Carbed small block chevy problem. What am I missing

79 corvette with a stock (as far as I know) 350 with a quadrajet. Owner complained it was low on power and asked me to rebuild the carb. The first thing I noticed when it arrived was oil in the air cleaner housing, which would have to be coming up through the pcv vent. I hooked up a vacuum gauge to the pcv hose, it only had 12-14" of vacuum at idle and idles rough. I also checked at other vacuum lines and confirmed the weak vacuum. I pinched all the lines off, no change. I sprayed carb cleaner at the intake and carb gaskets, no change. I put my thumb over the pcv hole and couldn't feel any vacuum there.  I tried advancing the timing to increase vacuum, no change. I went ahead and rebuilt the carb as requested, (I'm not a carb expert by any means, but I did this because it's for a relative.) Same weak vacuum, same slightly crappy idle. It smokes out the tailpipe. I dont know if it was before or not. When I rev it it blows smoke out of the crankcase vent, and the smoke increases out the exhaust. It underpowered going down the road, but not horribly so. I'm not sure what's causing these symptoms. Is the carb not right? Is there a mystery vacuum leak that somehow alluded me? Bad rings? That would fit with the smoke, could it cause low vacuum? Maybe timing chain jumped a tooth? I feel there's something obvious that I'm missing but I don't know what.

I'd say the cam has jumped a tooth OR one of the intake gaskets has failed on the underside of the manifold and it's sucking up oil from the valley.

>edit< Or it's worn right out.

Trans_Maro said:

>edit< Or it's worn right out.

One of my secret car guy fears is not knowing when to recognize it's just this.

Compression/leakdown test?

In reply to gearheadmb :

You state it’s underpowered - it only had 195hp from the factory, so don’t let that influence you too much. Being a Vette I’m guessing it’s not driven much? I wonder if it might have stuck rings & some Seafoam plus an Italian tuneup might help clear it up? Though honestly my gut says it’s just a worn out old SBC. 

Leak-down test will show if it needs an Italian tune up. 

It always surprises me how people treat their engines in an effort to preserve them.  I know of two people that killed their engines from carbon buildup, one was a rotary the other a Chevy 350.  Both were told to stop idling them in the cold every two weeks, but instead run them hot and hard before storage then leave them alone.

I was going to say start with a compression test and a leak down test. I may have also wiped a cam. 

Did the problems start suddenly?  Recently?  Or has it been slowly getting worse over a period of time?

Check TIMING PLUGS COMPRESSION/LEAKDOWN  A LEAKDOWN IS NICE BUT NOT NECESSARY ,MOST LIKELY TO FEEL PRESSURE WHERE PVC PLUGS AT VALVE COVER IN NOT VACUUM. SMOKEY MEANS RINGS SMOKE OUT THE EXHAUST MEANS VALVE SEAL. BE HAPPY, BUY A NEW ENGINE ,  MUCH BETTER TODAY THAN DOING IT YOURSELF.

PS how far out are the idle screws

jfryjfry said:

Did the problems start suddenly?  Recently?  Or has it been slowly getting worse over a period of time?

I don't know, and the owner bought it a few years ago and he barely drives it, it doesn't seem like he knows either.

In reply to GTXVette :

The idle screws were both at 5.5 turns out when it came in. I set them at 1.5 when I did the rebuild. It wasn't at all happy there so I spun them back to the 5.5 it got better. I tried tweaking them from there using a vacuum gauge and saw no improvement, so they are still somewhere near that 5.5.

had a similar issue with an old impala.  The pcv line was plugged into a vacuum port on the manifold.  When i plugged the port and installed a breather there was a world of improvement.

(you said you pinched all the lines already, but i wasnt sure if that included the pcv)

I spoke to my uncle  (the owner) last night. He is ok with taking it to a local shop at this point. The shop owner is an old timer that is good with carbed stuff and I know first hand that he is trustworthy. Besides an internal intake gasket leak just about any other possible culprit is going to mean engine coming out of the car, and if that happens it will be that it may as well either do a full rebuild or replace it. I don't really want to do that since I don't want to A) have my garage tied up for that long B) be responsible for it C) need to have it done on the current timeline. His son wants to drive it in his wedding in a month. 

I hate to do it but I think going to hand it off to the shop.

My first thought was timing. Easy to check.  Then tune the carb.  Plug the brake booster vacuum hose for a sec and see if anything changes due to a leaky booster.  Do the same with the trans modulator. 

