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951 Fouling plugs

CreekerMike

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#1
Greetings and salutations to all
I finally got my 98 Gtx Ltd' out on the river and i gotta say Im blown away by this thing! Everything seems to be in working order except for one small issue. Its fouling plugs. It ran amazing for about 2 hours when i was putting around and doing a few short WOT pulls. Finally on my way back in i opened it up and bluuup. The motor bogged down and shut off. I eventually got it started again and headed back to the ramp. When i got home the first thing i did was check the plugs. They were covered in oil. I replaced them and it fired right back up. Then fouled them too. Im running 40:1 premix with amsoil. Do you guys thing this is a carb issue? Maybe i need to lean out my oil? Or is there something underlying here? All input is apreciated! Thanks
 


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#2
As long as you're not experiencing hesitation, the carbs are probably fine internally. Does it run out fully approaching 7,000RPM or tend to fall on it's face, sputter on excess fuel (does it 4-stroke?) then finally clear out but doesn't run out fully? A couple things to consider:

Could be fuel fowling, or oil fouling? I think you should go back to a properly operating injection system, if possible.

1) The high speed screws should be closed fully, that is, lightly against the seat so as not to damage the seat. No need to crank down hard and distort the seat. The low speed screws might be set too rich, does it load up with fuel while idling and you have to periodically clear it out?

2) The flame arrestor should be installed.

3) The oil mix may be a little on the rich side for low speed operation, it's a tradeoff now so mix in anticipation of how you intend on riding, The oil pump system delivered no oil at idle, it relied on the residual crankcase oil. But you need sufficient oil at WOT, thus the tradeoff.

BR7ES in place of the BR8ES plugs will bump you up one heat range, might do the trick. I wouldn't go up 2 heat ranges in search of a solution though, although you might get away with it without damage, hard to say.
 

CreekerMike

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#3
As long as you're not experiencing hesitation, the carbs are probably fine internally. Does it run out fully approaching 7,000RPM or tend to fall on it's face, sputter on excess fuel (does it 4-stroke?) then finally clear out but doesn't run out fully? A couple things to consider:

Could be fuel fowling, or oil fouling? I think you should go back to a properly operating injection system, if possible.

1) The high speed screws should be closed fully, that is, lightly against the seat so as not to damage the seat. No need to crank down hard and distort the seat. The low speed screws might be set too rich, does it load up with fuel while idling and you have to periodically clear it out?

2) The flame arrestor should be installed.

3) The oil mix may be a little on the rich side for low speed operation, it's a tradeoff now so mix in anticipation of how you intend on riding, The oil pump system delivered no oil at idle, it relied on the residual crankcase oil. But you need sufficient oil at WOT, thus the tradeoff.

BR7ES in place of the BR8ES plugs will bump you up one heat range, might do the trick. I wouldn't go up 2 heat ranges in search of a solution though, although you might get away with it without damage, hard to say.
It seems to start to foul at about 5500-6200rpm, every rev range below that is fine. I cracked the carbs open and i have a 165 main so that seems fine, i was really expecting to find a rediculous size jet in there. As far as the high speed screws they were not adjusted properly. The manual calls for 1/4 out on the mag side and closed on the pto side right? Ill try going up a heat range on the plug and if that doesnt work ill lean my mixture a bit
 
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#4
Yes, as if there's an excess of oil collecting the crankcase then it comes flying up and immediately fouls the plugs when you rev it up? I wouldn't think 40:1 would do that, never had that happen to me at that ratio (generally speaking, not 951 specific) but never say never.

I think there's much less chance of fuel fouling at high speed due to over fueling.

The other type of fouling is aluminum from a melting piston crown due to detonation, hopefully that's not the case here but if it was there should be a hesitation warning.
 

CreekerMike

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#5
Yes, as if there's an excess of oil collecting the crankcase then it comes flying up and immediately fouls the plugs when you rev it up? I wouldn't think 40:1 would do that, never had that happen to me at that ratio (generally speaking, not 951 specific) but never say never.

I think there's much less chance of fuel fouling at high speed due to over fueling.

