84 720 runs great til i drive it!! carb issues????

[SlammedhappySteveO]

So i have a 84 720 i got back from a guy who threw a bunch a money at it but didnt help it....  this thing purrs @ idle... egr/ smog has been removed/ blocked off, has a webber dgav 32/36 but i dont believe it runs right... it has 2 inch exhaust and is slightly lowered!! i love this truck! good compression in all cylinders and from what i can tell there are no vacumn issues but i could be wrong... the minute you put i in gear it falls on its face n u have to mash the pedal to get it to move and stay running...

all the money and work this UTI grad did i have had to fix, repair or replace, right down to the brake shoes....

the carb was missing one jet from the main bowl, how i dunno... memory escapes me at the moment ill have to remember the jet size, both the main jets in the bowl are the same size ( dunno if thats good bad or indifferent?)

 

I would really like some help with recomendations for the right carb model numbers for this truck (8 plug z motor), what to eliminate for vacume with the carb recomended... Im pretty sure timing is ded nutz, but am also open to suggestions and recomendations here too!

 

I remember it use to be so easy to find parts for these but now i see theres a real problem trying to find tru triple chrome repro parts, IE bumpers, tail lights and corners.... trying to give it a facelift n some lovin !!

 

Ive had it back for a year and a half now and havent gotten newhere but mad :confused: .....  In advance thanks for any help, posts or otherwise!!

 

i will post some pics when i get off tonight, this thing is still intact and cherry! :thumbup:

[ggzilla]

So let me get this straight.

 

* The 720 runs great when someone else drives it.

* When you drive it ...  "i dont believe it runs right"

 

Sounds like it is not a carburetor issue. What symptoms does it have when you are driving it?

 

 

> I would really like some help with recomendations for the right carb model numbers for this truck (8 plug z motor),

 

Weber DGAV is one of our recommendations.

[SlammedhappySteveO]

Havent actually been able to drive it more than a block since I got it back.. At this point it wil start with a bump o the key and a couple pumps of the pedal.. And itl idle n kick down when warmed up but then will start to stutter much like a misfire or meybe a dead cylinder.... But ive got spark (consistant), and at last check all the cylinders had good compression..

Sooo ill rev it up a couple times , itll lean out n level off but as idle comes down it wants to stumble..

 

Had to move it a couple weeks ago n now the thing wont budge unless you floor it n drop the clutch, @ almost 2 grand on the tack but u reeally gotta hold it to the floor to get it to move without dying... Ive checked for vac. Leaks @ the carb n lines intake and exhaust... The coils seem to be working... Its got new plugs wires rotor n cap... I think the whole time ive had it back I only drove it one day and it was an overfueled day of nightmares and one call to AAA!

Im suspect of the ignition system, distributor, coils or the carb... And the deletion of egr and emissions and vacum ports..

 

Read ratsun threads all the time but never really post, definatel find good info here tho! And some great rides!

 

Hopefully I answered your question and thanks for the response!

[ggzilla]

Can you take it back to the mechanic who did the work, and ask him to fix it?

[SlammedhappySteveO]

Lol ,i owned this truck when I graduated in 90', traded it to my cousin for somethin else n he passed it to the guy I got it from..he's the one who did all the work or repairs or whatever. A uti graduate! Lol - I am a mechanic and have been for many years... I jus havent been able to figure out what the deal is , I know these motors can be A pain and cant help but to think Im missing something here.. I work from 330a.m. Til 630pm soo havent had lota time to get after it!

[ggzilla]

ALL motors can be a pain, even a chevy motor with holley 4-bbl. Especially one that is 28 years old.

 

 

So the symptoms are:

* Starts and idles fine

* Doesn't run good in nuetral if you get it any pedal, but "as idle comes down it wants to stumble"

* Doesn't turn good if you attempt to drive off. In fact it "wont budge unless you floor it n drop the clutch, @ almost 2 grand"

* It won't drive unless the pedal is floored

 

OK, these symptoms sound like either Coil or Fuel issues.

 

 

You said that when you got it from the UTI grad, the carb was missing a jet and now has two of the same size jets at the bottom of the fuel bowl? That's really wrong man. No telling what else he did to the carb, it could be totally messed up with linkage, settings, missing or wrong parts.

 

Since you are a mechanic, i'd advise take the top off the carburetor and double-check everything. Not a rebuild mind you, just ensure it is put back together correctly. Any chilton/clymer manual for the 720 or a the factory service manual will have the jetting specifications for mains,slows,air bleeds and emulsion tubes, etc. The factory manual has a carb breakdown too, if you are not familiar with how they go.

[Draker]

First isolate what system if failing.

 

If you think it's a spark issue.. look over this:

There are basically two ignition systems on the NAPS dual plug engines sharing a common ignition module inside the distributor. Power is supplied to the coil(s) and it is connected to ground through the ignition module which makes and breaks this connection to fire the coil. One coil (intake side) is powered directly from the ignition key while (for some reason known only to Nissan) the other (exhaust side) is supplied with power from the fuse box.

 

First, replace the fuse on the far left side of the fuse box located on the drivers side kick pad. I say replace, because you can't trust just looking at them. They are cheap. Check each coil for spark by putting an old plug in the wires going to the top of the distributor and laying on a grounded surface. Crank the motor over and look for a spark.This is the easiest and simplest thing that will cause the exhaust side plugs to not fire.

 

A word on the dual plugs:

 

Both plugs fire simultaneously and at all times in every gear under all conditions except one. Heavy throttle, low vacuum conditions will close a vacuum switch which grounds a special wire going to the distributor, disabling the exhaust side plugs. It's intent to to reduce engine noise under heavy throttle applications.

 

Under dual plug operation (which is just about all the time) because there are two ignition sources (plugs) and the fuel and air will take a shorter time to burn the ignition timing is reduced. Usually around 3-5 degrees.

 

Under single plug operation it will take slightly longer for the fuel and air to burn completely so the ignition module automatically advances the timing ( to about 8-10 degrees) so that performance is that of the dual plug system.

 

Here is a California distributor. Besides the regular three wires, two for the coils and one for power, you can see a separate plug half way down the length (white wire) that switches it to single plug operation.

 

 

If this plug is disconnected, the distributor cannot switch to single plug operation and will remain on dual plug AT ALL TIMES. So if you disconnect this plug and both plugs begin to fire then maybe the vacuum switch is at fault and is on all the time.

If you think it's a fuel issue, maybe check to see if you have good fuel pressure.. or the bowl isn't going dry under load.

 

Make sure you don't have any exhaust restrictions.

[Dawa]

obviously you are bad luck

[SlammedhappySteveO]

I appreciate all the responses! Ima run through it during my vacation in the next week and start with the basics and see if im not missin somethin... Im the mean time I scored another , an 85 king cab runner! Good shape ,and original ! Hope to get pics up this week! Thanks and happy holidays!

[blackwolf]

idk much about the weber carbs but i have a 84 720 z24 and the only time it stumbles (after tuning up right) is when a vacuum line comes loose on the pass wheel well or my fuel filter is clogged