Having some carb issues
Thread Starter
Drifting
Joined: May 2007
Posts: 1,491
Likes: 2
From: North Royaton Ohio
Having some carb issues
When I finished building my car 2 years ago, I ran out of money at the very end and had no choice but to skimp on the intake manifold. I had an old Edelbrock SP2P intake laying around the garage, so that's what went on the car. I topped it off with a fairly decent carb, this Jet Products QJet -> http://www.summitracing.com/search/b.../?autoview=SKU
I'm gonna clear this up now. There are no vacuum leaks on my car. I've checked every single vacuum line.
Until recently, the car ran alright. It had a hard time idling in gear, regardless of where the idle was set. When at a light, I would have to slip it in neutral to keep it running, at which point the RPM's would shoot up. If I would drop it in gear, they would fall about 200-300 RPM and stall out. The strange thing is that sometimes, without any warning, the car would idle just fine at a light in gear, and then all of a sudden the RPM's would drop drastically and the car would almost stall.
Recently, however, the car has all but refused to idle even when in neutral with the idle brought up considerably. It also has a tendency to want to stall when the brakes are applied.
What do you guys think?
I'm gonna clear this up now. There are no vacuum leaks on my car. I've checked every single vacuum line.
Until recently, the car ran alright. It had a hard time idling in gear, regardless of where the idle was set. When at a light, I would have to slip it in neutral to keep it running, at which point the RPM's would shoot up. If I would drop it in gear, they would fall about 200-300 RPM and stall out. The strange thing is that sometimes, without any warning, the car would idle just fine at a light in gear, and then all of a sudden the RPM's would drop drastically and the car would almost stall.
Recently, however, the car has all but refused to idle even when in neutral with the idle brought up considerably. It also has a tendency to want to stall when the brakes are applied.
What do you guys think?
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Having "good" vacuum lines does not guarantee that you don't have a vacuum leak. There are lots of ways to have vacuum leaks...in fact, most vacuum leaks are NOT due to a problem with the lines. Any of the components in the various vacuum systems (HVAC, wiper, headlight) might have an internal leak; the vacuum accumulator tank could be cracked or punctured; the vacuum advance can could have a bad diaphragm; the carb could have worn butterfly shafts, a bad gasket, or other leak sources; the intake manifold could have a leak in one of its gaskets or at the valley seal areas; etc., etc.
The only way to know if you have leaks in the vacuum system is to remove the entire system from the engine by pulling the main vacuum feed line and capping off that manifold fitting and doing the same for the dist. vacuum can and cap off the carb fitting. Then get a spray can of brake cleaner. Start the car and let it idle [as best it can]. Now, shoot little puffs of brake cleaner on the outside areas of the carb...near the base, particularly. If the engine speeds up after one of the sprays, that shows you that a leak path exists where you were spraying. Keep going to identify all potential areas of leakage. Then do the same while working around the intake manifold gasketed joints and the end joints. If the engine does not speed up with any carb or manifold checks, GREAT!!
Now you need to check out the vacuum systems. First, hook a vacuum gauge up to the main vacuum feed fitting [which you capped off earlier]. Now take a reading from the vacuum gauge. It should be in the range of 15-21" Hg with a stock engine. Record that value as the "base" vacuum reading for your engine. Now, hook the dist. advance can back to the carb fitting and check the reading again. If it drops significantly, the dist. vacuum can has a ruptured diaphram and needs to be replaced. If it reads the same as the "base" value, the can is good and can be left in place for the remainder of testing. OK, now, hook up the main feed line and 'patch' the vacuum gauge into the main feed line, as well. If the vacuum gauge reading drops more than 1-2" Hg when you connect the rest of the vacuum system, there is a significant leak somewhere in the system. Fix that and your idle problems will likely go away.
P.S. You perform detailed testing on the various parts of the vacuum system by removing one part of the system at a time and checking to see if the vacuum level raises to the "base" reading level. Keep peeling away the working parts of the system until you find the one [or more] item that causes the leakage.
This work is not difficult, but it is more involved than just looking at the vacuum lines.
The only way to know if you have leaks in the vacuum system is to remove the entire system from the engine by pulling the main vacuum feed line and capping off that manifold fitting and doing the same for the dist. vacuum can and cap off the carb fitting. Then get a spray can of brake cleaner. Start the car and let it idle [as best it can]. Now, shoot little puffs of brake cleaner on the outside areas of the carb...near the base, particularly. If the engine speeds up after one of the sprays, that shows you that a leak path exists where you were spraying. Keep going to identify all potential areas of leakage. Then do the same while working around the intake manifold gasketed joints and the end joints. If the engine does not speed up with any carb or manifold checks, GREAT!!
Now you need to check out the vacuum systems. First, hook a vacuum gauge up to the main vacuum feed fitting [which you capped off earlier]. Now take a reading from the vacuum gauge. It should be in the range of 15-21" Hg with a stock engine. Record that value as the "base" vacuum reading for your engine. Now, hook the dist. advance can back to the carb fitting and check the reading again. If it drops significantly, the dist. vacuum can has a ruptured diaphram and needs to be replaced. If it reads the same as the "base" value, the can is good and can be left in place for the remainder of testing. OK, now, hook up the main feed line and 'patch' the vacuum gauge into the main feed line, as well. If the vacuum gauge reading drops more than 1-2" Hg when you connect the rest of the vacuum system, there is a significant leak somewhere in the system. Fix that and your idle problems will likely go away.
