engine vacuum leak
Thread Starter
Burning Brakes
Joined: Sep 2009
Posts: 1,003
Likes: 48
From: hickory NC
C4 of Year Finalist (appearance mods) 2019
2017 C4 of Year Finalist
engine vacuum leak
Been chasing a rough running engine. At cruise speed (55-75) the engine will surge. Have gotten it better by upping the jets. (Holley) Still have a misfire thru the exhaust and will spit and sputter if I drop into first gear and accelerate. (Automatic)
Replaced the plugs, checked the plug wires, rebuilt carb, even checked all the wires inside the distributor.
Tonight I sprayed some carb cleaner and starting fluid around the hoses and intake, no change until I got to the runners for 1&3, the idle dropped when I sprayed that area.
Good indication of a bad intake gasket?
TIA
Replaced the plugs, checked the plug wires, rebuilt carb, even checked all the wires inside the distributor.
Tonight I sprayed some carb cleaner and starting fluid around the hoses and intake, no change until I got to the runners for 1&3, the idle dropped when I sprayed that area.
Good indication of a bad intake gasket?
TIA
Le Mans Master
Joined: Apr 2011
Posts: 6,284
Likes: 1,172
From: Hermosa
Sounds like you're on the right track. If it's just sucking vacuum and not oil or water you could just clean it up and apply some RTV sealer to that area. Something like permatex ultra black.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
Personally I would not use 'starting fluid'...it is highly flammable and a serious fire risk. If you do...just be real careful....that ether is wicked.
Also..when I do what you are doing.... usually the idle increases when I find a leak....and the idle not drop.
DUB
Also..when I do what you are doing.... usually the idle increases when I find a leak....and the idle not drop.
DUB
Thread Starter
Burning Brakes
Joined: Sep 2009
Posts: 1,003
Likes: 48
From: hickory NC
C4 of Year Finalist (appearance mods) 2019
2017 C4 of Year Finalist
As far as the idle, I always thought it should rise, not drop. Going to try some more tomorrow.
Some trouble I'm having is the idle is rough anyway with the cam that's in it. Kind of hard to tell when and if it smoothens out.
REALLY want to get rid of this surging and misfire.
Thread Starter
Burning Brakes
Joined: Sep 2009
Posts: 1,003
Likes: 48
From: hickory NC
C4 of Year Finalist (appearance mods) 2019
2017 C4 of Year Finalist
Probably be just a temporary fix anyway.
Race Director
Joined: Apr 2009
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From: Charlotte NC
OK...now the picture gets more in focus due to more information. Always seems to happen that way.
Now knowing that you have some type of cam in it that more than likely more performance minded than factory. That changes 'things' depending on how radical it is..
Not knowing what your engine vacuum is so the power valve in the Holley is/are correct...well that is another.
Re-jetting it but also still having backfiring in eh exhaust...I wonder if you have a fuel issue that may not have been addressed when the carb was rebuilt.
Not knowing what you timing is set at and how the advance is set up and what is reads....that is another.....and that is ASSUMING that the notch on the harmonic balancer has not spun and has been verified... UNLESS the harmonic balancer was replaced....so if you are using a timing light...it is worth while....because setting timing when a the balancer outer ring has spun....is pointless.
Also...removing all vacuum sources for the headlights and AC system and capping off that port and see if that helps...because I have worked on many Corvettes where the headlight actuator relays were shot and such a serious vacuum leak there...it effected how the car ran.
DUB
Now knowing that you have some type of cam in it that more than likely more performance minded than factory. That changes 'things' depending on how radical it is..
Not knowing what your engine vacuum is so the power valve in the Holley is/are correct...well that is another.
Re-jetting it but also still having backfiring in eh exhaust...I wonder if you have a fuel issue that may not have been addressed when the carb was rebuilt.
Not knowing what you timing is set at and how the advance is set up and what is reads....that is another.....and that is ASSUMING that the notch on the harmonic balancer has not spun and has been verified... UNLESS the harmonic balancer was replaced....so if you are using a timing light...it is worth while....because setting timing when a the balancer outer ring has spun....is pointless.
Also...removing all vacuum sources for the headlights and AC system and capping off that port and see if that helps...because I have worked on many Corvettes where the headlight actuator relays were shot and such a serious vacuum leak there...it effected how the car ran.
DUB
Thread Starter
Burning Brakes
Joined: Sep 2009
Posts: 1,003
Likes: 48
From: hickory NC
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2017 C4 of Year Finalist
DUH..I never thought about the headlights..will check that area tomorrow.
Vacuum reading is just about 20 inches on my VERY old gauge. I replaced the PV with a 6.5
Both idle screws out 2 3/4 turns for highest vacuum.
Even put my old plugs back in thinking I may have gotten a bad new one.(AC Delco R45ts)
I'm now fairly certain it's NOT a vacuum leak in the engine area. I did some "tests" today and idle did not change. May have been my wishful thinking/imagination using that starting fluid.
