LOW budget engine not working well....HELP!

[jberenyi]

Just fired up a newly rebuilt 1974 350 Corvette engine and the idle is rough and slightly unstable.  Above 850+ RPM and driving the car its great.  But in drive at idle its a low rough idle and it wanders from 650 to 850 rpm.  The idle mixture screws are about 3 turns out.  There is no hesitation either its just an unstable rough idle.  BTW, the engine is bone stock.  What could the problem be?  Do I have to wait for the engine to break in due to a lower compression till the rings seat?

[77cruiser]

Have you tried adjusting the mixture screws?

[Kenth]

A 1974 350 Corvette engine should idle like a kitten.
What camshaft is used? Part-#?

[Mudsport96]

Please elaborate on newly rebuilt.

Cam part number
Heads
Compression
Piston part number if available
Head gasket
Etc.

Could be in the boat i am. Not quite enough compression to get a solid idle but just enough to run fine everywhere else.

[Cliff Ruggles]

I looked in my QuickBooks program and it looks like that job was done back in 2017.  No details on the engine build and I don't remember the job.  It's lost in the shuffle as I've done close to 15,000 of them since I started keeping records.  We'll just have to wait for details.......

[jberenyi]

The newly rebuilt engine is a matching number 350 bone stock except for .030 over on bores.  Clifford built this carb from scratch because I did not have a carb for him to build.  This Vette uses  7044206.  When I got it back it ran just fine with the tired engine but now its a completely different animal.  I also had Cliff rebuild the original distributor and he re-curved it to be in at 36 deg at 2800 rpm which I have confirmed it is.  Initial timing is set to about 12 deg at 700 rpm.  Vacuum is about 15Hg at 1000 rpm and it will get better as she breaks in.  I confirmed there are zero vacuum leaks.  When I last spoke to you all I noticed that with 5 turns out on the mixture screws and cupping the airhorn with my hand the rpm would rise signifying a lean condition.  I turned the idle screws out to about 8 turns and and it got rid of the lean condition about 90%.  If she idles at 750 rpm and then cup with the hand it will go up to 800 rpm but that's it.  The carb was provided with 75 jets and a 44 primary which is concerning because the specs say it should a 46.  But me being at 4500 ft. above sea level maybe its okay. Anyhow, at 8 turns out she's running better but not perfect with respect to idle only in drive or park.  All other things like cruising, acceleration, etc. she's perfect.  I really think we have an IFR that needs attention on sizing.  Thought's anyone?

[Kenth]

In 1974 Corvette uses a std performance Qjet or a high performance Qjet.
The std 7044206/7 units uses 46B pri rods #75 jets .034" idle tubes .100" idle bypass air and CH secondary rods.
The HP 7044210/1units uses 43B pri rods .037" idle tubes .120" idle bypass air and DA secondary rods.
In any case i would open the idle needle holes to .095" and idle tubes .002" to start with.

[jberenyi]

In 1974 Corvette uses a std performance Qjet or a high performance Qjet.
The std 7044206/7 units uses 46B pri rods #75 jets .034" idle tubes .100" idle bypass air and CH secondary rods.
The HP 7044210/1units uses 43B pri rods .037" idle tubes .120" idle bypass air and DA secondary rods.
In any case i would open the idle needle holes to .095" and idle tubes .002" to start with.

Good info.  One thing to note, I know this carb's casting number is not the one I listed above.  Also, my baseplate is missing a port for the vent hose on my charcoal cannister!  After paying about $800 for this carb I'm disappointed its not working 100% on a spec build bone stock 1974 350. 

[Cliff Ruggles]

What are the engine specs?

You've mentioned "bone stock build", but no details.

What cam was used in the engine (specs). 

Overbore, dished pistons, flat tops, and what head gaskets?.....

[Mudsport96]

Sorry to the original poster/thread starter. I know you pm'd me, but i dont call numbers off of the internet man.  Post up your build info here and we can help. I know you said bone stock rebuild, but many parts are not to factory spec

[Mudsport96]

Also, being at 4500 ft you may need to throw a few more degrees of timing into it at idle.

