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Snowkilts
11-09-2017, 06:03 PM
Have any folks tried using a tailpipe sniffer to set the mixture on our TRs? Which one did you use and how did it go?

bobhustead
11-09-2017, 07:03 PM
There is a prescribed procedure that is based on real conditions at the needle and jet. It has worked well for over 70 years. Why try something else?
Bob

poolboy
11-09-2017, 07:22 PM
The problem with a sniffer or just one A/F sensor is that it can only report the combined effect of your carbs...One could be adjusted rich, the other lean, the results appear normal.

TR3driver
11-09-2017, 07:42 PM
Many years ago, I tried using an O2 sensor mounted into the exhaust manifold. It was the "narrow band" type, and as a result returned basically no useful information. The original SU carbs simply do not hold the mixture at the theoretical ideal mixture no matter how you adjust them; and the sensor I had (same as used in most modern cars) was not responsive either at the richer mixture that produces best power; or the leaner mixture that produces best fuel economy. But the slightest movement of the throttle would change from one extreme to the other.

As far as why, as I've written before, any change in how the engine breathes can cause a change in what needle is required to produce optimum mixture. That is why there are so many needles available, almist every car used a different needle. So bigger liners, a bigger exhaust, exhaust headers, a different cam and so on all may need a needle change for best performance.

There are also various defects, like a worn jet, that can cause the normal adjustment procedure to not produce best mixture. In retrospect, I believe that is the reason that my Dads TR3A would always overheat, and eventually swallowed a valve. It cruised so lean, and EGT was so high, that it eventually eroded an exhaust valve stem until it failed. At the time, we blamed it on the 4.1 axle and no OD, but I've since run that combination on my own TR3 and it did not overheat under the same conditions.

Snowkilts
11-10-2017, 12:27 AM
There is a prescribed procedure that is based on real conditions at the needle and jet. It has worked well for over 70 years. Why try something else?
Bob

I have never felt that I am getting the mixture right when I use the procedure. It says things like "turn the nut one flat, and see if the RPMs go up or down". OK, I turned it one flat. No change in RPM. Then 6 or 8 more flats. Still no change. I don't seem to ever get the results that the procedure says I should. I usually just mess around for a while and give up. Seems to run OK, although lately wanting to stall during warmup.

Snowkilts
11-10-2017, 12:28 AM
The problem with a sniffer or just one A/F sensor is that it can only report the combined effect of your carbs...One could be adjusted rich, the other lean, the results appear normal.

Sure, but wouldn't you just shut off one carb while adjusting the other one?

CJD
11-10-2017, 02:01 AM
I have never felt that I am getting the mixture right when I use the procedure. It says things like "turn the nut one flat, and see if the RPMs go up or down". OK, I turned it one flat. No change in RPM. Then 6 or 8 more flats. Still no change. I don't seem to ever get the results that the procedure says I should. I usually just mess around for a while and give up. Seems to run OK, although lately wanting to stall during warmup.

It sounds like your needle/jet are worn. If the carb is in good condition you will eventually kill the engine by turning the nut upward. If the needle/jet are worn, then the sniffer will have no better luck.

I’m not sure how you could turn a carb off? If you closed one completely, then you would have to open the other more to prevent the car from stalling. If you open the throttle more than the normal idle speed, then you are adjusting the carb at a larger opening, and once you reduce the opening to run on both carbs the mixture will, again, be wrong for the reduced setting.

TR3driver
11-10-2017, 04:47 AM
I have never felt that I am getting the mixture right when I use the procedure. It says things like "turn the nut one flat, and see if the RPMs go up or down". OK, I turned it one flat. No change in RPM. Then 6 or 8 more flats. Still no change. I don't seem to ever get the results that the procedure says I should. I usually just mess around for a while and give up. Seems to run OK, although lately wanting to stall during warmup.
When I get that, it almost always means that something else is wrong. Valve lash & ignition timing are the usual suspects, but I've also seen it with valve timing off by a tooth.

bobhustead
11-10-2017, 08:08 AM
I don't know how you could shut one carb off. If you could, as Randall said, everything else that is happening at the time you set it affects mixture. What happens, then, when you put the "off" carb back in the circuit. These carbs are not capable of perfect adjustment and the adjustment you do get is idle only. You don't look for the speed change by simply moving the nuts. Too gradual. The change you want is a bit of speed increase when you lift a carb piston a very little bit with a screwdriver. Your car is never going to idle at 500 RPM like some books say.
Bob

poolboy
11-10-2017, 10:00 AM
Pull your spark plugs after a extended drive, but one that doesn't involve a bunch of stop and go or extended periods of idling.
Get it out on the highway for 20 miles and bring along a spark plug wrench and a camera if you want to show us.

frankfast
11-10-2017, 12:47 PM
Pull your spark plugs after a extended drive, but one that doesn't involve a bunch of stop and go or extended periods of idling.
Get it out on the highway for 20 miles and bring along a spark plug wrench and a camera if you want to show us.

I agree. Forget the mixture at idle. It's my experience that if the mixture is adjusted at idle the car will run lean at speed. The idle should be set at least at 750RPMs or it will stall because it's too rich.

RonR
11-16-2017, 08:47 PM
I use an old Sun exhaust gas analyzer on both my TR3A and Austin-Healey 3000.
I always set the rocker arm gaps, point gap/dwell, and timing before attempting to set the carbs.
I get great results that last years.