How do you know if the carb is tuned right?

[Guest asinger]

My 1967 Mercury Park Lane has a 410 engine and aftermarket Holley 4 bbl carb installed by the previous owner. A year ago we had the carb adjusted pretty well--actually, it was adjusted by my father in law who is the car guy. I'm just a hobbyist.

Since getting the engine rebuilt, I've had to re-tweak the carb somewhat, using the info my FIL taught me regarding the accelerator pump and stuff I've read online about adjusting the two screws on either side of the carb.

Questions:

How do I know when it's "right?"

I had to adjust the accelator pump, opening the nut on top so the spring-thing spread out further. This lessened the "bogging down" problem a LOT, but it still bogs down and stalls if you hit the gas hard, or even just push the throttle down by hand. What could be the problem?

There are several vacuum ports on the carb. My FIL figured out which had to go to the distributor, but at least three are plugged. Why are they plugged--or why are they even there if they're just going to get plugged?

I have no idea what the previous owner did with this carb. How would I know if it needs to be rebuilt or not? Or even if it is right for the car? The manual says the engine uses an Autolite, which according to info online uses 600 cfm. The Holley, according to info online, also uses 600 cfm. I suppose in that regard it's the right carb.

Ideas?

[MCHinson]

The best answer is to replace the carb with a rebuilt correct as original equipped carb. That is probably not the inexpensive answer.

[impala]

Some suggestions: with the engine off look into the carb and make sure you're getting a good squirt of fuel as soon as you move the throttle linkage. If not the accelerator pump might be out of adjustment or the pump diaphragm might be ruptured.

Make sure the initial timing is set properly and that it advances quickly as soon as the throttle is opened.

The mixture screws on the side are used only for idle but to adjust them: Bottom them out then turn them out 1 full turn to start. With the engine warm and at a low idle turn each screw in slowly until the engine stumbles and loses RPM. Now turn the screw out until the engine smoothes and max RPM is achieved. Some use a vacuum gauge to get max vacuum but this is not totally necessary. These screws don't do anything above idle.

impala

[joe_padavano]

The "right" way to tell that the carb is properly adjusted is to put the car on a chassis dynomometer, connect an exhaust gas sniffer to the tailpipe, and verify that the carb is properly jetted for all driving conditions.

I suspect that is much more expense than you had in mind.

Ratcheting down the price scale, you can install a wideband O2 sensor and monitor the A/F ratio as you drive. If certain driving conditions result in A/F ratios far from stochiometric, adjust accordingly.

Even less expensive, but correspondingly less accurate and more work, is to read the plugs. Unfortunately, prolonged idling and stop-and-go driving will make it difficult to get an accurate reading. This method works best for race cars where you can make a run then shut off and coast to the pits.

[Dave@Moon]

The "right" way to tell that the carb is properly adjusted is to put the car on a chassis dynomometer, connect an exhaust gas sniffer to the tailpipe, and verify that the carb is properly jetted for all driving conditions.

I suspect that is much more expense than you had in mind.

Ratcheting down the price scale, you can install a wideband O2 sensor and monitor the A/F ratio as you drive. If certain driving conditions result in A/F ratios far from stochiometric, adjust accordingly.

Even less expensive, but correspondingly less accurate and more work, is to read the plugs. Unfortunately, prolonged idling and stop-and-go driving will make it difficult to get an accurate reading. This method works best for race cars where you can make a run then shut off and coast to the pits.

Somewhere between Joe's 2 least expensive options is using a Gunson's Colortune tool. It allows you to view the color of the flame front, and adjust the carb to something close to ideal combustion. They're very popular in the British car hobby.

Gunson | G4074 | Colortune Single Plug Kit

[Tom400CFI]

I agree w/Joe and Dave. If you aren't going to go the "dynomometer" route, then really, carb tuning comes down to meeting YOUR criteria, and using all the "clues" available to you, to determine the carbs' status. You can use a variety of tools to "read" A/F ratio, from plug reading, vauum gauge, to O2 sensors, to EGT probes and gauges.

For the value oriented, and someone doing it once however, reading plugs, using a vacuum gauge, noting fuel economy and driveability is sufficient. One thing to consider when reading plugs is that prior to reading, you need to run the engine under ONE CONDITION for at least 1 minute. Longer, the less load you're testing for.

SO:

For idle, I use my ear. If you don't have a good ear, use a vacuum gauge connected to MANIFOLD vacuum. You want to adjust the idle screws to obtain the absolute highest vacuum reading that the motor will make -or the highest, smoothest idle. As you dial this adjustment in, (if it is off) the engine will increase in RPM. Resetting your throttle stop to lower the idle back down, will make the idle circuit in the carb, even more sensative...allowing you to adjust again.

