Classic/Antique Car Repair: GT6 carbs, distributer cap, vacuum tubes

Question
Hello,

Ihave a question on my triumph engine. The engine has new plugs, points rotor, distributer cap and wires. I have replaced the timing chain and gears. The carbs have been rebuilt. The fuel filter is new, the gas lines and tank are new and the vacuum tubes and fittings are new. The fuel pump is electric.
The engine starts well, but has a miss. on idle before and after it is warm, it seems to loose some cylinders off and on. It is better with the choke on slightly. On acceleration the engine bogs and misses around 2000-2800RPM, then it will clear and run up to 4000RPM with power. I have had the carb tops off and cleaned the bowls. I am using a 94 octane fuel wich is the highest I can get in my area.
Because it is a 1970, it has the pollution control carbs.
Have you any suggestions on a method for tracking down the problem area.

PS, The info on my transmission shifting problem helped, thank you.
Joe McAskill

Answer
Hi Joe,
The sympthum you discribe is a lean fuel condition, but you need to eliminate ignition as a possible cause first.

First, check the ignition timing accurately.

After you have done this, set the idle at what ever speed the miss is most evident. Since you have a Kettering ignition (points etc) you can do a cylinder kill test by pulling one plug wire at a time to see if you can isolate the miss to one cylinder. (use insolated pliers) You don't need to leave the wire off but for a couple of seconds and rev the engine between each plug test to clear that plug of raw fuel. If you find a cylinder with no change in RPM with the plug wire off write it down and continue. If you find all cylinders cause an RPM drop then you can proceed to checking for a lean condition on the carbs. Don't skip this as the ignition must be checked FIRST. A faulty ignition will give you a false reading when testing the carbs.

Set the idle back to specs. Probably 750 to 900 RPM. Now feel under the lip of the top on the side of the front carb and you will find a spring loaded pin sticking straight down up close to the outer wall of the carb. lift that pin all the way up. Your carbs may not have these pins so if they don't remove the air cleaners and put a small straight blade screwdriver in under the piston to one side (not in line with the needle) and rotate the screwdriver so as to lift the piston while the engine is idling. The engine should slow down and start sounding like it is running on only one or two cylinders. If it stalls and needs to be restarted, either the carbs are not in sync. or the REAR carb is too lean. Do the rear carb after you rev the engine a few times to clear the plugs. If the rear carb also makes the engine stall you are for sure too lean. A lean condition can be caused by other things besides the carb adjustments. You could have a vacuum leak. With the engine idling and with it's miss, pinch off each vacuum hose one at a time to see if you get ANY RPM change, don't forget the large hose to the brake vacuum booster. Use a pair of needle nose pliers to do the pinching. If you brake a hose, you needed to replace that hose anyway. You should never get any RPM change by pinching off any hose.

Let me know the results.
Howard