CB550 carbs are 22mm bore and CB750 are 28mm bore. The rubber couplers are not interchangeable. And, would require special adapters or something creative to effect a seal between the smaller carbs and the larger couplers. Re-jetting the 550 carbs to compensate for the increased air velocity required of the larger motor will be way more challenging than correcting the jetting in your current carbs. Not sure, but I think the carb spacing is different, too.
I think we "talked" about this bike before. It's the chop, right? Do you know if the carbs are in stock form from the donor bike? Or, has someone already twiddled with the innards?
Are you reading the correct part of the plug?
See: http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
for what plugs should look like and to be sure your plugs have the center electrode insulator sooty.
Sooty plugs, regardless of how new, can become carbon fouled and fail to fire. Instead of jumping the spark gap, the energy travels through the soot (carbon) built up on the center electrode insulator of the plug. Sometimes you'll see a tiny bit of spark at the electrode only. But, it is not bridging the spark gap, and can cause failure to fire the cylinder.
Let's review basics. The cylinder needs compression, spark correctly timed, and a reasonable fuel air ratio, for the cylinder to produce a power stroke. For discussion, let's assume you do have correct spark. Before you can jump squarely on the carbs as the sole culprit of misfire, wouldn't knowing the results of a compression test be helpful. I mean, who wants to fiddle with carbs, if you have a valve problem?
Ok, if the compression is even across all 4 cylinders, and spark and timing are good, then those four beastie carbs are gonna be threatened with the pointy end of a screw driver. The shotgun approach of replacing the entire carb assembly only has a chance of success if you have the same type on hand and know they are jetted to match your engine's fuel mixture requirements. Otherwise, you'll be fixing the same problem (or a different one) on another set of carbs.
Carbs are misunderstood by many people, I think. Most of the SOHC4s use carbs that have mechanically linked slides, operated directly from the twist grip. (CV carbs have slides controlled by engine demand, not operator demand.) So, while tuning your carbs, temporarily mark your twist grip, so you have a reference for the slide position you are selecting for the bike during operation. Why?
Let's review how fuel is mixed with the air delivered to the engine. Let's start with the main jet. This is located near the bottom of the float bowls on the carb. Its orifice size must be large enough to provide the fuel needed for the engine at wide open throttle conditions. Since we need engine speeds lower than max RPM, we position the twist grip and the slides to restrict the amount of air going to the engine. But, less air also needs less fuel for a proper mixture, so attached to the slides is a needle valve, tapered to gradually diminish fuel flow as the slides are lowered. The needle valve is fed by the main jet. The needles further restrict the rate of flow from that source. For these carbs, the throttle valve (slides) and needle jet combination, effectively control the fuel mix between 3/4 and 1/8 throttle positions. Below 1/8 throttle positions, the throttle valve, can’t efficiently atomize the fuel for smooth operation. So, another slow speed circuit is present in the carb for these throttle settings, which meter fuel independent of the slide needle and main supply. This circuit gets fuel from the float bowl, too. It’s the slow jet, with a tiny orifice, usually located higher in the bowl so as to reduce sediment clogging. This jet along with either an air bleed screw or an Idle Mixture Screw, controls the fuel air mix when the sides are nearly closed and it’s needle is mostly shutting off fuel from the main jet.
Each metering circuit dominates the fuel mix relative to the throttle position. But, none are totally isolated at any throttle position. To some extent changing one will effect operation at other throttle settings.
So, with this understanding, and knowing that your plugs are sooty, tells us that the carbs are running rich. But, do we know if this is at all throttle positions? Do we change all the metering devices? Or, just the one we are dominantly operating on? A little more information is needed. If it is only midrange operation, perhaps lowering the needles on the slides a clip position may reduce the mixture over the 1/8 to 3/4 throttle positions to stop fouling plugs?
But, wait there’s more. Both the slow circuit and the main/needle circuit have emulsion tubes. What these do is premix air with the fuel prior to release into the carb bore. There are two air jets near the inlet of the carb bore, normally a drilled restrictor hole in the casting. Also, the emulsion tubes have cross-drilled holes that can vary in number and hole size to effect the premix of air into the fuel, and the ultimate fuel mixture delivered to the engine. An engine that is running too rich may have the emulsion tubes or air circuit clogged.
A few words about jet flow. Yes size matters, but also the pressure differential. A given orifice size will flow a given amount of fluid for a set difference in pressure from one side of it to the other. Increase the pressure differential and/or the orifice size, and more fluid will flow. (Given the same viscosity fluid.) The carburetor uses atmospheric pressure to push fuel through its jets. The float bowl reservoir is vented to the atmosphere, and this pushes the fuel in bowl through the jets it supplies.
The running engine lowers the atmospheric pressure in the carb bore. The resulting difference in pressure forces fuel through the jet orifices.
So what?
Well, if you change the pressure in the carb bore by closing the choke butterflies at the entrance to the carb, more fuel flows through the jets and into the engine because you are isolating a source of atmospheric pressure at the carb entrance. A similar, but not as drastic effect occurs with restrictions in the air filter. A carburetor ideally balanced for correct fuel mixture given a specific restriction at the carb entrance will go either rich or lean, given an increase or decrease in restriction offered by the air filter.
Oh yeah, you asked about synchronization. This is done so each carb slide is positioned in the same place relative to the all the others. Some where in the runner between the carb and the cylinder head is a screw plug. This screw is removed and a fitting is placed there and four manometers are temporarily attached to the fittings. The engine is run and the slide openings are adjusted so each of the manometers read the same vacuum pressure for all cylinders. I like to do this at idle speeds where the slide openings are very small and minute differences in slide position show up nicely on the manometers.
Ready to tackle those carbs now?
Cheers,
