1990 Jet-N-Cat powered by a 650 Yamaha

DMAL

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My Little jet boat is cutting out...spitting and sputtering, and won't get up to speed. It sound almost like it is blowing up when it cuts out, but there was no aluminum on plugs and the compression is 125 or so in both cylinders...is that ok? I hope it is just a fuel problem or something simple. Any suggestions?
 

dazk14

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Jul 22, 2008
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Fuel related problems are the most common issues for 2 stroke PWC engines. You can start by replacing the inline fuel filter as they get clogged.

The carbs themselves will clog over time and be accelerated by stale fuel. Carb rebuilds are normal maintenance.

Fairly straightforward job for Mikuni carbs. A Mikuni rebuild kit is available for a given carb and you can search the net, or go to a PWC forum for more detailed instructions. PWCtoday, greenhulk, riva are good ones.

Does choking it help when running? Be very careful if you are running fuel starved (lean) as that will put a hole in a piston in a hurry.
 

DMAL

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Sep 8, 2012
Messages
35
Fuel related problems are the most common issues for 2 stroke PWC engines. You can start by replacing the inline fuel filter as they get clogged.

The carbs themselves will clog over time and be accelerated by stale fuel. Carb rebuilds are normal maintenance.

Fairly straightforward job for Mikuni carbs. A Mikuni rebuild kit is available for a given carb and you can search the net, or go to a PWC forum for more detailed instructions. PWCtoday, greenhulk, riva are good ones.

Does choking it help when running? Be very careful if you are running fuel starved (lean) as that will put a hole in a piston in a hurry.
Thanks for the reply. I had a chance to work on it some. No, if I leave the choke on it will die when it gets warm. I put a carb kit in it and it didn't help. The filters in the tank on the pickups look good. I will have to check to see if I have an inline filter tomorrow. I did not find any gunk in the carb. The diaphragm looked a little out of shape. The spark visually looked good with the plugs out, but when I pulled the spark plug wires one at a time with the engine running I was able to narrow it down to the rear cylinder. The front cylinder ran smooth by it's self but the rear would die instantly as soon as I pulled the front wire.
 

dazk14

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Jul 22, 2008
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The plug wires are known to get internal cracks, causing random mis-firing. Sometimes cutting back the plug end of the wire cures it, but there's little excess.

The wires are sealed into the coil as an assembly($180). There is only 1 coil, which fires both plugs simultaneously.

You can test the firing on the rear cylinder using the front wire to see if it stays running.

I can explain how to DIY replace the wires that are sealed into the coil. Sometimes you can get away with a plug wire splicer and aftermarket plug wire and not go into the box

I'm presuming Jet-N-Cat supplied a single square body Mikuni carb.
 

DMAL

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The plug wires are known to get internal cracks, causing random mis-firing. Sometimes cutting back the plug end of the wire cures it, but there's little excess.

The wires are sealed into the coil as an assembly($180). There is only 1 coil, which fires both plugs simultaneously.

You can test the firing on the rear cylinder using the front wire to see if it stays running.

I can explain how to DIY replace the wires that are sealed into the coil. Sometimes you can get away with a plug wire splicer and aftermarket plug wire and not go into the box

I'm presuming Jet-N-Cat supplied a single square body Mikuni carb.
I will try to switch wires and see if it helps. Yes you are correct on the carb
 

DMAL

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Messages
35
The plug wires are known to get internal cracks, causing random mis-firing. Sometimes cutting back the plug end of the wire cures it, but there's little excess.

The wires are sealed into the coil as an assembly($180). There is only 1 coil, which fires both plugs simultaneously.

You can test the firing on the rear cylinder using the front wire to see if it stays running.

I can explain how to DIY replace the wires that are sealed into the coil. Sometimes you can get away with a plug wire splicer and aftermarket plug wire and not go into the box

I'm presuming Jet-N-Cat supplied a single square body Mikuni carb.
I switched the spark plug wires and there no change. Just as I was about to recheck the compression I decided to switch the spark plugs, and there it was... the problem moved to the front cylinder. I never would have guessed that a plug that looked good and had a spark that I could clearly see could cause a severe misfire.

Do you have any recommendations on what brand of plug that I should use?...and do you know what the jets on the carb should be set at?
 
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dazk14

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Jul 22, 2008
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966
Just an fyi - Jet-n-cat can have different specs for the motors from Yamaha, although often times the specs are same.

Since a jet boat can be run full throttle for LONG periods of time, internal cylinder cooling at WOT needs to be a double-check area.

Unless there is a forced water cooling restriction(weeds/sand), you should be fine as she was factory delivered. Idle RPM needs to be 1250, so the pump can circulate cooling water. Running the motor at lower rpm is very convenient for docking, etc., but it will overheat the motor - don't do it.

The Waverunner plugs used are BR8HS. B8HS can also be used, but you will get "noise" over the radio, etc.. Gap is .024. #8 heat range plugs are reasonable. There may also be a plug sticker on motor or area.

Carbs settings...All low/high needle settings are quoted as counter-clockwise turns out from a lightly seated position. I don't have the service manual in front of me...

Did you count where they were at when you rebuilt the carb? these should generally be fine tuned for peak performance.

For the low, set it at 1-1/16 turns out. For the high, set it at 2 turns out. The low is located low on Carb, the High is high on the carb.

The low is fairly easy to adjust when in water for smooth low end running and take-off and should ALWAYS be fine tuned before the highs.

