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Tachometer help


Arthur1947

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I have two of these. One clip goes to the distributor side of the coil, the other to ground, if memory serves me right. Not sure which clip goes where...I can get good tach readings from mine, though never sure about the dwell reading tho. Note that the lower blue bar is the scale for a 6 cylinder. If this doesn't work let me know and I will rummage around for my instructions.

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So I hooked up the tach and guess what? It was at the 8 in the dead center. That's not 800 rpm right? The reason I ask is that I tried to set my idle at at the 4 marker and it ran like garbage, then after about five minutes of starring at this stupid gauge (and it starring back at the stupid operator) it dawned on me that each even number was 100 rpms. Is that correct? My P15 ran beautifully at the 8 marker. I'm confused as always.

Best.

ARTHUR

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If the meter is set on Dwell and the needle was at 8 on the 6 cyl scale, it shows a dwell of 40 degrees -- or, if on tach setting then it is reading 800 rpm. If it ran badly at 400 rpm, it can mean a couple of things -- carb is running too rich or too lean. But you should be able to get it to settle into a nice purr at 400, if everything else is up to snuff.

Do you have a vacuum gauge you can hook up to the vacuum wiper feed of the manifold (assuming it is a car with vacuum wipers)? This is the most accurate way to set idle (and get a good reading on the internal condition of the valves, rings, compression, etc), shooting for a vacuum reading of a steady 19 inch of vaccum, but preferrable to a 20 to 25 reading on the guage scale. I'm assuming you know how to set the carb air/fuel mixture screw and idle speed screw and work them until you get the right idle speed and vacuum. If I am talking down, please forgive me.

Also, is your distributor timing correct for your engine? And if it is right, is your point gap correct? Both can throw off the idle setting.

Tackle it in logical steps. Start by checking the points gap and setting it to spec and then checking the timing and setting that to spec. To set timing right, get the engine to idle at its lowest possible, disconnect the vacuum line from the carb (or vacuum canister on the distributor) and plug the open ends with something (where you removed from the carb or from the vacuum cannister), and then check the timing. Set timing to the specs indicated in your service manual. Unfortunately, once you get the timing to spec you may need to readjust your point gap -- take your time, you'll get it!

Also, I have found that it can be a challenge to get these old carbs to lean out correctly. Most of them after years of wear now allow too much air around the throttle valve, unless the carb has been restored and even then many restorers don't do this part of the restoration. Pull the carb off and hold the mounting end up to a light and look through the throat...with the valve closed you should see no or just a hint of light coming around the curve of the valve and throat itself. If too much sunlight is coming through then too much air is coming through making it really hard to lean out the carb. If you can turn the air/mixture screw in entirely without killing the engine then air is getting around the throttle valve. Such a situation will always cause the engine to run rich. My guess is that if your engine is running badly at 400 rpm it is running way to rich but the extra 400 rpms at 800 rpm can handle the additional richness. Hope this helps.

Edited by jim leman
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If the meter is set on Dwell and the needle was at 8 on the 6 cyl scale, it shows a dwell of 40 degrees -- or, if on tach setting then it is reading 800 rpm. If it ran badly at 400 rpm, it can mean a couple of things -- carb is running too rich or too lean. But you should be able to get it to settle into a nice purr at 400, if everything else is up to snuff.

Do you have a vacuum gauge you can hook up to the vacuum wiper feed of the manifold (assuming it is a car with vacuum wipers)? This is the most accurate way to set idle (and get a good reading on the internal condition of the valves, rings, compression, etc), shooting for a vacuum reading of a steady 19 inch of vaccum, but preferrable to a 20 to 25 reading on the guage scale. I'm assuming you know how to set the carb air/fuel mixture screw and idle speed screw and work them until you get the right idle speed and vacuum. If I am talking down, please forgive me.

Also, is your distributor timing correct for your engine? And if it is right, is your point gap correct? Both can throw off the idle setting.

Tackle it in logical steps. Start by checking the points gap and setting it to spec and then checking the timing and setting that to spec. To set timing right, get the engine to idle at its lowest possible, disconnect the vacuum line from the carb (or vacuum canister on the distributor) and plug the open ends with something (where you removed from the carb or from the vacuum cannister), and then check the timing. Set timing to the specs indicated in your service manual. Unfortunately, once you get the timing to spec you may need to readjust your point gap -- take your time, you'll get it!

Also, I have found that it can be a challenge to get these old carbs to lean out correctly. Most of them after years of wear now allow too much air around the throttle valve, unless the carb has been restored and even then many restorers don't do this part of the restoration. Pull the carb off and hold the mounting end up to a light and look through the throat...with the valve closed you should see no or just a hint of light coming around the curve of the valve and throat itself. If too much sunlight is coming through then too much air is coming through making it really hard to lean out the carb. If you can turn the air/mixture screw in entirely without killing the engine then air is getting around the throttle valve. Such a situation will always cause the engine to run rich. My guess is that if your engine is running badly at 400 rpm it is running way to rich but the extra 400 rpms at 800 rpm can handle the additional richness. Hope this helps.

Jim,

All things considered this is where I'm at.

Resistor plugs set at 0.035

Points 0.20

float level on dual bbs 5/64

Timing is near tdc.

My throttle won't go back anymore so I'm guessing that I'm running too rich. Where should my float level be on a dual set-up?

Best.

ARTHUR

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I've since lowered my floats to 3/16" and double checked the timing and the dizzy points and they are spot on. It's now idling at approximately 600 rpms but not as nicely as when it was running at 800 rpms. I've messed with the idle mixture screw but adjusting it by ear is only guessing. Any thoughts?

Best.

ARTHUR

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Arthur -- Do I understand right that you are running a two-barrel on this car? Is this mounted on a stock 218 or 230? Are you running the stock cam? If so, perhaps this two-barrel is just too much carb for this engine...

That, I think, is what's causing this issue.

That aside, you might want to experiment with the timing. My P15 (running the stock 1-barrel Carter B&B) runs best with timing at about 4 degrees BTDC...has something to do the combustion engineers tell me with the faster burn cycle of modern gasoline...

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Arthur -- Do I understand right that you are running a two-barrel on this car? Is this mounted on a stock 218 or 230? Are you running the stock cam? If so, perhaps this two-barrel is just too much carb for this engine...

That, I think, is what's causing this issue.

That aside, you might want to experiment with the timing. My P15 (running the stock 1-barrel Carter B&B) runs best with timing at about 4 degrees BTDC...has something to do the combustion engineers tell me with the faster burn cycle of modern gasoline...

Jim,

This is a 218 with dual Carter B&Bs, stock cam. I going to mess around with it some more today.

Best.

ARTHUR

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