datsun 720 wont advance

[rjk629]

so i have an 83 datsun 720 4x4 with a 1981 z22 motor. it runs good and will rev up fine if you do it slowly but if you just hit the pedal it sputters out like the advance is not working on the distributor. i have new cap, rotor, plugs and wires. i pulled the vacuum line off and it has suction, i hooked a hose up to where the vacuum line connects and sucked on it and i can see the parts moving (with cap off) but for some reason it doesn't seem to advance. the strange thing is that i came out to it today and it started and revved up fine so it is inconsistent. i am desperate for any suggestions. this is my first post
thanks for any input
ryan

[PEZi]

sounds to me like a fuel issue more than anything... how old is the carb? has it been rebuilt? fuel pump? etc....

[captainzeros]

I agree, that screams of fuel. Specifically, the pump shot on the carburetor. When you slowly ease the pedal in, engine vacuum increases slowly and the fuel jets can keep up easily, but if you stomp it then it takes a second for it to catch up so all carburetors have a little mechanical pump on them that shoots a little extra fuel in when you hit the pedal firmly.

Not saying there ISN'T anything wrong with the timing, you can check that out with a timing light to be absolutely sure it isn't advancing, but my money is on the pump shot in the carb. I went through a lot of fiddling around with the accelerator pump on my truck myself to get it so it wouldn't stall out and die every time I jumped on the gas from an idle (like pulling away from a stop sign).

[rjk629]

so is it possible to change the accelerator pump without a new carb because this is actually another carb than the original cause that one had other issues. the fuel pump and filter are definitely good. both replaced in last 12 months. thank you for the help.

[flinterman2000]

You should be able to get the plunger for the pump separate from a carb kit. check your local parts store.

[captainzeros]

Yeah, the membranes and bits what make up the accelerator pump are generally included in the more complete rebuild kits, or can probably be found separately if you're pinching pennies. Doesn't hurt to have extra gaskets on hand from a rebuild kit if you're planning on using the carb for a long time and doing your own work.

[rjk629]

wow. thanks for the help guys. i would love more than anything to have my truck back on the road. its hard to test the accelerator pump right now because it is running great right now. its like i have to wait for it to break down again

[rjk629]

so i went out again today and it ran like s*** again. i am not sure the best way to test and accelerator pump. i looked in the carb when i pulled the throttle and it did shirt gas in the passenger side. is there any better way to diagnose this. i really do not want to buy parts if i dont have too

[flinterman2000]

Apparently there is something moving around in the pump chamber as it works good sometimes. Your best bet is to service the carb. Get a service kit and change whats necessary, but make sure all the passages are clear using a good quality carb spray.