The high speed adjuster provides the all important extra fuel at WOT settings to keep the pistons from melting down, so they cannot be arbitrarily leaned out.

Running too rich (2 turns out should be a tad rich), may not be fastest, but is a good start point.
 

DMAL

Seaman Apprentice
Joined
Sep 8, 2012
Messages
35
Just an fyi - Jet-n-cat can have different specs for the motors from Yamaha, although often times the specs are same.

Since a jet boat can be run full throttle for LONG periods of time, internal cylinder cooling at WOT needs to be a double-check area.

Unless there is a forced water cooling restriction(weeds/sand), you should be fine as she was factory delivered. Idle RPM needs to be 1250, so the pump can circulate cooling water. Running the motor at lower rpm is very convenient for docking, etc., but it will overheat the motor - don't do it.

The Waverunner plugs used are BR8HS. B8HS can also be used, but you will get "noise" over the radio, etc.. Gap is .024. #8 heat range plugs are reasonable. There may also be a plug sticker on motor or area.

Carbs settings...All low/high needle settings are quoted as counter-clockwise turns out from a lightly seated position. I don't have the service manual in front of me...

Did you count where they were at when you rebuilt the carb? these should generally be fine tuned for peak performance.

For the low, set it at 1-1/16 turns out. For the high, set it at 2 turns out. The low is located low on Carb, the High is high on the carb.

The low is fairly easy to adjust when in water for smooth low end running and take-off and should ALWAYS be fine tuned before the highs.

The high speed adjuster provides the all important extra fuel at WOT settings to keep the pistons from melting down, so they cannot be arbitrarily leaned out.

Running too rich (2 turns out should be a tad rich), may not be fastest, but is a good start point.
My low was set at a half at turn out and my high was set at 2 full turns and that is what I reset them at.
 

dazk14

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Joined
Jul 22, 2008
Messages
966
No problem, your welcome.

1/2 turn out is fine if it idles smoothly and doesn't hesitate on fast throttle take-offs.

The low does have a small effect on high speed fuel metering, but 2 turns out seems reasonable.

The ideal high speed adjustment procedure is with a digital tach.

Good luck!
 

DMAL

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I finally got around to replacing the spark plugs, But now it will not start at all or even fire The spark looks and feels weak. I am getting frustrated, before it would run on one cylinder, but now it won't even sputter. I performed most of the tests in this thread http://www.pwctoday.com/showthread.php?t=40840 except the ignition coil, my meter would not work for that test. The spark plug wires looked ok and the ohms reading on them was close, but I trimmed the end of the spark plug wires off anyway. Is there any other way to test the ignition coil? Thanks in advance if you could point me in the right direction.
 
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DMAL

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I have replaced the coil and it now has good spark, but now I have a no fuel issue. It will run if I prime the carb. I'm not sure where I went wrong... the carb was working after I rebuilt it.

Recap:
I rebuilt the carb, and there was no change in the way it ran (almost dead in one cylinder)
I replaced the plugs, But then there was very weak spark on both wires.
I replaced the ignition coil, it now has spark, but no fuel
I have not found any gunk or dirt in the fuel system at all any where.
I removed the filters for a test, and there was no change.
The pulse hose had a pulse
The supply/inlet barb had no noticeable vacuum (do the arrows indicate fuel in and vent out?)
I am able to blow air through the fuel line,on off valve and into the tank all at once
When I rebuilt the carb I did not replace the needle valves.
Is there a common mistake or something that I could have missed?
 
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DMAL

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I found a mistake that I made in the carb and corrected it, but now I am back to a spark issue. The spark issue was a new spark plug doing the exact same thing as the one that I replaced. I replaced the spark plugs and it ran great for 5 minutes and then it blew up. I have replaced the engine with a used engine. Now I have weak inconsistent spark. I tested the electronics and the charge coil seemed weak, so I replaced the stator. It started a few times and then I was back to the same or similar spark issues. I tested the cdi and suspected that it may be bad so I replaced it with a used one...again it started a few times and the spark went weak again. I then tested the regulator/rectifier and replaced it with no change to the spark issue. Now I am waiting for a new kill switch that I ordered...it is bad, but I have doubts that it will fix the spark issue. I tested the thermo switch and it seams to be fine. What have I missed?

The first question that I have is about the kill switch. It has a universal kill switch that is on a negitive circut. It has two wires,one wire goes to ground and the ignition switch the other goes to the ignition switch and then to the white wire on the CDI. When I disconnect the two wires from the kill switch and test them for continuity there is continuity between the two wires that connect to the kill switch. Is there supposed to be continuity between the white wire from the CDI to the ignition switch and ground that comes from the kill switch?

Here is a breif recap;
I replaced the ignition coil and retested (the secondary coil tests at 4120 ohms, but I can not get a clear reading on the primary with my meter)
Replaced the stator and rechecked:
Charge coil had 36V and 380ohms.
The pulse coil was 8 to 9 pulsing volts and 17.5 ohms. Both of these readings are higher than they are suposed to be. Can this cause my weak spark issuse?
The lighting coil is 2.85 V and 1.2 ohms. The voltage is low
Replaced the CDI with a used CDI, but I have not retested it, because it sounds like the testing procedures are unreliable
Replaced the regulator/rectifier, but have not yet retested it.
Ordered a new kill switch.
Questioned the continuity between the ground terminal on the ignition switch and the white wire to the CDI from the ignition switch?

Have I missed something or did I put a defective part in?
 
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