A couple of weeks ago, Bill fielded a cry for help from a CA77 owner, just up the road in Long Beach. After lengthy Q&A discussions about why his Dream would suddenly quit after a relatively short ride (enough to get the bike fully warmed up), the focus was on the carburetor as the possible culprit. He had cleaned the carb with an ultrasonic cleaner and changed the float levels around trying to solve a persistent rich condition at idle, plus this sudden “come to a halt” syndrome, which had also developed. It was hard to say if there was just one source for this problem set or not.
The spark plugs would come out blackened after his run and we both assumed that they were fuel-fouling to the extent that the bike would just quit running. He had to turn the mixture screws out over 2 turns (3/4 to one is stock setting) to keep it idling somewhat regularly and his gas mileage was way down from where it had been in the past.
We discussed ignition timing, points/condenser, compression readings, valve clearances, voltage to the points and the usual bullet points on the troubleshooting lists that come to mind when these symptoms occur. He had it pretty well covered, but initially shared that the compression readings were in the 120 psi range, instead of 150, where they should be. I offered that the cam timing could be a bit off, but the he had failed to check the compression with the throttle wide open. The WOT readings were indeed at 150psi, so that ruled out cam timing as a problem. I offered to ship him my spare “round bowl” CA77 carburetor which I had cleaned up and put up for sale once on eBay, but got no offers a few months ago. He was happy to try it out, so I shipped it up to him and it arrived in a day. Initial results were that the bike idle screw settings were happier at 1 turn instead of two, but after more test runs, the bike still would quit and not restart until it cooled down for a few minutes.
I urged him to take a spark plug socket with him on test rides and when the bike quit quickly pull a plug to see if there was spark and fuel getting to the plug. Sometimes, you just have to catch it when it is caught in the act of misbehaving, rather than trying to noodle it out in the shop afterwards. Having more or less eliminated the carburetor as the problem of the bike shutting down, he decided to change out the coil instead. Having a spare set of CB750 coils on hand, he creatively mounted and connected the CB750 coil set to the Dream and took it out for another run. This time… Success! The bike ran smoothly and evenly, never faltering or dying at idle. Measuring the resistance values before and after, they had shifted from 4.2 ohms on the primary of the stock coil to 4.6 ohms on the CB750 coils. Normally, you should see about 4.5 ohms on most 250-305 coils. It doesn’t sound like much, but when the values start tailing off of the 4.5 readings, there is probably insulation breakdown inside the coils, which probably worsened as the coil heated up during operation. In this case, the voltage dropped off at idle speeds when battery voltage is lower due to the way the charging system operates. Lower voltage in the primaries equals lower secondary voltage at the plug wires/caps and spark plug gaps. With low output voltages and fuel-fouling spark plugs, the power strokes started losing their punch.
So, in the end, the stalling issue was primarily a problem with the coil, but the secondary fuel-fouling problems still lay with the original “square bowl” carburetor. The carburetor air bleed ports in the throat hadn’t been physically cleaned with a wire or carb spray and despite the use of the ultrasonic cleaner, I feel like those airways were still plugged up which reduced the air available to the idle and main metering circuits. For proper fuel emulsion, both air and fuel must be mixed up in a proper ratio. The engines can’t consume big globs of fuel droplets; only a mixed fuel vapor will ignite properly in the combustion chambers.
My method of checking/cleaning the Dream carbs, which are equipped with a flame suppression screen, is to either poke the screen out of the inlet using a small screwdriver handle end or just poking a small hole in the screen with a scribe. The small hole will allow the introduction of a carb/brake cleaner spray can straw to guide itself down into each air bleed orifice. A quick, strong burst of cleaner will either blow back in your face (always use safety glasses) or will exit the idle and main jet circuits as each one is tested. A small wire can be inserted into the inlets, as well; however the passageways take a turn as they meet up with the fuel nozzle passages, so they will only do so much to clear out a blockage.
I remember owning a very low mileage CB77 that kept fouling the right side spark plug. I was going to blame the coil or condenser, as the idle and main jets were the correct size and clean. Finally, I removed the air filter and peered into the carb throat only to see varnish buildup in the main jet air bleed port. Blasting that passage open solved the problem, once and for all.
Generally, the carburetors are the last things you want to go “adjusting” when performance issues raise their heads, but once you have confidence in the engine condition and ignition timing setup, then the carburetor’s behaviors become the focus of attention. When the idle mixture screws don’t respond properly or are way off the normal settings, then you have to consider whether the idle jet is the correct size or the float level is incorrect. As mentioned, previously, the air bleed port in the intake is the other mixture factor. Beyond that you have to consider air leaks at the carburetor flange and/or carburetor insulator and their respective o-rings. Vintage 250-305 carburetor flanges tend to bend and warp, requiring some flattening with a large file and renewal of the o-ring. Insulators are known to disintegrate and break away around the o-ring channels. The carburetor top gasket must be intact to prevent unwanted air going past the slide, too.
Once you have sealed up the intake leaks, then idle/part throttle response is controlled by a combination of the idle jet size/mixture screw setting and the carburetor slide needle clip position. Honda issued a tune-up bulletin for the CB77s, which included raising the needle one notch to the 4th slot to remedy a part-throttle hesitation issue that arose. No other changes were recommended from the bulletin information, but that was all that was needed to solve the problem.
You have to keep an open mind when it comes to troubleshooting. It is easy to point to just one source, but sometimes it can be a combination of problems that can confuse and confound you, until you tease them apart and remedy each one separately.