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I am trying to increase the amount of fuel for each shot of the accelerator pump. I have seen the extended arm solution, but am wondering what problems there might be with the method I am trying. It seems to be working on the work bench. By adding the spacer in the picture, the arm of the pump lever is pulled back farther, allowing it to have a longer stroke when actuated. The end of the stroke stays the same with or with out the spacer. I checked and with the lever pulled back farther to start, it is still in contact with the metal "plunger" on the pump itself, so the plunger starts moving as soon the lever moves. Will I wear out the diapragm more quickly? Some other problem?
I am not seeing a problem at this point, but I can sometimes have less than 20/20 vision.
You are not really increasing the travel for the pump but just starting it early or late depending on how that moves. Not sure it will help that much. Also you need extra space between the AP housing and the carb body to increase the capacity of the shot. I think what you have won't hurt but I doubt it will have much increase in the AP shot or extra duration. The extended arm does both and the extra spacers on the AP housing will increase capacity even more.
Essentially what you've got there is the equivalent of screwing the adjuster nut way in; like t_g_farrell said, it doesn't increase the volume (by much, if any), you're just retiming what's already there.
I'm curious if your modification limits the end-point of the AP travel. If not, perhaps you've just run across an alternative to the extension arm.
The optimal setup consists of increasing the volume as well as the stroke. Theoretically the stroke isn't quite linear (due to a couple things) and so some fine adjustment ability may be useful, although likely unnecessary. I've found it works pretty well to max out the AP travel (using layered gaskets) and then find an extension length that covers nearly the full range of movement. If I recall correctly, my current extension is 1-1/4" and it's just about perfect. I believe the successfully documented extension arm lengths are in the 1-1/4" to the 1-1/2" range.
After looking at the results closer, it may not be making much difference. The thorough cleaning I just gave the AP circuit, gave me much more (and reliable) fuel flow. Since my carb is stripped, including all the unused linkage arms, my linkage may not work the same as others? By adding the spacer, the lever starts farther back (per the upper arrow in the picture) than is possible by adjusting the nut all the way down by itself. The lever is still in contact with the plunger on the pump diaphragm. As soon as I start opening the throttle, the AP lever starts moving, which starts moving the plunger on the pump diaphragm. Whether fuel starts moving right away, I really don't know, but the diaphragm is definitely starting to move. The lever still goes to the same end of travel as without the spacer and I assume that is the same place as normal. After a small amount of travel on the throttle, the AP lever has already reached its maximum. I am guessing that the extended arm, allows the AP to be "active" over a wider range of throttle application?
I decided I wasn't too keen on the angle of the actuator rod with the extra spacer, so I removed it.
As mentioned in another thread, this carb is on a race car so streetability issues do not come into play. I was only looking to get more fuel in quickly to get the engine started when cold. I have been assuming that the hard starting is due to the enlarged primary venturis. It turns out the hard starting is not completely related to the carb however. Checked the compression because of increasing difficulty starting and found it is down to about 60psi, pretty even on all pulses. Only about 8 hours on the engine, so not sure what happened. Didn't check compression when I first installed it, so don't know if it is worse or not. Pulling it tomorrow and hauling back to the builder so we pull it apart and investigate.
09-20-20, 09:37 PM
