1991 JS 650 carb tuning info needed

[Wheelz22]

I picked up a cherry low hours 1991 650 Kawi stand-up…it's bone stock, so I assume it has the Keihin 38 mm carb. It runs great but my 89 and 86 bone stock Kawi 550 stand ups (44 Mikuni) both just nip it in a drag race on glassy water.

My question…it has a little long cranking when it's been sitting for a week and cold. I want to check the initial carb high and low mixture screw settings…but I can't find the initial factory spec anywhere including the original owner's manual. It says...'don't touch, bring to dealer'. Any idea what the initial factory settings are for the 1991 JS 650 with Keihin 38 mm carb. I'd like to put them at that setting for starters and then tune from that point. Don't want to mod it with a primer kit. Thanks if you can spare a minute.

[Luv2JetSki]

Re: 1991 JS 650 carb tuning info needed

You can find that information in the STB technical articles section of this Forum:

http://www.sbtontheweb.com/forum/showthread.php?t=13981

[Wheelz22]

Re: 1991 JS 650 carb tuning info needed

Hey, I appreciate that...great site...wish I knew about it years ago!

Quick question though....from what I've read...the 88-90 Kawi 650 had a 28mm
carb and then in 91 onward they went to a 38 mm carb. If that's true...are the settings the same for both carbs ? The reference I was pointed to does not distiguish the two different model year ranges. Also, in general terms, if you put a K & N air filter/spark arrestor on any carb...should you richen the high and low screws slightly due to increased air flow over the stock unit ?

[Luv2JetSki]

Re: 1991 JS 650 carb tuning info needed

I believe the stock settings did stay the same across all the carbs. I can double check my manual when I get home tonight. As I recall even the Mikuni carbs used on some skis are nearly the same settings as the Keihins. Keihin carbs are actually set at the factory using a flow bench so the recommended settings are really just a place to start and will not be the settings your engine originally had when it was off the show-room floor. Seeing how your ski is running decent, I would not worry too much about the stock settings and instead just start the tuning process from you current settings.

As for the K&N filter question, this is my understanding. The filter increases flow and as a result reduces vacuum. Thus, the vacuum reached to pop your needle will not be generated until a higher RPM. Thus, your engine will run leaner than it did with the stock filter in the RPM band between when the stock pop off occurred and the K&N pop off occurred. You might be able to correct this to some extend by running the engine richer but the correct way is to lower the pop-off pressure.

Hope that helped

[Wheelz22]

Re: 1991 JS 650 carb tuning info needed

Dude....excellent info !! Tell ya the truth...I don't know much about the pop off pressure and how to adjust it on either a Mikuni or a K. I'l have to do some research unless you can tell me here in a quick response. If not, don't worry about it.

[Luv2JetSki]

Re: 1991 JS 650 carb tuning info needed

There is a great Mikuni manual floating around, do a search on the forum or google and find it. It is 22 free pages of great info. Although your carb will be somewhat different, it will provide a lot of background. The right way to adjust pop-off is by swapping springs and needle/seat to get the desired pressure. You can make small changes by clipping the spring. This is probably the most common method for making the adjustment after going to a higher flow filter.

To adjust you will need a pop-off tool, either the nice one made by Mikuni or a home made one. A home made tool can do a great job if you use high quality parts.

Read through that manual, if you have some more specific questions I may be able to help though I am no expert.

[Wheelz22]

Re: 1991 JS 650 carb tuning info needed

Thanks man....will do