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Quick and dirty SU Tuning

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It wouldn't run with the modded SM's so I put some modified N27's in today. Still wouldn't start. Since it ran with them before figured it must something else. Had a look at the points, they were rather played. The points cam was worn down, not enough gap. Bumped it around until it was on the cam and opened up the points a bit. It started right up and ran great, except for overrun when I shut it down after a drive.

Tomorrow will install a new set of points and set the dwell with the meter AFTER I get some points cam grease. Maybe the modded SM's were OK. I'll try putting  them back if it goes lean.

Idle was set high couple days ago so it would run. I forgot to readjust it, so today it was way too high once the engine warmed up. Maybe the cause of the overrun.

Edited by Stanley

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hey guys,

So, I spend the whole day on the car. The car starts right up with the fast idle screw and balance screw completely open and carbs are synced. I adjusted the float again but this time to the 70/71 specs. I ran the car until warmed and no overflowing. Now what I need is a tach so I can finalize the tuning.

I also received today a new coil, hoping that will fix my RPM guage but Iam having issues installing it. I posted in the electrical section.

Thank guys for your input.

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Can anyone comment on how strong the requirement is to keep mixture screws with the same setting for each side?  Certainly it's good to keep them as similar as possible, but even with the most perfect float level setting there may be slight differences for each.  I would assume slight differences are acceptable to get the mixture correct on each side?

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I disable one and get it to idle as smooth as possible with the mixture wheel. Let it clear up and then do the other one. Like Blue said "equally shitty".

When I'm happy with them the wheels are within 1/4 to 1/2 inch apart. I have one of those protruding knubs painted black on both carbs.

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You do have to compromise however.  One tuning item to note w/o air fuel meter::

70-73 domed SU's tuning:

1. have an air balance for idle (throttle valve stops)

2.  have air balance for off-idle (back carb rod screw)

3.  only have one fuel adjustment (jet height)

When tuning SU's, you are forced to adjust fuel height for either idle or off-idle.  Since you drive the car more than you idle, it is best to tune for off-idle.

The "equally shitty" stumbling (at idle when running on one carb) is good for initial tuning but it would be better to match the rpm drop (off idle... say 3000 rpm when running on one carb).

For this adjustment, use the high-speed screw to get the rpms up to ~ 3000 then balance air. Once balanced, disable the front carb and note lower rpm.  Repeat by disabling the rear carb and note rpm.  Compare rpms. The carb that was  carrying the load with the lower rpm needs its fuel level adjusted.  Repeat the test by balancing air then measuring rpm drop, then adjusting fuel until both are synced.  Revert back to idle to see how far off they are.  If the carbs are in good form and the needles are well profiled for the engine then you should be good to go.

Run the car for a while on the roads and note plug colour and exhaust soot.  Equally adjust each jet up or down to enrich or lean mixture to taste.

 

 

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There's nothing in the FSM that says both mixture nuts should be set the same. It has a graph that gives preliminary settings dependent on altitude and temperature. 2.5 turns down is about right for 70 F near sea level. Hotter ambient temperature and higher altitude require a leaner initial setting, richer for cold weather, but that's just the starting point. If the engine or carb needles aren't stock then the initial setting should probably be different. Then the mix is set per FSM or other more advanced or simplified methods. It's important to use syncrometer or similar and adjust balance screw to get them balanced. Since it's difficult to get the floats set exactly right, and since the carb pistons move differently depending on the oil level in each one, and maybe other differences, it's unlikely that the best mix would be where both mix nuts are set exactly the same. But if they're way off when it's running the best something's wrong, probably the float levels, and needs to be corrected.

Correct me if I'm wrong, but I think once the carbs are balanced and a test drive shows it's a little lean or rich, the carbs can be adjusted by moving the mix nuts of each the same amount, say 1/6 turn before re-testing, without rebalancing them.  Since you can't adjust them with the air cleaner(s) on, and air cleaners make it run richer according to British SU documentation, I set them a little lean and richen them up according to test drive, plug color and paper towel test. Air/fuel gauge would be better.

Edited by Stanley
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On 22/6/2017 at 0:32 PM, Mark Maras said:

 What color are the plugs at 4 turns down?

Ngk Bp6es Plugs at 3 turns clockwise both su carbs after some hard running and couple of traffic lights.  Should i richen or lean them?

20170730_114752.jpg

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 Sorry, I can't really tell judging by the pic. I'm assuming that's an old plug. The black portion indicates a rich mixture. The white portion indicates a lean mixture. You'll have to start with new plugs that have all white porcelain to get a proper reading. That said, I'd say that the black portion was from running the mixture too rich when the plugs were new. Then it looks like the plug was cleaned, the mixture and or floats were re-set to a lean mixture. Neither color is what you're looking for. You're shooting for a tan color. Normal driving.

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Is the tan color still attainable with unleaded fuel? I run 93 octane and its clear when the mixture is too rich as they are dark, but the lean condition seems like it might be harder to diagnose with the plug color using unleaded fuel?

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 Others have reported that the old standard tan color is not attainable with the fuel formulas currently in use. I can only assume that a much lighter tan color is what we're shooting for.

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