fishbob:
If a carb’d motor such as a Honda 90 stalls when the throttle is advanced from idle to mid- or high- rpm settings, it’s usually because the fuel/air mixture, which was already quite lean, has leaned out to the point that you don’t get combustion. In other words, too much air, not enough gas, motor hesitates or stalls. That can happen for several reasons. First, there could be problems at the fuel tank, or elsewhere, that result in the carbs being so fuel starved that they are only capable of delivering enough gas to support an idle. This could result from a vacuum in the fuel tank, partial plug in the fuel line, etc. Next, the fuel could be of such a poor quality, or sufficiently contaminated or degraded, that the effective octane rating is outside that for which the motor was designed. Finally, there might be sufficient quantities of adequate quality fuel delivered to the motor, but not on through the motor’s fuel filter, pumps and carbs.
Since the fuel delivery system begins at the tanks, and ends at the carb throats, it might help to isolate parts of the system, and see if they are a part of the problem.
Some isolation points have already been suggested: once a motor stalls, open the fuel fill cap, and listen for air rushing in. If you don’t hear any air, and the problem doesn’t go away, you’ve already eliminated the fuel tank vent.
The next isolation point I’d suggest is the fuel line quick disconnect at the motor. The next time a motor stalls, assuming you can do so safely, immediately disconnect the quick disconnect, and shut the motor down. Now you’ve got something you can work with, assuming you can safely do so. Since you’re going go be playing with the fuel system, turn off the other motor and shut down electrical appliances, bulge pumps, etc. when you can safely do so.
To check whether you’re getting fuel to the motor, use a plastic or wood tool (not a toothpick) to depress the ball valve in the part of the quick disconnect that’s in the fuel tank side of the line. It’s the one that you’re holding in your hand, not the one that’s built into the motor. I like to use the rounded end of an electrical cable strap, and gently push it slightly into the ball until it stays in place by itself. Gently and slightly are the operative words, since you don’t want to damage the ball, or have trouble removing ALL of the cable strap. The fuel line should be full right up to the connection, so hold it with a rag, above the top of the fuel tank, and pointed away from you before you depress the ball. It’s possible this part of the system is under pressure, so be very careful, because if there’s pressure, the fuel will want to spray. Lower the connection slowly, with the ball depressed, and fuel should run out of the connection. If no fuel comes out, you’ve got a problem somewhere between the tank pickup, and the quick disconnect. If fuel comes out, it’s time to start looking under the motor cover. Go ahead and reconnect the quick disconnect.
Remove the motor cover, and do a little more detective work. The first thing I’d do is inspect the opaque in line fuel filter, and see if it looked full or empty. Next, get a rag and flat bladed screwdriver, and beginning with the top carb, put the rag under the drain at the bottom/side of the float chamber, crack the drain screw in the bottom of the float chamber (it’s the one that’s about ¼ inch diameter on the side of the float chamber, at the very bottom, not the small one higher up) and see what comes out. You should get about ½ of a shotglass of something that smells a lot like gas – don’t stick your nose in it, but get just a slight wiff from a distance. You’ll know if it’s good. You can also catch the fluid in a clear container and look for water, but that’s easier said than done. Then, snug the drain screw back in place, and head on down the line, doing all 4. You should get the same amount of liquid from each, and each batch should look and smell the same. If you have a medical concern with the smell test, don’t do it – just look.
I’d strongly suggest going through this drill on dry land first, so you are familiar with the process, and have the rag, cable strap and screwdriver on hand for later. Just lower the motor into position, pump up the fuel bulb, and have at it. If you test the quick disconnect on dry land, be aware that if you pump it first, the gas will be under pressure, and squirt over everything, including you, if you depress the ball valve. Use extreme caution.
If the quantity of gas removed varied significantly from one carb to the next, you’ve got a float or fuel pump problem, and should consult a mechanic. If the gas was contaminated, buy fresh gas. Otherwise, with the motor cover still off, and the carb drain screws closed, reconnect the fuel supply line, pump up the bulb, and watch and listen to how the fuel flows through the motor’s in-line filter, and on as the carb bowls refill. If this happens smoothly, without your having to wrestle with the bulb each time you press it until the carb bowls are full, you’ve just confirmed that the tank, line, external filters, bulb and in-line motor filters are pretty much working. There might be defects in those parts, but they aren’t causing the problem you’re worried about. At this point, the focus should be on the carbs, and the diaphragm acceleration pump. There’s been a bunch written on the website about these, but since messing with the carbs is costly, whereas the diaphragm pump can be checked out for less than a case of beer, I’d focus on the diaphragm pump.
If you get to this point, and need more help, or want to discuss the diaphragm pump adjustment a little more, just do a post.
Either way, let us know what you find.
Good luck!
