I'm at a loss of sorts. I have a 4160 that was converted to use jets for secondary metering as opposed to a plate (block?). It's vacuum secondaries, dual feed. I rebuilt it when I rebuilt my engine and am now tuning. It started out with #68 primary jets which were way too rich. I ended up putting a 64 into the passenger side of the primary and now the four cylinders that correspond to the intake runners that get fuel from that side of the carb are where they should be. The problem is the other half, I'm down to a 56 jet and those plugs are still almost completely black. The plug that I checked is the correct heat range for the engine when it was stock but I now have .5 point higher compression and more timing so I know for sure that I'm not cold fouling and the problem is that I'm still rich. My concern is the huge difference in the two primary jets and also why I'm at a 56 and still rich. The engine is a mild 355 and I wouldn't think that I'd be using almost the smallest jet Holley makes. Any thoughts?

BTW, I set my idle mixture using a vacuum gage. I turned the screws to where I pulled the most vacuum, which turned out to be about 1 turn from all the way closed.

 

You should never split jetting on either side of the carb.It should always be even for the primaries or secondaries 68P/74S..56P/62S..something like that.As far as being to rich..I would look into a plugged passage in the carb from the rebuild.Idle mixture screw port has trash in it plugging it.

 

well i called holley and they said that my carb is too big for the engine, which i already knew, but the carb was free. it's a 750cfm and my engine is a 355 spinning to 5500ish.

also, why can't you run two different jets? i figured that's why there's two of them there instead of just one

 

Yeah the carb may be too big according to their inaccurate formula..but in the real world a 750 is perfect on a 355.Ask anyone on these boards.They sell two jets to a pack because there is two jet ports on each metering plate requiring you to run two jets of equal size on either end of the carb.If carbs were meant to be jetted individually on each circuit..jets would come one to a pack.Seriously..jet the carb evenly on either side..68P/74S..74P/82S..something like that and look elsewhere for your problem.A plugged passage somewhere in that carb is my guess.What intake are you running?

 

Most likely a blocked idle air bleed or downwell.
A 750 should run just fine with 68 to72 primary jetting.

Something is wrong with your carb.

 

Any suggestions on how to clean out a blockage? I ended up putting a 64 into both primaries and a 70 into both secondaries. Now I have another issue. I notice it most in sixth gear. If I'm just cruising at a constant speed it's fine but if I have to give it some gas like to maintain speed while going up hill it will lose power and feel like it's stuttering unless I give it more gas (enough to increase speed) at which point it fine again. I can see why people get discouraged, this is a pain in the neck lol.

 

Might have it a little too lean now. When I was muckin about with my 750 and had the 64/72 jets in, it was way too lean. I jetted up to 74/80 and it's running really good but probably could get it better. But of course everyone's engine likes different settings hehe.