Dang it, I typed this out once and when I posted it, I got an "error" message and lost all my text. I am rebuilding a 4781-2 that I bought on Ebay. It looked complete and ready to bolt on, but I decided to tear it down in case it had clogged passages or stuck needle and seat. The choke was gone, but I figured that if a previous owner had done some performance mods, that was fine with me. When I took it apart, I found it pretty clean, an the gaskets weren't stuck too bad, as if it didn't have many miles on it since a rebuild. I also found that it had no power valves--nothing where the front one should be, and a dummy valve (plug) where the rear one should be. I hear that's a racer trick. The shooters are 37s which also seems consistent with a performance enhancement job. Everything looked good inside except that it has no jets! First thought was "Can a Holley really run without any jets in it?" Second thought was "Maybe the owner wanted the jets out of it so he took them before selling me the carb." But his ad said nothing like "Carb not complete, is for parts only" etc. And the carb was torqued together like you'd expect a complete, working carb to be. So I have 2 mysteries to solve: 1. What would have happened if I'd assumed it was ready to run out of the shipping box and bolted it on my engine and started it up as is? 2. What jet sizes should I put in it to start with as I dial it in for my 430?
1 > it might have run for a sec but then major flood , stall , back-fire , etc maybe ? 2 > 4781-2 ( 850 cfm 4150 dbl pump ) - my holley book says jets front and rear were #80's . power valve #65 . that may be a good start out point and then tune from there . if someone here has had good luck with this combo they may get ya closer on jet size .
Thanks for the comments, Del. I bought a bunch of jets, and have been doing some reading--allegedly, Holley jetted this carb from the factory for power, not good mpg so some people changed to smaller primary jets (mid 70s) with good results. I am starting with 80s in the primaries, and 86 secondaries just for a baseline. I have a vacuum gauge, but not an air-fuel mixture meter so I won't come up with the perfect combo but as long as I can keep the thing from running too lean and it can pull hard, I will be happy.
thats not really an MPG style carb anyway . i'm sure that they were not worried about that . i usually just check/recheck the look of the plugs as i go along , using the oem recommended plug , and go from there in steps . if it runs crisp and the plugs look correct i think i'm close . i'm sure the air/fuel ratio meter etc is more accurate and alot faster .
You did not get a complete carb from the guy.... starting point,,, return the carb to holley specs and it should run good....
Thanks, Doc. I have the fuel cell and lines almost done, so when I get the thing running I will see how she goes and post an update in case it helps anyone else running an 850.
This always what I do when starting with a used carb. I use this reference chart: http://www.holley.com/data/TechService/Technical/Carb Numerical Listing.pdf Double pumpers can be made to cruise and idle lean and clean yet still run hard at WOT. The simplest and easiest trick is to drill out the power valve restrictor orifices and reduce the primary jet size. I would not attempt this without the aid of a wideband a/f ratio gauge though. I have a 950 double on my car and it idles at 14:1 and cruises at 14 to 14.5:1. Yet it still runs the same numbers at the track and maintains 12.7 to 13:1 at WOT. This was from using a wideband and playing with the PVCR and primary jetting.
