I have a 1999 Big Boss 6x6 sport 500 with a Mizuni carb.
Symptoms: over a few weeks lost power considerably, no guts, backfires with even a little load. It can reach max 29 mph if I let her run for a block or so. Craps out on a medium incline. If I start out from zero and push the throttle a bit hard, it will stumble or backfire or both. It idles smoothly. It has started backfiring more as time goes on. Usually backfires as I nurse it up to speed. will not backfire if I am very gentle. Use this multiple times a week all year long. NOT bad gas, use 91 octane unleaded, no ethanol. Carb does NOT leak gas, looks all good from outside. Exhaust and muffler sooty, black. Spark plug was very black before cleaning. Cleaned up fine though, no wear, gap 0.005" too wide, spec = 0.030"
Due to maintenance items and trying to fix this the following items have been done already:
-- changed oil / filter
-- new gas tank valve, leaked like a sieve.
-- did not change air filter but ran it with it off and no effect, cannot see light through though.
-- new spark plug
-- removed muffler, looks OK just sooty no plugging, mice nests etc, and no improvement in performance with it OFF, removed spark arrestor plug no sooty exhaust and no improvement.
-- simply cleaned the old spark plug and got some improvement, no further improvement with a new one.
-- changed fuel filter, no improvement
-- removed carb (with cables hanging on it) and dropped in amine based carb cleaning solution for 1/2 hr, did NOT remove any jets, shot carb cleaner through all exposed jets after the soak(1st removed top and bottom sections for access) NO improvement. Wondering if replacing the rubber diaphragm on top would have any effect; is that the accelerator pump? Did not remove the float assembly, just soaked.
(By now, I am REALLY familiar with how to access the carb but getting the dang remote fuel valve stem on is a major pain!)
Wondering if I should look somewhere else like timing or something major in the engine or keep focussing on the carb?
