Strange 2-Stroke Problem (SD 720)
David W Roach
I have a symptom in a Sea Doo 720 motor ('96 HX) that I have
never encountered in 25 years of working on 2-stoke dirt
bikes and watercraft. Once the engine RPM drops below about
3500 or so, the motor "loads up" and starts four-stroking
like an extreme over-rich condition exists. It only does this
while loaded/in the water, which makes troubleshooting in
the garage a tough proposition. Mid- to high RPM acceleration
and top speed appear normal compared to other HX watercraft.
There is no apparent problem with the performance of this HX
until you let the revs drop too low, in which case it always bogs,
goes into a four-stroking idle mode, and nothing you do with the
throttle has _any_ effect at all on the idle speed-- full throttle
or no throttle, it continues chugging along at about 1000 RPM.
The only way I've found to recover from this is to shut off the
fuel valve, wait about 20 seconds for the engine to start clearing
out again, grab some throttle and simultaneously turn the fuel
valve back on while the craft accelerates cleanly across the lake.
Thinking I had a pair of severely dribbly needle and seat assemblies,
I went through and completely rebuilt the stock Mikuni 38 carbs.
No change. I had access to another pair of like-new Mikunis off
a friend's HX, and again they didn't help a bit. So I'm convinced
this is not a carb problem. The fuel return line to the tank is
clear, and I've completely drained and filled the fuel a couple times.
Again, this thing always starts and runs great on the trailer, so
I have to make a trip to the lake just to see if the problem exists.
I'm now wondering if a mis-installed rotary valve could possibly
cause this problem. The previous owner had a shop install an
aftermarket (steel) rotary valve. I've searched for symptoms of
bad RV timing, but all I find are vague statements like "won't run
right" or "runs like crap". Anyone out there experienced what I
am seeing? I am not eager to tear this all apart again if the
rotary valve timing is not a likely suspect. I've not looked at
the ignition at all yet either, since it seems to be a fuel system
problem. Help! My local shops are backed up for weeks...
Dave R.
TTravelor
Tom
Beesley's Point SeaDoo
609-390-1113
Bill O'Neal
A couple of things come to mind here.
One, your description sounds very similiar to what was happening to a 720
motor I worked on last week. It had been re-jetted for flame arresstors.
When releasing the throttle from high rpm, it would not rev up again ( in
the water) , it would just 4 stroke. I put the next higher size springs,
95's in place of the 80's that someone had put in there and the boat
performed fine. Although I did not do it in this set of carbs, sometimes
stacking 2 plastic flapper valves on the backside of the jet cover helps
too. Are your flappers ( check valves ) in good shape ?
The second thing that comes to mind is the ignition. The 720 has the weakest
of all SD ignitions in it. I have solved similiar problems by gapping the
plugs to .021 and slightly advancing the timing by 1 degree to 22 degrees.
You did put a timing light on it didn't you ?
I have also seen bad magnets on a flywheel once, on an otherwise perfect
looking flywheel. This would be very rare, as I have only seen it once in 12
years.
First thing is to snip a quarter of an inch off the plug wires at the caps.
This often solves minor ignition problems due to a very weak spark from a
bad connection. This in combination with a wide gap and a weak ignition can
be the culprit.
If you doubt the RV timing, check it. It is very easy to remove the 4 RV
cover bolts and set the carbs out of the way. Make sure the closing edge is
at 65 degrees or close to it.
Bill O'Neal
WCM
David W Roach wrote in message <99464499...@cswreg.cos.agilent.com>...
