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82’ 720 carb replacement options?


RatRon

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Ok, so I’ve had this truck garaged for 5 years after it had been sitting under a carport in a field for 15 years. I don’t  know any past history on the vehicle except that it shows 78k mi, a manual tranny and it’s in pretty good shape(except the interior). The problem Ive always had with it is it seems to always be running too rich. I’ve had several people tweak the carb to find a good mix, but it always prevails. When it’s less rich, it won’t idle long before it dies. I bought a carb rebuild kit, and dis/reassembled it to the T. It seems to be running worse now. A senerio:  It starts up like a champ, idles great. When put in 1st it operates ok, but bogs down when you switch gears and dies. It will drive in 1st gear fine until switching gears. Before I rebuilt the carb, I could get it up to the 2nd gear fine until I went to switch to 3rd and then it would die. I would love to have this truck on the road, just am having difficulty figuring this issue out. I really think it’s a carb problem, but maybe it’s something else. 
 

There seems to be a large variety of carbs out there and want to make sure I pick the correct/best one. Everything is stock on the vehicle. 
 

I’m open to any suggestions or thoughts. Thanks. 

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Cheaper to fix what you have..... usually.

 

 Cut to the chase..... replace the fuel filter between electric fuel pump and the tank. Some early 720s had a mechanical pump on side of the head.... the filter is probably nearby. They are cheap, so if this doesn't help no harm done. They should be replaced every 3 years. If it plugged the truck may run but under load, when it uses more fuel, it can't get through the filter and the truck runs out of gas.

 

If this is not much help, look at the sight glass on the front of the carburetor. Fuel should be half way up the glass at the dot when it quits. If not at the middle or close the float may be stuck or set too high not letting the float chamber fill properly and the truck uses the gas faster than it can get in, and it runs out.

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On 1/2/2022 at 11:36 AM, datzenmike said:

Cheaper to fix what you have..... usually.

 

 Cut to the chase..... replace the fuel filter between electric fuel pump and the tank. Some early 720s had a mechanical pump on side of the head.... the filter is probably nearby. They are cheap, so if this doesn't help no harm done. They should be replaced every 3 years. If it plugged the truck may run but under load, when it uses more fuel, it can't get through the filter and the truck runs out of gas.

 

If this is not much help, look at the sight glass on the front of the carburetor. Fuel should be half way up the glass at the dot when it quits. If not at the middle or close the float may be stuck or set too high not letting the float chamber fill properly and the truck uses the gas faster than it can get in, and it runs out.

Listen to this guy!

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The Z22 uses a 32/34 vacuum operated secondary Hitachi. The 38/38 is a V6 synchronized carburetor and will work on a small V8. Arguably it could be used on a Z24 and would be better yet if it had a vacuum or perhaps a progressive secondary like the 32/36. It would still be too large but be better to drive. The fact that both barrels open together totally identifies it as suitable for larger displacement engines or smaller displacement at WOT. I feel the Z24 Hitachi (34/38) is slightly small to begin with and very restrictive if you have ever looked into the primary venturi. 

 

The 38/38 would outperform the 32/36 but is not as drivable and the 32/36 is not as drivable as the Hitachi. By 'drivable' I mean easy docile part throttle driving and anything other than WOT 

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I drove a 32/36 on a 510 and you have to actually 'drive' it properly. I guess a week would teach you where the proper secondary 'tip in' point is by a combination of engine sound and seat of your pants but it's tricky and in a hurry there's a tendency to hurry it up. Now owners might say that 'you just need to tune the carburetor' to fix this, or 'add more accelerator pump' or what ever, but the fact is a vacuum secondary would make the 32/36, (other than it doesn't fit the stock carburetor air filter, any emissions connections or even the intake bolt pattern) the perfect replacement carburetor. 

The 38/38 is a racing carburetor run at full throttle all the time suitable for a V6 or small V8 that moves lots of air. On a 4 cylinder it'll work, but even more work to 'drive' it than a 32/36. Just my opinion but you shouldn't have to guess at this. The worst carburetor is the one that is too big for your displacement.  

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