Secoundaries don't kick in till about 4500 rpm

[trjerm]

Hi I have a 1985 nissan 720 KC 4wd w/ a stock Hitachi dfp 384-5 2bbl. carb.(6 wire plug) and the secoundaries don't kick in or squirt gas till approx 4500 rpm. is there any way to adjust them so they kick in @ a lower rpm. The primaries seem to work fine and squirt gas @ appropriate rpm and the choke plate opens fully when engine gets to temp. Any help would be welcome, thanks Tom

[datzenmike]

Do you have a tach or is 4,500 an estimate?

 

The secondary opening is not by RPMs but by a combined vacuum from the primary and secondary venturis sent to the vacuum diaphragm. I suppose a weaker spring in the diaphragm would open sooner but no one ever does this. 

 

The secondary opening is factory designed to give added power only at higher rpms and load when the throttle is open more than 50 degrees. Sooner opening will give horrible mileage.

 

 

As the amount of air sucked past the venturi depends on how well the cylinders are filling I would check all the valve clearances. A tight valve will possible hold them slightly open and leak air. Worn, bent or broken valves or seats will do the same. A blown head gasket too.

[trjerm]

The 4500 rpm is an estimate as I'm manually operating the throttle while I look down the venturi to see when it actually squirts gas. It just feels like it doesn't have the pickup or hill climbing ability that it should. I've checked and adjusted valve clearances, changed oil & filter, added fuel system cleaner, flushed cooling system and refilled, changed all vaccum lines, installed new exhaust manifold and new front pipe, changed trans., both diff's and transfercase fluid, and installed a 82 z20 valve cover. The motor and trans were replaced w/ a used 16000 mile motor & trans by my brother before it sat  for years and he gave it to me. So far I've done all of the above plus upper/lower ball joints, all bushing, new shocks, brakes, 30" cooper tires on Tacoma rims, stripped interior and installed sound insulating layer,installed subaru seats, new front fenders, bumpers, installed trailer hicth, etc,etc,etc. The truck starts and runs good ,just feels like it doesn't have the throttle responce that it should, but I live in calif (passed smog w/flyin colors) and therefore can't install weber & header

[datzenmike]

First and foremost... California.  The emissions come first and performance last.

The Z24 is a high torque low RPM motor. Yes it will rev but it's power and torque are at cruising speed.

As the years went by on with the 720, the rear differential ratio was lowered repeatedly to bring up the MPG rating. I'm sure it did or remained at least the same in spite of weight gain and larger displacement. Again it plays havoc with acceleration and performance.

 

Pull the Hitachi off and save it, put a Weber 32/36 on. At emissions test time put the Hitachi back on. Otherwise replace the rear diff with a 4.11 or 4.375. The latter won't effect emissions.

[trjerm]

I'm starting to lean towards that direction, Which header do you recommend to go along w/ the weber?

[datzenmike]

Headers are a race only option. It's for very high end use where winning is worth spending $150 for a couple of hp. Your stock exhaust is more than enough for the street and most modifications will disrupt exhaust gas speeds and loose you some mid RPM torque. Headers are for use with a multi carb set up and a high(er) performance cam. I would settle for a larger muffler with bigger inlet and outlet pipes that has less restriction. Nothing straight through or flowmaster, maybe one for a much larger displacement motor so it's quieter and easier to live with. You may notice a bit more power when revved up but for the most part it's money better spent elsewhere..

 

For the weber get a 'direct replacement' carb that has an idle cut solenoid and an electric choke. Most of the feedback carb stuff can be just disconnected until you put the original carb back on. This is illegal to do so be warned.