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Dialing in the SU Carbs

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KVH

KVH

Obi Wan
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On my TR4A, I'm having trouble getting my SUs just right. I'm noticing what seems like a lot of soot out of the exhausts on cold startups (which I now understand may be normal), and the idle is very slow, almost to a stall, contantly needing the choke applied until the engine gets really warm, and then the idle is a bit too fast--about 1150 rpms with the choke completely released (off).

Last night I drove 25 miles and had to keep using the choke to increase idle speed at traffic lights. By the time I got home, the idle was around 1100 to 1150.

In all, the idle just seem irradic and unpredictable, with too much use of the choke to keep the car running. Though the car otherwise runs great.

Any ideas?
 
The last thing you set is the carbs. There is this old saying that what is wrong with you SU carbs is your Lucas distributor.
 
Hound, if your timing is OK, check the damper oil, then get the engine warmed up and check the mixture of each carb by lifting the air valve piston.

SU's usually have a provision for this. If your's dosen't lift the air valve an eighth of an inch with a thin screwdriver blade and see what the idle does. If it speeds up, you are too rich. If it stalls you're too lean.
Then synchronize the carbs by loosening the interconnecting throttle shafts link. You can synchronize the old fashioned way, with a length of heater hose. One end at your ear, the other at the mouth of the carb. Be consistant in where you place the hose. A little bit of difference in placement makes a difference in the sound (hissss)that you hear.
That would be a good time to see if the throttle disc is opening fully when you manipulate the throttle shafts. Do that when the engine is not running.
 
I have found the TR 4 cylinder to be a pretty forgiving motor as far as carb tuning, so it sounds like something is more than a little out of whack.

Hard to say what, make sure the jets aren't sticking when you take the choke off, don't know if you have SUs or strombergs, but both can have this problem, generally a push up on the jets from the very bottom of the carb, or or the linkage will get it back into place.

poolboy gives good advice about setting the mixture.

Also note when you are running these cars, especially in the cold the idle speed will go up as the oil warms and combustion works better with the engine warming up, and conversely, in really hot weather it will go down as underhood temperatures climb.

So the idle speed on these is a bit of a compromise, generally as slow as you can get it to run without stalling once warmed up. When I used to run the cars year round used to have to monkey with it a little bit depending on weather warm or cold out, also, shouldn't get too much soot in your exhaust even with choke on, make sure you have the throttle stops set so you get some devent throttle opening with your choke being pulled.

Good Luck!
 
Perhaps another possibility is a vacuum leak that changes as the car warms up and seals it off?
 
Have you checked that the pistons move freely (with the damper rod removed)? With the engine off, if you lift the piston and release it, it should land with an audible click (with the chokes fully off). I like to make this check with the mixture set fully lean, to ensure that there is a bit of clearance around the needle after the mixture is set. There should be no detectable resistance or binding anywhere in the range.

Don't know if the 4A suffers this problem, but on the TR3 the ball joints in the throttle linkage can wear oblong, which can cause binding just as the throttle close, leading to erratic idle rpm.

IMO, a certain amount of variation in idle rpm is inevitable; since the carbs suck air from under the hood. The large variation in intake air temp (40F or under at cold start; up to 200F+ after a hard run) causes a matching variation in air density, which causes the mixture to vary. (Carbs basically mix fuel/air by volume, but combustion chemistry runs on mass, so variations in the mass to volume ratios cause problems.)

What you describe is too much, of course, just making the point that the idle rpm was never as stable on these cars as on modern fuel-injected engines (which use a computer to adjust both mixture and idle rpm in real time).
 
I had a problem similar to yours once and it turned out that the vacuum advance in the distributor would not let the timing return and thus keep the idle high. However, part of your problem sounds like a lean fuel fixture. So perhaps you have two or more problems that are juxtaposition. Try enriching the mixture a couple of flats and check that the timing returns. In addition, and depending on what type of distributor, if you have points and the dwell is set at 45 degrees the idle will be higher than if you have the dwell at 60 degrees. The idle will come down that little bit between 1100 and 900 if you close the points some and open the dwell.
 
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