Questions About Weber Accelerator Pump

[frankc]

I was installing the Accelerator Pumps on my Webers after receiving the new PMO pump covers (separate thread), and noticed something odd on one of the housing.

Notice in the photo how the orifice on left (feeds right pump jet) has what appears to be a casting defect in it, or was in some other way damaged. My concern is that when the throttle is pressed and then released, it appears that the inner diaphragm is suppose to seal off these three holes so that the expanding housing chamber will draw fuel from the float bowl, and not try to suck fuel/air back out of the pump jets.

If my observation above of how the pump works is correct, should I consider replacing the housing? I have about 15K miles on the carbs and never really noticed any issue with the accelerator pump operation, but I'm not sure I would since the defect would only affect one cylinder.

I suppose the first thing to do would be to perform an accelerator pump volume test for all three jets, and see if they are in spec and equal. I see that PMO sells the pump bodies, so at least replacements are easily available if I need one.



Closer view:



And while I'm on the subject, the new diaphragms I received from PMO are different from the ones I received from them in the past. As you can see in the photo below, the new diaphragm (red) has a taller contact point for the pump lever cam. So will I need to re-calibrate my accelerator pump volumes now? The diaphragm travel should be the same distance, just starting from a different position, so I'm thinking no. But I wanted to get opinions from people who may have already experienced this.

[Steam Driver]

For what it's worth here is my experience with Weber accelerator pumps, based on long 914-6 experience. But that was a long time ago so I may be a little foggy on it.

First you will likely find that the pump quantity on the three throats will not be equal. There is no way to correct for this. The center one will likely be the strongest since it is closest to the pressure source (diaphragm). Don't worry about it; close is good enough. The pumps are only intended to compensate for the momentary lean condition caused when you dump the throttle open. The regular jetting quickly takes over. But this brings up another issue, the injection quantity itself.

When I had the -6 the contemporary tune-up specs, everywhere you looked recommended (and here's where that memory issue comes in) an injection quantity of something like 1 - 1.5cc/stroke. For my car at least this was WAY too much. The first time I cleaned the carbs and set them up that way the bog on trying to accelerate was horrendous. I kept leaning and test driving and finally settled on around .38cc/stroke.

So how do you know? If you get on it and the engine bogs and letting up on it makes it worse, i.e. the engine just has to "work through it," then the pumps are too rich (too much injection). On the other hand if you get on it and there is a severe bogging or hesitation that goes away a little or a lot when you let up on the pedal, then the pumps are too lean and need to be injecting more.

I believe I generally took my injection measurement at the center throat and let the others fall out wherever.

[1QuickS]

You can get squirts equal (we routinely achieve this) but everything needs to be in good order including: chasing he squirter nozzles clear, sanding the aluminum discs to be flat, servicing the inlet check valves so they close reliably and by resurfacing the back of the pump bodies as described below.

Obviously the nipple is damaged. You can correct this by:

• remove brass tubes by inserting the shank of a #34 drill into the inside diameter of the tube
• extract the tubes by gripping with side cutters (the drill shank prevents collapsing the tube) and levering out
• sand back of pump body until all three nipples are coplanar
• reinstall brass tubes
• blow sanding debris clear of fuel galleries

[frankc]

Steam Driver,
I know that I checked the pump volumes when I installed the carbs many years ago, but I don't remember now how close the volumes were across all three barrels.

Thanks for the tip on telling the difference between too rich and too lean on the pump volumes. I'll have to check for that when I get everything back together.

Paul,
Thanks for your advice, as always. The damage on the nipple looks rather deep to me. It appears that I would have to remove quite a bit of material to have the three co-planar again, and then need to bevel the ID.

If your opinion is that the damage will have a noticeable impact on the pump operation, then I am ok with simply purchasing a new part (assuming the part PMO supplies is compatible with the Webers). I just wanted some confirmation that this is something that should be either repaired or replaced, and I would prefer to do it now while I have everything apart.

Also, is the aluminum disc you refer to the one that presses against the inner diaphragm?

Thanks.

[1QuickS]

Bevel isn't a big deal, just sand pump body on flat, hard surface (glass plate, table saw top, etc.) with 120 grit or coarser until the nick is almost gone. Switch to 220 to finish it out. The gasket is complaint so it will seal fine. You can lightly sand the tips with some 400 grit to soften them as long as the ends remain coplanar.

The nick isn't so deep that the body is not serviceable.

This surfacing is something I always perform since the aluminum disc (yes the one that presses against these three nipples) needs to be flat AND it needs to pivot on the two little fingers below the three nipples. If the two little fingers are not coplanar with the nipples and the disc isn't flat then the hinge action will not be even and will thereby expose the nipples at different times which leads to unequal squirts.

Note that the spring that presses against the disc is not centered on the disc but between the three nipples and the bottom of the disc. The disc acts like a flapper valve and pivots at the bottom.

Another item leading to unequal squirts is if the ends of the spring are not parallel to each other. If not parallel then the force on the disc is not centered on the nipples and will again lead to unequal squirts.

The devil is in the details.

I just noticed the replacement outer diaphragm...the hex on the outer portion is too tall and will limit pump action. The one on the left (brass, cylindrical one) is the correct height. Trust me I know and Richard Parr knows this as well since I informed him of the issue. You can shorten the height by filing once you disassemble. The top surface needs to be fairly flat and perpendicular but not critical. The tall one is good for his PMO carbs but is not good for Weber accelerator pumps.

[frankc]

Paul,

Thanks once again for all this wonderful information, and for the description of how the accelerator pump aluminum disc (reaction plate?) operates in detail. I did notice that the spring was off-center, but did not notice the two little pivot fingers until you pointed it out.

And thanks for commenting on the new style diaphragms. I was concerned about the height of the hex contact point affecting the pump operation, which is why I posted the pictures and asked that question. I'll measure the height from the old diaphragms and use a Dremel to cut off the excess. Does your comment about the new diaphragm functioning properly in the PMOs but not Webers imply that the PMO AP housings and/or covers are different than Webers? If it's the covers, then perhaps I'm ok since I now have the PMO covers due to my plating debacle.

I was also concerned that the nylock nut on the back would bottom out in the housing (especially with the taller hex), but there appears to be sufficient travel.

[1QuickS]

I've had trouble with the tall hex in Weber top covers in the past. New hex height is about 0.030" (memory) too tall. Best thing to do is to assemble diaphragm with top cover onto the throttle body and check to see that the cam on the lever arm achieves full travel without anything bottoming out inside the pump.

There is adequate relief to clear the nut on the flip side which you have found out for yourself.

I checked PMO top covers and compared to Webers and was surprised to find they seemed similar...however I did experience the issue as I described and have been getting shorter hex heads in the diaphragms from PMO since then. Just reassemble accelerator pump as if all is OK and check to see that nothing binds up in the linkage. If you do have an issue then the Dremel is your friend. I do recommend disassembly so that any heat associated with grinding of the hex does not damage the diaphragm material.

[frankc]

Originally Posted by 1QuickS

...however I did experience the issue as I described and have been getting shorter hex heads in the diaphragms from PMO since then.

Paul,
Are you saying that PMO can supply the diaphragms with either short or tall hex heads? If so, this will be helpful to know for future orders.

Originally Posted by 1QuickS

I do recommend disassembly so that any heat associated with grinding of the hex does not damage the diaphragm material.

Yes, I had planned to disassemble them anyway to ensure I get a square cut. I guess that's one advantage to having the nylock nut instead of a rivet.

[1QuickS]

Yes, tall and short hex length. Best to assemble with tall hex and see if you get full stroke before shortening.