The other type of fouling is aluminum from a melting piston crown due to detonation, hopefully that's not the case here but if it was there should be a hesitation warning.
Ill try and get back on the river tommorow or thursday and see if the screw adjustment has remedied my problem, Have my fingers crossed that was the problem. Ill bring a set of br7es plugs with me incase the br8es foul out. If none of this has solved my issue then ill reaproach. Thanks for the help! Ill post an update soon.
 
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#6
The manual calls for 1/4 out on the mag side and closed on the pto side right? Ill try going up a heat range on the plug and if that doesnt work ill lean my mixture a bit
Yeah good point, I'm not sure on correct position the '98.

If the carbs have horizontal butterfly shafts I recall something about vibration at idle I believe, causing fuel control issues, they rotated the carbs later on to place the fuel chamber on the side instead of top where fuel just dribbled out of the chamber into the throat. Sheesh, seems like a no-brainer.
 
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#7
See if you can get a look at the piston crowns to see the wash pattern, normally a light carbon coating for most of the diameter except approaching 1/2~1/4 inch from cylinder wall the carbon is "washed" away and aluminum is the washed clean area.
 

mikidymac

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#9
Few questions..
1. Did you use genuine mikuni parts including needle and seats to rebuild the carbs?
2. Did you change the carb needle arm springs?
3. Did you replace the fuel block clear check valves over the jets?
4. Are you running the stock air box?
5. Are you only premixing or premix and injection?

Changing the plug heat range is not the correct thing to do, you have something else wrong if it is fouling plugs every time you get above 5,500 rpm. Also are you saying it fouled the plugs immediately again?
 
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#11
Add looking for exhaust leaks to the list. Not necessarily a cause for fouling but if the bilge is filling with carbon monoxide air for combustion is displaced. It's pretty common to discover exhaust leaks on the pipe joint on the 951, especially if someone has removed it recently or the two brackets have worked loose.

Compression - If compression is low, this can effect the engine is several ways, so it's advisable to confirm compression as part of the diagnostics.
 
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CreekerMike

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#12
Add looking for exhaust leaks to the list. Not necessarily a cause for fouling but if the bilge is filling with carbon monoxide air for combustion is displaced. It's pretty common to discover exhaust leaks on the pipe joint on the 951, especially if someone has removed it recently or the two brackets have worked loose.

Compression - If compression is low, this can effect the engine is several ways, so it's advisable to confirm compression as part of the diagnostics.
There are no exhaust leaks inside the hull. I know the v band on the tune pipe is a pita to seal but i managed to get it after a few tries, as for compression both mag and pto cylinder are at 135 cold. This motor was just rebuilt top to bottom.
 

CreekerMike

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#13
Few questions..
1. Did you use genuine mikuni parts including needle and seats to rebuild the carbs?
2. Did you change the carb needle arm springs?
3. Did you replace the fuel block clear check valves over the jets?
4. Are you running the stock air box?
5. Are you only premixing or premix and injection?

Changing the plug heat range is not the correct thing to do, you have something else wrong if it is fouling plugs every time you get above 5,500 rpm. Also are you saying it fouled the plugs immediately again?
Glad to see you chimed in miki, to answer your questions
1,2,3 I never rebuilt the carbs when rebuilding the motor. I popped off the covers and everything looked very clean so i left them be. The ski had always been taken to a dealer before i had it so presumably there all authentic parts.
4. I dont have the stock airbox on, i still have the k&n flame arrestors on.
5. I am only premixing at 40:1 with amsoil dominator.

For the first 2 hours it was fine, i was able to achieve 7k rpm consistently with no issues, then bam went to hit it and it bogged and fouled. When i got home and put new plugs in it fired right up. I cracked the throttle wide open for a split second and it fouled. Put a new set in and went back out on the river. Everything seemed fine. I tried a Wot pull and it opened up to 7k for a second then bogged, i managed to keep it lit and started to head back in again. I eased into the throttle and around 5500+ is when it seemes to start to foul. Could this all have been related to the high speed adjusters being wrong?
 
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#14
If by bogged, you mean hesitated and fell on it's face like it was running out of fuel, could be as simple as an air leak in a fuel line. Such an event wouldn't be a cause of immediately fouled plugs though.

If the HS mix screws were set too rich, this could foul plugs, certainly. But I doubt you'd see 7kRPM in that case unless your pump is worn, should be over 50MPH at that RPM, I'd expect.

