350 sputtering under load

[sbx22]

Engine is a mild street Chevy 350 with MSD 6AL2 box with pro billet distributor, 670 cfm Holley Street Avenger. 2 problems here.

Initial problem was the infamous hot start issue that gradually got worse. When cold, started up perfectly. When warm, it would take 3 turns/tries of the key, black smoke out of the exhaust with some struggle, then will idle out smooth. I had time this past weekend to order and receive a PowerMaster mini starter thinking it was a heat soak issue. I bought a starter insulating blanket and a heat shield. Wrapped the fuel lines adjacent to my long tube header in reflective insulating tape just to eliminate heat soak. My headers do have header wrap all around as well. I also used this tape to wrap the positive cable that runs from battery to starter as it runs close by the header as well. Have a 1" insulator under the carb already. Anyway, this didn't completely cure the hot start issue, still have some trouble although I would say it helped just a tad bit.

During the initial test drive, it developed a sputtering under load. It still shifted through all three gears (TH350) when cruising, but if you put mild to moderate pressure on the accelerator, it will sputter and act like it's starving for gas. This has not happened before. I wonder if it's coincidence or not, but I did take the header off for access around the starter, had to disconnect the spark plug wires. I also took out #2 and #4 spark plugs just to inspect them (they were dark and had mild wetness to them). These are some things I've looked at so far and some hints i've found:
    1. Total mechanical advance 35 deg BTDC with vacuum secondary disconnected.
    2. Gas squirts out of primaries normally.
    3. Electric choke is working like it should.
    4. There is black soot in moderate amounts I would say inside header all the way back to exhaust tips. Now that I look at it, the tips are starting to blacken.
    5. Primary Fuel bowl doesn't just trickle, it pours out heavily.
    6. I did a quick check, and I am getting spark from plug wire #2.
    7. Again, like said above, inspected plugs #2 and #4 and they both were black and with mild wetness to them.
    8. Vacuum is around 15 to 18 PSI.
    9. Fuel pressure is 8 PSI.
   10. Checked just in case, but plug wires are in correct firing order.

In summary, these are two different issues, but I wonder if they are related. Ideally, I know I need an A/F ratio meter to fine tune the mixture, but I feel like I'm running rich. My next step would be to size down the jets. Any thoughts?

[sbx22]

Forgot, these are what the plugs look like.

[78BIG-TEN]

Check the float levels on the carb,if ok it may have a blown power valve

[MannyDantyla]

Is the vacuum advanced line plugged into the manifold vacuum port or the timed vacuum port of the carburetor?

Most agreee it should be plugged into manifold vacuum port. read this: http://chevellestuff.net/tech/articles/vacuum/port_or_manifold.htm

[zieg85]

Plugs looking like that would cause it to stumble under load.  Clean em or change them and go from there.  Definitely running rich...

[sbx22]

I also switched from ported to full manifold vacuum this past weekend as well. I thought that this was what made it run worse. Last known to be running well was when it was running ported vacuum so currently that's what it's at right now.

Zieg, thanks for the input. That will be the next step. Gah, I'm not good at trouble shooting, would hate to do shotgun approach and buy new carb, coil, plugs and wires, etc.

[rich weyand]

If it runs better on ported than on manifold vacuum, then the idle circuit is rich as heck.  Switch back to manifold vacuum and lean out the idle circuit.

[rich weyand]

Let me explain that.  A rich mixture burns faster than a correct mixture.  Ported vacuum is late compared to manifold vacuum, which is set up to get the spark there at the right time for the chemistry to work.  Ported vacuum was something obnoxious they introduced for pollution control in 1968.  If it runs better on ported vacuum, that means the mixture is burning way faster than the correct mixture, and the only way to do that is for the mixture to be way rich.  So switch back to manifold vacuum.  Yes, it will run worse, because now you can see the full impact of the way-too-rich mixture, which ported vacuum was hiding.  Now lean out the idle mixture for best idle.

Also, which plugs are those?  That engine should want ACD R45TS, or Autolite 24.

