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I've synchronized the carbs with the common synchronizer tool and keep adjusting the mixture screws but can't seem to get the two front plugs to look the same as the two rear plugs. The front plugs are a very light gray (almost white) while the rear plugs are charcoal in color. I've adjusted mixture screws a quarter turn at a time but improvements have been minimal. They both started at a base point referred by the manual. Do I keep adjusting the screws in opposite direction or is there something else I need to know about plug checks?
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TR2/3/3A Plug Checks
- Thread starter frankfast
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Geo Hahn
Yoda
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Are you doing a traditional 'plug cut'? That is where you start with clean plugs, drive at speed, cut the engine and roll to a stop along the road.
If you let the engine idle before you shut it down it may compromise the result.
FWIW - I think a quarter turn is about an 8 thou change in jet height -- so changes will be slight but that's good as small moves are what you want.
If you let the engine idle before you shut it down it may compromise the result.
FWIW - I think a quarter turn is about an 8 thou change in jet height -- so changes will be slight but that's good as small moves are what you want.
Brinkerhoff
Jedi Knight
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If the plugs are like you describe the engine probably doesn't idle well? Float level and fuel pressure play a huge part in carb adjustment as they determine the height of the fuel in the jet. The common tool I use is a Unisyn to check for even air flow . When the mixture is changed , the air flow changes , therefore the throttle connection between the two carbs need be broken during adjustment and the air flow resynchronized. Lifting the piston ever so slightly should bring a slight bump in rpm . Once you get to that point the plugs should be burning evenly ( if a bit too rich or too lean) and you can make finer adjustments to suit.
CJD
Yoda
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The nuts are for idle mixture. The needles (and jets) and piston springs set the mixture off idle. Adjusting the idle mixture will not affect your plug color unless you spend most of your driving at idle.
Just a thought...
Try swapping your pistons/needles/springs from front to back and see if the color moves with the pistons. That will identify a worn needle or mis-fit of the pistons.
Just a thought...
Try swapping your pistons/needles/springs from front to back and see if the color moves with the pistons. That will identify a worn needle or mis-fit of the pistons.
CJD
Yoda
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By the amount of the idle mixture, you are correct. At idle that is a matter of a few extra drops per minute. When you are cruising at 60mph, how much difference do a few extra drops make? The needle and spring makes an order of magnitude difference in mixture off idle.
Another way to look at it...assuming your .008" per flat is correct (and I have no reason to believe it isn't), then compare that needle lift with the 1-1/2" of lift the piston provides. That kinda puts it in perspective.
Another way to look at it...assuming your .008" per flat is correct (and I have no reason to believe it isn't), then compare that needle lift with the 1-1/2" of lift the piston provides. That kinda puts it in perspective.
I agree.I can get plug color change with as little as two flats adjustment.
Lowering the jet richens throughout the range of piston movement.
THe problem is needles worn at the idle position causing lean mixture in the rest of the range.
Tom
Ps,don't waste your time adjusting jets if you haven't verified,by looking down the jet ,that the float level is correct and holding.
Tom
Lowering the jet richens throughout the range of piston movement.
THe problem is needles worn at the idle position causing lean mixture in the rest of the range.
Tom
Ps,don't waste your time adjusting jets if you haven't verified,by looking down the jet ,that the float level is correct and holding.
Tom
"Do I keep adjusting the screws in opposite direction or is there something else I need to know about plug checks? "
I am kinda getting confused with the terms (plus I am not trying to be difficult). The nuts are on the bottom of carb for fuel getting in at idle and the screws are on the shaft which lets in more air in at idle. Plus, I am not sure what you mean by opposite direction. Do you mean you turn one carb one way and the other opposite?
One thing that is important also is that the jet on the bottom of carb returns back to its base on the nut after each time the choke is used. These cars let more fuel in for a choke rather that shutting off the air to choke like many other cars.
I would suggest getting a couple sets of plugs then turn the “nut” on the bottom of the carb UP a couple of flats making sure the jet has returned to the nut after driving a few miles and see if the deposits change on the back plugs.
steve
I am kinda getting confused with the terms (plus I am not trying to be difficult). The nuts are on the bottom of carb for fuel getting in at idle and the screws are on the shaft which lets in more air in at idle. Plus, I am not sure what you mean by opposite direction. Do you mean you turn one carb one way and the other opposite?
One thing that is important also is that the jet on the bottom of carb returns back to its base on the nut after each time the choke is used. These cars let more fuel in for a choke rather that shutting off the air to choke like many other cars.
I would suggest getting a couple sets of plugs then turn the “nut” on the bottom of the carb UP a couple of flats making sure the jet has returned to the nut after driving a few miles and see if the deposits change on the back plugs.
steve
From this it will be seen that once the correct needle has been specified, the jet has to be set to a datum dimension on the needle to ensure that the desired mixture is obtained throughout the range of the engine operating conditions. This position is set when the carburetter is tuned for idling.
