it's a carb truck i just bought it, it won't go over 65-70 mph and takes it some time to get there. it's only gets about 10 mpg. i have replaced the spark plugs, and run carb cleaner though 3 tanks of gas and nothing has helped. i just took apart the carb tonight and couldn't find anything wrong, it was very clean inside and no gaskets looked to be bad. i haven't put the carb back on yet but i don't think it will fix my problem. there are a few vacuum hoses that might need to be replaced but don't look that bad. the truck has been sitting for about a year before i got it.

Has anyone else had this problem?

86 nissan 720 z24 4x4 carburetor

Timing off, stuck EGR, oil pump/distributor not timed correctly. Just some things to check.

is it the stock carb?

as rev mentioned... check those things

but the part i find odd is that it's only getting 10 mpg... you must be pumping LOTS of fuel to it without a whole lot happening cause even on the heavier 4x4 models the stock carb still gets 16 or so + MPG

PEZi720 wrote:is it the stock carb?

as rev mentioned... check those things

but the part i find odd is that it's only getting 10 mpg... you must be pumping LOTS of fuel to it without a whole lot happening cause even on the heavier 4x4 models the stock carb still gets 16 or so + MPG

yes every thing is stock i find it hard to belive that base timing is off, but maybe I have a bad vacuum pot or something I will check in to it.

You could have a bad vac leak somewhere.

sorry guys i havn't had any time to work on the truck i will let you know what i find whenever i get around to working on it. i think im going to start with the vacuum hoses, EGR, then check the timing, then adjust the valves. any other ideas?

i was gonna say clogged fuel filter or bad pump... but i don't think that would lower the gas mileage as much as yours is

that sounds like a combination of things. my friend got a beater 8 plug 4x4 truck and it got horrible mpg. check the vacuum and timing, then check for vacuum leaks, check the carb adjustments...does it have a cat? that may be clogged, catalytic converters don't like motors that run rich, and it would seriously bog down your motor. they usually smell like rotten eggs when they go bad, and you progressively can hold less and less speed on the highway depending on how clogged they are

The cat was the first thing I thought of so I pulled it off didn't make a diffrence

maybe its really tired, does it burn oil? can you do a compression check?

oh damn... i forgot one huge one

the secondary in the carb could be seized

that's of course after checking all of the above tho

well i just rebuilt the carb i'm going to put it back on tonight

BronkStang wrote:maybe its really tired, does it burn oil? can you do a compression check?

i don't have the tools to do a compression check yet, but it doesn't burn or leak any oil at all.

so i was looking at a water leak this thing has and it's coming from a block heater, does every nissan have a block heater on it or is it after market? i don't see why anybody would need it cause it starts just fine in 14 degree weather.

PEZi720 wrote:oh damn... i forgot one huge one

the secondary in the carb could be seized

that's of course after checking all of the above tho

thats exactly what i thought to and that's why i rebuilt the carb, how do i test it to see if it ever opens? cause i know it has to be under a load and higher RPM. i've looked down the carb and rev'ed it up but never saw it move, but i don't think it would move till you put a load on it.

loyalone wrote:thats exactly what i thought to and that's why i rebuilt the carb, how do i test it to see if it ever opens? cause i know it has to be under a load and higher RPM. i've looked down the carb and rev'ed it up but never saw it move, but i don't think it would move till you put a load on it.

above 3,000 RPM is when i believe it opens... don't know if it needs load or not

ok got the carb back on and a lot of vacuum hoses replaced, it idles a lot better and maybe a little more power on the low end but still won't get over 70 mph and it takes 1 mile to get to 65 mph. the engine sounds great doesn't ping or backfire or anything it's just really low on power. i'm going to drive it to work tomorrow and see how it does on gas.

I do see the secondary opening now if i snap the throttle, let it rev to about 4000 rpm.

Please let us(me) know what you find. I am in a similar situation with a 1984 Nissan 720 truck i just purchased(2.4z engine carbed). It has little under 200k miles, does not smoke, but will not get over 55. I at first thought fuel..Changed the fuel filter...no change.. so i replaced the plugs..they were not real bad, but some were real loose..but no change (this has me scratching my head because 2 were so loose i did not need a wrench just my fingers to remove-intake side). But, the truck still maxes out at 55.. the engine will rev high, but zero power under load. runs fine, but slow.

