I purchased a motorhome that had the original Generac genset replaced with a Onan Camp Power/Micro Quiet 4000 genset. The genset had only 22hrs on it and suffered from the much talked about carb problems from just sitting around without use. The genset would attempt to start but would die just after releasing the starter button. I did a internet search and found the carb issues (and replacements) well documented. Some folks have replaced as many as 3 carbs
With only 22hrs on the genset I was determined to fix the problem without having to spend as many as $300 (as read about) on a replacement carb.
Here's what I did: after disconnecting the battery power from the genset starter solenoid I removed the airbox cover and air filter (wingnut, cover wingnut, filter)). The filter has to be removed by pulling it straight out from the housing and then tilt the closed end downwards to clear the mounting post. You can then pull it up out of the housing. Next remove the fuel line from the carb. Mine had a crimp band that I cut off and replaced with a standard stainless gear clamp. You have to disconnect the anti-backfire solenoid wiring from the carb. Here's where I found the first problem with this genset. When Onan assembled this unit the ground for the anti-backfire solenoid on the carb was not correctly installed on the engine block. The star bolt used to attached the ground's ring terminal was not threaded all the way into the engine block so the connection was loose from the git go. They also placed the ground on the outside of the painted support bracket the holds the stop/start switch with only a star washer to try and ground the solenoid. Between the bolt that did not tighten down correctly and the dismal surface contact for the ground it was not surprising there were solenoid issues. Even though I found this issue I felt it was important to fully remove the carb and give it a good cleaning. Unplug the positive side of the anti-backfire solenoid.
You need a 10mm socket on a 1/4" drive ratchet extension to remove the two retaining bolts for the carb/air filter housing. Reach through the air filter housing and loosen/remove the two long 6mm-1.0 thread bolts (10mm hex head). There are flat washers on each bolt so don't loose them! After removing the two long bolts compress the air filter adapter into the sealing foam to get some clearance between the carb and the filter housing. Don't try to yank the carb out before you do this or you chance bending the choke and or throttle plates on the carb. Reach up and close the choke plate (pull the rod toward the choke spring element mounted on the right side of the engine) This will allow the air filter adapter to slip out to the side of the unit until the filter mounting post hit the inside of the filter housing. This is enough clearance to allow the carb to clear the intake manifold... be careful not to force the carb as the throttle plate may be caught on the intake manifold adapter.. (you don't want to bend it). Once the air boot adapter is removed the carb is just retained by the choke rod, throttle rod and throttle rod spring. Do not bend these rods to remove them from the carb! Just tilt the carb forward from the back to disengage the two rods and the spring.
Once the carb is in your hands you will need a 3/4" wrench and a 11/16" wrench to remove the anti-backfire solenoid and the float bowl for cleaning. Using the 11/16" wrench unscrew the brass solenoid assembly out of the bottom of the carb. There is a copper washer to seal the bottom of the bowl against the brass solenoid adapter... don't lose it. At this point the flaot bowl can be removed from the carb. Be careful not to lose the aluminum washer that is on the inside bottom of the float bowl which the brass solenoid adapter threads through. Set the carb and float bowl aside. Grab the 3/4" wrench and the 11/16" wrench and unscrew the solenoid from the brass adapter. There is an aluminum washer between the two.... keep track of it. The solenoid plunger extends up into the brass adapter and down into the solenoid. The smaller part that actually does the sealing work for the main jet circuit is plated steel. Make sure this shaft and flat end do not have green deposits from our wonderful ethanol based fuels on them. Use carb cleaner and/or scotch brite to clean the post from all deposits. Attached to this steel plunger is the grey/black iron solenoid actuator. This is the part that retracts into the solenoid housing, when it is energized, which pulls back the metal plunger allowing fuel to flow through the main jet circuit on the carb. Unfortunately this plunger assembly is at the bottom of the carb where any and every piece of dirt that makes it's way into the carb and the remains of evaporated fuel ends up. If this plunger can't open your engine can't run. Clean this iron actuator with carb cleaner and/or scotch bite and make sure it moves freely in the solenoid housing. Remove the small return spring from the housing and give it a good cleaning along with the housing itself... being at the very bottom lots of gunk can be trapped there. Use plenty of carb cleaner to make sure the solenoid is spotless. Grab the brass section and make sure all of the fuel feed holes that are radially drilled around it are clean and free of debris. Once everything is clean drop the spring into the solenoid housing, install the iron end of the plunger into the housing next. Make sure the aluminum washer is on the housing and screw the brass section of the assembly onto the solenoid. Tighten the sections together with the 3/4 and 1/16 wrenches. Now focus on the float bowl and float. If your carb is really bad you might want to remove the float pin and drop the float assembly out of the main carb body to clean it completely. Be gentle with this assembly making sure not to bend the float valve arm which will change your float level and affect engine operation. Once this section is clean and the float is reinstalled on the pivot pin clean the float bowl itself. Here's where surging of you genset can manifest itself. Surging is caused by a lean condition.. If your travel limited altitude adjustment tapered needle in the float bowl is covered in green deposits from ethanol fuels your engine WILL run lean and surge when running!
