Is this idle normal (video)
06-19-2016, 09:43 AM
#1
Instructor
Thread Starter
Is this idle normal (video)
74 base engine (350/198). New plugs/wires/points/carb rebuilt. Prior to carb work the engine was lean on tip in which is much better now. However previously the car idled more steadily around 7500 rpm. Attaching a video here (unless i screw it up) that shows closer to 8500 but bouncing a bit betwen 8500 and 6000. Is this normal? Chance the recent tune is good but maybe the idle is indicative of a valve sticking or vacuum issue - or did my tuner just not tune the car right ?
Btw - there is exhaust leak as well that i hear - not sure video picks it up
Btw - there is exhaust leak as well that i hear - not sure video picks it up
06-19-2016, 11:23 AM
#3
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St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I can hear the video but it's kind of hard to tell what is normal or not. If it's a stock camshaft go look at the engine it should be shake free for the most part and smooth. If you suspect a vacuum issue throw a gauge on it and see what it reads.
Typically we called this the old fart vacuum chart. Or what does my vacuum gauge tell me.
On a typical engine:
Steady 17-22 hg = normal engine operation.
Low but steady = Late ignition, valve timing or low compression.
Very low = Vacuum leak.
Needle fluctuates as RPM’s increase = ignition miss, blown cylinder head gasket, leaking valve or weak spring.
Gradual drop at idle = excessive back pressure in exhaust system.
Intermittent fluctuation at idle = Engine miss or sticking valve.
Drifting needle = Improper idle mixture adjustment, carburetor not synchronized, or minor intake leak
High but steady = Early ignition timing.
Typically we called this the old fart vacuum chart. Or what does my vacuum gauge tell me.
On a typical engine:
Steady 17-22 hg = normal engine operation.
Low but steady = Late ignition, valve timing or low compression.
Very low = Vacuum leak.
Needle fluctuates as RPM’s increase = ignition miss, blown cylinder head gasket, leaking valve or weak spring.
Gradual drop at idle = excessive back pressure in exhaust system.
Intermittent fluctuation at idle = Engine miss or sticking valve.
Drifting needle = Improper idle mixture adjustment, carburetor not synchronized, or minor intake leak
High but steady = Early ignition timing.
06-19-2016, 12:26 PM
#4
Race Director
I can hear the video but it's kind of hard to tell what is normal or not. If it's a stock camshaft go look at the engine it should be shake free for the most part and smooth. If you suspect a vacuum issue throw a gauge on it and see what it reads.
Typically we called this the old fart vacuum chart. Or what does my vacuum gauge tell me.
On a typical engine:
Steady 17-22 hg = normal engine operation.
Low but steady = Late ignition, valve timing or low compression.
Very low = Vacuum leak.
Needle fluctuates as RPM’s increase = ignition miss, blown cylinder head gasket, leaking valve or weak spring.
Gradual drop at idle = excessive back pressure in exhaust system.
Intermittent fluctuation at idle = Engine miss or sticking valve.
Drifting needle = Improper idle mixture adjustment, carburetor not synchronized, or minor intake leak
High but steady = Early ignition timing.
Typically we called this the old fart vacuum chart. Or what does my vacuum gauge tell me.
On a typical engine:
Steady 17-22 hg = normal engine operation.
Low but steady = Late ignition, valve timing or low compression.
Very low = Vacuum leak.
Needle fluctuates as RPM’s increase = ignition miss, blown cylinder head gasket, leaking valve or weak spring.
Gradual drop at idle = excessive back pressure in exhaust system.
Intermittent fluctuation at idle = Engine miss or sticking valve.
Drifting needle = Improper idle mixture adjustment, carburetor not synchronized, or minor intake leak
High but steady = Early ignition timing.
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06-19-2016, 04:18 PM
#6
Instructor
Thread Starter
Yes sir - Everything stock on the car to my knowledge.
When i mentioned vacuum it was stabbing in dark but my primary question which i'm still unsure of - on a standard engine with no mods- should the idle rpm be steady, and if so what rpm is to be expected?
When i mentioned vacuum it was stabbing in dark but my primary question which i'm still unsure of - on a standard engine with no mods- should the idle rpm be steady, and if so what rpm is to be expected?
I can hear the video but it's kind of hard to tell what is normal or not. If it's a stock camshaft go look at the engine it should be shake free for the most part and smooth. If you suspect a vacuum issue throw a gauge on it and see what it reads.