Secondaries are a popular fail item. You can check if they are seized by trying to move them by hand, it's controlled by a vacuum actuator next to the cold start step cam if I remember correctly. The linkage and throttle plate should move by hand, if not then that is likely causing you those symptoms. I have an 85 FSM I can check for you if you need me to.

"Secondaries are a popular fail item. You can check if they are seized by trying to move them by hand, it's controlled by a vacuum actuator next to the cold start step cam if I remember correctly"

I have no idea what your talking about. I am a Ford man, and a semi decent mechanic. (more on the semi side) This truck has the most complicated looking carb. i have ever seen. Can you be more specific for me?

Ok.. found this site.. but i still have questions - if it is my secondaries: (1) how do i check this, (2) how do i fix it?

http://dimequarterly.tierranet....html

Not sure how to check secondaries because mine is throttle body.

Are both coils firing? Possibly running only on the secondary coil? how is the idle quality?

Mississippi1976 wrote:"Secondaries are a popular fail item. You can check if they are seized by trying to move them by hand, it's controlled by a vacuum actuator next to the cold start step cam if I remember correctly"

I have no idea what your talking about. I am a Ford man, and a semi decent mechanic. (more on the semi side) This truck has the most complicated looking carb. i have ever seen. Can you be more specific for me?

If you are a Ford man then you should know, it's basically a 2bbl carb that runs on 1 barrel most of time till you go WOT, that's when the 2nd barrel (ie secondary barrel) should open up.

Reverend D wrote:Secondaries are a popular fail item. You can check if they are seized by trying to move them by hand, it's controlled by a vacuum actuator next to the cold start step cam if I remember correctly. The linkage and throttle plate should move by hand, if not then that is likely causing you those symptoms. I have an 85 FSM I can check for you if you need me to.

I stopped fooling with the truck. I took it for a 250 mile trip and now it only runs on 3 cylinders, I should have replaced the plug wires when I did the plugs, I think it's the wires or internal engine damage. But yeah sounds like your having the same problem as I am.

Or anything ignition related, cap, rotor, coil etc.

Ok..i am about 95% sure that my secondary is not coming in..I can rev the engine to 50% then manually actuate the secondary. The RPMs increase alot.. seems to be what i am missing - power wise. Now my problem. I can not tell what actually jets this. I can manually raise the linkage, but is is suppose to be done by a diaphragm (around 1.5 inch in diameter and sits towards the back of the carburetor behind the secondary) It seems to me that a vacuum is to form on top of the diaphragm and this causes the linkage to raise, which jets gas into the throat. There are No external vacuum lines hooked to this diaphragm/actuator.

Anyone have knowledge of this. I am not rich so a new carb is out of the question, but i'd love some insight.

Mississippi1976 wrote:Ok..i am about 95% sure that my secondary is not coming in..I can rev the engine to 50% then manually actuate the secondary. The RPMs increase alot.. seems to be what i am missing - power wise. Now my problem. I can not tell what actually jets this. I can manually raise the linkage, but is is suppose to be done by a diaphragm (around 1.5 inch in diameter and sits towards the back of the carburetor behind the secondary) It seems to me that a vacuum is to form on top of the diaphragm and this causes the linkage to raise, which jets gas into the throat. There are No external vacuum lines hooked to this diaphragm/actuator.

Anyone have knowledge of this. I am not rich so a new carb is out of the question, but i'd love some insight.

ok the secondary diaphragm is bolted to the carb and opens on ported or venturi vacuum (can't remember which one) there are no vacuum lines going to it, there is a passage where it bolts the the carb. the only time it opens its when your driving the truck and accelerating (engine under heavy load) and throttle is past 50%. so if the linkage is not binding when you move it then most likely it's working fine. i completely rebuilt my carb and it has over 200,000 miles on it, it was in perfect condition.

what i would suggest to you is do a compression test on your truck, i just did that to mine today and found cyl #1 160psi #2. 50psi #3. 50psi #4. 160psi, and i pulled my radiator cap while it was running and found bubbles coming up though there. so it looks like i have a blown head gasket. new gasket is only $20 so im going to tear in to it soon and find out for sure whats going on.

rebuilt the carb. I think i have the wrong carb(or wrong base) on it because the base where it bolts to the intake manifold seems undersized. I am sure the secondary is still not opening even though it should be. The linkages are releasing it to open but does not appear to be building up the necessary vacuum to actuate the diaphragm and open the throat linkage.

