Super Jet Engine Stops With Throttle Use

[Chest Rockwell]

Hey y’all,

First post here. I have a ‘90 SN with a stock 61x in it (for now).

The problem that I am experiencing is that when I apply throttle the motor immediately stops. It will idle fine, but as soon as I pull the trigger, things fail in a way that gives a sound like it’s not getting enough air/fuel.
However, occasionally, right after startup the motor will run at elevated rpms without any throttle applied. Additionally, I checked the compression and it was 90psi on the shaft side and ~75psi on the stator side.
I did purchase the motor 2nd hand, so I do not know how old it is.
I am already certain that I will need to rebuild the top end, however, I want to double check if there’s other things I need to check.

Can y’all help point me in the right direction?

 

[Chest Rockwell]

Hey y’all,

First post here. I have a ‘90 SN with a stock 61x in it (for now).

The problem that I am experiencing is that when I apply throttle the motor immediately stops. It will idle fine, but as soon as I pull the trigger, things fail in a way that gives a sound like it’s not getting enough air/fuel.
However, occasionally, right after startup the motor will run at elevated rpms without any throttle applied. Additionally, I checked the compression and it was 90psi on the shaft side and ~75psi on the stator side.
I did purchase the motor 2nd hand, so I do not know how old it is.
I am already certain that I will need to rebuild the top end, however, I want to double check if there’s other things I need to check.

Can y’all help point me in the right direction?

Sorry, I forgot to mention that the carburetor is functioning properly.

 

[harrymank]

I think you need a better compression gauge. That is really low. Like it should not start.

 

[Big Kahuna]

Sorry, I forgot to mention that the carburetor is functioning properly.

Errrr, Your carb is not functioning properly by what you described.

1. turn your idle screw back to where it does not idle. then slowly turn the screw in until it idles nicely. If you keep getting runaways you have an air leak either at the carb to manifold gasket, intake gasket or base gasket. https://www.x-h2o.com/threads/runaway-motor.111672/

2. Where are your Main and High Speed screws set at? I would start about 1 turn out and see how it idles. Sounds like you are way to lean right now. https://www.x-h2o.com/threads/tuning-tips-for-rookies.39846/

read these 2 links.

 

[fastgtfairlane]

Do a leakdown test before doing anything else. You need to know for certain that your engine doesn't have an airleak. A big enough air leak can cause the compression psi numbers to be off. Remove the carb and exhaust and plug both holes then pump thr engine up.to 8psi. Should hold 8psi for at least 10 min. An old engine that sat for a while us susceptible to leaking crank seals.

 

[Chest Rockwell]

Thank you all for your input and suggestions.

The compression testing unit is new one from NAPA with stainless fixtures. However, user error could be a factor, though I did use a neighbor’s unit and got similar results.

Regarding the carb, it is a new carb from Blowsion, though I haven’t tested it yet, I trust that it’s working properly. That being said, I will try testing the tuning and see what happens.

Regarding the leak-down test; can failed crank seals (or other seal failures) cause the engine to stop with applied throttle?

Thank you again for your help and suggestions.

 

[Big Kahuna]

Thank you all for your input and suggestions.

The compression testing unit is new one from NAPA with stainless fixtures. However, user error could be a factor, though I did use a neighbor’s unit and got similar results.

Regarding the carb, it is a new carb from Blowsion, though I haven’t tested it yet, I trust that it’s working properly. That being said, I will try testing the tuning and see what happens.

Regarding the leak-down test; can failed crank seals (or other seal failures) cause the engine to stop with applied throttle?

Thank you again for your help and suggestions.

Yes, when you pull throttle you are demanding more air fuel mixture, if you are sucking air on your seals, you just took your air fuel charge and leaned it out to where it will not run due to not enough fuel.

 

[E350]

Believe it or not crank seals are not that hard to do. If I can do them, you can do them. Buy the manual for your ski and read the procedure. You will be ok.

These gurus here will help.

 

[Chest Rockwell]

Thank you for the words of encouragement E350. I have seen plenty of the YouTube videos rebuilding 2-strokes, so I feel fairly confident, though this will be my first time doing so. Though, I want to do the leak down test first, just to confirm. I will keep y’all posted.