How to tell if the intake manifold is bad
#1
How to tell if the intake manifold is bad
I bought a 390 S code intake and installed it on my truck. Truck doesn't seem to run as good as it should. engine misses here and there and shakes a lil at idle. ccould the intake be causing this. How do I test the intake manifold to see if there is anything wrong with it.
#3
Also could be tuning. In your other thread you mentioned the PO kept it running good with the 2bbl. If you just slapped on a 4bbl intake and carb without any tuning, it's possible even with a brand new carb that it's out of tune. Most carbs are sent out jetted a little on the rich side. If you're at any kind of altitude, then it'll run even more rich than the same carb would at sea level. It may just need some tuning. Does it smell like it's running excessively rich? Enough to cause your eyes to water, or any black smoke especially when under a load?
If all ran fine before the swap, then one or both of these new parts are your problem. Check for vacuum leaks on the intake first, then move on to tuning the carb.
If all ran fine before the swap, then one or both of these new parts are your problem. Check for vacuum leaks on the intake first, then move on to tuning the carb.
#4
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#8
If the gasket is leaking you can find it by spraying carb cleaner along the sides of the intake where the gasket is. If the engine speed picks up at any time then that spot is leaking. You don't need to hose down the intake, just easy light spraying around the gasket seam. Don't forget to also try that at the base of the carb.
Also look down the throat of the carb with the engine running, you should not see any droplets of fuel going into the manifold. If the fuel is visible then the carb isn't working properly. The only time you will see fuel is when you open the throttle, that is coming from the accelerator pump and is normal, once you hold the throttle steady again you shouldn't see any fuel dripping. PLEASE wear safety glasses when looking down the throat of a carb on a running engine!
Also look down the throat of the carb with the engine running, you should not see any droplets of fuel going into the manifold. If the fuel is visible then the carb isn't working properly. The only time you will see fuel is when you open the throttle, that is coming from the accelerator pump and is normal, once you hold the throttle steady again you shouldn't see any fuel dripping. PLEASE wear safety glasses when looking down the throat of a carb on a running engine!
#10
If the gasket is leaking you can find it by spraying carb cleaner along the sides of the intake where the gasket is. If the engine speed picks up at any time then that spot is leaking. You don't need to hose down the intake, just easy light spraying around the gasket seam. Don't forget to also try that at the base of the carb.
Just a tech tip... an alternative to using carb cleaner is to hook up a hose to a propane bottle and lightly crack the valve. Wave the hose end along the intake's mating flanges and vacuum fittings to locate a leak if the RPM increases.
Indications of a bad manifold included cracks (internal or external), mating surfaces that aren't machined flat, out of square or parallel head mating flanges, and bad threads.
#11
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Really hard to test an Fe manifold since you only have the end you can check, The front of the intake has the water jackets so coolant would possibly leak there.
Check your timing do you even have a timing light??
Hook your vac. gauge up for full manifold vacuum. You new gauge comes with instructions sheet & a chart on what everything means with what's happening on the Ga.
Check your vac advance make sure it's not leaking & sucking air..
Orich
Check your timing do you even have a timing light??
Hook your vac. gauge up for full manifold vacuum. You new gauge comes with instructions sheet & a chart on what everything means with what's happening on the Ga.
Check your vac advance make sure it's not leaking & sucking air..
Orich
#12
Vacuum gauges come with instructions. There are a lot of charts on how to read them. I think they must have made posters back in the day, as advertising, and then included the handy chart. They are the "poor man's Sun machine", or kind of a steampunk scan-gauge for old-school iron.
Can determine engine compression and overall health, valve issues, vacuum leaks, exhaust restriction, ignition timing problems, also tune carburetors, the list is pretty extensive.
They are always connected to a constant source of manifold or "always on" vacuum. Like the brake booster, or a tee on the manifold itself. Try to find one with a large face, the needle or "hand" is more precise. I bought this one from Sears many years ago when I had a brand X truck, can't even remember why I thought I needed one. It's really allowed me to dial in my Y block to a high state of tune. Well, mainly FTE knowledge base.
Can determine engine compression and overall health, valve issues, vacuum leaks, exhaust restriction, ignition timing problems, also tune carburetors, the list is pretty extensive.
They are always connected to a constant source of manifold or "always on" vacuum. Like the brake booster, or a tee on the manifold itself. Try to find one with a large face, the needle or "hand" is more precise. I bought this one from Sears many years ago when I had a brand X truck, can't even remember why I thought I needed one. It's really allowed me to dial in my Y block to a high state of tune. Well, mainly FTE knowledge base.
#13
Vacuum gauges come with instructions. There are a lot of charts on how to read them. I think they must have made posters back in the day, as advertising, and then included the handy chart. They are the "poor man's Sun machine", or kind of a steampunk scan-gauge for old-school iron.
Can determine engine compression and overall health, valve issues, vacuum leaks, exhaust restriction, ignition timing problems, also tune carburetors, the list is pretty extensive.
They are always connected to a constant source of manifold or "always on" vacuum. Like the brake booster, or a tee on the manifold itself. Try to find one with a large face, the needle or "hand" is more precise. I bought this one from Sears many years ago when I had a brand X truck, can't even remember why I thought I needed one. It's really allowed me to dial in my Y block to a high state of tune. Well, mainly FTE knowledge base.
Can determine engine compression and overall health, valve issues, vacuum leaks, exhaust restriction, ignition timing problems, also tune carburetors, the list is pretty extensive.
They are always connected to a constant source of manifold or "always on" vacuum. Like the brake booster, or a tee on the manifold itself. Try to find one with a large face, the needle or "hand" is more precise. I bought this one from Sears many years ago when I had a brand X truck, can't even remember why I thought I needed one. It's really allowed me to dial in my Y block to a high state of tune. Well, mainly FTE knowledge base.
#14
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