Carb. was actually in great shape and seemed to be in rebuilt condition already.

I have now 1) changed the fuel filter,2) added 8 new plugs and wires,3) rebuilt the rebuilt carb. 4) cleaned the rotor button and distributor cap.5) changed the oil...was very black but did not have a fuel smell.6) checked for bubbles in my radiator.

My next step will be in trying to gain a few more hp and I will rod out the cat. converter. I may first just remove it and see if this might be causing back pressure.

I now max out speed wise at 57 mph. I am actually happy with this speed, but it take like 3 miles to build up to it, and do not get me started on inclines.

loyalone wrote:
i don't have the tools to do a compression check yet, but it doesn't burn or leak any oil at all.

so i was looking at a water leak this thing has and it's coming from a block heater, does every nissan have a block heater on it or is it after market? i don't see why anybody would need it cause it starts just fine in 14 degree weather.

for a moment i was worried you had a diesel and were throwing regular fuel in it cause diesel motors usually have block heaters because the ignition temp of diesel is higher than that of gasoline because it is less volatile so in colder areas up north and in every firetuck in the USA block heaters are used to aid in easier faster starts the first time every time. but you wouldnt have a diesel cause you don't got glow plugs but ive heard if you put diesel in a gasoline truck it can in lower concentrations prevent proper power from being transfered to the road and in high enough concentrations seize a block but thats all useless info i think for your problem...What kinda person/persons have owned this rig sometimes people abuse things and your rig might be one of em. Talking to people about trucks ive learned that the general concensus is that "Our" trucks are fun lil throw-a-way trucks that are ment to be harped on and trashed cause they were "cheap" then thrown in a junk yard i would ditch that hitachi tho that things exploded diagram was scary i think a ford engine has less to it than those things

See if this helps anyone can use it if you want hopefully they dont get pissed but go http://www.alldata.com login under User:Takko1988 PW Time2500 its the everything you could need for a 1986.5 nissan HB it might not have the carb in there cause the 86.5 is a EFI but it lists stuff for older models too

Also Ive been reading about other vehicles that had similar problems that you are having one such incidence was a part of the inner wall of the exhaust pipe just after the flange of the bottom of the manifold had broken loose and slipped to one side blocking off half of if not more of his exhaust which prevented him from accelerating hard prevented the timing from retarding but he was still able to reach high speed if he slowly accelerated, the other problem that seemed to be a re-occurring in other vehicles 1) don't use seafoam or anything like that to clean blocks it messes with sensors 2) some people where having problems with their distributors or the shaft inside the motor being rotated 180 degrees off. I thought i was having that problem for a time but i discovered that my input shaft off the dist was a semi circle and could only go on one way but the year before had a full circle with a rectangle taken out of the center which could allow for it to be put in backwards which could change the firing order of your cylinders. But even if thats not the case keep on it its gotta be either fuel spark vacuum or timing just be meticulous in everything its real gay to spend a bunch of money for something you missed in step 2 oh and if you think this is a bunch of reading it took me 45 min to read what these poor bastards had to go through just cause they're mechanics at Midas missed the problem 2 times while replacing cats and mufflers on this guys blazer in the first two posts he listed.

Modified by Takko1988 at 1:56 AM 2/22/2010
Modified by Takko1988 at 3:13 AM 2/22/2010

Check compression first! Before you spend any more money. I've done every thing you've done and I have the same problems your having. Your secondarys wil not open if the motor doesn't have the compresion to create the vacuum to open them. Compression gauges are cheap and it's very easy to check on these trucks.

My truck was having some problems like that, I replaced EVERY vac line, used carb cleaner, and replaced my O2 sensor - now I get around 25mpg and reasonable power.