[sbx22]

Thank you Rich for the reply. That makes some sense to me. When you say idle mixture, that is the screw on the side of the carb right? I had originally set the idle mixture screw to read the highest vacuum pressure reading. And there are 2 of these screws on my carb, on both sides of the primary side. I assume I would do the same after going from ported to full manifold vacuum? Also FYI, the primary fuel jets are 65 and secondary jets are 68. Both sides are turned 2 1/4 turns FYI. Plugs are AC Delco R44T. Does this all sound right?

[rich weyand]

Yeah, see, you tuned the idle mixture to match the ported vacuum, which made it rich as a dog because the late spark is going to want a richer (faster burning) mixture.  So when you switch to manifold vacuum, it runs worse.  Switch it to manifold vacuum, and lean it out for best vacuum, and it will run much better, give you better mileage, and not carbon up at idle.

Yup, it's the two idle mixture screws, one for each side of the carb.  Adjust them together (same number of turns out) to best vacuum.  You are going to be screwing them in from where they're at.  (I think.  Idle mixture screws are usually fuel metering, not air.  Not sure about the Holleys.)

R44T is probably one notch too cold for a mild 350.  Too cold plugs will (ta-da) foul.  I think R45T would be the correct plug, which is one notch hotter.  Go one hotter on the plugs and lean out that idle mixture on manifold vacuum, then let's see what we've got.  There may be tuning on the other carb circuits, but I don't know the Holleys, so hopefully someone else here can help you on that.

But try those things first and see see where you're at.

[78BIG-TEN]

This engine is way to rich.Pull plugs off the side of the float bowls and verify the float level is correct.Fuel should not run out of hole,if it does fuel level is to high.Adjust accordingly.Look down throttle bore to see if fuel is dripping from the boosters at idle.You need to get the mixture under control.Check your oil for fuel contamination.If it has fuel in it drain it out.If your running a flat tappet cam it will wipe the lobes in a hurry(ask me how I know).If the float levels are ok and fuel is dripping from primary boosters only then more than likely your idle set screw is adjusted past the idle transfer slots into the main metering circuit.This will cause the primary boosters to drip cause there is not enough air flow to atomize the fuel entering the engine

[sbx22]

*fuming* 78Big-Ten probably nailed it. I'm suspecting worn cam lobe. There is fuel in the oil! Now I think of it, there was a weird sound coming from passenger side head that was never there before. Not exactly ticking though. Spent so much on this truck (my first American muscle) to have it fail. I've only had the rebuilt engine for 2 years or so and I only drive it on the weekends, so I hadn't had much time with it. Sorry, just fuming. I do consider myself a newbie to V8 muscle and carbed applications. I don't want to blame Holley but they say it is ready to run out of the box for MOST applications so I never thought of fine tuning it. Such an expensive learning process.

My next step is to confirm if the rockers are moving. I've drained the oil/fuel mixture out and will be pouring in new oil, start it up, and look at the rockers. My last oil change, I didn't notice fuel smell in the oil however. Working 2 jobs, 2 kids, with my single household income, this might take me a year to get the truck driveable again.

[rich weyand]

Well, that sucks.

[78BIG-TEN]

Sure hope you caught it in time,I know it sucks ,I just went through the same thing with 0 miles on my truck.I can't tell you how many carbs I've put on new out of the box that didn't need something done to them.I just put a quick fuel carb on a Pontiac that floats were way off

[sbx22]

Quick follow-up. I can't call it a victory yet but I'm 98% sure this is the problem. See pics. One picture shows a primary fuel jet blocked off by a thin layer of caramel looking substance. I took off the fuel filter, which I'm guess has only been used 200-300 miles city driving. This caramel looking substance was also found in the fuel filter. When I shake it, it rattles loose. I cut it open and you can see for yourself. I believe this caramel looking substances came from the cardboard ends of the filter element. It just came apart, deteriorated.

What I learned from this:
1. I'm never going to buy these cheapo filters again.

New Wix filter put on, tune for air/fuel mixture screw for max vacuum then will test drive tomorrow.

Also, the hot start issue described above was due to fouled plugs. Ran the R44T and am now running R45T, mostly as I drive city and want the hotter temp for short drives. These were fouled as the float setting was set too high I believe from manufacturer. This has cleared up thus far, starts each and everytime beautifullly now even when hot.

Hope this helps others out there.