P.S. You perform detailed testing on the various parts of the vacuum system by removing one part of the system at a time and checking to see if the vacuum level raises to the "base" reading level. Keep peeling away the working parts of the system until you find the one [or more] item that causes the leakage.
This work is not difficult, but it is more involved than just looking at the vacuum lines.
Thread Starter
Drifting
Joined: May 2007
Posts: 1,491
Likes: 2
From: North Royaton Ohio
Having "good" vacuum lines does not guarantee that you don't have a vacuum leak. There are lots of ways to have vacuum leaks...in fact, most vacuum leaks are NOT due to a problem with the lines. Any of the components in the various vacuum systems (HVAC, wiper, headlight) might have an internal leak; the vacuum accumulator tank could be cracked or punctured; the vacuum advance can could have a bad diaphragm; the carb could have worn butterfly shafts, a bad gasket, or other leak sources; the intake manifold could have a leak in one of its gaskets or at the valley seal areas; etc., etc.
The only way to know if you have leaks in the vacuum system is to remove the entire system from the engine by pulling the main vacuum feed line and capping off that manifold fitting and doing the same for the dist. vacuum can and cap off the carb fitting. Then get a spray can of brake cleaner. Start the car and let it idle [as best it can]. Now, shoot little puffs of brake cleaner on the outside areas of the carb...near the base, particularly. If the engine speeds up after one of the sprays, that shows you that a leak path exists where you were spraying. Keep going to identify all potential areas of leakage. Then do the same while working around the intake manifold gasketed joints and the end joints. If the engine does not speed up with any carb or manifold checks, GREAT!!
Now you need to check out the vacuum systems. First, hook a vacuum gauge up to the main vacuum feed fitting [which you capped off earlier]. Now take a reading from the vacuum gauge. It should be in the range of 15-21" Hg with a stock engine. Record that value as the "base" vacuum reading for your engine. Now, hook the dist. advance can back to the carb fitting and check the reading again. If it drops significantly, the dist. vacuum can has a ruptured diaphram and needs to be replaced. If it reads the same as the "base" value, the can is good and can be left in place for the remainder of testing. OK, now, hook up the main feed line and 'patch' the vacuum gauge into the main feed line, as well. If the vacuum gauge reading drops more than 1-2" Hg when you connect the rest of the vacuum system, there is a significant leak somewhere in the system. Fix that and your idle problems will likely go away.
P.S. You perform detailed testing on the various parts of the vacuum system by removing one part of the system at a time and checking to see if the vacuum level raises to the "base" reading level. Keep peeling away the working parts of the system until you find the one [or more] item that causes the leakage.
This work is not difficult, but it is more involved than just looking at the vacuum lines.
The only way to know if you have leaks in the vacuum system is to remove the entire system from the engine by pulling the main vacuum feed line and capping off that manifold fitting and doing the same for the dist. vacuum can and cap off the carb fitting. Then get a spray can of brake cleaner. Start the car and let it idle [as best it can]. Now, shoot little puffs of brake cleaner on the outside areas of the carb...near the base, particularly. If the engine speeds up after one of the sprays, that shows you that a leak path exists where you were spraying. Keep going to identify all potential areas of leakage. Then do the same while working around the intake manifold gasketed joints and the end joints. If the engine does not speed up with any carb or manifold checks, GREAT!!
Now you need to check out the vacuum systems. First, hook a vacuum gauge up to the main vacuum feed fitting [which you capped off earlier]. Now take a reading from the vacuum gauge. It should be in the range of 15-21" Hg with a stock engine. Record that value as the "base" vacuum reading for your engine. Now, hook the dist. advance can back to the carb fitting and check the reading again. If it drops significantly, the dist. vacuum can has a ruptured diaphram and needs to be replaced. If it reads the same as the "base" value, the can is good and can be left in place for the remainder of testing. OK, now, hook up the main feed line and 'patch' the vacuum gauge into the main feed line, as well. If the vacuum gauge reading drops more than 1-2" Hg when you connect the rest of the vacuum system, there is a significant leak somewhere in the system. Fix that and your idle problems will likely go away.
P.S. You perform detailed testing on the various parts of the vacuum system by removing one part of the system at a time and checking to see if the vacuum level raises to the "base" reading level. Keep peeling away the working parts of the system until you find the one [or more] item that causes the leakage.
This work is not difficult, but it is more involved than just looking at the vacuum lines.
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Joined: Dec 2004
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I would start looking at the ignition system. There's nothing in the carburetor that changes going from a load to no-load condition. The only thing the carb needs to do is have an operating choke (during cooler weather), provide fuel through a properly operating idle circuit, and add a shot of fuel under acceleration.
The ignition system, however does change.
Intermediate
Joined: Aug 2009
Posts: 28
Likes: 0
7t1 vettes info was righton. But no vac leaks. Now I'm not a big fan of q-jet i like my secondarieas to open mechanically. but what you describe could be alot of things, including the carb. You could just have a little peice of crap clogging one of the venturies in that q-junk. I doubt its the coil cause they either work or not. youre Worst case scenario but it could be youre torque converter is staying locked up for some reason. The front pump could be worn and causing the converter to bind ever so slightly so the tranny stays engaged. not an easy check but if you keep hitting that wall thats were i would look
