1995seadoo
New Member
Hello, I have a 1995 sea doo spx with the 657x motor. It’s backfiring out the exhaust and carbs like a machine gun. Here’s the story: Just bought it for $500, seller said it ran rough. Get there and the second you rev it, it backfires. I take it home and find out he’s running 20:1 (oil injection deleted). Check the spark and it’s extremely weak. Drain the old fuel and put new spark plugs in, and she’s running beautifully(for a few minutes) Run it a little bit longer (on the trailer with water hooked up) and it starts backfiring again. This time only backfiring at high rpm’s, mainly the rev limiter. Called a local sea doo repair shop, they said it’s 100% normal for a 657x to backfire when the engine isn’t on a load and what’s happening is the engine is cutting the spark at the rev limiter and it’s causing a build up of fuel which causes a backfire. It makes sense to me but why has this never happened to me before with my other skis? I’ve had 2 other 657x’s and never had this problem, so the fact he’s saying it’s completely “normal” sounds a bit weird to me. So far what I have done to try to fix this: New gas (40:1 ams oil dominator), new Ngk Br8es plugs, rotary valve timing checked, carbs cleaned/rebuilt with oem kit, checked flywheel for sheared key (looked fine), compression is around 110 per cylinder but my crappy harbor freight tester reads low, I also checked the rectifier, was told you check the volts of the battery as you rev it and as long as the volts are 15 or lower your good, mine stayed in the 12 range so i’m assuming it’s fine. Looked for any potential air leaks, found none. Tried adjusting carbs in all different ways, nothing. Although my rotary valve and rotary valve cover do have some scratches in them, the scratches on the cover do slightly catch your finger nail. I heard this is bad but causes bogging in the water, not backfiring. The head of the motor has an SBT sticker on it, and from what I heard that doesn’t help the situation. I’ve never tested it in the water yet. I seemed to have tried anything and everything that could possibly go wrong and everything seems to check out. What other things am I missing? Is there a way I can attach a video of how it’s running?