Re-jetting

[mecjr22]

I haven't been able to get a good spark plug reading with my spark plugs, with my high speed adjusturs nearly all the way out on my 95 750 slt with triple fuel pump and OP flame arrestors. It runs great now, but I fear running it too lean. I bought some 102.5 jets for high speed. Will this most likely solve my spark plug reading, and what should my high speed adjustments be set at? I'm premixing at 40:1, and don't know if this has an effect. My last question is if going to a bigger jet will effect performance negatively.

[mecjr22]

Re: Re-jetting

Do any sbt staff have a comment on this situation.

[Technical Support]

Re: Re-jetting

I have no idea what jets you need to go with - contact whatever shop sold you the flame arrestors. If you can not get color, you are too lean, period.

[mecjr22]

Re: Re-jetting

I bought the ski with the OP flame arrestors on there. I went to 102.5 inserted now, but I will take it out and update this to see how it is doing. Thanks for the quick reply staff.

[Riixxii]

Re: Re-jetting

Take the plugs out and look at the tops of the pistons paying close attention to the edges. On the outer 1/4" of the piston should be "clean" with the blackish carbon build-up on the rest of the piston crown. The closer to center the "wash" gets the leaner you are burning. Sometimes its easier to read the piston than the plug.

[mecjr22]

Re: Re-jetting

What do you mean by the "wash" are you talking about the clean part or the carbon build up part. Thank you

[addicted]

Re: Re-jetting

more air= more fuel. By not putting in bigger jets you are hurtung performance. Ther are probably no known baseline setting for that combo. Everyone's ride, altitude, ect are different. Besides, polaris is not the name most people think of when they think performance watercraft. Snowmobiles, yes., hence the lack of acceptable responses. I would try the HS needles at the max, 3 turns out, and work our way down from there just to be safe. One other question. If it came with the FA's on there when you got it, and the jetting was not correct, you may have internal damage. Have you checked compression?

[mecjr22]

Re: Re-jetting

The waverunner rides great, so I don't think there is internal damage. When I bought it, it wasn't running becuase the stator was bad. I rebuilt the carbs and set the adjustments to factory without knowing about the ocean pro flame arrrestors. Since then the plug readings seemed alright, but then I was haning a problem with bogging, and that was due to my flywheel not torqued enough. While I was figuring out that problme I decided to go premix, and since then I haven't been able to get a reading on the high speed screws. I turned the m all out nearly all the way and still nothing. So tuesday I put in a bigger size high speed jet, becuase I read on a forum of someone with a similar set up doing the same. I guess I will just try it out today when I get done teaching and see how it runs. If the performance is lacking, would I be able to go down to 90 high speed jets but change the premix from 40:1 to something like 45 or 50:1? Thanks everyone for the insight and help.

[addicted]

Re: Re-jetting

Dont change the pre mix ratio!

I would try it first and see. You should get good color and about 6200 rpm if all is well. Also, is the prop stock? What type of oil are you running. I think i read somewhere that the high end synthetic stuff is so clean that you can't get good plug readings.

[Riixxii]

Re: Re-jetting

The wash I refer to is the clean portion around the outside edge of the piston. The farther in to center the wash gets the leaner you are running. I like my personal motors to run right on the edge, not customers, so i adjust their machines to stock settings or even a little richer for longevity. Here in MN we are actually considered on the edge of high altitude. My old 750 had stacks and I used the factory jets and ONLY adjusted with the needles and it is still running strong. Starting 3 turns out probably won't even run but you will at least start out safe and see improvements with each adjustment. Put your stock jets back in tough. you only need to go up in jet size if you get pipes.

[addicted]

Re: Re-jetting

He said he couldn't get good readings with the stock jets. I had the same problem.

[mecjr22]

Re: Re-jetting

Well, I put the 102.5 jets in and turned the screws out and still no reading on the very top of the spark plugs. The threads are nice and oily all the way up to the very top, however the very top flat part is still clean. I've been turning it out with no change. However, the more I have changed it there seems to be some build up toward the top threads. Could it be that I'm getting to rich, but I'm burning everything off of the very top flat part of the spark plugs. I'm testing at top throttle since I don't have an RPM reader. Should I test high speed at a lower speed than full throttle.

The only other thought I have that could be causing this is with the high speed adjustment. Where does the air enter into the system from there. Could I have the high speed check valve to tight not allowing air in. I'm not sure if the air enters into the area below the diaphram or if it goes into the carburetor throut.

The oil I'm using in Quicksilver. I'm going to go to Penzoil semi-synthetic this weekend to see if anything changes. I have also made changes to the position of the fuel restrictor. Before I changed it, the restrictor was right off the carburetor and I moved it right before the gas tank. I'm moved it back today to where it was before to see if that changes anything. The only reason I did that, is becuase part of the choke comes off a bit after where it was before. I'm not sure if the choke relieves some pressure and doesn't allow enough fuel into the carburetors or not.

[John Kubiak]

Re: Re-jetting

The threads are nice and oily all the way up to the very top, however the very top flat part is still clean. I've been turning it out with no change. However, the more I have changed it there seems to be some build up toward the top threads. Could it be that I'm getting to rich, but I'm burning everything off of the very top flat part of the spark plugs.

The threads of the spark plugs are oily??? This can only happen if they are not properly torqued into the cylinder heads....Read this excerpt from another post that I replied to a while ago.


Many times, in the many years I have been working with many different types of vehicles and engines I have found a common installation mistake made by many people; novice and expert alike.


THEY DO NOT PROPERLY TORQUE THE SPARK PLUGS!!!!!

The specs are 17 foot-pounds or 1/2 to 2/3 of a turn after the crush washer touches the cylinder head.

This can and will have a direct effect on spark plug life, and the plug's ability to resist fouling.

This is quoted directly from the NGK Spark Plug website:

Athough this website does not specifically state "for PWC" the same data applies.

http://www.ngksparkplugs.com/techin...1300&country=US

"Torque is one of the most critical aspects of spark plug installation. Torque directly affects the spark plugs' ability to transfer heat out of the combustion chamber. A spark plug that is under-torqued will not be fully seated on the cylinder head, hence heat transfer will be slowed. This will tend to elevate combustion chamber temperatures to unsafe levels, and pre-ignition and detonation will usually follow. Serious engine damage is not far behind.

An over-torqued spark plug can suffer from severe stress to the Metal Shell which in turn can distort the spark plug's inner gas seals or even cause a hairline fracture to the spark plug's insulator...in either case, heat transfer can again be slowed and the above mentioned conditions can occur."

Make sure to properly gap/install/torque your spark plugs.

[mecjr22]

Re: Re-jetting

thanks for the info, I will be sure to torque them down more. I just had a problem with the spark plug busting off inside, and didn't want to torque them too much. I looked at the pictures in the teck articles about reading spark plugs, and some of those pictures showed a mixture on the threads. I guess I will just torque it down a bit to make sure. Thanks for the help.