http://www.youtube.com/watch?v=mNy430NSkb0


This is an 84 z28 HO, computer carb with the computer removed(came this way) What could cause this? the car starts right up and runs fine, but it dies right afterward. I havent had a chance to check the fuel pressure but it doesnt seem related because it runs and drives at high rpms fine, only idle and under 2000 its a problem. The only way it idles is if i pump the gas. Is it the lack of computer control hooked up to the carb? I have a non computer Q-jet from an 83 pickup 305 that im going to rebuild if it is the carb. Or what else could it be? i have the timing set to what im semi-sure is TDC, no advance or retard. I did it by rotating the crank to the tdc mark on the piston then setting the distributor and putting the number one pin directly over the rotor. Could it still be the timing? if i advance or retard it it runs the same just if i go too far it wont start.

 

Is it warmed up if not it sounds like the choke isnt working. If it is are all the vacuum lines hooked up properly?

 

most of the vac lines are blocked or good. It does the same thing even when i let it get hot, but i havent let it get really warm since i changed the plugs and wires. Ill have to check that. anyone else in the mean time?

 

seems like its not getting enough air to me, but its hard to tell for the video. check on the cat it could be clogged. id also suggest checking out the fuel filter and seeing what the psi is on the fuel rail.

 

You should get into more detail about the carb, but if I assume its the same style as the one you took off here is the list. The order of the this list is by what is most common. Or atleast the order I would check it.

Choke pulloff:
When you engine is cold the choke closes all the way. This gives you a very rich startup. Once you have engine vacuum a small diapham opens the choke a small amountso the engine can get enough air to remain idleing. Your choke is also hooked to your throttle so it will stay open as long as your foot is on the gas.

Stuck EGR Valve:
EGR valve is suppost to open while you drive under load. This allows a small amount of exhaust gas into the intake to be reburned for better emissions. If that valve is stuck open far enough the car will not idle.

Vacuum Leak:
As stated above.

Idle circuit in the carb is pluged:
There is a seprate part of your carb that is just for idle it hands off the operation to thefloat/metering rodsonce it reachs a certain level in carb vacuum. If the idle circuit is pluged the car will not run well under 1200 RPMs.Common cause for this is sitting, the gas turns to varnish.


Time chain jump: This is very rare.Most of the time when the chain jumpsand the engine still starts it will idle and have no power. I have seen it a few time do the oppisite. I think it has to do with which way it jumps which would depend if it happen on accelor deccel.

Now if this is one of those rare Carburated cars with a fuel rail I can not help you, its just out of my league.
(sorryAlih I could not resist)

 

Another thing hit me last night. Can you keep the car running if you hold the gas down pedal down 20-30%? Or do you have to pump it to keep it running? If you have to pump it this could mean that only the accelerator pump is working and fuel is coming from the fuel bowl as it should. Thereis a few things that can cause this but it means a carb rebuild/replace is in order.

Also if vacuum hoses are hookedup wrongthere is no telling what could cause it. I have seen manifold vacuum hooked to fuel bowl vents, purge valves hooked up wrong,,,,, just to many things to list.

 

oh yeah sorry, dont really work with carb'd cars that often, kinda forgot about the whole carb setup!

 

i have to keep my foot down for it to run, but under 2000 or so i have to pump it which could mean it will only run off the accelerator pump. Does anyone have a vac diagram for an 84 HO 305 Q jet? Im pretty sure there all hooked up where they were. Oh and my EGR valve isnt attached, im going to get a blockoff plate for it though. Could that have something to do with it? Based on Gorns info i would say vac leaking or bad idle circuit...how can i check for this? The car was sitting before i bought it. Im gona mess around with the vac lines tommorow if it stops raining! i took off work to work on my car and its been raining all day!

 

If the spring in a disconnectedEGR valve where to break it it could cause this. If you take the EGR valve off and spray some carb cleaner in the roundend with the valve and see if it will hold a puddel, If so it's sealed and most likly not your issue. Block plate is a good idea if it is ok in your area. You can try car Fuel Injection cleaner in your gas to see if it will break up the varnish. Also some of the high grade gases have a weak detergent. I have seen them where nothing short of overnight ultra sonic bathof the carbbodywas the only thing that would clean out.If I had to guess 60% of the time the cleaner and a tank of the good gas will clean it out.

Good luck.

 

Alright ill try that when i get the car insured again. The egr valve and system is just missing, so its shooting exhaust out the intake manifold and there is nowhere for it to get into the carb unless its done internally. It seems like it just takes exhaust out of that passenger side port and pumps it right into the air filter. thats how it was on my 2.8v6. But im gona get some FI cleaner and see if it works.