David Butcher's Datsun Roadster SU Carburetors

Thu Sep 05 22:47:00 PDT 1996

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:

  1. When the dashpot damper plunger or the dashpot oil reservoir walls wear, the carbs will become momentarily too lean upon acceleration. This is due to the requirement to provide a richer mixture upon acceleration, which is satisfied by the restricted air opening created when the dashpot is struggling against the damper action. If the dashpot opens too rapidly, the vacuum will diminish and less fuel will be pulled from the bowl.
  2. If the dashpot damper oil is low, the carbs will become momentarily too lean upon acceleration.
  3. Dashpot function does not affect mixture at a constant speed.
  4. Dashpots can close too rapidly upon deceleration with no negative effect, other than a slightly richer mixture, but opening too fast on acceleration can seriously affect performance and drivability. "Bogging" and backfiring are both symptoms of suddenly lean mixtures during acceleration.
  5. When the needles and seats controlling fuel flow wear, the carbs will gradually richen.
  6. When the throttle shaft bushings wear, the carbs will gradually become lean.
  7. At full, constant throttle and maximum RPM's, the only significant factor affecting fuel mixture will be the fuel flow, as the venturi size is fixed by the design of the carburetor. Fuel flow is affected by seat and needle size, seat and needle wear, and fuel level in the float bowl, and carburetor adjustment.

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.

Other Tips:

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


This information is provided in good faith but no warranty can be made for its accuracy. Make these modifications at your own risk! If you notice something incorrect or have any comments, or information to add to these pages, feel free to mail me.
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David Butcher
davidbu at www.los-gatos.ca.us
Tel (408) 978-5495
Los Gatos, California 95030
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