97XP 787 mag side backfire off idle

[Micped]

Hi
97 XP fires right up and idles nicely. Hit the throttle and it backfires out mag side carb. It was backfiring at idle until I cut back plug wire and reinstalled. Could my coil just be weak and I'm only seeing it on one cylinder. All new fuel lines and carbs cleaned up. Please help me!!

 

[Mekanix]

You could cut both wires and I believe you can cut the ends inside the coil as well.

Swap the plug wires and see if the problem moves to the other cylinder. If not I'd look at synchronizing the carbs. Then maybe checking ignition timing followed by rotary valve timing.

 

[Micped]

Still having trouble with this thing. Ran perfect in the driveway on the garden hose no sputtering or backfiring.
I threw it in the water and it fired up and idled alright until I hit the throttle and it died. Pulled carbs today everything was still clean. I lowered my pop off from 40 to 20 and it idles even nicer now but still sputters and backfires once in a while. What am I missing? It runs excellent one day and crappy the next. Is backfiring a sign of lack of fuel?

 

[Strizzo]

Ok, let's back up. How long have you had it and what all have you done to it lately?

 

[Mekanix]

Every time you pull the carbs out you have to synchronize them again ! Not being synchronized will cause backfiring as well.

 

[Micped]

A little background on this seadoo. When I got it the countershaft bearing was gone and it destroyed the crankcase. I purchased another 787 with 140 pounds compression on each side. (Yes I checked it with my own tester). Motor reinstalled with new rotary valve. Fuel tank pulled and clean. All tempo grey fuel lines replaced. Carbs pulled and cleaned up( they were pretty clean). Pilot jets are 70's and low speed adjusters are out 1 1/2 turns now (I've tried anywhere from 1 to 2 1/4 turns. Has 2.0 needles now. Stock is 1.5 which I tried but could not get pop off below 40 with any spring. Last week it was backfiring and missing until I played with adjustments and then it was running really well so I thought I would take it on a lake run. It started up and idled but as soon as I hit the throttle it died?

 

[Strizzo]

Since you changed the rotary valve, did you get it timed up right? If the timing is off it can make it run crappy on the low end and backfire. Also check the rv cover clearance.

 

[Strizzo]

Also 40 pop off is "in the range" for the 97 xp with a 1.5 n/s. LS screw factory setting is 1 turn if you still have a working accel pump.

A wise honda dr once said "it should run fine on the factory settings, if it won't, something else is likely wrong."

 

[Micped]

The carbs are out of sync a bit I think. The butterflies should both be directly in line with the Venturi correct? I'm sure the rotary valve is timed correctly. There were a couple if timing marks from the previous owner and when I put my degree wheel on I got close to these marks. I will go sync the carbs now. With my low speed adjusters out 2 turns am I flooding it and going the wrong way? The air box is gone and it has flame arresters. Stock pilot jut is also 64 which is smaller than the 70 that was in the carbs.

 

[- BiRkS_]

Well without the stock airbox, you will have to play with the mixture settings. So slightly richer on the low end makes sense. I'm against any aftermarket airbox as the stock one breathes quite fine, and you don't get the "Waa" sound from the carbs.

My 951 had the same problem yesterday with your symptoms. Try unplugging the stator housing connection, clean it then plug it back in and push it in firmly. See if it cures the problem.

 

[Micped]

So I unplugged the wiring harness from the stator housing cleaned it up and put it back in tight. Put it on the hose and fired it up and still backfiring. So I decided to tear in and check rotary valve timing. I unhooked fuel return hoses and lots of gas poured out. Then I opened fuel tank cap and relieved pressure. I put return lines back on and left fuel cap open. I fired it up with no hose and it would not backfire??? What the hell?? Put it back on the hose fired it up and it ran good for a while now it's starting to backfire again just not as much and mostly at higher rpm. Anyone got any ideas. Am I flooding the carbs cause I can't get fuel back to the tank on the return lines?? I checked tank pressure relief valve and it seems to be working.

 

[- BiRkS_]

Are you sure your lines are routed correctly? Normally a backfire is due to a lean condition, or advanced timing. And there should be no pressure other than the tiniest hiss when opening your gas cap. Loosen your gas cap, or have it removed and take the ski for a run. See if the problem is fixed. If not, Double check rotary valve timing.

