I posted this up on MTF a few years ago. I haven't seen a guide on how to fix the surge in 75% of the Duraforce's that I have come across. I can't take credit for any of this. It is a compilation from posts at MTF and my experiences fixing over 40 of these mowers. Some with a slight surge and some that were almost like off and on.
Now I have had my 10323 for a long long time, and have used it so much. It has never surged at all, just ran so amazing. now my grandpa's 10323 has never run right. I will get to that after a bit.
Anyway I got a 10550 a while back and it surged a bit. Nothing terrible like my grandpas, which will barely run. But it did surge. So i have been digging around this forum quite a bit about all the DF surging issues.
So I ordered up the Briggs & Stratton 42.5 Jet, part number 801308.
I also ordered up a new throttle plate Lawn boy part number 98-7047.
Total parts bill was $4.42.
First i looked at the pilot jet, it was the 37.5 jet, so i put in the 42.5 jet. This really didn't make too much of a difference. So i finished up mowing up my leaves in the back, and thought I would check to see if I had one of those smaller holes in my throttle plate. Took off the carb and looked and yeppers, the hole in my throttle plate was A LOT smaller then the replacement one I ordered, 98-7047. So i took that smaller one out and put in the replacement one. Really easy to do!
Started it up and it smoked a bit and ran what seemed to be too slow. So i turned up the RPM 4 clicks, and bam sounds so good now. Just like my old 10323 that i recently sold. Soooo much power. WOW. That is all I can say is WOW. Its like running the mower on idle before and now its all opened up. Its night and day difference. Im so excited!
Can't wait to tackle my grandpas 10323. I have all the parts for his as well. If anybody has a Duraforce that surges even a bit, I would highly suggest looking at your throttle plate, and getting the proper one and installing it. Also getting the larger B&S pilot jet and installing that as well.
So a step by step cure for the surge. This has worked on 100% of the Dura forces that I have come across.
1. Clean the carb. Including taking out the pilot jet on the side and clean it. It is under a sticker. This will let me verify everything.
2. I make sure the float is the right height, 1/2" above the base of the carb when upside down.
3. Replace needle and seat if needed.
4. Verify that the throttle plate has the proper vent hole. It's the plate at the back of the carb, closest to the engine. If its the tiny hole, then take a pair of plyers and pull it out. Replace it with the correct part number. Yes they made a mistake and put in the wrong one. Make sure that you note the position of the flat part of the throttle plate
5. Put it all back together and fire it up. Let it warm up, use it for a while.
6. If it still surges a bit, put in the B&S jet. Its on the left side, when you are mowing, of the mower on the outside of the carb. It is under a sticker originally. Put in the new larger jet.
7. This will fix it every time! If you think your mower is now running too slow, then adjust the governor collar 2-3 clicks up to increase the RPM.
8. Mow what ever you want now. Look out because you will not be able to stop this mower with grass. It will simply smooth out the taller and thicker grass you mow.
9. Seriously the power is un-real. Gas consumption may go up a bit but it is mostly un-noticeable in my opinion.
Other guys have had success with drilling the main jet. I have never tried it as this "system" has worked every time.