Engine makes high pitched noise when running

I

ironmike

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Joined
Dec 17, 2003
RO Number
12572
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13
Greetings,
I have a freshly rebuilt small block chevy (350) counter clockwise rotation motor, setup with crusader exhaust. I wanted to start the motor before installing back into the boat so I have it setup on a wooden platform in the garage. The motor has a closed cooling system (anti-freeeze) so I ran it for less than a minute without the raw water pump belt on. I spun the engine with a drill and the distributor tool before starting to make sure I had oil pressure. When the engine was running, I noticed it was making a loud, high pitched noise which seemed to be coming out of the carburetor. It sounded light some sort of vacuum leak. The engine has a Rochester quadrajet carb installed with the mechanical choke mechanism but does not have any vacuum ports or hoses. This is an 80's setup. Additionally, the frame arrestor is not installed. The motor cranks over without making the noise. I tried cranking the engine with the coil wire disconnected. I hear the noise while the engine is running. The carburetor is the same one that came of the engine before it was rebuild, which seemed to be working fine, and was setting in the garage will the motor was being rebuilt. The engine ran before the rebuild but the performance wasn't very good. In the off season, since the other motor was removed, I decided to take it to a machine shop for inspection. When it was disassembled, the machine shop informed me that the pistons were not installed backwards, which is required for counter rotating engines. A lesson for anyone buying counter rotating engines off the internet. Any idea what the origin of this noise could be? Thanks in advance for your help.
 
Sounds to me like a normal unsilenced air intake through the carb. I remember my old carbureted cars being kinda 'whistly' when the air cleaner housing was off. When you say that the engine was running before the rebuild, but not very well, it's likely that it had low vacuum. Now that it's is fixed, it's sucking more air through the carb throat. Have you tried it yet with the flame arrestor on?

BTW, the 'standard' rotation of a GM small block is counter-clockwise. Rotation direction (clockwise vs CCW) is determined by facing the back (flywheel) end of the motor. If you're facing the front of the motor and it's running clockwise, that again is a standard rotation, CCW engine.

My .02
 
Backing up John's rotation description:

http://www.arcomarine.com/xhtml/tech determine marine starter rotation.pdf

Counter-clockwise (Standard LH ) rotation and counter-rotation ( CW / LH ) are not the same engines.

You might try tightening the carb mounting bolts in case of a vacuum leak below the carb. And /or spray soapy water or wd-40 at the carb base.
If there is a 2nd belt on that engine, you could check whatever was turning (alternator, circ. pump?) for degraded bearings, or loose or glazed belt noise.
Sometimes failing bushing bearings on the dist. shaft will screech when engine is running but not at cranking speed.
Does it make the noise with the choke open?
Marine engines don't have vacuum advance but some have vacuum-influenced external choke mechanisms. It doesn't sound like your Q-jet uses that though.

In spite of oil pressure at the sender and alarm switch, an internal bearing my be starved if a passage is clogged with debris.

Vacuum leaks are generally more of a whistle type noise. Bearings tend to screech. Belts squeal. But different people interpret those differently. Screech is more expensive.
 
Thanks for the responses and clarification. I get them mixed up. Based on the definition supplied, the engine is RH since standing at the front of the engine (harmonic balancer) the engine is spinning counter clockwise. I do have the LH engine setting next to it. Sandy, you bring up a few interesting points. I want to say it's a whistle but it could be a screech. I think of a screech as more of a metal scratching sound. Only 1 belt is installed since Im not running any water through the raw water pump. I said the carb has a mechanical choke and I may have misspoken. The choke is the coil mechanism which bolts to the intake which I believes expands and contracts based on the intake temperature. Since the engine was cold, approximately 65 degrees outside, I think the choke was open. It doesn't sound like a belt squeal. The engine was professionally rebuilt so I'm hoping it's not a crank or rod bearing. I can remove the distributor and check it. Of course it was the same distributor on the motor before it was rebuilt, a prestolite pointless model, which appeared to be working fine. How do I check the distributor? I will also try running it with the flame arrestor as well as spraying soapy water around the carb base.
Thanks again.
 
Use a section of hose as a stethoscope to locate the sound source
 
What Bruce said. A long dowel or screwdriver can sometimes work the same way, or a backwards transmission funnel.

If it is the distributor, you could just put your finger on the housing and you would likely notice something was amiss. If the distributor was removed, the rotor shaft would have noticeable resistance to turning and rotation would not be smooth. I've replaced 2 or probably 3 semi-seizing marine distributors and the screech definitely got my attention each time.

Do you know if the rebuilder test ran the engine or did you just get the block back and add back the bolt-ons?

Hope it's just an inexpensive suction leak and you are able to track it down soon.

I used to replace those bimetal choke coils every couple/few years on my old carbed Crusaders as they seemed to get tired.
 
We had a case on an RV forum where the carburetor was loose on its gasket to the intake manifold.

Since this was just rebuilt and carb re-installed, could it be the mounting gasket is wrong? If it's got a leak under the carb, it's going to howl.
 
The engine wasn't tested by the rebuilder. I received a long block and bolted on the remaining parts so it could have been something I did. I have done this a couple of times so I thought I had it perfected, including timing the engine. At any rate, I will examine over the weekend and pot back to the forum the results. Thanks
 
There's a lot of vacuum at idle. Use a hose to locate a vacuum leak if there is one. Wouldn't get noise from carb throat. There's vacuum take-off on the front/base of some models of Q-jet that will whistle. Could be you don't have a marine carb. There's also a vacuum port between the carb and dist on the intake manifold. It either has a pipe plug or 3/16" hose fitting with a cup type plug.

Another possibility is a loose belt or frozen pulley on a bad ALT or water pump.
Those are the only possibilities I can think of short of a change of description.
 
quote:
Originally posted by j-d

We had a case on an RV forum where the carburetor was loose on its gasket to the intake manifold.

Since this was just rebuilt and carb re-installed, could it be the mounting gasket is wrong? If it's got a leak under the carb, it's going to howl.

Click to expand...


This.

I had the same noise when I switched carbs from a 4010 to a 4160. On mine, the nuts holding the carb to the intake weren't tight enough because I didn't have the right tool for the job handy. I needed a box end wrench and tried to save time by using a crescent wrench which didn't allow me to torque the nuts enough. This caused a vacuum leak at the carb base which caused the carb base gasket to make a high pitched squealing noise.

I'd check that for sure before tearing anything apart or spending any money.
 
Spray some carb cleaner around the base of the carb and the edges of the intake manifold, you will find out pretty quick if you have a vacuum leak.
 
Problem solved. I removed the carburetor and flipped the spacer plate and the gaskets that were installed between the carburetor and the intake. This plate levels the carburetor since the engine is angled back about 15 degrees. I seem to recall that the LH rotation engine had an indication on the plate which side should be touching the intake but this one didn't. The other thing I noticed on the LH rotation engine was that the gasket was sticking out a little bit. The gasket on the RH rotation engine fit the carburetor dimensions. Of course the LH rotation engine was not making this noise. I also believe I purchased this gasket from a different source. Maybe the LH rotation engine was marine? At any rate, this solved the problem. Many thanks to all who responded. The forum comes through again!
 
Makes sense. Congratulations on solving the issue and for the good followup post!
 
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