SBC 350 Craptastic IDLE

Symptom I’m having is a very Rough very shaky idle, the engine shakes so hard at idle it makes the whole car shake. Runs ok off idle but has stalled while sitting at red lights. Also the exhaust has a very heavy gas smell.
The Carburetor air fuel adjustments don’t seem to make any impact on how the car idles and I’ve checked for intake leaks and didn’t spot any issues.
I’ve tried multiple timing settings with little impact to the rough idle. All of the ignition components have been replaced.
My vacuum pressure is very low about 14pds at 800rmp, it was around 10 but I just readjusted my valves.
This is what I find strange, if I open up the vacuum port on the intake the idle picks right up and smoothes out, it seems to want more air coming into the engine, this makes me think I don’t have an intake leak even though I have symptoms of a leaky intake.
I can’t say much about the engine I have except that it has about 5,000 miles and it is a Mexican block and heads crate engine, the CAM was listed as an RV CAM (I will have to hunt to find the CAM specs). Transmission is a used 350 and the intake is an Offenhauser tri power manifold currently only running the center with the outers capped till I get this squared up. The distributor is a Mallory mechanical daulpoint with a Protronix kit.

 

what do the spark plugs tell you?

is that the highest Vac. reading you can achieve with timing and idle mixture adjustments?

 

the carb could be leaking, running rich regardless of your adjustment. check your plugs. I had an AFB fail last week, just started gushing fuel out the p*** side venturi.

 

Check for dirt in carb, secondary throttle blades leaking, weak spark, fouled plugs and/or wiped cam lobes. Conduct a compression test while the plugs are out. Introduce yourself in the fng thread.

 

What kind of Carb. You have discribed a rich at idle problem. By opening up the vacuume port and letting in more air it just proves it is tooooo rich. So tell us what type of carb it has ???

 

Find the vacuum leak or source of weak spark or mis-timed spark.

Just like 90% of statistics are ********, 90% of running rich problems are not the carb.

good luck

 

Take the air cleaner off and look down the throat of the carb while its idling. Do you see fuel dripping or running off of the venturis? This could be caused by several things,
Too high of fuel pressure, gunk stuck in the needle and seat, too high of float level a**** other things. Pull a plug and look at the color, is it black?

Sure sounds like its running pig rich and the fact you can screw the air/fuel screws all the way in just confirms that.

 

Never mind the suggestion about the secondary throttle blades leaking, you said yours are capped off. From the symptoms you decribed, probably time for a carb rebuild.

 

What he said......maybe a cracked float?

 

Thanks for all the quick replies. The plugs are brand new and the carb is a Rochester 2G. I put in the Speedway tri-power base setup that rebuild kit for the center and I will be using their bases for the outers once I get it running correctly on just the center. Compression did check out I can't remember the exact amount but it was pretty standard for a crate and all cylinders where within 8% of the same range.


Introduction l have two rides, a 58 oldsmobile supper 88 with the 371 running tri-power that tri power was just a pain in the *** to get going. I said I would never do tri power again but I damn if it don't look cool and I don't like the idea of not having it hooked up. This is a 31 Ford 5 window coup that I'm trying to get on the road before summer ends here in the Northwest. I've always messed with cars but all I learned was from my Stepdad, friends trial and error and reading.

 

As was said. look to see if there is any fuel dripping out of any where in the top of the carb

 

just because the plugs are new..doesnt mean they arent black as coal now..take a few out and look at em

every spark plug tells a story..there is a lot to be learned about an engines health by what the plugs look like

also as said above ,,..is the carb pukeing fuel ?
if so maybe the float is stuck, or has a hole in it keeping it from floating, thus pouring raw fuel down the throat

oh and because its a tri-power set up..any one of the 3 carbs could be causing this or have a stuck or filled float..ask me how i know..i just went thru this on my tri-power set up..take out the floats and shake em...if they slosh..its time for new ones..they cost about 8 bucks each

 

***

 

I'll check my plugs and look at the float when I get home and report back.

Thanks
JR

 

electric fuel pump?
if so, maybe the fuel pump pressure is overtaking the needle and seat

 

I pushing 11 pds out of my mechnical pump, fixed that and replaced the intake manifold gasket. I running smooth now. Thanks for all the help.