Rochester 2g accel pump shot
 

Working on my 6 carbs before spring hits. Want to try bigger accel pump shots to launch my 3500 lb car. My edelbrock x-2 intake has 2 big plenums, plenty of air but could probably use more fuel for launch. I have read that you can drill out the squirt holes in the venturi cluster where the pump shot squirts out and get a bigger shot. Circle track guys seem to mess with these carbs more and I have read on almost all the modded 2g suppliers they say the accel pump shot has been increased. Has anyone messed with their tripower carbs in this way and care to share any details? I read somewhere to enlarge holes .005 at a time until 60' times stop improving. Sound right? Impressed with Slowbirds dads tripower gto. Anyone know what he has done? I know about using holes closest on the pump arm.

I have owned and worked on a fair number of tri-power set-up over the past 30 years. I hope my comment isn't too vague to be any good. I remember using accelerator pumps from late 1970's Buick V-6 engines. The old 231 engine before they went to the dual jet. The advantage was they were shorter and had a stiffer spring on the pump body. This would position the plunger higher in the bowl and the stronger spring would push more fuel before compressing the spring. I wish I could be more specific on that pump. I will look in the next few days and see if I still have one. I do remember this mod was helpful on my 421 HO car with a fairly hot cam.

Opening up the holes in the cluster doesn't increase fuel delivery, it dumps the fuel sooner.

The shorter pumps increase fuel delivery compared to the longer ones.

We sell the shorter pumps here. I would also recommend eliminating the pump feed system, as they have a bad habit of leaking a lot of fuel back to the bowl when the pumps start moving down.......Cliff

The last few years,it has been almost impossible to get a decent accellerator pump for the 60-65 center carbs. They are all junk. Been meaning to call you Cliff to see if you sell any.

Thanks for all responses.
Cliff, can you elaborate more on eliminating pump feed system. I might try both larger holes and shorter pumps.

Cliff can correct me if this is wrong but I believe the feed system he is referring to is the bottom fill that uses the inlet check ball to seal the feed when the pump pushes on the fuel. There is also a fill slot that will fill the pump area from the side/top but you must use the proper type of pump for that to work. In order to fill from the side/top only, you need a pump with a "floating cup". This allows the fuel to fill over the top when the pump is at rest, Most of this design does not use a garter spring in the cup. Works much like a master cylinder as far as how it fills. My guess is that the fill area at the bottom of the well gets corroded and leaks after 40 years of use, and that is Cliff's concern. To block that feed and eliminate the possible leak, I think you could lightly tap an extra outlet check ball in the inlet. It is larger and would block that passage. I have seen that done and always wondered why it was blocked, now I know. The system works fine with that blocked and a proper pump. As Cliff said, opening the cluster just delivers the same shot faster. That may give you bog, stumble if the pump shot runs out before the main system and power system are flowing enough fuel. I would concentrate on a larger pump shot, with Cliff's shorter pump and possibly a slightly higher float level to richen all the circuits a little.

If you take a small steel ball bearing that is approximately the same size as the pump inlet check ball you can restore the pump seat (after glass beading the corrosion away).

You drop the steel ball into the Pump inlet check location, you then use a thing long drift punch to smack the steel ball on the seat, and then you remove the steel ball and replace it with the proper aluminum pump inlet check ball.

No messing around with chebby type pumps, possibly putting the wrong pump in the carb after blocking the pump inlet passage and having no pump shot at all, and finally having to precision set the pump location in relation to the feed slot.

All that bs, you might as well put a chebby center section on the carb and run the engineer designed chebby parts.

Tom Vaught

ps If the pump check ball was driven into the passage effectively sealing it, you can drill the casting from the bottom and remove the ball and then pean over the drilled hole. No one will ever know that it was drilled.

I like the idea of restoring the bottom feed. Pumps with the garter springs seem like the best idea to avoid leaking around the pump wall. Other than a shorter pump, any other ideas to improve, enlarge his pump shot?

When he hits the tri-power he will have 3 pump shots delivering fuel to the engine, more than enough. With a properly calibrated functioning center carb pump shot he should have excellent drive manners during part throttle accelerations with a stock set-up.

Tom Vaught

I am using 6 small base 2g carbs. These are all bastardized various gm carbs. The 4 outer ones have been converted to progressive secondaries. It's like running 2 sets of tri powers. The carbs have no check balls under the accel pump. The inlet to the well is on the side of the bottom of the well. The carbs have been working pretty good, I just want a harder stronger launch and better 60' time. I have an adjustable clutch pressure plate that is set pretty loose for launch and then has weights on the arms for more pressure as rpm's are increased. Any tighter on the clutch and it bogs. