Need help jetting holley 4150's on a stock 4th generation 454. The 454 is in a early truck with a turbo 400 trans. Have 2 different holley 4150, 2 metering block vacuum secondary carbs. One is a 4118-s 725 cfm that I jetted 72/78 with 6.5 power valve. Second is a 3310-1 780 cfm jetted 72/80 with 8.5 power valve. Both carbs are re worked and run. Can not get jetting correct. 7118 runs lopey and has a dead spot off idle. 3310 runs rich and has a dead/rough running spot at 2000 rpm. I thought I was pretty good with holleys until now. Thanks for help.
Put them back to stock jetting and start over- many times previous mods were ill-advised. A dead spot off idle is usually an accelerator pump adjustment, or timing slightly retarded
I have a 3310 I got from a buddy,he said it doesn't run right .I took it apart it looked fine.I have had it on a 383,283 and now a 427 sbc. No changes it has worked fine.Check timing and advance curve.Try setting it up with a vacuum gauge,both timing and mixture.
Jetting is to the fuel ratio of the venturi. An 8.5 to 1 comp ratio of a 283 could easily perform fine on a 454 with the same comp ratio. One will run out of air at a different time depending on the CFM of the carburetor. If you look into purchasing a Holley body only without the choke tower from one of the many suppliers. The 650 cfm will come with #62 jets, the 750 cfm will come with #72 jets and an 850 cfm will com with #82 jets. These are starting points. Higher compression will normally need more jetting.
Like MeanGene said, check and adjust the acceleator pump to insure a squirt at very little movement, then plot the timing curve to insure it is correct, "distributer machanical advance could be sticking" or you can find out if heavy advance springs were installed once upon a time that open to late, "after 2000 r.p.m. When this is done, try a vacuum reading to find the correct new power valve. The method I use is to subtract the vacuum reading at hot idle by 1/2 and add .5 as the starting point. Example: 14 inches of vacuum at idle would start with a 7.5 P.V. 14 devided by 2 = 7 then add .5 will get you close
Most carb problems are in the ignition, including stumble off idle and running rich ( especially at idle). For example if the ignition timing is off , and therefore the intake manifold vacuum is low, the power valve will stay open. Common carb problems which cause running rich are too high float level, leaking needle and seat (internal leak ), and damaged power valve. Check all air leaks, advance and timing , including the accuracy of the mark on the damper , i.e that the outer ring on the damper has not moved. You say the 454 is stock , if that includes a stock intake which originally had a Q jet fitted, there is a high likelyhood of an air leak at the base gasket. 3110 is jetted standard at 72 78, I would use a 6.5 powervalve with a stock cam. The 4118 s is a very special and rare carb. Its probably worth more than a new Holley. Only ever fitted to a 1968 Shelby Mustang . Its an original not a later Holley replacement carb. Someone will be looking for one.
The Jalopy Journal
