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Old 11-18-2010, 08:15 AM
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0 Cranking Vacuum

New GM 350 crate motor. New Edelbrock manifold and carb, MSD 8362 ignition. Compression is 150 in 7 holes and 140 in #7. All vacuum connections on the carb (second new carb I have tried) are plugged. Starter turns engine well. Absolutely no vacuum while cranking (vacuum gauge is on port under the throttle blades). Spraying carb cleaner around the intake manifold gasket areas has no effect. Engine will try to start if fuel is introduced into the carb and then immediately die.

Any thoughts are appreciated.

John

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Old 11-18-2010, 08:25 AM
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if it is making compression then it has to be sucking air.

Engines don't pull much vacuum on the starter.

does the carb squirt any gas when you open the throttle? Carb should be full by now, right?

have someone crank the engine so you can set the timing. You could be way off.

Is this a new problem? has the engine run before then just started doing this or is this a new engine that you are trying to get started for the first time?
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Old 11-18-2010, 08:37 AM
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Thank you for the quick reply. Carb does squirt gas. There is a steady 5 psi at the carb. Timing was set while cranking @ 10 degrees BTDC. This is a new motor that has never been started.

I agree that you probably won't see much vacuum while cranking but I would think you would need some just to get the carb to function.

John
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Old 11-18-2010, 09:26 AM
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So, you need to pour gas down the carb to get it to fire? and the carb squirt isn't enough to fire it by itself?

are you sure the timing is right? did the engine come with the timing chain cover? You could have a mismatch in balancer and timing chain cover timing marks.

Try advancing the timing until the starter has a hard time turning the engine then back off a bit.
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Old 11-18-2010, 09:27 AM
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John, here is an article I wrote for the wiki. Maybe it will help.
http://www.crankshaftcoalition.com/w...valves_SBC_BBC
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Old 11-18-2010, 09:33 AM
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No vacuum at all? Plug off the pcv holes in the valve covers and see if there is any vacuum there. A thought that comes to mind is the wrong intake or gaskets which won't be visible outside. It's a stretch, but is the exhaust open? No bananas in the tail pipe?
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Old 11-18-2010, 09:49 AM
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hooking gauge up to the vacuum port that doesn't see vacuum til throttle cracks open? (ported vacuum?) Try again while just cracking the throttle.
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Old 11-18-2010, 10:24 AM
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Quote:
Originally Posted by sampanjohn
New GM 350 crate motor. New Edelbrock manifold and carb, MSD 8362 ignition. Compression is 150 in 7 holes and 140 in #7. All vacuum connections on the carb (second new carb I have tried) are plugged. Starter turns engine well. Absolutely no vacuum while cranking (vacuum gauge is on port under the throttle blades). Spraying carb cleaner around the intake manifold gasket areas has no effect. Engine will try to start if fuel is introduced into the carb and then immediately die.

Any thoughts are appreciated.

John
Use the DRIVER SIDE vacuum port- it is manifold vacuum, or use a port on the intake manifold, or the large center carb port. Any of those will be manifold vacuum.

It's likely a case of improper ignition timing. DETERMINE TDC then MAKE A TIMING TAPE.

Once you know the TDC marks are correct on the engine, set the timing advanced about 10-12 degrees and try it again.

The temporary timing tape will allow you to see what is going on w/the total timing, w/o needing a dial back timing light.

I would use (sparingly- per the directions) spray starting fluid w/top end lube added to it to "prime" the engine with. Using raw gasoline is dangerous and can wash the cylinders of the oil needed to keep the ring and cylinder wear from being excessive.
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Old 11-18-2010, 11:20 AM
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Replies to questions - Engine came with timing chain cover. Timing mark verified with piston stop.

Tried to find vacuum at both the ported and full vacuum ports with throttle both closed and cracked. Still no vacuum.

Engine will briefy run when starting fluid is used then shuts off when fuild is stopped.

Mystery seems to be how I can have specified compression (150) and still have no cranking vacuum.

By no cranking vacuum I mean that I can move the vacuum gauge more by sucking on it than the engine does while cranking. It seems the problem must be more than just a leaking intake gasket to give no vacuum at all.

Thanks

John
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Old 11-18-2010, 07:02 PM
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You didn't say what kind of intake your using(single or dual plane) or what size cam your using. If its a large cam, there is a lot of valve overlap that can greatly reduce any vacuum signal at the carb. If its a single plane intake it will make the signal much weaker still. One cylinders exhaust valve will be partially open to the intake plenum when another's intake valve is partially open.
I can't say that I've ever noticed the vacuum reading on a gauge at cranking rpm, but I'd bet that its nearly nil. Just because its not measuring on a gauge doesn't mean that its not enough to vacuum draw fuel vapors into a cylinder.
I'd think you have carb/fuel delivery problems. Just my thoughts,
ssmonty
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Old 11-18-2010, 07:19 PM
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Thanks for your thoughts

Cam is very mild. 383/401 with 112 degree seperation that comes standard in 260 hp GM crate engine. Intake is Edlebrock Performer. Fuel pressure is steady 5 psi at the carb and I have tried it with 2 different new Edelbrock carbs.

I would expect the cranking vacuum to be low but can the carb draw fuel if there is no vacuum ?

John
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Old 11-18-2010, 08:01 PM
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"Engine will briefly run when starting fluid is used then shuts off when fluid is stopped."


I'm not saying that there is enough vacuum to draw fuel from the carb during cranking, but enough to draw vapors into the cylinder from the plenum as evident from your results from the starting fluid. Do you get a shot from the accelerator pump when you twist the throttle by hand? I usually have to give mine at least one good pump shot when its cold to get it to start.
I actually have a vacuum gauge mounted in the dash on my ride, but I don't recall what the reading is at cranking, which makes me think its nadda or I would remember. Its cold and dark and I'm not dressed for it or I would go out and try. I'll try to remember to try tomorrow when I get home from work, but no guarantee as I usually bring home a hot meal for supper on Fridays and like to eat it while its hot, but I'm still betting that its nil on the gauge.
ssmonty
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Old 11-19-2010, 08:02 AM
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try opening up the idle screw.

or pumping the gas while you are stating it.

sounds like a fuel issue, since it runs on starter fluid.
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Old 11-19-2010, 08:16 AM
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[QUOTE=ssmontyI actually have a vacuum gauge mounted in the dash on my ride, but I don't recall what the reading is at cranking, which makes me think its nadda or I would remember. [/QUOTE]


Same here, I can say I have never seen any vac when cranking. I know this for sure because I spin the motor to get oil pressure before I light it up, the only gages reading are the volt and oil pressure . I have two hotrods configured this way and the action is the same. One has a very radical cam and the other a performer, same gage activity, 0 vac spinning the motor.
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