98 GTX Limited Carb Tuning

[tommyadrian5]

I recently installed a SBT motor in my 98 SD GTX Limited. I got it in the water yesterday, started it up. When starting it needed to be given gas before it would catch, and once started it was idling at 1220 rpm. Also from around 2,000 rpm to 4,000 rpm the motor made a muffled pop which sounded as though it was misfiring. Also, when WOT was applied from a standstill the motor would bog down for 1-2 seconds before taking off and accelerating. Above 4,000 Rpm the motor was very smooth. The motor is in the break in period. This brings me to my 2 questions.

1) I believe the idle speed should be 1,500, and needs to be increased. Which screw (is it only one) does this, the one on the Mag side of the carb which hits the throttle linkage plate? How much farther should I screw it in to gain 200-350 RPM on my idle?

2) Bryan told me this sounded like a rich condition on the low end. I am wondering how exactly i fix this. Where exactly are the low speed screws on this carb, and should i turn them in or out(and how far).

Thanks,
Tom Adrian

[ April 21, 2003, 09:20 AM: Message edited by: tommyadrian5 ]

[Technical Support]

The idle speed screw is located right where the throttle cable attaches tot he carb brace, it has a thumb screw head.

If you hold the carbs vertically, the low speed screws are towards the bottom of the carbs, the highs are on the opposite side, more towards the top of the carbs.

The lows should be out 1 1/4 turns.

[Bill O'Neal]

For what it is worth,
If you are tuning carbs, and are cruising along at 1800 to 3000 rpm and you are hearing a muffled popping sound, you are too LEAN on the low speed circuits.

If you stab the throttle and it hesitates before taking off, you are too LEAN on the low speed circuits.

Peolpe need to be able to differenciate between BOGGING and HESITATING.

Bogging ( a rich condition) is when you give it gas and it takes awhile to clean out and run good. The sparkplugs will be black and wet if you don't check them after you run it on top end rpms, but check them when it bogs, before gaining rpms.
Hesitating ( a lean condition) is when you nail the throttle and it seems like it is out of breath, and you may possibly have to feather it to get it going, or it just dies when throttle is applied. The sparkplugs will not be wet and black, but dry.

It is possible to have a rich or lean bottom end adjustment, but still be lean or rich on the high speed adjustments ( or jetting). So reading the plugs after a high speed run is only indicative to the high speed jetting or adjustments, and may not indicate what is going on with the low speed circuits.

When adjusting carbs, always start with the low speed adjusters. Get the motor to accellerate without hesitation or bogging, then check the idle speed rpm (1500 rpm in the water for most motors). After getting the low speeds working correctly, it is time to go adjust the high speed circuits. After you get the motor adjusted for high speed running ( nice brown sparkplugs, maybe a little bit wet around the edges, you can go back and readjust the low speed adjusters, and idle speed, if necessary, to get it as perfect as possible.

Never check your sparkplugs after idling back to shore, unless you want to know what they look like after idling. It will not be indicative of your jetting when running the motor hard.

[ April 21, 2003, 10:22 AM: Message edited by: Bill O'Neal ]

[tommyadrian5]

Sorry for my ineptitude but I am still confused as to which screw is which. When you say "holding the carb vertically, which way vertically do you mean?) If you could identify which screw is which from these descriptions of the magneto (front) carb it would probably be easier.

1) The screw closest to the magneto, close to the fuel line entry into the carb, yet on the opposite side(horizontally boatwise) of the carb from the fuel lines.

2) The screw on the PTO side of the carb, towards the top of the carb.

Is the idle speed adjustment the screw on the bracket where the throttle cable leaves its housing?

thanks

[tommyadrian5]

In response to bill, is there any way to tell which it is, if it takes right off after the initial hesitation. If throttle is applied slowly it will accelerate fine, however with the muffled popping noise. Is there any way to tell if it is lean or rich from this, i didn't look at the spark plugs this time i wanted to see if there were any major glitches with the new motor before i started tuning it.

[Technical Support]

[tommyadrian5]

Awesome, thanks for the picture, that is worth not one but 2 million words, for my inept carb tuning self.

[Bill O'Neal]

Tommy,
Like I said in the previous post, if it needs to be feathered, it is lean. Open the low speed adjusters (they have small tee handles and are at the bottom of the carbs, hull side) about 1/4 turn and try it again. Adjust until you get rid of the lean condition and it accellerates correctly.

But, if you are idling at a low idle speed (it should be 1500 in the water), it may just need the idle set up to the correct idle speed. Try that first.

[tommyadrian5]

Bill, Thanks for the info. But I am confused, you tell me that the low speed screw is on the bottom of the carb, but on the picture posted above by technical support #2 it appears that when the carb is mounted on the boat the high speed screw would be on the bottom of the carb. On my carb the choke assembly is on top of the carb, and in the picture it says that the low speed screw is on the same side as the choke assembly. Either this is wrong, a different carb than is in my boat, or I am missing something here. Any suggestions.
The boat is a 1998 GTX Limited.

[tommyadrian5]

I found some pictures of carbs for a 951 on ebay. Is the low speed screw this little bronze one by the mounting bolt in the first picture.

http://cgi.ebay.com/ebaymotors/ws/eb...gephotohosting

http://cgi.ebay.com/ebaymotors/ws/eb...gephotohosting

can one of you take these pictures and paste them into the thread with notes on where everything is to clarify. Thanks

[Technical Support]

Top - means holding the carb like the picture, to the top of the carb - NOT top as is facing up whem mounted on the ski. Carbs can be mounted in a variety of positions and angles on skis. When describing needle locations it's always relavent to the carb itself.

Top is towards the air intake. Bottom is towards the engine.