Copied this from a description for SU carbs. I was also under the impression that jet adjustment changes the mixture throughout the rpm range. Doesn't the tapered needle move within the jet allowing a metered amount of gas to enter the chamber?
CJD
Yoda
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Yes, the "idle" jet adjustment changes the mixture throughout the rpm range. But...if you are using the nut to adjust your cruise mixture you are going about it in the wrong way.
1) Set your best idle using the jet adjusting nut. You're done with the nut!!!
2) Unless you idle all day long, your plug color is determined by the cruise mixture...or your wide open throttle mixture if you are doing throttle cuts on brand new plugs.
3) Cruise mixture is determined by the jet and needle selection, as well as the piston spring strength.
4) Transition from idle to higher rpm is determined by the piston spring and dashpot oil. Transition has little affect on plug color.
So, we are back to the initial question...are you looking at the plugs after driving the car under all conditions...or are you doing full throttle cuts on brand new plugs?
1) Set your best idle using the jet adjusting nut. You're done with the nut!!!
2) Unless you idle all day long, your plug color is determined by the cruise mixture...or your wide open throttle mixture if you are doing throttle cuts on brand new plugs.
3) Cruise mixture is determined by the jet and needle selection, as well as the piston spring strength.
4) Transition from idle to higher rpm is determined by the piston spring and dashpot oil. Transition has little affect on plug color.
So, we are back to the initial question...are you looking at the plugs after driving the car under all conditions...or are you doing full throttle cuts on brand new plugs?
I'm looking at the plugs under all driving conditions. It seems to me that if they are properly set, the plugs should look the same. The car is not always cruising at sixty mph. The carbs may be running in synch at that speed but not at slower speeds. I would think that transition from idle to cruising speed does have affect on plug color. Plug checks at full throttle are useful for race cars but not for everyday driving. Plug color should be the same no matter how one uses the car. I agree that there are other things to look at beside the mixture screw.
CJD
Yoda
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OK, so we are talking about just general driving, it sounds like the rear carbs are too rich. The first thing I would do is to check the gas level at the jets (I think someone already mentioned that, I think Brinkerhoff). To do that, remove the domes and pump fuel using the pump lever. They should be equal level in both carbs, and just a bit below the level of the top of the jets. If the rear is higher than the front...you have found the issue. Lower the float level.
If the fuel levels are the same, I would try swapping the domes/pistons/springs while they are off...to see if the rich plugs change with the domes. That'll tell you if one of the jets is worn from rubbing on the needle, a spring issue, dashpot issue, etc.
If the fuel levels are the same, I would try swapping the domes/pistons/springs while they are off...to see if the rich plugs change with the domes. That'll tell you if one of the jets is worn from rubbing on the needle, a spring issue, dashpot issue, etc.
CJD
Yoda
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If they load up at idle...use the nuts to set the idle mixture. If the idle is great, but they load at transition or cruise...then using your idle adjustment to attempt to fix those is backwards. You would be throwing idle mixture out of wack trying to compensate for the real issue...i.e. spring, jet, needle, dashpot.
This is probably not your problem, but I have seen a situation like yours with an engine in which the "oil rings" were worse on the back 2 cylinders and left a charcoal look, but when the plugs were being cleaned part of the problem of oil deposits.
Anyways, the cure was to pull the head and drop the pan and pull the pistons and install new rings.it was labor intensive but did not cost too much. I guess the problem was the rings were made out of Nickle or something. The new rings were made of something else I believe-- Iron. It was a long time ago and I could be wrong about the material, but it fixed the problem. Plus the engine had ok compression.
steve
Anyways, the cure was to pull the head and drop the pan and pull the pistons and install new rings.it was labor intensive but did not cost too much. I guess the problem was the rings were made out of Nickle or something. The new rings were made of something else I believe-- Iron. It was a long time ago and I could be wrong about the material, but it fixed the problem. Plus the engine had ok compression.
steve
charleyf
Luke Skywalker
Online
[QUOTEIf the fuel levels are the same, I would try swapping the domes/pistons/springs while they are off...to see if the rich plugs change with the domes. That'll tell you if one of the jets is worn from rubbing on the needle, a spring issue, dashpot issue, etc.[/QUOTE]
John,
you refer rather casually about swapping the pistons and domes. After having some real issues with centering the needles in some cars, I would be concerned about doing any swapping. You might get a lot worse result than you ever started with.
Charley
John,
you refer rather casually about swapping the pistons and domes. After having some real issues with centering the needles in some cars, I would be concerned about doing any swapping. You might get a lot worse result than you ever started with.
Charley
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