07-26-2012, 01:02 PM #1
1997 SL780 Stumbles around 3K, and will not go past 4.5k
Referred to as the "Purple Flash", well not to flashy right now.
Here is the run down.
Ski shows up, nasty gas in it, been run on the "nasty gas" PTO 130, Center 130, Mag 125
It idles fine, stumbles all over itself from 3K-4.5K, just burping throttle-4.5K is max, (Can see a very even fuel mist in carb throats, but just stumbles like on a limiter).
So... I order 3 Mik rebuild kits, including needles and seats, and carb filters
New fuel line is in order, and of course a Mik Fuel pump rebuild.
So I go to work, pull tank, scrub it out, perfectly clean and dry, also cleaned pickup assembly(Aluminum bar on this model no tank hose and weight deal)
Pull new fuel lines, including impulse
Rebuild fuel pump complete clean out and soaking
Pull Water sep filter, clean, and reassemble
Tank on off valve, flushed for a ridiculously long time
Rebuilt carbs, tested popoff wet with WD40, 23 across the board
Adjusted Low and High speed needles (Low 1.25, High 1/
reassembled the newly rebuilt and cleaned parts.
Fired up ski, put on hose, idles like I knew what I was doing.... rev up, falls on it's face same place.
PS reeds were checked and found to be fine.
Visual inspection of intake tract while running, no extra fuel, throttle up, nice even center mist. Looks all happy in there, but it is not.(Same issue) no fuel flying out in the reverse damaged reed kind of way.
So to start trouble shooting, I move low speed needle slightly further open, all seems happy, High speed is +- an additional 1/8 per service manual, so I mess with them a bit... I get a 6K run up, then a calm back to idle routine with no throttle input from me. ( I also hear what sounds like cyl's dropping out, so I throw a timing light on it, hold trigger and watch for a flame out... nothing can be preceived in light as a drop. The in and out is very temporary and you have to listen very close)
I also notice a fairly large bubble at fuel pump inlet. So I watch it as I put throttle input into machine... bubble disapears and reappears. The machine has been run at this point 15- 20 min total, in 2 min run, 15 min off intervals. Also removing hose prior to kill of motor.
New plugs NGK correct for this machine.
Checked indexing, hillbilly style with dowel rod and tape measure. Nothing appears out of order, ski shows zero signs of hydrolock. I am actually suprised that a $600 ski, looks show room new, however I am starting to think it looks so new because nobody could figure out why it runs like crap.
My next order of buiness is to check all lines are tight, pretty sure that is the case, look at water sep black ring make sure it is not cracked, and Oring etc is sealing. I am going to run a new pump inlet line bypassing the On/Off valve. I will check for the 10th time I did not put a hose on wrong... did a one for one exchange with a sharpie marker recording position... so I am feeling pretty good about line placment.
So if anybody has any ideas let me know. I also checked that carbs were in the correct place... I kept track myself... but just in case some other idiot was in there, I looked at low and high jets and matched to service manual. All is correct there. I also rechecked all three carbs by a second disassembly and double check everything is in it's place. I have 5 evenings in this so far, I have the 780 rash on my forearm from taking those damn carb nuts off many times... So boys your thoughts would be welcomed, cause this dude is stumped..... PS I also have a 12 MM wrench that had a visit with the chop saw... that works well.... now I wonder if that Craftsmans warranty will cover it... cause after this one is done, the next 780 that shows up here will go somewhere else!!!
07-26-2012, 04:16 PM #2
- Join Date
- Oct 2006
- Cleveland OH
Did you try disconnecting the gray wire inside the elec box?
Did you try disconnecting the MFD and see what happens?
07-30-2012, 07:35 AM #3
Tried both... we thought we had it whipped. Sent the ski south, report from Norris lake, same bull crap... got up this morning... sometime in the night the Purple flash snuck in from Norris lake... it is back....
07-30-2012, 11:06 AM #4
Im sure this is not your problem, but I have a 96 slt780 that I had months into, engine out, tried everything in the book 5 times. Ended up being one of the spark plugs had a failed insulator.
07-30-2012, 11:22 AM #5
When I bought my carb rebuild kits from Zigler he told me the manual is wrong for the pop-off pressure. 22-24 PSI is too high for the free-flowing F/A and it doesn't draw enough manifold pressure to open the needle. I thought the manual has to be right, so I used my original springs and set PO to 23PSI and put the LS needles at factory 1 3/8 turns.
Sure enough, I got on the water and when I nailed the throttle it just fell flat on it's face. No stumbling or gurgling, just dead flat. I opened the lows to 1 3/4 and bingo, it was just fine.
So, Zigler was right, that pop-off pressure does need to be lower than stated in the manual if you want to use the factory LS needle settings.
Is it falling flat at 4500RPM or is it gurgling and burbling?
Here are a few other things I would check:
1) Is the return fitting in the MAG carb completely clear? If there's any blockage it may be causing over pressure in the carbs.
2) Did you synchronize the carbs? maybe the butterflies are out of whack causing over/under fueling on one of the cylinders.
That's all I have for now.
07-30-2012, 04:29 PM #6
Yes carbs were set with air flow meter, all three spot on. I will open the lows a fuzz more and see what happens. It just blubbers, that on the limiter sound.... and yes the return fitting is clear... I assume you are talking about the small restrictor... it is clear.
07-30-2012, 04:29 PM #7
07-30-2012, 05:26 PM #8
- Join Date
- Oct 2006
- Cleveland OH
Get it warmed up, then put the spark testers on it. See if you're dropping a cyl after the magneto gets hot.
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