Carburetor Help
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Freshman User
Joined: Apr 2014
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I addition to suggestions above, a basic ignition tune-up may help. On mine, replacing the 35 YO factory carb with a reman fixed about 80% of the idle and off idle roughness. But it still wasn't 100%. New cap, rotor, wires and plugs got her to 100%. The old wires did not have a lot of miles but lots of years. It seems wires may deteriorate with time almost as much as mileage.
Also agree the vacuum gauge idle tuning approach works well. I'm pretty sure mine is more than 2 or 3 turns out and it idles great.
Also agree the vacuum gauge idle tuning approach works well. I'm pretty sure mine is more than 2 or 3 turns out and it idles great.
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
It's simple. First you need a gauge of course, and a source of (constant) manifold vacuum. Usually a port is available on the carb, below the throttle plates Disconnect and plug distributor advance vacuum hose. Engine warmed up. Idle RPM at spec. Make sure your float level is OK., timing is already where you want it, etc. If you change your initial advance, adjust valves, etc the idle mixture setting will probably move so save this for last. Even a new fuel pump would might change float level, that would bugger idle mixture, etc.
If the engine is tuned well the sweet spot will be a narrow defined adjustment band. Use the tach and vacuum gauge, but trust your ears at that point. Simply adjust the idle mixture screw(s) equal amounts for the highest vacuum indication achievable, and back off a little for smoothest idle, you'll hear it. (Favoring a slightly rich mixture.)
You'll likely have to adjust RPM again once you get the mixture set closer, a back and forth kinda deal. Using a gauge will give a good indication of engine health, timing, and carb setup. They are a must-have in my book.
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If the engine is tuned well the sweet spot will be a narrow defined adjustment band. Use the tach and vacuum gauge, but trust your ears at that point. Simply adjust the idle mixture screw(s) equal amounts for the highest vacuum indication achievable, and back off a little for smoothest idle, you'll hear it. (Favoring a slightly rich mixture.)
You'll likely have to adjust RPM again once you get the mixture set closer, a back and forth kinda deal. Using a gauge will give a good indication of engine health, timing, and carb setup. They are a must-have in my book.
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Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
While I sprang for a full engine build it got spendy for me. New pistons, rings, rods, ground crank, oil pump, etc, but it never ran right. A Pertronix perked it up nice. Still not there, though. A new Holley, it liked that! About the same improvement again roughly. Setting it up correctly, even more. Finally... New dizzy, plugs, rotor, battery THEN the engine was ready for carb tuning and distributor curving. The Y blocks have about the smoothest thumpety-thumpeta idle you ever heard. Whoever said carb problems are 90 percent ignition related was exactly right! Coulda saved all that time by doing it right from the git go!
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Freshman User
Joined: Apr 2014
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That was my experience. Where I messed up was, kinda cheaped out on the rebuild at the end. Big mistake. Used a junkbox carb and dizzy and slipped damper. Nobody could time it with a light at that point. While I sprang for a full engine build it got spendy for me. New pistons, rings, rods, ground crank, oil pump, etc, but it never ran right. A Pertronix perked it up nice. Still not there, though.
A new Holley, it liked that! About the same improvement again roughly. Setting it up correctly, even more. Finally... New dizzy, plugs, rotor, battery THEN the engine was ready for carb tuning. Whoever said carb problems are 90 percent ignition related was exactly right! Coulda saved all that time by doing it right from the git go!
Sent from my iPhone using IB AutoGroup
A new Holley, it liked that! About the same improvement again roughly. Setting it up correctly, even more. Finally... New dizzy, plugs, rotor, battery THEN the engine was ready for carb tuning. Whoever said carb problems are 90 percent ignition related was exactly right! Coulda saved all that time by doing it right from the git go!
Sent from my iPhone using IB AutoGroup
Postmaster
Joined: Nov 2011
Posts: 2,553
Likes: 10
EGR in general reduces tendancy to detonate. If switching vacuum lines for EGR caused the ping that may be why. Both the carb and distributor advance curve need re-tuned to run without EGR.
I wonder if the EGR system has a vacuum leak or other problem?
I wonder if the EGR system has a vacuum leak or other problem?
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Freshman User
Joined: Apr 2014
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Alright, time for an update. I solved the pinging issue when I discovered that my timing light was completely shot, and that the distributor was set at nearly 20 degrees for the base timing (No clue how it was still idling well). Going to vacuum tune the carb tomorrow. If that doesn't solve the hesitation/bog issue, I'll have to assume that there was a mechanical failure inside the carb and inspect it as such. Either that, or I have a phantom vacuum leak, which I doubt.
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Freshman User
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Another update:Vacuum tuned the carburetor and correctly set the advance. The bottom end hesitation is now more of a rolling bog than the engine dying outright, but it's still there. It's still dying on WOT upshifts too. It'll rev to redline, shift, then cough and choke before dying completely. Seems like the knock is back too. My money at this point is a vacuum leak, but again, I'm not sure.
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