PS. You did try new spark plugs right?

 

[Micped]

I will double check my fuel lines and rotary valve timing tomorrow and let you know how things go!! Thank you all for the help so far

 

[Micped]

K so rotary valve timing was out a bit. Fired it up and still doing same thing????? Do I have an ignition problem?? What do I check next. I'm running out of ideas. It starts and idles wonderful and if I slowly squeeze throttle it's ok for a bit then bogs and backfires. It seems that nothing I do with the carbs helps. Should I get a new coil and stator or can I test them?

 

[Strizzo]

there are specs for the resistance of the coil and stator ohm readings in the manual.

are you sure its coming back out of just the mag carb? on an x4 or 3 seater 787 ski, you could mark the pto for TDC and check the timing with a timing light.

another thing - has this motor been hydro-locked before? since the crank is pressed together, if one cylinder gets locked, and the rest tries to keep going, it can get the crank out of phase so the timing isn't right on the inside even though it seems right on the outside. the test for this in the manual is to take a longish and a shorter screwdriver, and get one cylinder near TDC so the other should be near BDC. put the screwdrivers in the cylinders and rotate the crank by hand, looking to see if anywhere in the rotation both cylinders are going up or down at the same time, indicating the crank has slipped.

also again double check that you've got the fuel lines routed in the right direction and on the right places and all, and that the pump pulse line is in good shape. you might also think about hopping on ebay and getting an oem airbox and flame arrestor screen setup to get some consistency to your tuning challenge.

 

[Micped]

Nope it's backfiring out both carbs after further investigation. Because it is backfiring out of both does this mean that crank timing and straightness is ok?

 

[Mekanix]

Could the flywheel have spun on the crank and only slightly shearing off the pin but not enough to throw it way out of wack ?

I'd say your next check should be with a timing light to see where its firing or take the cover off and take a look. I wonder if the rotor is maybe 1 bolt off giving you a super advanced ignition timing ?

 

[Strizzo]

Could the flywheel have spun on the crank and only slightly shearing off the pin but not enough to throw it way out of wack ?

I'd say your next check should be with a timing light to see where its firing or take the cover off and take a look. I wonder if the rotor is maybe 1 bolt off giving you a super advanced ignition timing ?

i know some motors have been known to shear the key but not slip far enough off to not let the motor fire at all, just make it backfire like crazy and run like crap, so its worth a check on the timing light.

 

[Micped]

So I pulled flywheel cover off again and checked to make sure magneto rotor was installed in proper position and it is, the magnet in the outside lines up with the hole in the flywheel. Is there anything else I need to check while this cover is off? Am I able to check the stator and pick up while its on the bench? I personally installed the flywheel on the crankshaft and all looked well with that and it has not been run in the water since. I did buy this engine used though so I'm not disregarding that the crankshaft could be twisted. I just want to make sure that I do everything I can before I put this cover back on.

 

[Robj]

May want to pull the flywheel to make sure the key has not sheared.

 

[Mekanix]

If the crank spun, the mag side wouldn't backfire It would be the pto that would.

 

[Micped]

Both cylinders are backfiring. Originally I thought it was just one but after I ran it in the dark you could see that they both are. Does this mean its probably something common to both cylinders like sheared flywheel key, stator, pickup or coil?? I will pull flywheel and double check. I tried 2 rods in spark plug holes and the 2 cylinders never go in the same direction.

 

[Strizzo]

If the crank spun, the mag side wouldn't backfire It would be the pto that would.

If te flywheel sheared at the keyway, both cylinders would be off the same amount relative to the ignition timing. If one side of the crank spun, yes it would most likely be one or the other backfiring (probably one is worse than the other).

 

[Coastiejoe]

There have been more Han a few that have sheeted the key way.

Probably worth looking at.

 

[Micped]

Pulled flywheel off and keyway is in place and flywheel was fitting over top of it where it belongs. Now what?? Can the mpem be programmed too much out on this different engine? Am I thinking correct that all 787's are the same or not? I'm going to check all wiring harnesses again and make sure all coil connections are clean and tight. Any other ideas?