Too much fuel will keep RPM's limited, so you might notice four-stroking. But let's say a big wet slug hit the plug insulator, that's bound to solidify onto the hot insulator and immediately short the plug out. As a benchmark, a new set of plugs in my 951 the insulators will take on a yellow tinge following a 1/2 throttle run of 15 minutes, further running they begin turning cardboard brown.

135psi is very good.

One thing I've noticed about having excess oil is it never leads to melted pistons or scored bearings, but can lead to fouling the plugs earlier.

As Gordon Jennings explained years ago:

"There are a few rules that may be used for guidance by those who have yet to acquire experience - or by the many whose experience has left their ignorance largely undiluted: First, you should know that it is all but impossible to read anything in the appearance of a spark plug unless the engine has been cut clean after having been brought up to operating temperature and given a long burst of wide-open throttle. Very experienced tuners will see the signs they're looking for under the layer of soot, oil and fuel that 'accumulates so quickly at idle, but even they vastly prefer to work with clean-cut plugs. Second. get the right heat-range before you try to read mixture strength, and my recommendation is that you always use the hottest plug the engine will tolerate. You'll know a plug is too hot when you observe signs of blistering around the insulator nose (which will also be scorched white) and on the electrodes. A too-lean mixture will also give you a whitish insulator, but will not usually produce the burned, pitted appearance of the electrodes that is characteristic of a too-hot spark plug. Also look for signs of melting along the sharp edges at the ground electrode's end any sharp corner will get hotter than other areas along the electrode, and trouble will first be revealed there. A plug that is too cold simply looks, and is, wet. Plugs of the correct heat-range get hot enough to burn away oil, and soot, and will have only dry, brown to tan deposits on their insulators after a hard run. As noted before, the correct mixture strength will be only slightly leaner than that which is just lean enough to keep the engine from four-stroking. "
 
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CreekerMike

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#15
If by bogged, you mean hesitated and fell on it's face like it was running out of fuel, could be as simple as an air leak in a fuel line. Such an event wouldn't be a cause of immediately fouled plugs though.

If the HS mix screws were set too rich, this could foul plugs, certainly. But I doubt you'd see 7kRPM in that case unless your pump is worn, should be over 50MPH at that RPM, I'd expect.

Too much fuel will keep RPM's limited, so you might notice four-stroking. But let's say a big wet slug hit the plug insulator, that's bound to solidify onto the hot insulator and immediately short the plug out. As a benchmark, a new set of plugs in my 951 the insulators will take on a yellow tinge following a 1/2 throttle run of 15 minutes, further running they begin turning cardboard brown.

135psi is very good.

One thing I've noticed about having excess oil is it never leads to melted pistons or scored bearings, but can lead to fouling the plugs earlier.

As Gordon Jennings explained years ago:

"There are a few rules that may be used for guidance by those who have yet to acquire experience - or by the many whose experience has left their ignorance largely undiluted: First, you should know that it is all but impossible to read anything in the appearance of a spark plug unless the engine has been cut clean after having been brought up to operating temperature and given a long burst of wide-open throttle. Very experienced tuners will see the signs they're looking for under the layer of soot, oil and fuel that 'accumulates so quickly at idle, but even they vastly prefer to work with clean-cut plugs. Second. get the right heat-range before you try to read mixture strength, and my recommendation is that you always use the hottest plug the engine will tolerate. You'll know a plug is too hot when you observe signs of blistering around the insulator nose (which will also be scorched white) and on the electrodes. A too-lean mixture will also give you a whitish insulator, but will not usually produce the burned, pitted appearance of the electrodes that is characteristic of a too-hot spark plug. Also look for signs of melting along the sharp edges at the ground electrode's end any sharp corner will get hotter than other areas along the electrode, and trouble will first be revealed there. A plug that is too cold simply looks, and is, wet. Plugs of the correct heat-range get hot enough to burn away oil, and soot, and will have only dry, brown to tan deposits on their insulators after a hard run. As noted before, the correct mixture strength will be only slightly leaner than that which is just lean enough to keep the engine from four-stroking. "
I dont think it was or is an air leak. Like i explained to miki, when i put the new plugs in it fired right up then right when i blipped the throttle wide open for literally a second...voula engine shuts off, pull plugs and they're wet with oil. Im gonna get some new plugs later today after work. Ill post an update then.
 

mikidymac

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#16
It's your carbs dumping a ton of fuel in on top end. Rebuild them with the correct Mikuni kits and needle and seats.

