If you are not using an A/F meter than you are just guessing. Take it from someone who has burned or broken more than my fair share of parts. A meter is the only fool proof way of knowing what is going on inside the motor. A good fast meter like the one I own is less than a tank of fuel. At 400-500 it sure sounds like the best way not to melt a 10,000 - 50,000 dollar motor.

First let’s discuss how a carburetor works:

Carburetors work based upon pressure differential. As your engine draws air through the carburetor it creates a low pressure in the center of the carburetor. The difference of this pressure, and the higher pressure (from the atmosphere) pressing down on the fuel in the bowls is what causes the fuel to flow through the carburetor into the engine.

Now with that out of the way, if you have to jet up excessively from a carburetors baseline it usually indicates one of two things. It could be the carburetor being too large for the combination, or your fuel system not keeping up with the demands of your engine.

With that being said, going to a larger carburetor is not going to help you at this point. If your carburetor was too small it would actually run rich, not lean. The first thing to look at would be your fuel delivery system. A 110 GPH fuel pump would be on the small side for a supercharged engine.

As far as your original question, jetting by vacuum most likely isn't going to work on your current combination.

Yes, I am using single 850 set-up. The primaries are now 96 with 98 secondaries. I just changed them to 98 primaries and 100 secondaries but have not been able to get back on teh water. My bet is that it will still pop.

I know pressure and volume is two differrent things but, the fuel pressure (at the inlet manifold to carb) guage indicates 6 psi when running at 4500 rpm. Theses "pops" seem to occur over 4000 psi and sporadically. I know its a lean condition from previous expreience; this is idicatical to what was happening after rebuilding teh run out engines, prior to the installation of blowers. Every one on this fomr seems to be seeing at least 1050 carb set-up with the 250 blowers.

Wish I had during the off season. Hate to miss the Tickfaw this coming weekend. I would take ti over to them but I'm afraid it would be weeks before they got to it. May just bit teh bullet and order new 1050 carbs. Don't know anyone that has a set here locally do you?

Jetting by Vacuum readings was obtaine doff the holley carb site. Thanks for the spark plug reading article; I've been looking that the procelain as the main indicator forever.

i'd be willing to say he has a set or 2 there..

all i can say is that i wouldn't take the risk of a meltdown ...seems you have a issue and imo you should get it resolved before you have a scrap pile of parts or someone hurt...

think about it ,is it really worth risking???...

Hey Al im running a 250 B&M And i have DUAL 750 on a 509 BBC 1st if you have a single 850 you are running lean and you cant push enough fuel to get that not to run lean. I am running 85 and 90 in mine and its still borderline lean runs great but nees a little more fatting up. Ive talked to many people on here and that is what i have learned. The 1050 for a single carb blower is the only way to go. But my self, im running twins so was a pain in the balls to tune. The 1500 CFM's do run great though. But let me know what you ant for yours i may just up grade to those and save a little hassle of pushing the 750's to the limit
Mike

Ok guys the "rookie" is back with an up-date. Spoke to the guys at Barry Grant today. First, no one mentioned the fact that these 850 carbs had "blower referenced" power valves on the primaries; apparently there should be vacuum hose running from under the primary bowl to the manifold under the blower. Without it, as Im's sure most of you expreienced guys already know, the 4.5 PV will never open increasing the jet size by 6-8 jet sizes.

After considerable conversation, the Barry Grant (matine) guys believe (rather strongly) that the 850 blower carb is plenty of carb when connected properly and (very important here) it is supplied with enough gas; as Doug from Grant mentioned in a reply above, the 130 gph mechancial fuel pump is more than likely not able to provide enough fuel to the carb under substain high rpm conditions. If they had sold me the carbs, one might think they were simply defending the sale however, they didn't recommend or sell me the carsb so I'm thinking it would have been very easy for them to say "wrong carbs for the application" and quickly sell me another set of bigger carbs, if they really felt that was the problem.

The lowest that I've ever heard the pop is between 3500 -4000 rpm. At 3500, the vacuum reading is -5; at 4000 rpm, the vacuum reading was -4. If the PV opens in that range and increases available fuel (6-8 jet size bigger) perhaps we are no longer lean.

Anyway, I've added the vacuum line, have installed #98 in the primaries and #100 in the secondaries. Will drop it in the water tomorrow afternoon and let you know what happens.

Thanks for the input. I'm going to try these a bit more, especially after speak with the Grant guys today and learning of the needed feed between the primary and the manifold; simply never knew of the need. If I don't run the carbs out of gas and they pop, I will pursue the 1050 carbs and see what we can work out on a reasonable price for these 850's, if your still interested of course. Thanks again for the information.