QuestionQUESTION: Good Morning and a question to start the day ! Have 79 Midget that runs very well except it has a stumble, cut out or a skip for lack of better term. Realize many possibilities , however, looking for the most likely and downward the list. Occurs at 2500 RPm and 40-45 mph when loafing. Never when being put to speed. Car has newer plugs, plug wires, cap, rotor ( Yep has points system not the electronic) condensor, points. Timing set correctly. Carb has newer metering needle , diaphragm etc. and is the orginal Stromberg Zenith unit. Oil level in carb is correct and piston moves freely. However a conflict with my knowledge or changes in manufacturing ? The needle appears to be non adjustable ? Simply fits in the assembly and held by the grub screw ? Contray to Victoria British carburetion diagram for 150 CD unit. Has someone simply left this part out in past endeavors or a changed detail ? The terminal and lead ( 25 D distributor ) has a frayed insulation cover , cloth, for the wire going from the Diode to the points assembly but the very fine wires inside seem secure and connected. Only questionable items I can see. Best guess and advice and THANK YOU.
ANSWER: Hi Mark,
79 was a second tough year for BLM trying to meet US emission standards so it has been even tougher for the owners of 78 and 79 cars to make them perform.
A stumble at 2400 RPM can be many things but usually carburetor. However, with a non adjustable needle you are faced with a difficult time in diagnosis. A stumble under no load conditions is a common fault of a lean condition.
Question, can you note the poor running setting still at 2500 RPM? If so there is a simple method to see if a lean condition is the cause. Just set the engine speed at the poor running RPM and take a propane torch (soldering torch) and allow a small amount of propane to enter the intake (not lit of course) This richens the mixture as the engine don't know the difference between propane and gasoline. If it does not smooth out then it is not fuel. If it does smooth out then take a piece of mechanics wire and bend a loop in one end and remove the top damper and put the bent loop of wire down into the piston so a piece of the wire is sticking out of the top and set the RPM at the poor running speed and note where on the wire the edge of the top of the carb is at. This gives you a exact position on the needle where the lean condition is at. remove the piston and the needle and put the needle in a drill and take a piece of emery paper (fine sand paper) and sand the area where the needle is in the jet. You need to sand a smooth area about a 1/4 inch so that you are making a smooth transition in the area. This needs to be done a little at a time so as not to cut it down too much as you can't add, only subtract.
If the propane does not smooth out the poor running you should change the ignition timing about 5 deg at a time in each direction to see if ignition timing could be the cause. If ignition timing corrects the problem at 2500 RPM you should check ignition timing at 1000 RPM increments all the way up to 4K to get a picture of the advance curve so a change in the advance curve may be necessary rather then just a change in timing (which changes the timing of the whole range)
I hope this sheds a little light on a complex problem.
Howard
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QUESTION: Thanks, I will try your suggestion with the propane and followup on it. While this remains to be done, to keep it simple with questions one at a time, I will ask the following as you mentioned the advance. This car had the electronic system removed and a 25 D distributor with points installed at some point in it's past. The vacuum line was blocked off at the flame trap connection from the carb and does not travel via a pipe to the distributor. This modification was done by a professional repair shop ( have old W.O. ) but do not know if that is really the correct way to do the modification. What the mechanical result of this being done ? Is the mechanical advance in the unit enough to do the job ? Would lack of the vacuum advance result in the symptom I have ? Or is it creating a effect I am not even aware of ? As you can see this is about where my knowledge level disappears. Thanks !!!!
ANSWER: Look at the vacuum unit and if it is a retard unit you need to put manifold vacuum to it and if it is an advance unit you need ported vacuum to it. Most are easy to ID, if the vacuum port is on the distributor side of the vacuum unit it is a vacuum retard unit. If the vacuum port is facing away from the distributor then it is a vacuum advance unit. Some Triumph Spitfire distributors came with both advance units and retard units on the same distributor and the factory (BLM) blocked off the retard side. Your engine is a Triumph Spitfire. Putting points in place of the electronic system was a common practice as the original electronic ignition system was very unreliable.
You need to first do what I told you to ID what the cause is before making a bunch of changes. The tests I gave you are valid tests to see if it is fuel or ignition.
Howard
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QUESTION: Mr. Fitzcharles, I ran the car to warm up to normal and , parked in driveway, ran engine to 2500 rpm and stayed there. Engine was smooth , without being able to detect defect. Exhaust note was even and constant when heard from the tailpipes. As everything was smooth did not run with propane- as I would not have been able to detect anything smoother than what I had. Ran car on the road and stumble was there in same RPM range regardless of gear. Tried several different timing settings , as you suggested, from 2 degrees to 14 degrees. No difference in stumble at the RPM range I have discussed. Left timing setting at 10 degrees, points at .15 and plugs at .25. Any ideas as it seems that both tests show neither to be the culprit ? I do thank you for taking the time.
AnswerAre you judging a "stumble" by the exhaust note? If so you need to put ear plugs in and ear muffs on and drive the car and see if you can detect the stumble by feel and not be influenced by the sound. However, the test for a lean mixture by using propane was not a valid test if it don't stumble at 2500 sitting still. The fact that the stumble is not affected by timing proves it is not a timing issue. Now you need to add fuel at the stumble speed to determine if it is a lean condition. You could add fuel by putting a hose that fits snug onto the propane pipe (without the air nozzle on) and run the hose in under the hood and into the intake or into a vacuum port to be able to add fuel at the stumble point. You need to have everything secured so as not to end up lighting you car on fire.
If the stumble is only an exhaust note problem you need to look for a exhaust leak at the manifold to head or manifold to pipe or any holes in the pipe or a leak between the muffler and the manifold.
Howard
