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Egt sensors on 951

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1998 Sea Doo XP Limited
2000 Kawasaki Ultra 150
#1
Hello all,

Does anyone know if egt sensors permanently installed on a 951 exhaust manifold will affect performance due to turbulence or slight restriction caused by the sensor?

Thanks in advance for any info.
 


mikidymac

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#2
No, it will not, or at least nothing you would ever notice.
 

seadoosnipe

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1997 Challenger, 1997 GTX , 1991 SP and a shop with 2 spare 787cc's and a bunch of parts laying arou
#3
Hello all,

Does anyone know if egt sensors permanently installed on a 951 exhaust manifold will affect performance due to turbulence or slight restriction caused by the sensor?

Thanks in advance for any info.
951? DI, RFI... naturally aspirated? I agree with Mik.. but, if it is a DI or RFI, there could be a few issues. The EGT is basically a temperature sensor. If there is a problem, then you should get a warning of some kind. In the RFI and DI engines, the EGT sends a signal to your MPEM to let your ECU know whether your running to lean, or if your engine is cold, will automatically enrichen your fuel mixture till your engine heats up, then it'll lean it back out. But, that's only if you have the RFI or DI motor. The natually aspirated engine shouldn't be affected at all.
 
Messages
56
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2
Location
central florida
Water Crafts
1998 Sea Doo XP Limited
2000 Kawasaki Ultra 150
#4
Thanks for the relies. Its a 98 xp limited carb engine. I was thinking to install the egt's just to catch a problem before serious damage occurs in the event of a lean condition or gasket failure. I had a head gasket fail this past summer and it caused some slight wear to the MAG cylinder. It still had good compression, but oil was seeping out under a head nut. It made it through summer after a new gasket, but that piston started to melt by september. New top end now, so I'm watching it closely. The question is whether the gasket failed and started the wear process, or if it was a lean condition that made the gasket fail? It would be cool to just look at a guage instead of pulling plugs and raves while out riding. I was thinking to get a dual EGT/CHT guage to keep an eye on it.
Thats interesting to hear about the DI and RFI already having the EGT sensors. I didn't know the MPEM regulates mixture on those boats using an EGT as opposed to an O2 sensor.
 

Matt Braley

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I can't keep up with it anymore. I've got some really cool skis!!
#5
I run egt sensors on my keeper skis. The probes use 1/8" NPT and must come in from the bottom diagonally on the 951 manifolds. Even in that spot there is not a lot of "meat" so you have to be careful not to hit the water jacket and then to not strip out the threads. It can be done but if your machine is stock and passed the pressure test then you will be fine if your fuel system is up to speed.
 
Messages
56
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Location
central florida
Water Crafts
1998 Sea Doo XP Limited
2000 Kawasaki Ultra 150
#6
Thanks for the reply. Ive been opening the hatch and feeling the temperature of the plugs after runs at some different rpms. So far so good. The MAG plug feels slightly hotter than the PTO, but just a little. The engine passed the pressure test when I put it back together and the fuel supply is good. Ive got an extra exhaust manifold that I just might tap into for the EGTs for the next time I have it apart.
 
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Location
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1998 Sea Doo XP Limited
2000 Kawasaki Ultra 150
#8
I had set both HS adjusters 1/4 out when I assembled it this last time, and ran it with the stock jets (80 lows and 160 highs) that have worked with no problems since I put on Tau Ceti pro series flame arrestors about 15 years and 150 hours ago. For the last 15 years, it has run with the HS adjusters closed.
I've now rejetted to 85 lows and 167.5 highs, low adjusters set at 2&3/4. High speed adjusters are now set at closed with the bigger jets. I'm still getting lean plug readings after full speed short runs, and the MAG piston is starting to melt in the center. I believe the melting occured when the smaller jets were in, but it may still be happening with the larger ones.
I'm going to install a clear fuel line from the strainer to the carb to see if there are bubbles in the fuel. Ive already checked the tank fuel filter, checked the selector valve for blockage, isolated the fuel strainer and pressure tested it, but I haven't tested any of the components under vacuum.
I'll post up what the problem is when I get it figured out. Thanks for the replies.
 

mikidymac

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#9
If you have a piston going and 150 hours on that engine it’s on borrowed time.
 
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Location
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1998 Sea Doo XP Limited
2000 Kawasaki Ultra 150
#10
This engine is #3 in the life of the boat, and probably has about 30 hours on it. The second engine I put in the boat lasted about 11 years with the stock carb settings and the tau ceti flame arrestors. That engine lost a cylinder shortly after a head pipe leak, so I assumed it was from water ingestion that time. Since it had over 100 hours, I replaced the whole thing instead of doing a top end. Since this one has relatively low hours, I decided to fix it. It may turn out that I have a front seal leaking at higher rpm but passing a pressure test on the bench. I don't know yet, but I'm going to try a new piston one more time. I'd hate to do a whole engine to find out that wasn't the problem.

