720 tuning issues - 1997 Speedster

[Kanyon]

I have a 1997 speedster with 720s. I am having issues with the Port side engine tuning and I can't find my particular issues already discussed. Compression at 130-132. Single Mikuni carb rebuilt using genuine Mikuni parts. cleaner flows through all holes in the carb well. I can not find the low and high needle settings for the speedster. The pilot jet is 67.5. The main jet is 182.5. The initial setting I used is Low: 1 High: 1.5. Idles set at 1500 in water. Pop-off 36 psi.

Here are the issues I need help with:

• The engine dies when throttled back to idle. Must add throttle to restart. will stay running at 1900 RPM.
• The engine will smoothly go up to 4000 RPMs. Approx. 1/2 throttle. No change in RPMs through 2/3 throttle. Then a jump to 6000 RPMs. I can then back down the RPMs to about 4500 RPMs without feeling a surging in the motor. I am assuming this surging I am feeling at 4000 - 4500 RPMs is the pop-off turning on and off.

What are your suggestions to get this engine running more smoothly through its RPM range?

 

[kenneth.mccoy1]

for tuning the idle you want to be able to blip the throttle and then see the rpm’s drop back to where they were before blipping the throttle. You do not want them to go below then back up, if they go below then back up that means you are rich. If you blip the throttle and then it holds an rpm for a second or two then goes down it’s lean. Whenever you turn the idle screw in and out in 1/4 increments listen for engine rpm raising or becoming more steady. When you hear the rpm raise a little bit, back the idle screw out just a little and that would be a good idle setting. To verify this, let the boat idle and then kill it, let it sit for 10 seconds, then start it. It should start almost instantly and return to normal idle rpm without using the throttle. As for the high speed, you need a digital tach to tune. If you are lean you will feel power loss on a wide open pull, and at wide open rpm will drop 10-20 rpm every 2 seconds, if you see it consistently dropping rpm DO NOT KEEP HOLDING IT OPEN, you will score a piston or two, ask me how I know lol. If you are rich, you will accelerate with normal power, but your rpm’s will be lower than expected. Make sure you are patient with it, I know how frustrating it is, but it is something we have to learn by doing it. Also make sure you tune in smooth water. Groupk has a lot of good stuff on their website about carb tuning

 

[kenneth.mccoy1]

And you can only tune once you are certain you have no leaks in your fuel or air delivery

 

[Kanyon]

Thanks. I will try your suggestions this week.

 

[burtshaver2021]

Did you visually check the size of your jets? I have 98 Challenger, also has twin 717’s 167.5 main and 72.5 pilot, with accelerator pump, sounds like the same set up, manual for 98 Challenger calls for low speed 1 1/4 plus minus 1/4 turn and high closed

 

[Kanyon]

Yes,
I did. 182.5 Main and 67.5 pilot. had to use a magnifying glass