TR2/3/3A TR3 Carburetor Adjustment

[fpstude]

TR3 Carburetor Adjustment UPDATE

I am part of a small group that volunteers at the California Auto Museum (Sacramento). We help maintain vehicles owned by the museum.

We are currently working on a 1957 TR3 that has been inoperative for awhile. Our current problem is rough acceleration. At speed it is smooth. However, the engine is very rough as the RPM increases.

We have replaced leaking gaskets in one of the carburetors. The points are set at 0.015". Dwell is about 30 degrees. Spark plug gap is 0.025". It has SU ACU786 carburetors. The operation of the chamber piston is smooth with the correct level of oil.

Any thoughts on what we may have missed or done wrong would be appreciated.

 

[Don Elliott]

I use a 20 inch length of garden hose and put one end on the flat rim of the carb intake flange (with the air filter removed) and the other end to my ear. I listen to make sure that the air sucked in is equal for both carbs. I move quickly from one carb flange to the other. If the "HISS" sounds the same, then the carbs are in sync. If the hiss is not the same, you will have to disconnect one of the links between the two carbs. Just loosen the nuts and see that the joining link is free. - but leave it in place.

Then adjust the idle screw on one carb or the other - either up or down - till you get equal hissing with the hose technique. Get the idle about 700 RPM at this time, then re-tighten the nuts of the joining link between the carbs,

Re-check the hissing again. Sometimes the adjustment will go out a bit while tightening up the nuts on the link.

Good luck

 

[TR3driver]

My first thought is distributor problems, caused by excess wear inside the unit. Try hooking up a timing light and watching the timing mark as you slowly rev the engine up to 3500 rpm or so and back down. The mark should move gradually in proportion to rpm. If it doesn't move, or jumps around, or the light flickers; you've found the problem. (Or at least "a" problem, could be others.)

Another thought : Where are the mixture nuts set? If they have to be set noticeably leaner than usual (about 2 full turns, or 12 flats, from full lean) to get the correct idle mixture, you may still have an internal leak in the carb, causing the mixture to go rich at idle (and hence lean at other throttle settings). A worn needle or jet can do the same thing. Most common leak is probably the float valve (often caused by a bit of debris from the fuel lines, the original soft lines were not ethanol-compatible); followed by the upper cork gland and rarely, the copper washer between the upper jet bearing and the housing (generally because the washer got left out by accident). A simple experiment here is to back off (richen) both nuts by 3 flats or so and take a test drive.

On the carb pistons, I assume you can feel the resistance when lifting them. If you then let them fall, do they land with a distinct 'click'? If not, you may have jet centering issues, which can both upset idle mixture directly and also lead to wear in the jet & needle as above.

Oddly enough, I've never made it to the California Auto Museum. I'll have to come visit some day soon.

 

[sp53]

See if the spark plugs fowl out quickly. If so, examine them and report back. In addition, make sure the choke disengages and the jets return to the base of the carburetors.

 

[fpstude]

Thanks for the input. We will give the ideas a try this week. I really appreciate your time.

 

[fpstude]

After several adjustment attempts we decided to check the ignition wires. We checked the resistance in the wires and the meter indicated an open circuit in the carbon filled wires. We installed the spare distributor cap and wires I carry in my TR4A and the engine runs much stronger upon acceleration. We have not replaced all of the carburetor gaskets yet, that is next

 

[TR3driver]

A common problem in my experience, when using carbon core wires with the original insulation-piercing distributor cap. Suggest either using copper core wires with discrete resistors in the plug boots (which is what I do), or else a cap with push-in terminals (readily available for MGB, fits TR dizzy).