I might have found what has burning the piston. I unhooked the carburetor/fuel strainer fuel line from the fuel strainer end and spliced in a one foot long piece of clear line. That revealed a small consistent stream of air bubbles. Since the strainer was hanging by the lines, I turned it upside down a few times to make sure any residual air inside the strainer got carried downstream. The air bubbles kept coming. So I then took out the fuel selector valve and pressure tested it out of the boat. It stood the pressure test and the soapy water test. Then I spliced a clear line into the "on" line coming from the tank, and plugged the "reserve" line. I hooked that line to the fuel strainer. There were no bubbles. Then I spliced in the fuel selector valve into the clear line. No bubbles. So, I decided to splice in a short clear line between the carb return outlet to the return line to see what was coming out. At first, there was alot of air coming out of the carbs and going towards the tank. Then just a few bubbles would hang out at the high point of the line near the return nipple of the carb, but they remained stationary. So, I took it out for a ride on the lake and after some riding even the bubbles in the return line disappeared.
Before the MAG piston melted down a couple of months ago, I was getting an occasional slight surge while cruising between 6000-6300 rpms. The surge cleared up at full throttle though. So I rebuilt the fuel pump and went through the carbs. I didn't see any visible leaks in the lines and the fuel strainer was clean. So, I did a top end thinking the problem was head gasket. I didn't see anything wrong in the carbs, so I thought since the head gasket had leaked a few months before, that was the cause of the melt down. When I had replaced the head gasket, I felt very slight wear in that cylinder and decided to run it because the compression was still good.
So, the new top end made it through break in with no issues. Once I started getting into that 6000-6300 rpm range, it started that little surge again. So, I'm thinking head gasket causing water ingestion. This time, the compression dropped.
Since I caught it early, this second time I just honed it out and did the one MAG piston. This time the dry head gasket failed the pressure test on the bench. So I copper coated it and it passed the pressure test.
So I run it through the break in again, and everything is good. First day out after break in, I notice an occasional slight surge at cruising speed. This time its so slight, I can't even tell if it was a surge or just my thumb pressure varying the throttle. Then it started getting a lean hesitation at take off. It also had that muffled popping sound in the exhaust under load and under 4000 rpms. So, I back the low speed adjusters out to 2&3/4 to get rid of the lean hesitation. That got rid of most of the popping too, but not all of it. At this setting, the plugs would show wet with fuel after idling for a while. So I increase the jet sizes, but that doesn't get rid of the occasional popping. So I was thinking since the ski ran good all these years with the smaller jets, its probably not carb settings.
So, finally I hook up the clear fuel lines and see the bubbles. The surging and lean condition is consistent with bubbles in the lines, so I'm hoping I found the culprit. That MAG piston does have the beginning of a melt through depression in the center, so its time to replace it again. I'm willing to install another piston and try again now I that discovered there were bubbles in the lines.
Oh, so the line between the selector and the strainer got clipped 1/2" on each end. The ends looked a little dry. That must have been the source. I will post up whether or not the next piston holds up. It might be a couple weeks from now.
An EGT would have been nice through all this. Now I'm also thinking maybe a detonation sensor with an audible alarm.
 

mikidymac

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#11
Sucking air and running lean will always destroy pistons. A 951 is already lean from the factory and adding aftermarket air filters with stock jetting is a bad idea and asking for engine damage.
 
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Location
central florida
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1998 Sea Doo XP Limited
2000 Kawasaki Ultra 150
#12
Yes, I see now. The flame arrestors were put on by a shop when they put in engine #2 and replaced the fuel lines back around 2004-5. The stock jetting worked good after that for many years though. Come to think of it, it did always seem to have a little of that occasional muffled popping at low speeds. Now I know that is a lean condition but at the time I thought it was normal for the ski. Now that I have been doing the work on the skiis myself and studying them, i realize it was probably running right on the edge of too lean and the small bubbles put it over the edge. I'm thinking the bubbles hit the MAG carb worse because its first in the circuit before the PTO, and caused that front cylinder to keep melting. The PTO cylinder has been fine. I took it apart yesterday and the PTO piston was rich with deposits and the MAG looked like the melted center was starting to get coated with rich deposits and not dry anymore. Im definately going to keep the 85's and 167's in there and turn in the adjusters in a little to start. I remember when the ski was new, the dealer recalled it to put flaps on the airbox to keep water out. They probably rejetted it down to the 80's and 160's then. The manual says it should have 82's and 165's stock.
 

mikidymac

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#13
Keep in mind the piston deposits are a little counterintuitive. If it is black it is lean and if it is clean it is rich on a 2 stroke. You want to learn to read the piston-wash.
download.jpg

Also on these carbs aftermarket air cleaners can actually make the mid-throttle lean condition worse as they reduce the vacuum signal which can lean it out no matter how rich the jetting whereas the stock air box keeps the vacuum signal high like they were designed.

I will always maintain on a stock engine you can't do better than the stock airbox. It is not restrictive and will not limit power and will keep water out which is the single biggest killer of the 951. When you start adding pipes and porting then change air cleaners.
 
Messages
56
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Location
central florida
Water Crafts
1998 Sea Doo XP Limited
2000 Kawasaki Ultra 150
#14
Good advice. I saved my airbox just in case. Maybe it will have to go back on. My pistons did look like the one on the right.
 

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