Experience Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.
Organizations Associate member SAE
EAA member
Publications Import Car magazine
Education/Credentials ASE Master Auto with L-1 certification up to 2000
Question QUESTION: My 1974,V12, E type will miss fire when left to idle for a few minutes after the engine has reached running temperature. The 3 plugs on the left side front will then be totally sooted up. I tried to balance the stromberg carbs with a Gunson carbalancer but the carb feeding these three cylinders is difficult to set. There is no suction when idle speed at 650rpm. The bypass valve diaphram on this carb is torn and I have ordered a new one - will this cure the problem?
ANSWER: Hi Ian,
The bypass valve is a decel valve and will not cause no vacuum. To disable it for test purposes, you can just put some kind of packing in place of the spring to hold the valve closed at all times if you like.
You need to run a compression test on all cylinders (be sure to hold the throttles open during the test)
Is the main top diaphragm good on that carb? Also check for vacuum leaks either with a oil gun or if you use propane keep a fire extinguisher handy because any small spark could light the propane.
Howard
---------- FOLLOW-UP ----------
QUESTION: The diaphragm is good on this carb. I will do a compression test. What would be the best way to do a vacuum leak test, and do I only do it on this carb.
Answer There are several methods that mechanics use to test for vacuum leaks and some use a combination of several to be sure.
A simple one is to use a piece of hose in your ear and search for hissing noise. Another is to squirt oil on all the joints while at idle and listen for an RPM change. (this one is messy and requires a wash afterward) Another one is to spray propane from a unlit propane torch and listen for an RPM change. (This one should only be done outside and with a fire extinguisher handy incase a spark ignites the propane. It has the advantage of getting into areas that are difficult to reach) Another method that is commonly use is to let the engine idle and very slowly restrict the available air going into the intake. If there is no vacuum leak you should be able to slowly slow the RPM down as you restrict the air. If at any point the RPM climbs up a little or smoothes out the RPM then there is a leak somewhere.
let me know,
Howard
