Have you ever wished you could cure that annoying hesitation and backfiring on acceleration? I believe I have the cure.....
Before I get to the details, let me open up a discussion of how SU carbs wear. I believe properly adjusted carbs will run well, so this discussion only applies to carbs that have been properly adjusted and STILL don't run smoothly.
Here are the debate points:
That is all I can determine at this point. The focus of my research was on the hesitation and backfiring I experienced on acceleration. I am sure the problem is caused by a lean mixture, because leaving the choke out cured the problem.
I narrowed the problem down further by observing the action of the dashpots when the throttle was "blipped." This can only be done when the back plate of the air cleaner is removed from the carburetors, allowing you to look right down the throat of the carbs. I noticed that when the throttle was opened rapidly, one carburetor opened in half the time of the other. I switched the dashpot damper plungers, and the problem switched to the carburetor with one of the plungers. I had found the problem, listed as Number 1 above. Now the question was, how to fix it?
One answer is to replace the carbs. Of course, you already KNEW that answer ;-) The alternate solution I chose was to enhance the damping action of the plungers by machining brass washers to a slightly larger diameter than the plungers, and adding them to the existing plunger assemblies. Here is a drawing to illustrate the modification:
Top of plunger |======| || || || || || || || || || || ==== | | | | | | ------ <-- This is the new washer ||I have exaggerated the relative size of the brass washer and the existing plunger. The real difference in size was about 4 thousandths of an inch. I made the washers with a drill press an assortment of files.
As far as I can tell, the work paid off. The initial hesitation is almost totally gone, and the engine can be pushed hard from 1000 RPM on (testing only, hard acceleration at low RPM's is very hard on the engine). Backfiring has totally stopped. Drivability is MUCH improved.
I use LYNX (Mercury/Ford) air filters. They are larger than the stock filters, so they should flow better. They will also not seal as well as the original filters between the air filter plates. I slit a length of aquarium tubing lengthwise (wear heavy gloves, and count your fingers when done!), joined the ends with vinyl cement, and pressed the resulting gasket over the sharp edge of the plates to form a better seal. The I sandwiched the lynx filter in where the factory original was, and voila! I have a weird clutch master cylinder that the filter hits when the engine is really torqued over, but it just "dents" the edge of the filter, a cosmetic problem only.
Air Filter: FRAM CA3597
David Butcher davidbu at www.los-gatos.ca.us Tel (408) 978-5495 Los Gatos, California 95030