View Full Version : GT40P head questions
06-20-2012, 04:20 PM
I've been reading up a lot on this head swap and have gotten a lot of good information between this site and the correctcraft site. One thing I haven't been able to figure out is carb tuning. How much adjustment to the 4160 carb is required for this? Between the new intake and better flowing heads, I would've thought the jetting or something else would have to be adjusted but I don't see anyone talking about that anywhere. People seem to only be concerned with the 7/16" to 1/2" hole enlarging of the new GT40 heads. Any info on this?
06-20-2012, 04:57 PM
Jet size, accelerator pump volume and stroke, and power valve application number all play a part in utilizing the new-found ability to breath. I haven't gotten far enough into my To Do List to get to the GT-40P heads yet, but it will be fun doing the tuning.
06-20-2012, 05:31 PM
I have these heads on my current engine. I still tweak the 4160 as I have for the last bunch of years for the standard heads. I have not noticed any difference in performance in either setup using the same 4160 baseline.
06-20-2012, 06:09 PM
The stock marine 4160 jetting is overly fat. No need to adjust anything with a basic P-head upgrade, despite the increased flow. You actually may pick up power by jetting down (leaner), but that is best confirmed with a plug chop (or a rigged up O2 sensor).
06-20-2012, 06:18 PM
Awesome thanks guys! Might have to add this to the "to do list".
I upgraded my GT40s to GT40Ps and have been very pleased. I needed no carb adjustment, just the slightest advancement on timing.