03-20-2015, 09:56 PM #1
Getting whooped by a carb'd 787 (97 gtx)
Lets start at the beginning. Ski was brought to me with air box and flame arresters off and I was told the ski has not run since last november. The ski sounds like its out of time or trying to run on one cylinder while trying to start. If I convince it with a fresh set of plugs and fogging oil to start and run it will run and idle for a bit but will not tk throttle. The ski has new fuel lines, the fuel filter is clean, primer is installed and the fuel is fresh.
I have performed the following.
Overhauled carbs (twice just to be sure)
Settings are Mag Pto
Low 1 1
High 0 1/2
Checked Rotary Valve timing. (147 degrees)
Verified proper Rotary Valve
Bypassed Pet Cock (running reserve line str8 to carb)
Battery tested good (slaved in another known good battery to be sure)
Compression is good
Rave Valves move freely
Spark on both cylinders (cut back 1/4 inch on wires and reinstalled caps)
Tried to verify Crank index in hull by marking edge of Rotarty valve while mag piston is at tdc then again while pto piston was at tdc and making sure there 180 degrees out but the rotary valve has 5mm slop measured on the outer edge of the valve. My understanding is this is not out of spec and unit should still run
I have also pulled the rotary valve shaft and verified the brass gear is in Good shape.
My thoughts are that I have leaking cranks seals and there causing my plugs to foul and creat a lean condition from the carbs or the ignition timing is off. I have no way of testing the timing cause I dont have the special tool. Can I make one?
03-22-2015, 08:26 AM #2
Have you tried just cranking the engine without the sparks plug in, if the seals are leaking you should see oil spraying out of one of the cylinder hole when cranking.
As for the timing tool, I would just google the service manual instructions on the web and see if you can find a diagram of the degree wheel and if you can somehow expand the picture and print it... that may work ? although, you will still need a timing gun ...
03-22-2015, 06:47 PM #3
I have turned the motor over without the plugs and there is no oil spraying out I have seen SBT engines with this problem but I was thinking (likely incorrectly) that maybe there only leaking slightly as the engine is running. As far as the timing goes I was referring to the dynamic ignition timing. I have the degree wheel for the rotary valve timing and that is timed correctly. For the dynamic ignition timing there is a tool that bolts to the case that points to the pto and you make a mark at tdc and use the computer and a timing light to test/ make adjustments to the ignition timing. Are there any other known problems with these skis that i may be missing? i.c faulty cluster's causing problems?
03-23-2015, 08:04 AM #4
- Join Date
- Feb 2011
What is the compression readings? If you spray premix in each hole does it run well till it burns? I've fought a fre RFI 800 and can tell you the timing is critical. You could be one tooth off and it would make a big difference.
03-23-2015, 08:57 AM #5
The Compression readings are good. Spraying fogging oil in the carbs helps at first but plugs begin to foul immediatly.. i have comfirmed that the stock carbs have the stock jets set at the stock settings. the valve timing as been verified with the proper degree wheel several times due to the fact it sounds so much to me like a timing issue....Im concerned with the slop in the rotary valve and the ignition timing.
03-23-2015, 12:46 PM #6
Electrical systems on these are pretty picky. Measure ohms of stator to any two yellow wires. Should all be less than 1 ohm and none should read continuity to ground. Also make sure ground reads to ground. Sometimes the pickup coil will actually break off it's mount inside the front cover. You should pull the front cover and check it out. Also verify voltage at battery while cranking.
03-24-2015, 09:54 AM #7
the stator charging leads test good as far as ohms go.
whats the spec for the pickup coil?
i got 243ohms
battery test good.
and have slaved in another just to be sure.
03-24-2015, 10:51 AM #8
Then I would pop the front cover to see if the pickup mount is broken.
03-24-2015, 05:53 PM #9
Just thought I'd post an update on this. I pulled the front cover a fair amount of metal shaving in front cover. I also noticed that 3 of the magneto bolts were finger loose and the other three were very easy to break loose with a 1/4inch drive ratchet. And one of the bolts had been replaced with a hex head instead of the allen. This concerned me but was not enough to convince me this was the problem so i built a pressure tester and performed a pressure test. Problem solved, The block would not hold pressure!
03-26-2015, 09:11 AM #10
What do you call "good" compression numbers. What are the actual numbers?
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