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How to adjust rich SUs

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Luke_Healey

Luke_Healey

Jedi Trainee
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On the 'new' Sprite, I have the carbs set according to the manual when idling, but the car is running very rich and not putting out a whole lot of power. The plugs are sooted up with carbon.

I sound synced the carbs at idle and used the starting point on the jet nuts. The adjustment on the jet nuts was started at 12 faces turned down. However, to achieve fastest idle and pass the rise and fall RPM test, I had to turn them out further quite a bit (richer than the starting point.)

Is there a secret to getting them to the point they are making the best mixture as well as the best idle?

I'm sure this is asked all the time and varies car by car, but I figured I'd fire it off anyway.

My other Sprite is running seemingly properly according to the plugs and definitely has more get up and go. It also gets over 35mpg fuel mileage even with me whooping its butt.
 
Hmmm, I'm kinda confused my your proceedure.

I usually, start 12 flats down like you said, then sync mine with with a unisyn, then lock the linkage. After it's locked I use the "lift a pin" method to test the mixture strength. Once I'm satisfied with the mixture, then I use the idle screws to acheive what ever idle I want. I ajust the screws the same number of turns with the linkage locked.

I definitely DO NO adjust the idle by changing the mixture.
 
My problem is that I can never get the lift pin method on this car to the sweet spot.

It always fails the test, erring on being lean.

ie: you raise the lift pin 1/8" and the car sputters and idles down as if it wants a richer mixture. This can go on until you screw the mixture jet nuts right off of the bottom of the carbs.

However, there is a range there.. I got it to run too lean tonight at about 20 flats out.

I don't have an exhaust sniffer.

My next step is to do what I should have done first before I got digging into the carbs, which is check the ignition timing. For some reason, the advance/retard adjuster screw on the distributor is frozen. I also need to check point gap.

So perhaps I jumped the gun. I'll return after I've got the timing checked over.
 
Luke_Healey said:
So perhaps I jumped the gun. I'll return after I've got the timing checked over.
Click to expand...

Definitey! Nevef touch the carbs 'til the timing is spot on. Don't forget the valves.
 
Timing is set at 3dbtdc, so that was OK. I didn't verify that the vacuum advance was working though since I was alone. However, I am just going by the spark plugs now and getting it dialed in.

It's running better now. The problem tonight ended up being that I got it dialed in with the air cleaners off, and reinstalling them enriched the mixture noticibly. I'll have to back off of the jet nuts tomorrow and get it buttoned down. I'm running Napa's 2158 paper air cleaners, which are new.
 
Mybe it's just me, but I've never been able to tune SUs accurately using the lift pin method. Now I use a color tune, and it seems fine, at least as indicated by the plug color and lack of soot. I use a unisyn to sync the carbs.
 
Oddly enough, the lift pin test passes AFTER I adjusted the carbs by looking at the plugs. Doing it incrementally by adjusting the jet nuts with the car running? I couldn't make it work.

I need to look into a color tune and/or a mercury balancer. I could use one of those on my motorcycles as well.

Someone online mentioned an onboard exhaust sniffer for about 100 dollars, but I have not found what they're talking about. I assume it hooks into the exhaust via an O2 sensor threaded portal? Do any of you know where to buy a digital analyzer that I could use onboard while driving the car? I am going to be driving the car at high RPM and don't want to be running it so lean that I damage things.
 
Two colourTune units and a UniSyn will get you where you need to go in minutes with a bit of practice.

The on-board gas analysers I've seen have been threaded into an O2 sensor port just abaft of the cast manifold, in the downpipe. Can't recall nor find the references to the $100 units, tho. Sorry.
 
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