Briggs and Stratton 42A707 surging at idle

swiltseJuly 27, 2010

Briggs and Stratton 42A707 2238E1 9801145B Engine

Bought a used riding mower last summer with this engine. Seemed to be running fine last summer, when I went to run it this spring it wouldn't maintain a smooth rpm when idling at no load. First thought was bad fuel, so I replaced the fuel with fresh fuel treated with fuel additive, replaced the fuel filter and one of the fuel lines, replaced the air filter, changed the oil and sprayed the carb with carb cleaner. I'm getting good fuel flow from the tank to the carb and the carb seems to be getting adequate fuel. Engine still surging. So I replaced the plugs and it seemed to run fine for a few mowings and then it started surging again. When it surges if I lower the throttle enough it will eventually stall.

I replace the plugs and started troubleshooting again and thought I had a problem with the coil because I was having trouble running the right cylinder (if sitting on tractor facing engine) by itself but I could run the left cylinder, so I put the ignition wire from the left cylinder on the right and was able to run the right cylinder by itself (so i assumed the cylinder/pistons/valves are ok). Couldn't test the right ignition wire on left cylinder because it was to short, but when I tried to test for spark on that ignition wire I couldn't get one. When running on the one cylinder it seemed to stop surging, so I figured I needed a new coil. Put on new coil and engine seemed worse. Had trouble getting it started, especially once the engine was hot. Couldn't get it to run on one cylinder. So I put the old coil back on and it seemed to start better and now I can get either cylinder to run on their respective ignition wire, seem to be getting good spark on both wires, so I'm not sure if there's an ignition problem or I just wasn't testing properly, but the engine is still surging.

At this point I'm not really sure what is causing this. I've removed the carb and started cleaning it, will be getting a carb overhaul kit today and hopefully putting that all back together tonight. But the carb doesn't look dirty and I found nothing stuck in the jet. When the engine was running I could see fuel spraying into the carb, so I'm not really to optimistic that the carb is the problem. It seems to be getting good fuel, so it doesn't appear to be a fuel restriction problem. It seems to be getting good spark, so I'm not sure it's an ignition problem, unless the flywheel key is damaged and the timing is off. At this point I'm wondering if it has something to do with the governor or valves/pistons. Any suggestions?

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Well there are several things here. First while there are two spark plug leads, there is only one coil. I don't see where you cleaned the carb float bowl. Remove the hex head plug on the lower right side (when facing the carb) be aware that gas and perhaps crape will run out. IF this doesn't help, you may need to remove the top of the carb and clean the bowl. Caution: the gasket must come off with the top, this gasket will be very hard to find short of an overhaul kit if damaged.

Before removing the carb top, you may want to check compression on erratic cylinder. It is possible a valve is sticking OR a valve seat is hopping out. Also, there is an adjusting screw just below the air cleaner sticking straight out at you. Turn the screw 1/8 turn counter clockwise.

Walt Conner

    Bookmark   July 27, 2010 at 3:35PM
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Thanks for the advice Walt.

I removed the carb last night, disassembled and have been soaking it in carb cleaner. I got a carb overhaul kit and will be putting the carb back on tonight.

I will check compression on both cylinders.

I'm not sure what adjusting screw you're referring to. I'm assuming you're referring to an adjusting screw on the carb. The only screws on the front of the carb that could be adjusted are the Idle Speed screw and the Idle Mixture Jet screw. The Idle Speed doesn't seem to be my problem and according to the Briggs and Stratton manual for this engine, the Idle Mixture Jet screw is nonadjustable.

    Bookmark   July 28, 2010 at 11:57AM
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So I put the carb back together with the new parts from the rebuild kit and put everything back together on the mower. Started up fine and ran steady at full throttle. Tested low and medium throttle and seemed to run good. So I decided to take 20 mins to mow the front lawn and see how it ran. Everything seemed good while I was mowing, so when I got done I lowered the throttle to about half speed and it started surging again. Brought it in the shed, checked for good spark on both cylinders and ran engine on each cylinder separately. Fired it back up with both cylinders and ran fine at all throttle levels. Mowed the back lawn the next morning and no problems at all. Ran it at low, medium and high after I got done mowing and no surging.

I am still a little concerned though because it did surge after the first mowing when i put it back together. Do you think my problem is fixed or is there something else I can check?

The funny thing is my friends mower does the same surging when he runs at half throttle, but it runs fine at full throttle. He has a different model, but its a Briggs and has the same basic configuration. Is there a known problem that will cause these engines to surge when not at full throttle?

    Bookmark   August 2, 2010 at 1:45PM
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You are esentially seeing the effects of the governor trying to find a "sweet spot" in lower RPM range that it is not able to find when the engine has no load on it.
The surging is actually quite common to the opposed twins and is not really a "problem", it's just a characteristic of an engine that was built to do some work at full open throttle and full fuel flow through the main jet. At low RPM and no load, the governor has to hunt for satisfaction
that it can't seem to find.

    Bookmark   August 3, 2010 at 1:52AM
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