• Hey Guest!
    British Car Forum has been supporting enthusiasts for over 25 years by providing a great place to share our love for British cars. You can support our efforts by upgrading your membership for less than the dues of most car clubs. There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Upgraded members don't see this banner, nor will you see the Google ads that appear on the site.)
Tips
Tips

TR2/3/3A Carburetors again, and again, and again

Share this page

S

STeve 1958

Jedi Hopeful
Offline
I found that I was pushing the button on the SUH6 too far, giving me a false lean reading. When I only push it a 64th of an inch I get a rich reading. Am I right that a 64th is far enough?
Now the real question... Has anyone bought a color-tune and was it a good investment at $31.00
 
I have had a Colortune for many years. It is very easy to diagnosis rich or lean conditions without the Colortune. I found that the Colortune is difficult to read ,but admit that I used the Colortune to solve a problem I had on my car. It is just another way to determine the rich / lean status of each individual cylinder.
 
I always push the whole button?!?

Honestly, I always thought of the button as a cutesy little "gimmick" on SU's. With all carbs, all engines, if the engine runs roughly and gradually slows down at idle...you are rich. If the idle is smooth, but surges so you can't get a sustained steady idle speed, then you are lean.
 
John..method not much help with twin carbs., where one might be rich and the other lean. I get dieseling at shut off indicating lean, and black suit on plugs indicating rich. When I push the whole button it says it's lean, when I push half the button it says rich. Car runs great but idles rough due to a suspected cam. Car also runs great in the morning but not as well in the afternoon which would indicate rich.
Maybe time for a color-tune
 
Back to basics...my method works on all carbs, all cars. With multi's you must first start at a base with equal draw from both carbs and identical mixture flats set. Adjust them equally together and all will go well. At the very end, when you have gotten the best mixture with equal adjustments on the multi's, ONLY THEN try to improve a tad by individual adjustments, and even then only a flat at most difference. As my master taught me..."you must feel the engine, Grasshopper. It talks and you must listen to understand it's needs". Rough is rich...always. Surge is lean...always. This applies to both Cox TD.049 model engines and 4,000HP Donovans, and everything in between. The button is a mere gimmick to aide the unenlightened or unobservant.

If you keep getting conflicting results as you adjust equally, then you have another issue afoot. Loose butterfly shaft, vacuum leak, bad float setting, worn jet, worn needle, worn diaphragm piston, bad cylinder rings, burned valve, blown head gasket, etc. If you go off on a tangent with a single carb, you will invariably result in one carb providing the air and the other providing the fuel, and the diagnostics conflicting with themselves.

Now, go forth and tune young Luke...or Steve, I mean!?!
 
Your spark plugs will give you a pretty good indication of mixture....If the plugs are old go for a 30 mile drive; try to avoid stop and go and prolonged idling, keep the rpms up, use lower gears find a place to pull over and remove at least one plug being served by a carb.
 
Remember there is more than one mixture. Idle and high speed. Above I am talking idle...and the plug reading doesn’t tell idle mixture. Plug reading is helpful for high speed mixture. While idle mixture in SU’s has a small affect on high speed, the primary high mixture is controlled by the needle number and jet size...along with the piston springs. Then there is transition mixture, controlled by the dashpot oil and piston springs. You can see...there is overlap in all the mixtures, with a primary control, but the others are affected when any one is changed.

But, get the idle set and most of the other mixtures fall in line...so long as you jets and needles are good.

Edit...Oh, with the carbs off you should be able to see the fuel level just below the top of the jets with the pistons lifted. If you do, and it’s not so high that fuel is spilling out, then your floats are set well. They should be as close to the same level as possible.
 
A lot of thought went into creating needle profiles to give proper mixture throughout the range of rpms and load....at least with ZS carbs so you'd think SU's as well....but if you must make a choice as to where to set the jet, set it where the mixture is best for what conditions you drive the most.
If you spend most of your time sitting around letting the engine idle, set it there if you will; but if you drive the car, that's where you set your mixture to handle those conditions.
 
I started over from scratch and got a much better tune up. Goes like a bat out of **** and even idles smoother.
Still not perfect but close. Thanks for all the help.
 
Sounds like you got it—I would add the distributor plays a big part in performance. The spark needs to advance and return smooth. Or you can get surges and fowled plugs and all kinds of things.
 
This morning "She" didn't run so good and needed a lot more choke. Maybe that's because she was too rich before and now is more correct. Or maybe too hot when I tuned her up before.
Anyway once I got the choke idle down to a reasonable level all is well. "She" is a temperamental B....
 
Problem solved. After resetting the mixture to neutral and retuning the other day, the jet got slightly off center. This was hanging up the damper causing a lean mixture. When I pulled the choke the damper would drop and sometimes stay down, until the next time it hung up.
Totally intermittent but easy to find once I took the air cleaners back off. We'll see how it goes tomorrow.
 
Well I went on a 40 mile trip today and made it 35 miles before losing half my power. I figured the throttle linkage had come loose between the carbs. So I pulled over and found the I hadn't tightened up the air cleaners very well. One bolt vibrated out and the air cleaner dropped, blocking the intake. Felt stupid but great to get back on the road so easy. The bolt was even still there waiting for me. All in all a very fun trip from Livermore Ca. to Danville all on country back roads.
 
Back
Top