1Boatnut

Ok,I would like to know what methods guy's use to check the jetting on their carbs...
example:take a blast at a certain RPM and read the plugs....
Exactly what do you do and how do you do it? Inquiry minds want to know

John B

Nut, On a new combination (new stock carb), I start 4 sizes larger and run for max speed. Then go up by 2 and see if you gain speed. You may have to down size but go up first. Find best combination, under the same load conditions. You can do this on a stocker also. We have never cooked a motor setting it up this way.
JB

powerboat

Run the boat wide open then shut it down without idiling(Don't sink the boat by letting water over the back try slower speeds first) Read a couple of plugs 1,7, etc If they look like nestle quick that's perfect. White is lean, black is rich.

Audiofn

I just read a artical in a mag (sorr can not remember what one it was) that said that reading the plugs is very hard to do. I gathered from what they were saying is that even a good running engine will take time to get the plugs to color up so it can be hard to tell what is truely going on. They also recommended the runs at speed and looking for the top speed. The only thing that concerned me about that is wont a slightly lean mixture produce your fastest speed?

Jon

RumRunner

First off reading plugs is very difficult if you're not sure what you are looking for. You need a good spark plug light (designed for your type of plug), and then it's a matter of time learning what your specific combination wants. You're looking for a fuel ring down around the bottom of the porcelin, then you want to see how much heat is on the electrode, and threads, you are obviously looking for signs of detonation peppering, or shinny metal (your pistons !!)

As far as how I tune a carb, I first get the idle set properly. If the engine doesn't Idle right it will never perform to it's optimum. Next I'll work on the accelorator pump circuit, so I can hit the throttle without any stumbles or hesitations. Then tune in W.O.T. The more information you have to do this the better. What I mean is this you can tune just by spark plug color, or speedo, but if you combine that with EGT, or O2, and engine Temp., and RPM this gives you a better chace of getting it right. REMEMBER 99% OF THE TIME YOU ARE SAFER BEING RICH !!! Once I've got the idle & W.O.T. tuned I'll check the cruise speeds the boat will travel at. I check the engines vacuum so that I can tune the Power Valve size to be closed at low cruise speed to hep with economy. From there depending on how energetic I get I will in some cases work on the PVCR (Power Valve Channel Restrictor) size. This controls the amount of fuel that comes through the power valve circuit when it opens. You can richen this up, and take some main jet out to help your fuel economy even further. Not something that most people will play with, but it works extremely well.

1Boatnut

Thank's for all the input guys. I already have the carb set up.Power valves,squiters,engine vaccum ....all set. I threw in a new set of plugs and I'm heading to the water to try to get the jet settings dialed in. I was told to do 3 passes....1 at 300o rpm,shut down and pull #1 # and 6. Then at 4000 rpm,and then WOT.Just wanted some different ideas........Working under the hatch on the water when it's 100 degrees out is not fun,but it's part of boating......Thank's again for all the different styles.