Originally Posted by WTNFDXJ
im bringing this back again... my 89 came with the holley on it.. it has a 4.2 0.60 over 4.0 head nutter mod.. it has no balls sucks down gas like a champ. and is horrible to start in the AM.. im getting it rebuilt hopefully monday.. but one thing i noticed is that my throttle butterflys only open 3/4 of the way my throttle cable only allows that much.. its an automagic.. if i have it rebuilt to your specs on the first page you think it will help out my set up?
Each motor is a tad different, use his specs as a guideline.
A carb is nothing more than a metering device, X amount of air through it will cause X amount of fuel to be "metered" through it. The things that will effect your fuel metering are the jet size, fuel level in the bowl and barometric pressure.
When Holley sends thier carbs out, they have the jets for metering the air fuel ratio just a tad on the rich side for the recommended fuel level in the bowl. First thing is always to get your fuel pressure correct for the application. While the floats are made to push the plug up to stop fuel from entering the bowl when full, the fuel pressure can over come the plug and push more fuel in. So your bowl level changes. Holley sells different plug sizes, the larger the area of the plug, the easier it is to have the fuel pressure over come the force of the floats. Don't put a bigger plug in thinking it will help, keep it stock.
Next get yor linkage correct. If it only moves 3/4 of the way then you need a taller linkage on the carb. Some times a little fab work is involved.
Then with the carb fully closed, check your accelerator pump squirt, it's adjustable. What I see more often than not is the pump squirt bottoms out too early in the throttle travel usually caused by the pump setting being too early and you're not getting the full amount of pump squirt. Look at the linkage and the pump cam and you can figure out just where it can be for full pump squirt. Don't change the squirt nozzle, that's for real fine tuning, stay with what you have as long as it's the stock sized nozzle.
Once you get it running, check the vacuum at the manifold and the vacuum at the timed port (the one on the side). Which one you choose to use is up to you, but understand that no two manufacturers carbs will have the same timed port vacuum nor will they be the same from one sized carb to another. A greatly overlooked problem for carbs is the vacuum to the distributor. The vacuum advance in your distributor or an EFI with a carb relies on the vacuum signal for timing. Lets say your stock carb produced 12" of vacuum at idle at the timed port, so the manufacturer used a vacuum advance module in the distributor to reflect the vacuum readings that reflect that of the carb timed port. Change carbs and the timed port vacuum at idle may be higher or lower than the original, your timing is going to shift.
It's also the same with a power valve. Lets say you have a 300 CFM carb and you go to a 400 CFM carb. The air flow in the engine hasn't changed but the carb openings have. A larger carb will drop the vacuum signal in the manifold quicker and further. The power valve works off the manifold vacuum signal. If it opens too soon you're dumping too much fuel, not soon enough and you'll get a hesitation when you hit the throttle. Holley uses a rule of thumb, half the vacuum signal at idle in gear for automatics and not in gear for a manual. It'll get you in the ball park. But depending on how well the engine is using the carb will determine where you really want to be. I always go a size or two larger than the hesitation. That is if I have a 6.5 in there and my pump squirt is correct and I can feel the hesitation, then I'll go to a 7.5 or 8.5 to open sooner. If it's opening too soon, you'll see a yellowish stain below the carb where the fuel is dumping, but not being atomized or taken along with the air flow.
Jet size is hit and miss with out an O2 meter in the exhaust. I've added them in there for fine tuning. But if your carb came with 58 jets, you're close. It's rare to go up a jet size running a stock or slightly modified vehcile. Remember it's a metering device, if you suspect your not getting enough fuel at cruising speeds, it's probably a timing issue. Lean mixture burns slower than a rich mixture so you need more advanced timing at cruising speeds. So you see folks add larger jets (richer mixture) to compensate for the lacking of advancement. It runs better, but your burning more fuel than you need to. Matching your advancement in the distributor or electronics would have cured the problem.