Thread: 400 SBC problem - ignition or carburetion?

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Symptoms - rough running, feels as if cylinders are missing, poor power.

History - my wife and I built the car (1929 HiBoy roadster) in the early 1992. I replaced all battery cables February 2023 because the engine wouldn't crank. That problem was solved, but the engine wouldn't start. I tested all ignition wires for voltage drop, and found none. I replaced the plugs (see below) and the engine runs, not great, though.

As I normally do in preparation for each driving season (I live in PA, so that's April through October), I ran Seafoam through the engine in April 2023, but that didn't have the usual salutary effect. Thinking that the carburetor may have been affected by ethanol (as I've found in the past), I used an Edelbrock kit to rebuilt the 1405, carefully following the factory instructions and settings.

Further information:
1. Initial timing set at 12 degrees BTDC. Vacuum advance distributor.
2. Spark plugs: Accel copper core 576S, gapped at .030.
3. Plugs in cylinders four and six are perfect - clean and light brown.
4. Plugs in all other cylinders are dark and wet.
5. Using a spark tester, I confirmed that all plugs are firing.
6. Other than an aluminum intake manifold, the engine is a stock mid-70s 400 SBC.

Any suggestions or observations? What have I missed?

Thanks in advance for your time and help. I will follow this thread and respond to comments ASAP.

Dorsey

The first thing I'd do is replace the plugs, but get something other than Accel, which seem to be mentioned a lot when guys are having problems. I like NGK's, other than my Jeep which runs Champions. You checked the plug wires in February so they should be good. If the plugs don't do it I'd get a new coil, and probably a new distributor cap, too.

Thanks, I'll take both of those suggestions. Our local Advance Auto Parts has NGK for this engine in stock, so I'll pick those up today and post the results.

NGK are not the least expensive of my options, but price is less important than performance and longevity. I also noticed that NGK recommends gapping at .045 rather than .030. I'm pretty sure that I had found the smaller gap in a tune-up chart for that engine (400 SBC). Pardon my ignorance, but is the gap manufacturer-specific? I thought it more a function of other factors such as timing, dwell and coil strength.

The engine runs smooth as silk again with new NGK UR4 plugs at the manufacturer's suggest .045 gap! I haven't taken the car out on the road yet to check the power, but because the other symptoms are gone I expect that will be gone, as well.

I'm always reluctant to just replace parts until a problem goes away, but at least one other person suggested replacing the plugs I did that.

Thank you very much.

Sorry for not responding, Dorsey! I was at the practice day for the KC Air Show, to see the F22, F35 and Blue Angels fly!! Glad the new plugs worked, and I think you'll be pleased with the NGK's!
Thanks for closing the loop and letting everyone know what worked!

Dorsey, I'm glad you got it figured out! Roger pics or it didn't happen. Side note, the local fueling company here at the Des Moines International Airport got the military contract for re fueling. Signature recently moved over by us. We get to see all kinds of cool planes. I don't think there are any air shows happening in Iowa any longer.

Originally Posted by 40FordDeluxe

Roger pics or it didn't happen. ...... I don't think there are any air shows happening in Iowa any longer.

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I didn't even take my camera this year, as I tend to watch the show through the lens and miss the big picture. The only picture I took was my neighbor in front of BA #7 for her to send to her kids. #7 is the Advance Plane that brings the PR gal, and as a 2 seater it does the media/local hero rides and also which acts as the spare.
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You missed the Quad Cities Air Show in Davenport June 24 & 25?? The Thunderbirds were there!

I did miss the quad cities air show. I did see a super hornet here in DSM on Saturday though.

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Back to the topic, although that's a great pic, the NGK UR4 plugs are sooty after about 30 miles of mixed driving. I had to locate these via "make, model and year" which I approximated as 1973 GMC Jimmy, then selecting the 400 SBC option.

Is it worthwhile trying a hotter plug, or should I look elsewhere for the cause of that plug reading?

Rather than a hotter plug I think I'd tighten the gap a bit, say to 0.035 and take it out for a long drive to see how they look before letting it sit at idle. You said in your initial post that you carefully returned the Edelbrock carb "to Factory settings". Did you do anything to check that the settings were right for your engine setup, setting idle mixture by the highest vacuum using a gauge, checking plug color immediately after a WOT run (starting with clean plugs and shutting off before going to idle - kick to neutral, shut it off and coast to a stop)? It sounds to me like you're running fuel rich in the mid-range, unknown at WOT and idle.
The Edelbrock 1405 Calibration Kit comes with 6 sets of Metering Rods, 5 sets of Main Jets and Metering Rod Springs. Not sure what the rebuild kit comes with, but thinking none of the above, just gaskets & O-rings.

Tightening the gap is an easy thing to check and test and the other road tests as soon as possible. Thanks for your quick response.

The rebuild kit contains gaskets, floats and needle valves and a few other assorted pieces, clearly not the same as a calibration kit.

Fresh data:

By "factory settings," I mean the float levels and accelerator pump height. No other changes to the internals.

I reset the initial timing to 8 degrees from 12 BTDC and also (don't laugh) moved the distributor vacuum line from the manifold to timed side. That's been wrong for over 20 years, but the engine ran fine, anyway. Go figure.

Manifold vacuum is 17-18.

Closing or opening the idle jets has little effect - manifold vacuum rises slightly, but engine speed barely changes, even with the jets completely closed! I sprayed carb and choke cleaner around the carb base for leak detection and found none; the idle speed didn't change.

As I had reported above, the engine ran great after installing the new NGK plugs, but quickly (less than 30 miles) returned to running poorly; it sounds and feels as if one or more cylinders are not firing. After gapping down to .035 and road testing as suggested, plugs 1, 4 and 6 are clean and light brown. Plugs 2, 3, 7 and 8 are sooty, 5 is actually wet!

Before I take the carburetor off yet again, does this appear to be a carburetor or ignition problem? The distributor is tucked into the firewall and very difficult to access, but not impossible. I can't remove it without pulling the engine, but I can probably replace the cap and rotor, and have replaced the points and condenser less than 10,000 miles ago, but that was in the mid-2000's.

The carb has been on the engine since 1988 and I'm not averse to replacing it with a shiny new 1405. I'd opt for the calibration kit if it weren't for the fact that this is a very recent problem.

Carb issue. if you are closing the idle jets with little change, there is gas getting in somehow. And if you are fouling plugs, it's a sizable issue. If it were me, I'd get the calibration kit and a rebuild kit and remove / disassemble the carb and search every inch of the interior for a potential fuel path. Even the jets / needle valve could be leaking. my point oh two fwiw

I replaced the carb-to-manifold bolts with longer ones so I could apply more torque without fear of stripping the threads and now have idle jets that actually respond as expected.

However, there's now a loud whistle coming from the air horn! This is the first time I've experienced that in 50-odd years of casually working on cars, motorcycles, or small gas engines.

This thing is driving me crazy - I feel as if I'm playing whack-a-mole.

LOL at your whack-a-mole!

A whistling sound through the carb is indicating a vacuum leak, and it's often the carb base gasket, or loose hold down nuts. Can also be a hose, but check your carb nuts first. Spraying starting fluid or carb cleaner around connection points can help pinpoint the leak. WD40 works too, but is less dramatic.
Good hunting!!