Carb question
Thread Starter
Intermediate
Joined: Mar 2011
Posts: 45
Likes: 0
Carb question
so im having an issue with my carb. when i fire her up cold, she starts right up and runs for 15 seconds on its own and then starts sputtering and turns off. Any ideas on where to start. also, i did some tweaking and now when i stand at a red light the vette wants to drive away on me. when i let up of the brake it gets up to 15mph by itself. Any ideas? thanks
Pro
Joined: May 2011
Posts: 581
Likes: 5
From: BC
Your problems are most likely going to be choke related, not the actual carb itself.
You always start your diagnosis with a cold engine, ideally one that has sat overnight. Remove the lid from the air cleaner and watch the choke valve as you crack the throttle open. It should snap shut and be closed completely with a moderate tension on a 18 degree (C) morning. The next test is best done with a handheld vacuum pump. There is a small vacuum diaphram on the rt side of the carb. This is the choke pull-off. It's purpose is to crack the choke open a pre-determined amount as soon as the engine starts. A choke pull off with a ruptured diaphram will not pull the choke off enough and the engine will "load up" with fuel. This is where the engine will slowly start chugging, spewing black smoke and slowly die. Restart happens easiest if you hold the pedal to the floor while cranking it. To test with a vacuum pump attach your pump at the vacuum hose connection. The rod coming off the diaphram should retract fully, you shouldn't be able to push it in any further with your fingers. If it's doing that you now need to make sure it's adjusted properly. There is a dimension in your service manual that tells you how far the choke plate should be opened when the pull-off is bottomed. You check this dimension with the appropriate size drill bit to measure between the choke valve and the air horn. Too small a gap, the engine will "load up". Too large a gap and the engine will starve and die suddenly (without the chugging, rolling idle leading up to it).
Once you have determined that the choke is closing properly and the pull-off is working and properly adjusted you can start it up. Your problem with the stalling after 15 seconds will be cleared up in the above section. What we're looking for now is to make sure the choke is coming off as the engine warms. If it isn't that will be the cause of your high idle. As the engine warms the choke valve should gradually move to the fully opened position. As it does the fast idle cam should drop out of play. If one or the other, or both, fail to happen you need to find out why. If the choke is coming off nicely in 15 mins or less but the idle is not dropping, check for a gummed fast idle cam or someone having adjusted the curb idle on the fast idle screw. If the choke isn't opening you want to check all the sources of heat responsible for opening that choke. I don't recall if 79 uses the 5/16ths heat tubes that run into the intake crossover or a divorced choke or an electric combined with either of those. A common problem with no heat is an inoperative or removed heatriser valve in the exhaust manifold. If the car has headers and therefore no heat riser you won't get the choke to work right. Go to an electric and learn to live with it.
There, that ought to get you started.
Steve g
You always start your diagnosis with a cold engine, ideally one that has sat overnight. Remove the lid from the air cleaner and watch the choke valve as you crack the throttle open. It should snap shut and be closed completely with a moderate tension on a 18 degree (C) morning. The next test is best done with a handheld vacuum pump. There is a small vacuum diaphram on the rt side of the carb. This is the choke pull-off. It's purpose is to crack the choke open a pre-determined amount as soon as the engine starts. A choke pull off with a ruptured diaphram will not pull the choke off enough and the engine will "load up" with fuel. This is where the engine will slowly start chugging, spewing black smoke and slowly die. Restart happens easiest if you hold the pedal to the floor while cranking it. To test with a vacuum pump attach your pump at the vacuum hose connection. The rod coming off the diaphram should retract fully, you shouldn't be able to push it in any further with your fingers. If it's doing that you now need to make sure it's adjusted properly. There is a dimension in your service manual that tells you how far the choke plate should be opened when the pull-off is bottomed. You check this dimension with the appropriate size drill bit to measure between the choke valve and the air horn. Too small a gap, the engine will "load up". Too large a gap and the engine will starve and die suddenly (without the chugging, rolling idle leading up to it).
Once you have determined that the choke is closing properly and the pull-off is working and properly adjusted you can start it up. Your problem with the stalling after 15 seconds will be cleared up in the above section. What we're looking for now is to make sure the choke is coming off as the engine warms. If it isn't that will be the cause of your high idle. As the engine warms the choke valve should gradually move to the fully opened position. As it does the fast idle cam should drop out of play. If one or the other, or both, fail to happen you need to find out why. If the choke is coming off nicely in 15 mins or less but the idle is not dropping, check for a gummed fast idle cam or someone having adjusted the curb idle on the fast idle screw. If the choke isn't opening you want to check all the sources of heat responsible for opening that choke. I don't recall if 79 uses the 5/16ths heat tubes that run into the intake crossover or a divorced choke or an electric combined with either of those. A common problem with no heat is an inoperative or removed heatriser valve in the exhaust manifold. If the car has headers and therefore no heat riser you won't get the choke to work right. Go to an electric and learn to live with it.
There, that ought to get you started.
Steve g
