787 fouling plugs at idle

[Minton33]

I have a 96 challenger with the 787. Still running the stock oiler. Will run all day long with no issues. Aside from some burbling around 2 or 3k rpm. Which is a pretty useless rpm on this boat anyway. But if it idles for more than 15 or 20 seconds when you go to take back off it will sputter and then eventually clear out and take off. The longer you idle the worse it is. It also does this on the first takeoff of the day. I know this and have been trying to avoid it by holding it above idle and giving random bursts of higher throttle. and it’s fine aside from sputtering when I take back off, although pretty annoying

But my girlfriend was piloting the boat with our friends on it the other day while o was riding our gtx and through the course of the day it fouled 2 plugs after idling. Thankfully I keep extra on it. They were all on the same cylinder, believe it was mag side but don’t remember for sure.

My first thought was crank seal but it’s strange to me that it will run fine all day long with no fouling as long as it’s above roughly 2000 rpm. So I was wondering if it may just be too rich on the fuel or even possibly the oil at idle.

 

[burtshaver2021]

See what others say but I have never fouled a plug on one of these Seadoo’s from to much oil and I’ve even used premix besides the oil injection so essentially running approximately 25:1 I would say you may be right with leaning the mag cylinder just a hair. Try turning the low speed screw in just a hair, say 1/8” turn. What is it at now? And actually you may want to wait to see what others say, I don’t want you to lean out a cylinder on my advice.

 

[Minton33]

See what others say but I have never fouled a plug on one of these Seadoo’s from to much oil and I’ve even used premix besides the oil injection so essentially running approximately 25:1 I would say you may be right with leaning the mag cylinder just a hair. Try turning the low speed screw in just a hair, say 1/8” turn. What is it at now? And actually you may want to wait to see what others say, I don’t want you to lean out a cylinder on my advice.

So I was checking some things this morning before we took it out and I noticed that the oil pump was adjusted completely wrong. It was pulled like 1/4 of the turn at idle. Fixing this seemed to help significantly although I never gave it the chance to foul a plug today.

I am normally always either at idle or above 5,000 rpm and had noticed it sputtering some at lower rpm before, but today I was going some awkward speeds following someone and it was running terrible between 2000 to like 4500. Sputtering and missing. On either side of that it runs pretty good.

 

[burtshaver2021]

So I was checking some things this morning before we took it out and I noticed that the oil pump was adjusted completely wrong. It was pulled like 1/4 of the turn at idle. Fixing this seemed to help significantly although I never gave it the chance to foul a plug today.

I am normally always either at idle or above 5,000 rpm and had noticed it sputtering some at lower rpm before, but today I was going some awkward speeds following someone and it was running terrible between 2000 to like 4500. Sputtering and missing. On either side of that it runs pretty good.

On a 2 stroke you can’t run them clean like you can with a 4 stroke. At this point it’s proven that something is not right. Best to start diagnosing and repairing or risk damaging the engine. It’s ok to take it for a water test, but the moment it falters I would take it back to the trailer and look at it again. No fuel equals no lubrication and lean mixture equals high heat equals lubrication burnt off or worse. Little things, have you checked that the high and low screws are at factory settings? Have the carbs been rebuilt in awhile? Has the selector valve been changed? Don’t buy from Amazon. Has the boat been sitting for any length of time? Sorry if this info is in previous posts

 

[burtshaver2021]

Just re read your first post, sounds like it could be rich, check the screw settings

 

[Minton33]

On a 2 stroke you can’t run them clean like you can with a 4 stroke. At this point it’s proven that something is not right. Best to start diagnosing and repairing or risk damaging the engine. It’s ok to take it for a water test, but the moment it falters I would take it back to the trailer and look at it again. No fuel equals no lubrication and lean mixture equals high heat equals lubrication burnt off or worse. Little things, have you checked that the high and low screws are at factory settings? Have the carbs been rebuilt in awhile? Has the selector valve been changed? Don’t buy from Amazon. Has the boat been sitting for any length of time? Sorry if this info is in previous posts

it had been sitting when I got it. It had been rebuilt and rode for a season and then set.
I was thinking about going ahead and rebuilding the carbs before the next time I take it out. I’ve had luck with the sbt kits in the past and that’s what they put in them at the dealer where I work. I work at a Seadoo dealer but I do not work on skis there. I have not messed the carbs at all but being that the oil pump was not adjusted properly I would not be surprised if the screws have been tampered with. I’ve been unable to find a free manual so I’m not sure where the factory settings are

Also what is a selector valve?

 

[burtshaver2021]

The fuel selector valve, that has on, off reserve. I would buy the back to OEM kits from OSD Marine osdparts.com I have gotten factory settings from seadoosource.com but I’m not sure if they have them for Seadoo boats or not. I would stay away from the SBT kits, they may be fine but a lot of kits are not and I know the OSD ones work. If you do de use not to buy from OSD make sure they are genuine mikuni and not just compatible. The others don’t work. Theres a really good carb rebuild thread on here, just look under Mikidymacs profile for it. At first I thought it was just at idle but if it’s stumbling at various rpm’s probably should clean and rebuild the carburetors

 

[burtshaver2021]

Just looking at another thread to see if I could find your factory settings, I can’t but do you have accelerator pumps on the carbs? You should have if there stock, twin 787’s right?

 

[burtshaver2021]

This is from the 1996 Seadoo jet boat manual, it says for your Challenger, 1 turn out for the low speed screw. When your turning it in, don’t turn it in tight, just until a slight resistance is felt

 

[Minton33]

View attachment 67843This is from the 1996 Seadoo jet boat manual, it says for your Challenger, 1 turn out for the low speed screw. When your turning it in, don’t turn it in tight, just until a slight resistance is felt

Thank you much. It is a 96 but only with a single 787. And not sure if it has accelerator pumps or not. I’m very familiar with bike carbs but only done a few sets of Seadoo carbs

 

[burtshaver2021]

Thank you much. It is a 96 but only with a single 787. And not sure if it has accelerator pumps or not. I’m very familiar with bike carbs but only done a few sets of Seadoo carbs

This manual would be for your model. Just for fun let us know how many turns it takes to seat the low speed lightly when you set them