For Cruising: There are two ways that you can approach this; the most accurate way, IMO is to read plugs. The other way is to use a vacuum gauge and speedometer.

For reading plugs (the better and easier way, IMO), simply find a nice even grade (highway) and run the car for over a minute (maybe 2-3 minutes) at as consistant a throttle setting as you can hold, at a "cruise". After 2-3 minutes, shift into neutral and shut the motor off, quickly. Coast safely to the side of the road, keeping in mind that your power accessories (steering and maybe power brakes) won't be helping you. Remove several or all of the spark plugs and note the color of the porcelin, and look for color change on the ground electrode. Porceline should be LIGHT tan, and ground electrode should have a color change at the 90* bend. The closer the color change is to the tip of the ground electrode, the richer it is running. The closer the color change is to the threads, the leaner.

Using the vacuum gauge method, find a section of road that is utterly consistant grade (flat, slight up hill, doesn't matter as long as it's consistant) and get the car going an absolutely consistant speed. Use a vaccum gauge to read manifold vacuum. As you change jetting (main jets on the primary metering block) you should see different vacuum readings at the same speed, on the same grade. The higher the vacuum reading, the more efficient the engine is running. You could use both methods at the same time to verify your findings.

Power mode: Using your vacuum gauge, again, connected to manifold vacuum, Find a nice consitant grade, and run the car at a consistant throttle setting, up the grade, for 1+ minute, at a throttle setting that gets you 8" of vacuum or LESS...but not enough to open the secondaries (I assume it's a 4 barrel). -Probably about 1/2 to 3/4 throttle here. Again, run it under this condition for over a minute, then cut it off clean and pull over. Pull the plugs again and read; you're looking for the same colors/changes as above. To make adjustments to this range of the throttle travel, you'll be changing the "Power valve".

WOT: Again, find a place where you can run the car at WOT for over a minute. Maybe up a hill while riding the brakes some? You can start from a stop for this one, and simply floor it for a minute, then cut if off clean again and read plugs again. This will tell you how your jetting is on the secondaries. To adjust this part of the throttle range, you need to change the "main jets" in the secondary metering block which is between the rear float bowl and the main body.

After that, you're done, and the carb is "tuned right". And tuned very well if you've paid attention to detail through out the process.

Assumptions: Your engine is in good mechanical condition, and your spark timing is set and operating correctly. The carb is a Holley Vacuum secondary, 4 barrel.

Vacuum ports. The reason why there are blocked off ports on the carb are that the carb was designed for many different applications. It is a fairly universal design and depending on the application, one would use various ports and not others. There should be at least two manifold vacuum ports, and at least one, if not two "ported vacuum" ports. If you want to know, I can tell you what they're for and how to tell which is which.

Bog; As "Impala" said, you need to verify accelerator pump operation. W/the engine off, look down into the primary (forward) two barrels and stroke the throttle. You should get two HEALTHY shots of gasoline into the barrels, for the entire duration of the throttle stroke. If it is weak at the begining, then gets going, or if it peters out toward the end, you need to replace your pump diaphragm and check valve.

Hope this "novel" helps. Ask if you have more questions.

-Tom

[Guest asinger]

Tom,

Thanks for the information. Your testing procedures are a bit more than I could do right now, however, I did shine a flashlight into the barrels and watched for gas. In each front barrel, I saw a tiny stream of gas squirt down as I advanced the throttle. The gas came out from beginning to end.

Perhaps the vacuum? Can you tell me where the vacuum lines go?

[joe_padavano]

I saw a tiny stream of gas squirt down as I advanced the throttle. The gas came out from beginning to end.

That would be the accelerator pump shot, which is a short-duration squirt of gas to prevent a bog while the added vacuum in the venturi overcomes the inertia of the column of fuel in the main jet circuit. The stream of gas means virtually nothing as far as steady-state tuning is concerned.

[Tom400CFI]

I think the comment about the accelerator pump performance is in reference to this issue that the OP is having, from the first post:

I had to adjust the accelator pump, opening the nut on top so the spring-thing spread out further. This lessened the "bogging down" problem a LOT, but it still bogs down and stalls if you hit the gas hard, or even just push the throttle down by hand. What could be the problem?

I'm not sure on the vacuum lines. One would go to your dist, and that should be ported vacuum. One might go to your HVAC controls, and your "Thermac" Heat riser valve. Manifold vacuum for the PCV valve. Is there still a vacuum diagram sticker under the hood?

Other than that, I can't think of much else that a '67 would have for vacuum lines.