I don't kike the K&N filters at all and especially on a 951. If they get even a little water on them they flow zero air and you go full rich.

I think Amsoil Interceptor is the oil actually suggested fro the seadoo's.
 

CreekerMike

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#17
It's your carbs dumping a ton of fuel in on top end. Rebuild them with the correct Mikuni kits and needle and seats.

I don't kike the K&N filters at all and especially on a 951. If they get even a little water on them they flow zero air and you go full rich.

I think Amsoil Interceptor is the oil actually suggested fro the seadoo's.
Ill look into rebuilding them with a good kit, im gonna toss in a new set of plugs and hope maybe the mixture screws fixed my issue. If not ill follow your advice. As far as the k&n ive read that too. Im still looking into either the stock airbox or the prok with outwears. As far as the oil goes i chose dominator for the added film strength (figured itd help because the 951 is so stressed) also something to note the needle seat is orange, the stock one is black right? Maybe the dealer put in a 2.3 and thats contributing to my issue?
 

mikidymac

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#18
They are brass/Gold in color and even the junk aftermarket ones are too.
 
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#19
I don't expect an aftermarket seat of the correct orifice diameter will cause these carbs to over-fuel on the upper end/WOT, orifice diameter is independent of manufacturer. If the carbs are dumping tons of fuel, it seems unlikely the engine can make enough power to run out to 7,000RPM

I think there's a case to be made the 951 is sensitive to having too much oil stored in the crankcase, I believe I've experienced similar plug fouling a couple of times after fogging mine perhaps too aggressively.
 
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CreekerMike

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#20
Update: i adjusted the high mix screws and re installed the carbs, the ski started right up. I let it run for about 10 seconds and blipped the throttle wide open. The plugs didnt foul! So im getting somewhere. I wont know until tommorow when im out on the river wether my issue is solved or not though. Ill post an update then. Thank you all for the input and advice!!!
 

mikidymac

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#21
What are your highs set at?
The book calls for 0 on the front and 1/4 on the rear. Even with the K&N it really isn't going to flow much more air than the stock airbox.
 

CreekerMike

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#22
What are your highs set at?
The book calls for 0 on the front and 1/4 on the rear. Even with the K&N it really isn't going to flow much more air than the stock airbox.
Thats what i have them set at, previous owner had them both at an 1/8th of a turn open or half way inbetween the rubber stopper.
 

mikidymac

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#23
Even then, it shouldn't cause it to shut off and foul the plugs as soon as WOT is reached.
 

CreekerMike

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#24
Even then, it shouldn't cause it to shut off and foul the plugs as soon as WOT is reached.
I was worried someone would say that. If that is the case (asuming i go on the river tommorow and foul more plugs) the next step is rebuilding the carbs right? I really have my fingers crossed that was the simple issue... guess ill know in the first 15 minutes or so. What buggs me is the fact that it was fine for almost 2 hours and then fouled. That makes me think the oil pooling would be a more logical reason... But if that were the case wouldnt the oil still be pooled today and have fouled the plugs? Given itd be a bit more settled at the bottom of the case then when im out riding. Wouldnt it still sling up?
 
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#25
[QUOTE="CreekerMike, post: 620177, member: 101857"That makes me think the oil pooling would be a more logical reason... But if that were the case wouldnt the oil still be pooled today and have fouled the plugs? Given itd be a bit more settled at the bottom of the case then when im out riding. Wouldnt it still sling up?[/QUOTE]

Oily yes but do you recall the color of the plug center electrodes, were they fuel soot black, cardboard brown, yellow, white? Even water in the case can short a plug and steam-clean the insulator to white. Was there water trapped behind the fuel pump diaphragm on the crankcase pulse side? FWIW, I noticed a couple droplets of water in mine both times I had carbs off, the first time was salty tasting like NY harbor (where I bought the boat).
 

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