The rubber side of the washer...
The rubber side of the washer must be sanded to 3/16-inch thickness.This will be at the point where its small step is removed.
There is a small red rubber flapper valve in the pump cavity. It issucked open when the accelerator pump arm releases the diaphragm withthe closing of the throttle. This is what fills the pump chamber. Whenthe throttle is opened again, the pump arm depresses the diaphragm. Thiscauses pressure to close the flapper. Fuel is then pumped through acircuit to the squirters. That is how it is supposed to work. I was nowconvinced there was nothing in the manifold or the airflow in the carbcausing the problem.
On some engines, mainly small four-cylinder ones, there can be avibration frequency at some rpm that vibrates the flapper. This occursin a sequence where the flapper is rapidly activated, thus pumping fuelinto the squirters even though the pump arm is held motionless by thethrottle arm. I believe that curing the vibration patterns would have tobe done by experimentation at the factory or testing at laboratorylevel, something well beyond the means of most of us.
The best reason for using...
The best reason for using this particular washer is that one surface ismetal and the other rubber. The metal side can be joined to the metalpart of the diaphragm with a good epoxy. The rubber surface that willclamp the flapper shut won't cut through it. Be careful to center thewasher on the plate before the epoxy sets so that the spring won't bind.
The next step was to attack the symptoms rather than the disease. Iwasn't going to beat the vibration. Since the vibration was diagnosed asthe disease, and no antivibration pills were available, the solution wasto stop the flapper from vibrating. Obviously, the flapper had to workon demand; we couldn't just shut it off. In other words, its operationmust be positively timed.
A 1/4-inch long piece of 5/16-inch...
A 1/4-inch long piece of 5/16-inch OD steel fuel tubing may be needed insome cases to space the spring on the throttle arm. When all iscompleted, the pump should operate only in the first third of thethrottle opening. After that, the washer will clamp the flapper shut andthere will be no more pump discharge until the throttle is released.There will be no pullover or vibration-induced pumping discharging fuelthrough the squirters. Additionally, a large No. 37-No. 40 squirtershould be used. This is necessary because the washer has decreased thepump volume. Note the 5/16-inch piece of tubing wedging the throttleblades open. At this throttle opening, the pump adjusting screw has beenadjusted to hold the pump diaphragm fully depressed with the washerclamping the flapper shut. The spring will take up the additionalmovement of the pump cam.
The fix is in. The accelerator pump was disassembled. Carefulmeasurement determined that when the pump arm was depressed, a 3/16-inchspace between the diaphragm and the flapper remained. A 3/16-inch spacerwas attached to the bottom of the diaphragm with epoxy (I used J-BWeld). It needs to be small enough in diameter to fit easily inside thediaphragm return spring. It will need to have a 1/4-inch hole to fitover the diaphragm rivet. I used a small rubber bonded to steel valvecover hold-down washer from a Honda. The rubber side had to be sandeddown to the right thickness. Although a metal spacer could probably beused, I preferred this one with a rubber face to prevent cutting theflapper while the metal on the other side could be attached to thediaphragm with epoxy. The thickness is not extremely critical as long asit can reach the flapper. When the first step is sanded off of therubber on the Honda washer, it is the right thickness. I'm sure thereare other things that can be used for the spacer. This one was handy andworked well.
Once the spacer is installed, the throttle arm screw that activates thepump arm must be adjusted. To do this, adjust the screw till it justtouches the pump arm at the idle setting. Now the pump will work tillthe spacer contacts the flapper. After that point of throttle opening,no fuel will go through the accelerator pump circuit, whether fromvibration or pullover.
As the throttle arm continues to move, the spring on the adjusting screwwill absorb the additional movement of the throttle arm. The flapperwill now be held shut by the spacer on the diaphragm when the throttleis more than about 1/3 open.
With this system, you might want to try a large squirter in the Holleycarb. I have found a No. 37 to be about right in my application. Thisdumps the pump capacity quickly for good throttle response. Differentsize squirters will adjust throttle response. If the engine coughs, thesquirter is too small. If the engine hesitates a little before pickingup speed, the squirter may be too large. Now the carb can be jettednormally. The 71 jets seem to be about right. The 2.5 power valve workswell controlling the top end mixture.
The overall effect of these changes is to mechanically shut off theaccelerator pump after 1/3 throttle. The carburetor can then function asit was designed to, using the main jets and power valves for fuelmixture control through the rpm range.
Isn't it interesting how the simplest things are often the mostdifficult to trace?