I cant figure out my problem with my 84 longbed 720

[michaelaguirre]

Hi people i have aproblem with me truck that was my grand fathers . The problem is two things i think frist problem is when i get to abought 55mph im at 3000 on rpm gage at 65 almost 4000 and cant get over 70 and am pushing 4pluss on my gage . i let off on the peddel and can do 65 at 3500rpms. so first problem is why so high on rpms in 3rd gear cause it is a automatic? second is i just had the carb re built and the float bowl will not fillup fast enough in high rpms. checked pump its at 4psi and that pump is rated at 2-5 , new fual filter. 0 to 50 runs like a champ good in the city, get on the freeway and i think im giong to blow it up. I need this truck to get to work. IS there a high speed pump in the tank by chance .can anyone help me thx 720 owners

[fastboatman29212]

Do you have the vacuum lines removed and ports plugged correctly? If you still have all the vacuum lines, are there any leaks? New vacuum lines should help a lot if you have not already done that. I have no ideas about the high RPMs at highway speed, sorry. If the truck runs fine at high RPMs during the first few minutes of operation, but later starts to be staved for fuel, then you may be building up vacuum in the fuel tank so check the vent line to the tank and/or see if you can hear/feel any suction at the gas filler cap. Of course, you should trace all fuel lines and flush them out to be sure you don't have any blocked lines. I'd be interested in what others suggest.

Let us know what you find.

[Rev_D21]

Is the truck getting starved for fuel at higher speeds? Is the float set properly at idle? Sounds like the rebuild was botched and needs to be revisited. I'd also make sure the fuel filter is new. No sense running a rebuilt carb on an old and dirty filter. Start at the basics and work your way up from there. Don't throw parts at it unless you can condemn the parts you are about to replace. There is a wealth of knowledge here at NICO, you have come to the right place.

[michaelaguirre]

checked float at iddle and it will idle all day ,give it gas say 3500rpms and it drops till it is gone , cant fillup fast enough

[Rev_D21]

Pinched lines? Plugged filter? Incorrect jetting? I know you said you have good psi at the pump but what about volume flow rating? You don't want too much because it will flood but for some reason it can't keep up with demand the way it is now.

[michaelaguirre]

went back over the carb and found that the carb float was not droping downfar enough to let enough fual in to the foat bowl ajusted the float and now runing correctly can keep it floored and got fual still. now need to find out abought the rpms. should it be att 4000rpms at 70mph?

[Rev_D21]

That's rather high but it's a 3spd slushbox. Might want to invest in a 5spd swap somewhere down the line.

[michaelaguirre]

a five speed out of what and do i have to change the diff if it is geared 1to1

[Rev_D21]

5spd out of another 720 would be a good way to go. As for the rear axle, both used H190A axles. The 5spd version came with 3.700 while the auto version came with 3.889 ratios. The lack of an overdrive is what is making your RPMs so high at 70mph.

[michaelaguirre]

thx man your great help. what year has the 5ive speed in it and is it a direct swap? will it hert the moter running at high rpms ? do tou know of way to incress power with out a hole lot of money? once again thx reverend d21 you rock

[Rev_D21]

You aren't hurting it but you sure are making it earn its fuel. The 720 series trucks began in 1980 with the first models being built in 1979. I had an 81 that was a 5 spd so I know they are out there. Yours being a z24 equipped model allows you to use early D21 Hardbody transmissions. You will need the clutch, transmission and a number of incidentals to make the two work together. You are in luck if you decide to go this route, I have a clutch pedal assembly for a 84 720 sitting in a parts box inside my garage if you need it.

[michaelaguirre]

thx again but right now i dont have yhe money to make that type of change to my truck at this time well at least it runs good and gets better gasmilage than my v8 jeep Do you know of any low costing things i could do to get more power and better fual milage this truck is all stock

[Rev_D21]

Tune up, synthetic oils and keeping the tire pressures correct can help MPGs. Keeping the existing parts in tip-top shape is key.

[michaelaguirre]

could i do a 4speed automatic

[Rev_D21]

I'm sure you could if you come across one.

[lambohaha]

I`m gonna chime here lol. Is this a 2wd? And what size tires are you running? Reason I ask is I have an `85 720 and it had 195/75`s on it and it sounded like it was wound up tighter than d!ck hat band. I went to a 205/75 at the corners and that helped out alot. Its not overkill in tire height but enough to help. Good luck and I`m gonna stay tuned as I need options too.

[michaelaguirre]

I've got 205/70's on it and yes it is 2wd . what size moter do you have ant what transmission?

[lambohaha]

Z24 5 spd

[michaelaguirre]

do you know of a automatic trans that not 3 speed that i could use so i dont have to change it to stick inorder to have a 4 or five speed. i also have the z24 any tips on it

[lambohaha]

My wife used to have a `89 I think it was that was an auto with OD but it was a V6. Not sure they would interchange. Maybe you could find one of those in salvage yard or u-pull-it somewhere.