06-01-2012, 08:36 PM #1
787 backfiring through flame arrestors at 3/4+ throttle
1998 SPX with 787 engine. Fairly new machine to me.
Has MSD ignition enhancer, PROK flame arrestors, and PSI exhaust. It also has a brand new SBT rebuilt engine in it. (I know... I know... lets don't argue that here. Its already bought and in the watercraft.)
1.) Put the engine in late last year. Don't know if I can trust my pressure gauge totally... but reading 150 psi in both cylinders. (Wish it was slightly higher pressure, but no difference in them which is good.) Broke the engine in at the end of last year with light riding, then parked the watercraft for the winter.
2.) I'm using the standard MSD dip switch settings.
3.) I don't remember the exact carb jet sizes I used (because I have 4 other skis I worked on at the time)... but I remember them being larger than normal.
4.) I rebuilt the carbs with Mikuni parts. Pop off pressure was in spec. Cleaned the whole fuel system. It does not have the grey fuel hoses. I swapped those out early last year before my original engine went belly up. (Cooling hose had popped off.)
Took it to the river yesterday. It runs great up 3/4 throttle. Anything above that it didn't really rev up any more and ran rougher. Would smooth back out if I left off the throttle and held it to no more than 3/4 open.
After putting new spark plugs in, I put in on the water hose this evening. Noticed that its backfiring through PTO side carb flame arrestor at any throttle over 3/4 open. (The old plugs were too black. Not wet... but not the good dark brown either.)
What do I check next?
Thanks in advance.
06-01-2012, 09:38 PM #2
- Join Date
- May 2008
- Elephant Butte New Mexico
I'd bet the rv is off a tooth or two
06-03-2012, 01:32 PM #3
Yeah... I was wondering about the RV too and second guessing myself. I used a new RV and checked the degree settings 3 times before I buttoned it all up and installed the engine. I called it "good" all 3 times. (At TDC.) I've done that procedure many times on other skis I own without any problem.
I'm wondering more about the MSD ignition enhancer being set correctly. I've got really good spark, but the timing is adjustable over a wide range with those dip switches. However, if spark timing was off it would affect it over the whole range of throttle, right? Not just 3/4 +.
06-03-2012, 01:40 PM #4
06-04-2012, 09:31 PM #5
Def sounds like timing. Find the directions for msd and start at stock.
Also, with 1st back fire, possibe it sheared brass gear, r.v. Could be off just enough
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