[Cliff Ruggles]

99 if not 100 percent of the "stock" engines I see are not actually built to stock specs.

This is why I ask for details. The statement was made below which makes me think changes were made or the engine is not actually built and blueprinted back to the original "stock" specifications:

 "When I got it back it ran just fine with the tired engine but now its a completely different animal."

Right to start with it is difficult to obtain a real factory "stock" camshaft for the early 350 engines.  Those engines with the stock cam made close to 20" vacuum at idle speed without much timing in them.

Another problem is that no one will use the correct .020" steel shim head gaskets nor will they verify that the pistons used are only .015-.018" in the holes at TDC.

Instead they purchase "rebuilder" pistons that can end up .030" or ever further in the holes at TDC and then throw a thick "builder" head gasket on the engine.  So instead of having .035-.038" quench they end up with .060"-.070" or even more in some cases.

This LOWERS the static compression ratio WAY below where it was on the original engine and idle vacuum and quality will suffer.  So basically the owner thinks he's duplicating the stock engine and compression ratio is 8 to 1 or so when it can actually be much lower.  Not uncommon to see a SBC build with 882 heads (stock in 1974 on those engines and complete pieces of bovine excrement....I would NEVER use them on one of my engines even if I was hired to build it back to "stock" specifications......FWIW), thick head gaskets, pistons WAY below the deck at TDC, and an "RV" cam substituted for the stock one.  With the compression ratio now in the 7 to 1 range and camshaft being used having 10 degrees more duration than a stock one the engine will not make nearly as much vacuum at idle speed as it should be.  This may require MORE fuel from the carburetors idle system than a stock carburetor can deliver, or you may find yourself backing out the idle mixture screws till they are nearly falling out of the carburetor to get decent idle quality.

I have no idea if that's what's going on here as we aren't getting any details. 

I'll ask once more, provide the "details" of your new engine build and we'll get to the bottom of the issues and see if we can help get a good end result here.......Cliff

[jberenyi]

Regarding the engine specs here are the details.  When I bought the car (which I sold 2 years ago to a friend) it was somewhat rebuilt at .030 over.  The heads were original and untouched and so was everything else other than pistons and rings of course.  This was all I could find out about the engine when I bought it.  Fast forward, one month ago my friend let the oil get low and spun a bearing so we had the block cylinders honed and replaced the rings, crank, crank bearings and one connecting rod.  While at it we ground the valves and put new seals in which were original still and in sad shape.  The cam is the same one when I bought it.  No other changes were made to the engine and this is all I know.  Other than these few changes I don't understand why the carb has a rough idle in park and drive when before it was fine.  The only thing I can think of is maybe wait for the rings to seal better and break in the engine more.  She only has about 15 miles on her.  The lean condition this engine has makes no sense to me considering the changes noted above.

[Cliff Ruggles]

The correct .020" steel shim head gaskets would NOT have been in your gasket set. 

So most likely you installed a "rebuilder" set of head gaskets instead.  This would LOWER the compression ratio close to half a point.

Lowering compression would make for a lower vacuum reading at idle speed. 

You may have to advance the timing at idle to compensate.  Even so I doubt is lowering the compression with the thick head gaskets would make for  "rough" idle quality.

Another possibility is that there is a vacuum leak someplace.......

[old cars]

"Do I have to wait for the engine to break in due to a lower compression till the rings seat?"
No. Should be no difference

"one month ago my friend let the oil get low and spun a bearing "
Why would the engine oil go low on a good engine?

"While at it we ground the valves and put new seals in"
When you say we I hope you mean a Professional

"The cam is the same one when I bought it."
A used cam?
 
"I don't understand why the carb has a rough idle"
Have you ever thought it might not be the carb. Why was the carb fine before the motor was taken apart 
"Vacuum is about 15Hg at 1000 rpm"
Maybe you should be checking your engine repairs / valve job.