Lean condition at idle only

[jberenyi]

On a Rochester 4MV, what other things can you do to richen the idle circuit other than drill out the idle tube restrictors and downflow restrictors?  I verified the diameters of both and the idle tube is .040 and the downflow is .055.  I can cup the air horn at rough idle and it will smooth out nicely so it needs more fuel at idle but I'm afraid to open the listed restrictors based on what I have read other people using.  Seems like mine maybe maxed out.  Mixture screws are at 8 turns.  What I also fear is that even if I get it to smooth out with more fuel in the idle circuit I won't be able to get the idle lowered to an acceptable level like 700-750 rpm.  The latter statement I have yet to prove though...just a fear. 

[Cliff Ruggles]

What is the carburetor part number?

What is the application, engine, type, CID, compression ratio, camshaft, etc?

What has been done to the carburetor up to this point?

What distributor, what has been done to it, initial timing, etc?....

[jberenyi]

Application: Stock street use

Engine: 350 chevy

Type: Stock other than bored .030 years ago

CID: 350

Compression: 150 psi all cylinders with 8-10 cranks

Compression ratio: 9:1

Camshaft: L48 stock camshaft Intake: 195 Duration @ .050" , Lift .390" Exhaust: 202 Duration @ .050" , Lift .410" Lobe Separation 112

Carburetor: 7044213 built correctly by you. Only thing done is verify size of idle tube restrictors and downflow restrictors which are .040 and .055 respectively.  Jets are 74, prim metering rods are 44. CH sec rods. I haven't checked the diameter of the idle bypass air ports yet.

Distributor: rebuilt by you.

Timing: 36-degree advance just before 3000 rpm

[Cliff Ruggles]

What is the initial timing set at?

How much vacuum is it making at 700-750rpms?

[jberenyi]

I believe initial timing is 12 and the vacuum at 750 is about 10-12.  But when I cup the airhorn it rises up to 15-16.

[Cliff Ruggles]

Still a tad "low" for that camshaft if you truly have 9 to 1 compression in the engine.

In a stock 350/300hp engine with just a tad over 10 to 1 true compression and only 4 degrees initial timing you'll see at least 20" vacuum around 650-700rpms.

In a 9 to 1 350 with 12 degrees initial I'd expect at least 16" if not 18-19".

The carb has PLENTY of idle fuel for the engine specifications listed.  The 7044213 does have a pretty decent size upper IAB but it is offset by the .040" idle tubes and DCR's just over .050".

Do you see any improvement backing out the idle mixture screws a bit more?

If not might want to check the size of the holes under the mixture screws and open them up just a tad, to .86-.090".  Some of those later emission years carburetors had tiny holes to limit how much fuel you could add at idle speed even when they didn't put limiters or covers over them........

[jberenyi]

Thanks Cliff.  I'll check how big those mixture screw exit holes are when I get home after work.  BTW, do you typically open those up when you build someone a carb or reserve that option to be done later if needed?

[Cliff Ruggles]

I typically leave them at the stock size for stock or very close to stock applications.  If I get the carb on the test engine and it acts like it's limited for idle fuel deliver I'll take a look there before taking the top back off.

For what you are doing unlikely you'll need larger idle tubes or DCR's, at least if the cam is actually the correct part and intake isn't closing too late from a stretched out timing set, etc....

[jberenyi]

I opened up the mixture screw exit holes to .089 and it runs much better at idle now.  If I cup my hand over the airhorn it barely raises the rpm with mixture screws out about 6 turns.  I was thinking of opening it up a little more but my friend was afraid to let me LOL.

[jberenyi]

I might go to .093 allowing a little more flow and less turns on the mixture screws this weekend.  Thoughts Cliff?

[Cliff Ruggles]

I would go no larger than .096".  Usually .089" is enough with the early carburetors with the 10-32 pitch mixture screws.  Some mixture screws have short/blunt tips, others long with a lot of taper so plan accordingly.....