I'm also having visions of a new cam needing to be installed.
And/or a valve job.
Just went back and reread my original post, when I mentioned it misfires in exhaust, I left out(thought I added this) that it backfires thru the carb also. Sounds like a shotgun going off.
Vacuum reading is just about 20 inches on my VERY old gauge. I replaced the PV with a 6.5
Both idle screws out 2 3/4 turns for highest vacuum.
Even put my old plugs back in thinking I may have gotten a bad new one.(AC Delco R45ts)
I'm now fairly certain it's NOT a vacuum leak in the engine area. I did some "tests" today and idle did not change. May have been my wishful thinking/imagination using that starting fluid.
I'm also having visions of a new cam needing to be installed.
And/or a valve job.
Just went back and reread my original post, when I mentioned it misfires in exhaust, I left out(thought I added this) that it backfires thru the carb also. Sounds like a shotgun going off.
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Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
DUH..I never thought about the headlights..will check that area tomorrow.
Vacuum reading is just about 20 inches on my VERY old gauge. I replaced the PV with a 6.5
Both idle screws out 2 3/4 turns for highest vacuum.
Even put my old plugs back in thinking I may have gotten a bad new one.(AC Delco R45ts)
I'm now fairly certain it's NOT a vacuum leak in the engine area. I did some "tests" today and idle did not change. May have been my wishful thinking/imagination using that starting fluid.
I'm also having visions of a new cam needing to be installed.
And/or a valve job.
Just went back and reread my original post, when I mentioned it misfires in exhaust, I left out(thought I added this) that it backfires thru the carb also. Sounds like a shotgun going off.
Vacuum reading is just about 20 inches on my VERY old gauge. I replaced the PV with a 6.5
Both idle screws out 2 3/4 turns for highest vacuum.
Even put my old plugs back in thinking I may have gotten a bad new one.(AC Delco R45ts)
I'm now fairly certain it's NOT a vacuum leak in the engine area. I did some "tests" today and idle did not change. May have been my wishful thinking/imagination using that starting fluid.
I'm also having visions of a new cam needing to be installed.
And/or a valve job.
Just went back and reread my original post, when I mentioned it misfires in exhaust, I left out(thought I added this) that it backfires thru the carb also. Sounds like a shotgun going off.
I do not know if it is your problem.....but if you are getting spitting back up through the carb...that is a good indicator that you have an exhaust lobe worn off your cam or something going on in an exhaust valve somewhere. Usually requires pulling off the valve cover and killing the ignition and spin the engine over and WATCH your rocker arms and see if there is one not acting/moving the same.
DUB
Thread Starter
Burning Brakes
Joined: Sep 2009
Posts: 1,003
Likes: 48
From: hickory NC
C4 of Year Finalist (appearance mods) 2019
2017 C4 of Year Finalist
It seems to be the consensus that when you get a GOOD backfire through a Holley...the power valve(s) just got ruined....and may need to be checked.
I do not know if it is your problem.....but if you are getting spitting back up through the carb...that is a good indicator that you have an exhaust lobe worn off your cam or something going on in an exhaust valve somewhere. Usually requires pulling off the valve cover and killing the ignition and spin the engine over and WATCH your rocker arms and see if there is one not acting/moving the same.
DUB
I do not know if it is your problem.....but if you are getting spitting back up through the carb...that is a good indicator that you have an exhaust lobe worn off your cam or something going on in an exhaust valve somewhere. Usually requires pulling off the valve cover and killing the ignition and spin the engine over and WATCH your rocker arms and see if there is one not acting/moving the same.
DUB
BTW..got any good heads laying around at the shop? Not sure if Gary has any but I also know someone with a set of Vortecs.
Race Director
Joined: Apr 2009
Posts: 19,294
Likes: 2,754
From: Charlotte NC
Yes I do have cylinder heads but they are my 'keepers'.
I would be careful in putting in some serious heads...and not 'possibly' addressing any compression issues or poor ring drag on the engine. I am in the belief that if a wicked set of heads are put on an engine that has a cast crankshaft and weak rods and cast pistons...it is a recipe for disaster. If your bottom end is not stout...putting on some small cc heads will raise the compression and cause for something to 'possibly give way'. I am NOT writing that you would have a problem.....but it just makes sense to me to get all the parts to the strength they need to be if you are building a strong engine.
DUB
I would be careful in putting in some serious heads...and not 'possibly' addressing any compression issues or poor ring drag on the engine. I am in the belief that if a wicked set of heads are put on an engine that has a cast crankshaft and weak rods and cast pistons...it is a recipe for disaster. If your bottom end is not stout...putting on some small cc heads will raise the compression and cause for something to 'possibly give way'. I am NOT writing that you would have a problem.....but it just makes sense to me to get all the parts to the strength they need to be if you are building a strong engine.
DUB