiring to reach back behind the carburetor rack. The new coil wires were correct solid-core units, but were now about three feet long, instead of less than one foot each on the original coil sets. The wires were all zip-tied together, all in two groupings of parallel bundles. From ancient teachings in 1960s auto shop classes, there was the mention of “cross-firing” in spark plug wires which ran parallel with each other for a long run. The power pulse of the original plug wire induces a secondary voltage in the adjacent wiring, causing it to “fire” the spark plug out of correct timing. From the looks of the wiring setup, cross-firing would seem to be a high priority condition to address, but before that occurred, the last step was to check the spark plug caps for proper continuity and resistance values. Using a volt-ohm meter, the spark plug caps, unscrewed from the wires show resistance values of: infinite, infinite, thirty-three thousand ohms, thirty-three thousand ohms, thirty three-thousand ohms and the final one at the specified five-thousand ohms. AH-HA! It all made sense now, especially when the rest of the systems had been checked and setup to specifications. A set of long-necked NGK-branded, 5k ohm spark plug caps were ordered up and received quickly from a nearby eBay vendor. Once the new caps were installed, the engine started up on the first couple of revolutions and positively barked out of the exhaust pipes with clear, resonant tones. No more backfiring, no hesitation, no part throttle hesitation, just a smooth-running engine hitting on all six of its happy cylinders. The Benelli Sei was now a much happier machine than it had been in many years.
The old, metal-jacketed plug caps were also a product of N-D, but the tiny internal resistors held inside were all toasted, mostly due to the high cylinder head temperatures and the confined spaces they were held into in order to reach the ends of the spark plug tips. Only two of the six resistors were still colored their original blue hues, but just one of those was still reading a normal 5k ohm level.
It has been observed that more and more of the spark plug caps, dating back into the 1970s, are now found to be failing or failed altogether after 30-40 years of service. Often, the engines will idle decently, as the spark requirements are low at that speed. Once you crack the throttle open, a huge rush of air is inhaled by the carburetor where it is mixed and sent down the intake manifold to feed the intake valves as they open up. The air-ratio on this throttle opening goes lean, momentarily, then richens up as the metering circuits catch up to the air flow demand. The lean mixture is difficult to burn, requiring the ignition coils to dish out a greater amount of spark energy in order to keep the process under control. If the spark fails to jump the gap due to excessive resistance in the secondary wiring (plug wire, plug cap and spark plug), then a misfire occurs.
On an ignition coil, which is supplying spark for two spark plugs at the same time, excess resistance affects both spark plugs, one of which has reversed polarity. One bad spark plug cap is bad enough, but a pair on the same circuit will fail to fire the engine much past idle speeds. That is what happened to the Benelli’s ignition system. Despite the installation of performance coils and new wires, the whole system was compromised by the reuse of the forty-year old spark plug caps. If you are fighting a misfire in your engine and all the fuel systems are known to be clean and functioning to specs, it is time to back up and check your ignition timing and the condition of the spark plug caps.