Typically we called this the old fart vacuum chart. Or what does my vacuum gauge tell me.
On a typical engine:
Steady 17-22 hg = normal engine operation.
Low but steady = Late ignition, valve timing or low compression.
Very low = Vacuum leak.
Needle fluctuates as RPM’s increase = ignition miss, blown cylinder head gasket, leaking valve or weak spring.
Gradual drop at idle = excessive back pressure in exhaust system.
Intermittent fluctuation at idle = Engine miss or sticking valve.
Drifting needle = Improper idle mixture adjustment, carburetor not synchronized, or minor intake leak
High but steady = Early ignition timing.
Typically we called this the old fart vacuum chart. Or what does my vacuum gauge tell me.
On a typical engine:
Steady 17-22 hg = normal engine operation.
Low but steady = Late ignition, valve timing or low compression.
Very low = Vacuum leak.
Needle fluctuates as RPM’s increase = ignition miss, blown cylinder head gasket, leaking valve or weak spring.
Gradual drop at idle = excessive back pressure in exhaust system.
Intermittent fluctuation at idle = Engine miss or sticking valve.
Drifting needle = Improper idle mixture adjustment, carburetor not synchronized, or minor intake leak
High but steady = Early ignition timing.
06-19-2016, 05:42 PM
#7
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St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Yes that looks pretty normal to me, it'll vary a bit. I'd still put a gauge on it.
06-20-2016, 07:08 AM
#8
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The idle spec will be in your tuneup specs. You set it with the set screw or idle mixture screws. Then if it fluctuates you track the problem from there. My 68 has a 72 motor and the idle set for that motor is 900 rpm
06-21-2016, 09:18 PM
#9
Instructor
Thread Starter
06-21-2016, 10:30 PM
#10
Instructor
06-22-2016, 11:39 AM
#11
Burning Brakes
Maybe a stupid question, but does it do the same if you hold the gauge in your hand? It kind of looks like it's just the engine vibration that shakes the needle.
It looks like the engine shakes quite a lot?
Also that port is the 5/8 runner, in order to get a proper reading you should find a manifold vacuum port on the carburetor.
It looks like the engine shakes quite a lot?
Also that port is the 5/8 runner, in order to get a proper reading you should find a manifold vacuum port on the carburetor.
Last edited by Danish Shark; 06-22-2016 at 11:41 AM.
06-22-2016, 02:43 PM
#12
Instructor
Thread Starter
I will retest with a port off the carb and also test under throttle
The jittery gauge acted the same both in my hands as well as when laid on the engine (when video was shot). I will try again to verify the gauge isnt just transferring shake from engine.
I was told the engine mounts could stand being replaced which i attribute the shake to. Does the shake appear 'bad' to you ?
The jittery gauge acted the same both in my hands as well as when laid on the engine (when video was shot). I will try again to verify the gauge isnt just transferring shake from engine.
I was told the engine mounts could stand being replaced which i attribute the shake to. Does the shake appear 'bad' to you ?
Maybe a stupid question, but does it do the same if you hold the gauge in your hand? It kind of looks like it's just the engine vibration that shakes the needle.
It looks like the engine shakes quite a lot?
Also that port is the 5/8 runner, in order to get a proper reading you should find a manifold vacuum port on the carburetor.
It looks like the engine shakes quite a lot?
Also that port is the 5/8 runner, in order to get a proper reading you should find a manifold vacuum port on the carburetor.
Last edited by vmsns007; 06-22-2016 at 02:43 PM.
06-23-2016, 02:27 PM
#13
Burning Brakes
Difficult to see on the close up video, but a stock engine shouldn't shake unless there something wrong or someone put a silly camshaft in it.
Whether the mounts could be causing it I don't have enough experience to say that. If they're bad then replace them and see if it makes a difference.
Yours appears to shake about as bad as my new big block and I'm definitely not happy with that one.
Is the car shaking? Mine is.
Whether the mounts could be causing it I don't have enough experience to say that. If they're bad then replace them and see if it makes a difference.
Yours appears to shake about as bad as my new big block and I'm definitely not happy with that one.
Is the car shaking? Mine is.
Last edited by Danish Shark; 06-23-2016 at 02:32 PM.
06-27-2016, 12:37 PM
#14
You stated the carburetor was rebuilt, who installed the carburetor and was it adjusted on the engine while running?