Cleaning these deposits with carb cleaner is important! I use Seafoam... it's awesome stuff for carbs and will remove these deposits over time even in a diluted state mix with fuel. It is far better also for storing your genset than just a normal fuel stabilizer. Once the float bowl assembly is clean stick the solenoid assembly up through the bottom of the bowl making sure the copper washer is on the brass assembly between the bowl. Now drop the aluminum washer onto the brass assembly and into the recess in the bottom of the bowl. Install the float bowl/solenoid assembly onto the carb body and screw the solenoid assembly into the carb body. Align the float bowl drain/altitude adjustment towards the front of the genset on the carb body and tighten the brass section of the solenoid assembly with the 11/16 wrench. Don't go crazy here tightening.... brass, aluminum and copper don't need you to show off your muscles.
You are now ready to reinstall the carb... first install the filter mount post assembly in the filter housing. Then tilt the carb forward back end up to install the choke and throttle rods and the throttle spring. Move the carb into position while compressing the soft rubber seal on the the carb/airbox adapter being careful not to scrape off/damage the gaskets on either side of the carb. Align the carb on the manifold and slip in the ack carb/airbox retaining bolt... make sure the flat washer is on the bolt before installing it. You'll have to move the carb around a bit to find the threaded hole in the manifold. Once it's starting into the hole a few threads align the other hole and install the remaining bolt. Tighten both bolts and install the air filter and cover. Hook up the solenoid to the spade connector and make sure the ground side has good contact to the engine case when you install the retaining bolt and star washer as noted above. Prime the carb a few times until the fuel pump quiets down. Make sure your choke spring temperature assembly is adjusted properly... mine was set way too lean so the genset would surge like crazy until it finally warmed up enough. Loosen the 9/32 nut and turn the choke adjustment plate counter clockwise to richen the choke. Again, don't go crazy here.. I moved mine 3 marks on the choke adjustment and it solved the surging when cold issue. Tighten the 9/32 nut while holding down on the spring assembly so it doesn't rotate up and bind the choke rod. Make sure both the throttle and choke rods/valves move freely without binding. You are ready to give the genset a go! If you are at sea level and the genset still surges when it is fully warmed up and you are sure it is not a fuel supply related lean condition (fuel line air leak, pump issue, plugged fuel filter) then you need to open the main fuel circuit altitude adjustment needle further than what the black travel limiter will allow. I trimmed my black plastic limiter off on the rich side so I could open the needle valve a bit more. (turning the adjustment screw further counter clockwise than the travel limiter would originally allow) This ended my surging issue that I was having. Once I got the genset running for an hour or some running the air conditioner I readjusted the altitude adjuster a bit leaner making sure the genset still responded to loads without surging or stumbling. So now I'm a happy camper without spending $300 for a new carb. Seafoam should prevent this in the future... but if not I know what needs to be done.