QuestionQUESTION: I cannot get both carbs scronized, fast idle set with proper gap, oil same in both, replaced both diaphragms (one had a crack) car idles smoothly at aprox 1200 rpm, exaust manifold gasket was just replaced...had a blown gasket.....front carb shows about twice the vacuum as rear....checked shafts with carb fluid when running and no change in idle....pistons appear to be at same height...any ideas?
ANSWER: Hi Jim,
First thing you must do is run a "Dry" then a "Wet" compression test on the engine.
A "Dry" compression test is just a normal compression test (Throttle open & at least 4 or 5 revolutions of the engine) write down the results and run the "Wet" test. (about 4 squirts of engine oil into the plug hole and throttle open and 4 or 5 revolutions of the engine)
The "Wet" test will be higher on each cylinder but it must not be more then 10% to 15% higher then the "Dry" test.
Any one or more low cylinders can be rings, valves or gasket. You didn't say anything about over heating so you should be able to forget gasket (unless two cylinders side by side have low readings Dry) The "Wet" test is a test for rings. Generally, if you have a low Dry reading and the Wet test did not raise the reading, you most likely have a valve problem. If the Wet reading brought a low cylinder up much more then 15%, you most likely have a ring problem.
1200 RPM is a fast idle and can't be used to gauge the engine. You should be much slower (800 to 900 normal idle)
If you are using a "Unisyn" gauge to show the amount of air going into each carburetor you first need to get both throttle plates very close to the same.
The method that worked best for me in the dealerships I worked in is as follows.
Back off both throttle stop screws and the fast idle stop. loosen the linkage between the carbs so each work independent of each other. You can confirm by feel that both throttle plates have closed all the way. (most of the time you can also tell by opening a throttle and let it slam shut. It will make a different sound when it is closing all the way)
Now, Lock the two linkages together and hand open the throttle and look that there is a free play before the throttle plates open when you open the throttle by the link from the pedal. You also want to open the throttle by hand using the link form the pedal to see that it opens both throttle plates exactly the same time.
Now you have the linkage tight and if you slowly open the throttle by the pedal link, you first have a free play then both open at the same time.
Now turn both throttle stop screws down to just touch a piece of paper (or a very thin feeler gauge) between the stop screw and the throttle linkage. Now you have the linkage in sync and the throttle stop screws in sync but not touching the throttles yet.
Now turn each stop screw down one and one half turns exactly. This all gives you a tuning starting point.
If you have the adjustable needles in the pistons, you should set both at the same height.
Now, pull the choke all the way out and start the engine and set the "Fast Idle" screw ONLY. Set it at 1200 to 1500 RPM (which ever you prefer)
After the engine temp is up take the choke off (confirming that the fast idle screw is NOT touching the linkage)
Now, use the throttle stop screws to set the idle at 800 to 900 RPM. (BUT, YOU MUST TURN EACH STOP SCREW EXACTLY THE SAME AMOUNT EACH TIME YOU MOVE THE STOP SCREWS)
As soon as you get an 800 to 900 RPM idle, check the amount of air going into each carb with your Unisyn gauge (or a hose in the ear or what ever method you use to balance carbs with)
ONLY now can you adjust one stop screw at a time.
The final test is to lift the piston on one carburetor with the "lift pin" (some carbs don't have them. in these carbs I use a small flat blade screw driver to insert under the piston and rotate the screwdriver to get an exact amount of lift of the piston) If the engine picks up speed and stays fast, the OTHER carb is too rich and needs the needle reset leaner. If the engine dies, the OTHER carb is either too lean or you were not careful enough balancing the throttles.
When you have everything right, the engine will drop down from the 800 to 900 RPM to running like an old John Deer tractor. Rev the engine to clear it out and check the other carb the same way.
My method of lifting the front piston to check the OTHER carb is not the method stated in most manuals. But is has served me well for over 15 years working on as many as 4 or 5 multi carb cars a day for 15 years in dealerships and my own shop totaling 50 years.
Howard
---------- FOLLOW-UP ----------
QUESTION: Prior to gasket change all compression was 170 or 180 range one was 150 did not check compression as advised (will have to borrow one).... But did notice front piston is a bit higher with bigger gap....doesn not change regardless of any adjustment.......car ides approx 1000 rpm but front still twice vacuum as back.....will check compression as advised but any other thoughts
AnswerThe height of the piston is totally controlled by the volume of air entering that carb. If more air is entering and the piston is higher then it proves that the carbs are not in sync and that throttle is open too far.
Some models had a spring loaded round valve in the throttle plates for emission control and these failed often allowing air in, even when the throttle plates were closed. So you should look to see if your carbs have those valves. Most mechanics either soldered them closed or replaced the throttle plates with solid ones. It does not effect the emission tests of most states.
That is the only other reason I can think of to make more air enter that carb then the other.
Howard
