It is a solid engine these days but headed for the scrap heap. The Quantum Power began in the late 80's and went through a period of trouble with the valves. The fuel tanks have also gone through troubles and still have some. Initially the problem was with the shut off valve which they originally had from the factory. After several failed attempts to correc they just eliminated the valve. But ever since they have had problems with seam seprations causing leaks. Still do. They also now have some plain old tank cracks on the inside corners. The engine valve problems have ong since been corrected. The tanks with fuel shut off valve troubles were placed on a safety recall. Still good for those who have one. The seam and crack leaks although not on a safety recall will still be warrantied as long as the failure is not caused by obvious damage. The engine has been the mainstay for the Briggs rotory mower line up for years and except for the occasional continueing saga of fuel tank failures[which by the way is also seen on Inteks using the same tank]the Quantum Power has been an excellent engine. It is on the way out though due to emission regs. It already is history in Kalifornia and as soon as EPA standards meet Kalifornia's CARB regs this great old flathead will be gone altogether. EPA regs are currently slated to come in line with CARB in 2010 but there is some talk of bumping it up the end of this [07] year. Time will tell.

There may be some defect in the carburetor (such as a small crack) that is causing the leak. It is also possible the needle seat has a defect and even with a new needle, the seat is responsible for the leak. If the original fuel line was reconnected to the engine, there is a possibility rubber particles from a deteriorated fuel line are entering the carburetor. Since you removed the carbureter yourself, the shop could not verify their repair. I don't know if you can expect them to perform additional repairs. The vacuum operated fuel pumps supply very little fuel pressure - perhaps only 2-3 inches of pressure. When the engine is shut off, the pressure would drop to zero very quickly. I would not expect a fuel pump problem. My thoughts are to verify the condition of the entire fuel system by removing the fuel tank for cleaning. I dump all old fuel and spray the tank interior with carburetor spray and allow this to drain in a plastic container. If there is anything left, I draw it out with the Thomas vacuum system. During this process verify the cap is venting so pressure will not build in the tank due to heat. New fuel lines and hose clamps would reduce the possibility of rubber particles entering the carburetor. A new 35 micro fuel filter would trap smaller dirt particles. Allow a little fuel to flow through the system before connecting the fuel line to the carburetor. The carburetor will probably need to be rebuilt again or replaced. If you have a fuel tank that is higher than the engine, your best bet is to install a shut off valve.

I have a 8hp tecumseh on my snowblower, 10hp Briggs intek on my chipper, 18hp Briggs Vangauard on my generator, a 25hp Kohler Command on my tractor and a honda 5hp lawnmower. All start easy and run great. None of them burn oil. The honda is the quietiest but also has a biggest muffler compared to the others. I will say that I'm disappointed in the honda mower. The underside of my deck is completely rust. I also had to replace the cable to engage the tranny, but had to buy the whole assembly as a kit to get the cable. My deck clogs easily whether I use mulching blades or regular blades and the cut isn't that great. The engine my be great, but the rest of the mower is not worth the 400 paid for it. With proper car, air filter changes, oil changes, new spark plugs, and good gas, any engine will last a long time. I know a lot of people that just put gas in and go. Then the engine breaks and the equipment is the biggest piece of junk. For the average homeowner, I don't think the engine should be of a concern. I think the unit its mounted too should be of more importance.

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?