US1 Fountain

Full of questions tonight!

How many of you guys determine your jet sizes by reading the plugs at WOT?

According to the motor Bible(Dennis Moores BBC Marine Book), you are supposed to jet you carb at CRUISE, at which point you have 100% flow thru the jets.
And you control the WOT mixture by drilling the power valve metering holes larger if more fuel is needed at WOT, when the PV is open. Page 159-160. This gives the best A/F mixture and economy at cruise, and still having the needed A/F mixture at WOT. By jetting only at WOT, you may have a poor cruise mixture,= poor economy.
I'd be very interested if someone else would read that article and post their thoughts.
Jerry

traviss

if it pops, it's lean.. if it blubbers, it's rich.

formula31

I never run below WOT. And I set my jets there. My secondary power valve opens as soon as the secondarys do. What I would call cruise speed would be arounf 4000rpm and my secondarys havent opened yet. I suppose the size of the boat and load on the engine would have some bearing on that. I read the Holly high performance book. Interesting question though, have to look at that a little more closely this spring.

turbo2256

Ok it goes some thing like this set your jetting to your highest cruse speed power valves closed or blocked off use a vacuum gage to deturmin full cruse vacuum. then put a power valve in that wont open untill a less vacuum is reached about one or two inches lower if your reading is 7 go with a 5 or a 6. Now you guys with more radical cams need to check idle vacuum to se if the vacuum drops below the reading found at your cruse speed or the power valve circut will open at idle causing a rich condition. So in this case a power valve of a lesser value must be used than the idle reading. If it idles at 4 you might need a 3 or 2.5. Ok back to top end. Run it up to top speed kill the engine at that point dont idle down read the plugs. If they show lean jet up... rich jet up. What ever it needs untill it runs and reads right. Now its runing good there her comes the fun. If you had to richin it up in a holley chart find the flow data for the jet ( cross section area) find the differance between the best top speed jets and best cruse jeting. Measure the power valve feed restriction dia. Figure its area and add to it the differance found in your high speed and cruse jeting find the drill dia that will give you the hole dia needed to accomplish this. REMBER YOU ARE WORKING WITH AREAS AND RESULTING HOLE SIZES NOT DRILL SIZES OR YOU WILL GET THINGS ALL SCREWED (<<< BEING NICE HERE) UP. If you need smaller get a bushing to fill in the hole and drill it to size or JB weld it and drill or may be a small lead weight out of the fishing tackle box tapped in the hole and drill. Before you start this process aquire HP book on holley carbs and Peterson READ AND STUDY THEM BEFORE STARTING THIS. WHAT EVER ELSE YOU DO YOUR BETTER A BIT RICH THAN LEAN. Mabe a good pro mechanic might be more help.

Mr Gadgets

bopp,
Good explaintion! Welcome to the board!
You need to balance the jets with the PV orfice to get what you want at any rpm range. You can do the same thing with EFI, except you use the lap top instead of drill...
Sorry just couldnt resist. Been there done that sort of thing. The best advice is to read that book several times. Then go slowly at any changes..

RumRunner

The key thing when you are doing this type of tuning would be:
1) make sure you have enough fuel at WOT
2) double check your vacuum at idle to make sure the PV is closed at idle (so that your engine will idle cleanly)
3) make sure you check the amount of vacuum your engine produces at your cruise speed this will determine if you PV is closed at this speed and this type of tuning will help you
4) before getting into any type of calibration changes in a carburetors metering circuit make sure it is performing properly. Doing this to cover up another problem will not help
5) make sure the idle circuits on your carburetor are set up and working right before doing anything else. If an engine doesn't idle clean, you can't expect it to do anything else correctly. Make sure that your butterfly's are positioned properly before you start.

mxz800

Damn i really love my EFI now

formula31

And it gets worse. Now figure out how to do that with twins when you cant even get at the center plugs, and unless youre real high out of the water or running dry pipes, the engine will hydrolock when you shut it off at wot. Been there, done that.

Crazyhorse

The perfect way to jet your Holleys is to send Bruce a round-trip plane ticket and buy a box of jets.

US1 Fountain

Crazyhorse, I'm 2 steps ahead of you. Already have the Holley jet pack, AND had Bruce work over my carb this summer. Do to funds, only the carb made the round trip.