SUPERJET 720 Cylinder issue

Whatton

Whatton

Location
Uk
Hi guys, I think I have my ski running on one cylinder. Recently built by a garage.

Took the ski out today for the second time since a fresh build and it was running great in the sea, first time being in a lake. It ran for roughly an hour, hitting waves etc.

However it randomly felt like it had lost power... Almost like it was slipping... Revving up but very little thrust. On the way riding it back to the beach, I would let it sit and idle and then slowly build the revs up, it would run fine and go full power again. IF I gave it full throttle right away it would run terrible, like half the power.

I got it back on the beach and turned it over, left it to idle and the engine would vibrate like crazy! Prior to this there was very little engine vibration at idle.

Has anyone had this issue before?

I will provide a youtube video link for reference.

Many thanks

Nathan
 
S

Storbeck

Location
Minnesota
Are you getting revs but no thrust, or are you not getting revs?

From your description it is not clear which, or if it is sometimes one and sometimes the other.

It would be two completely different troubleshooting processes.

Revs with no thrust would likely be a pump issue, could be as simple as something stuck in the intake grate or pump, or leaky pump seal.

Not getting revs is probably a motor problem, not a pump problem. Could be running on one cylinder.

If it's running on one cylinder, it is for a reason. Fuel, spark, compression, timing. One of those is missing on one cylinder.

Easiest to check are spark and compression (just stick your thumb on the spark plug hole and turn the motor over, if it has enough compression to blow your thumb off it will run on that cylinder, it it doesn't, it won't). Spark just look at the spark plug and see if it sparks.
 
DylanS

DylanS

Gorilla Smasher
Location
Lebanon Pa
Check your motor mounts first before anything.
you have a single carb setup, fuel delivery issues would cause issues across both cylinders consistently, not just a single cylinder.
I would be checking compression and spark first.
Just because you see spark with the plugs out doesn’t mean you can rule out a intermittent spark issue either, you could have a bad wire, etc.
take your air cleaner off, look down the throat of the carb at idle, see if it’s leaking fuel like crazy. That would also cause a very rough idle.
I would have someone who knows what they’re listening for stand on shore and tell you if it’s losing a cylinder or if your having a fuel related issue on both cylinders. Or even record a video and post it. If you actually were down a cylinder it wouldn’t rev right up. What you’re describing sounds like two issues, primarily cavitation, and possibly a broken motor mount.
 
Whatton

Whatton

Location
Uk
Apologies for the slow reply guys and thank you so much for the help, it's greatly appreciated!

Unfortunately I've not been near the ski since making this post, however I did do a check on the mounts and they're all fine, no play or movement, also had them replaced with new ones.

I did a thumb compression check on each cylinder. My spark colour on each plug was a weak yellow and wet, so I swapped my plugs our for new ones which gave me blue on spark.

When I removed both plugs I did notice smoke coming from the cylinder closest to electrical / ignition box. No smoke emitting from the front cylinder.
I've checked the pump also and can see no visual signs of anything stuck or wedged in there.

Whilst out on my ski, once the problem arouse of losing power ... if I let the ski sit and settle back down into idle and then start to slowly build the revs up.. sometimes, not every time, it would run and pull fine again. This did make me think at the time it may be an issue with the pump or cavitation issue.
However... Would this cause the engine to vibrate so much out of it water at idle if it was a miss alignment or pump issue? As you can see it doesn't vibrate as much with revs... Just on idle.

I'm fairly new to skis so I've got a lot to learn. Thank you for all the advice above, I will check all of the above out at the weekend when I've got a day with the ski!
 
DylanS

DylanS

Gorilla Smasher
Location
Lebanon Pa
Whatton said:
Apologies for the slow reply guys and thank you so much for the help, it's greatly appreciated!

Unfortunately I've not been near the ski since making this post, however I did do a check on the mounts and they're all fine, no play or movement, also had them replaced with new ones.

I did a thumb compression check on each cylinder. My spark colour on each plug was a weak yellow and wet, so I swapped my plugs our for new ones which gave me blue on spark.

When I removed both plugs I did notice smoke coming from the cylinder closest to electrical / ignition box. No smoke emitting from the front cylinder.
I've checked the pump also and can see no visual signs of anything stuck or wedged in there.

Whilst out on my ski, once the problem arouse of losing power ... if I let the ski sit and settle back down into idle and then start to slowly build the revs up.. sometimes, not every time, it would run and pull fine again. This did make me think at the time it may be an issue with the pump or cavitation issue.
However... Would this cause the engine to vibrate so much out of it water at idle if it was a miss alignment or pump issue? As you can see it doesn't vibrate as much with revs... Just on idle.

I'm fairly new to skis so I've got a lot to learn. Thank you for all the advice above, I will check all of the above out at the weekend when I've got a day with the ski!
Click to expand...
No mounts broke with a vibration that bad at idle..
i would Check compression, pull the head if your feeling ambitious, then check midshaft and pump bearings.
I’d personally be tweaking and ripping the motor down to inspect crank bearings and the whole gambit. Would not overlook a crank issue.
 
Sparkplug

Sparkplug

Location
Red Bluff CA
If the pump seal is bad or the pumps shoe is cracked it can pull from behind the pump and cause a extreme amount of cavitation. Which in turn causes extreme vibration.

Being able to slowly get into it, like a clutch, is 99 percent of the time the pump.

Sent from my SM-G970U using Tapatalk
 
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