'76 620....bad hesitation

[dinnc]

Hello all, newbie here. I just purchased a 1976 620 longbed with an automatic. The truck has 142k and very little rust. It came with all the original paperwork and the factory manual. Its bone stock and all original. I love it..but I have an issue I cant figure out. The carb has been rebuilt, new fuel pump, new filter, plugs, wires, etc...when you start to accelerate the truck bogs BAD, and will cut off. It get better when its warm, but not great. If you push the pedal down,it acts like its flooding, if you let off and then get back into the throttle, it does OK. Given what has been done already, I am at a loss of where to go next. I am a bit confused by the air pump, and all that stuff. Hope someone can shed some light...thanks!

[datzenmike]

With engine off and air filter off... look down the carb and pump the gas. You should see a strong squirt of fuel into the primary barrel. (one closest to the valve cover) If weak or none the accelerator pump isn't working. The accelerator pump helps the motor transition from the idle circuit to the primary circuit by providing extra gas.

 

Can I assume when you said 'etc.' that you also set the timing and checked the valve lash?

 

Ignore the air pump. Weather it's working or not, it has no effect on running. All it does is pump air into the exhaust manifold to help burn off emissions. It draws very little power to run the pump but you can just remove the belt to disable it.

[herculesinwyoming]

i agree sounds like a carb issue, the accelerator pump is a good place to start, and an easy fix

[dinnc]

I was thinking accelerator pump. I have the receipt from the PO for when the carb was rebuilt, but it does not say anything about a new accelerator pump, only a price quote for a carb kit(gaskets and jets I assume). I have not checked the valve lash or timing yet, they are on the short list of things to do. Thanks for the help. I will check the fuel flow shortly.

[herculesinwyoming]

carb kits usually have a new assembly for the accelerator pump. the biggest part of a good carb rebuild is meticulus cleaning,

 

i just finshed rebuilding my factory carb yesterday, it took 2 days because i soaked the carb body parts almost 24 hours

[datzenmike]

I was thinking accelerator pump. I have the receipt from the PO for when the carb was rebuilt, but it does not say anything about a new accelerator pump, only a price quote for a carb kit(gaskets and jets I assume). I have not checked the valve lash or timing yet, they are on the short list of things to do. Thanks for the help. I will check the fuel flow shortly.

 

Depends who or how well it was built. It's easy enough to put a new accelerator pump in but forget the tiny BB that blocks the pump flow from reversing into the fuel bowl instead of squirting into the primary. It happens.

[dinnc]

Hmmm...I dunno about how thorough the carb rebuild was. It was done at a shop, and the PO was charged and arm and a leg for it, but the receipt give no details. I am going to go out tomorrow and look in the carb and see what I see. I am also going to check the timing. I have read were lots of folks put a Weber on instead of the factory carb...are the factory carbs known to be problematic?

[datzenmike]

The stock Hitachi is best for mileage and good performance. The Weber can with proper tuning get close to the same mileage and is better performance wise. Just better. Usually by the time someone buys a good new Weber the original Hitachi is so worn out and in need of replacement that the new Weber isn't a fair comparison. If you compare a good new Hitachi to a good new Weber the difference is not as great but I would give the Weber an advantage over it on performance only.

 

With the Hitachi all the hoses and electrical connections work.... with a Weber you often get one that doesn't have the idle cut solenoid or auto choke and so you have to jury rig something. Often the stock throttle cable doesn't fit exactly and a manifold adapter plate is needed. I would assume that a new in the box direct replacement would avoid all this except the adapter.