All the smoke/blow-by does lend ones thoughts to the engine being tired, but you haven't stated the mileage, or if you replaced the PCV. I would check timing again with all vacuum ports plugged and with Vacuum advance disconnected (and plugged) at idle and 3500rpm, note your readings at 3500 should be at/above 30 deg and verify the mechanical advance weights are free moving. There are a few different vacuum pots on the distributors, 14, 16, 18 and 20 degree advance, but yours might be leaking as this is not uncommon. See if the vac advance can holds vacuum while you're at it...

Qjets are decent carbs, but the aluminum base plate wears out at the primary throttle shafts quite readily - spray around there with the carb cleaner while its running and the idle should go up - or just physically move it and you'll see how bad it is. Its really hard to tune a carb with .040" play on both sides of the primary shaft, and you may never get a decent idle because of that alone... That being said, carb cleaner used to have really good chemicals in it like 1,1,1 trichlorethylene and that would make the engine react, I have no idea what passes for carb cleaner today and it may not be as useful a tuning aid as it used to be.

Also, due to the complex nature of Smog Qjets, there is a chance a part is missing/stuck/not replaced/installed incorrectly, its quite an easy oversight. They can also have 2 or 3 internally and externally ported vacuum dash-pots on them and yours could have all the diaphragms cracked and leaking internally or the internal ports are plugged with crap. Does this one have the idle screws in the baseplate in front of each primary throttle shaft blocked or plugged? GM did that to stop mixture adjustments and limit the service life due to the new smog rules and making one of those carbs 'work' can be a challenge.

If possible,  i'd also try to put a known good carb on there to see if its better...even a holley with an adapter plate just to see how the motor runs with a good carb.

Best of luck!

matt

IT'S TRYING TO RUN ON THE IDLE JETS NOT THE LOW SPEED CIRCUTS, THE PRIMARY BLADE MUST BE IN THE MIDDLE OF THE SLOT IT MAY BE CLOSED TOO MUCH

brass float in carb?   if so, i'd be checking that float to make sure it still floats.   a pinhole will allow fuel to get inside the float, and it won't rise high enough to close the needle and seat, leading to fuel trickling out the vent and into the venturi, leading to a rich condition and stumbly operation.  a quick check is to bend the tab up so the float level is lower than it should be, and see if it runs better.

I think you have more going on than a tune up.

If it wasn't smoking, I would say ignition module or coil.  Leakdown test isn't necessary to confirm my guess... cracked ring, head gasket, or hole in a piston.

Also quite possible that TransMaro is right: Manifold gasket sucking oil from the bottom.

GTXVette said:

IT'S TRYING TO RUN ON THE IDLE JETS NOT THE LOW SPEED CIRCUTS, THE PRIMARY BLADE MUST BE IN THE MIDDLE OF THE SLOT IT MAY BE CLOSED TOO MUCH

he's talking about the idle *mixture* screws, not the idle *speed* screw.  your statement re throttle blade position is correct, but in this case i think it may not be applicable.

In reply to AngryCorvair :

The float is black plasticy stuff, not brass, and seems to operate properly.

In reply to Kramer :

I squeezed all the vacuum lines with no improvement. I moved timing a bit with the vacuum gauge hooked up, no improvement. 

In reply to Matt :

I did replace the pcv valve and the hose going to it because it was rock hard. As far as the possible carb issues, that is were I'm going to duck out and let the local Old Pro take a crack at it. I'm really not very carb literate.

Curtis said:

I think you have more going on than a tune up.

If it wasn't smoking, I would say ignition module or coil.  Leakdown test isn't necessary to confirm my guess... cracked ring, head gasket, or hole in a piston.

Also quite possible that TransMaro is right: Manifold gasket sucking oil from the bottom.

The internal intake leak fits the symptoms, but i hate to "throw one at it" since I'm not feeling vacuum at any at the valve cover holes. The more I think about the stream of smoke out pcv vent keeps leading my mind to a more insidious issue.

I think you did the right thing handing this one off given the considerations you listed. Unless you were looking for a labor-intensive learning experience, i dont think this thing was going to go in your favor in the end. You generously did everything you were in a position to comfortably do for free. 

AngryCorvair said:

GTXVette said:

IT'S TRYING TO RUN ON THE IDLE JETS NOT THE LOW SPEED CIRCUTS, THE PRIMARY BLADE MUST BE IN THE MIDDLE OF THE SLOT IT MAY BE CLOSED TOO MUCH

he's talking about the idle *mixture* screws, not the idle *speed* screw.  your statement re throttle blade position is correct, but in this case i think it may not be applicable.

yes I Misworded. the air bleed screws being out 5.5 turns tells me its not getting fuel at the slot

In reply to GTXVette :

It's a Quadrajet, some of them (smog ones) need about 4 turns out on the screws to idle properly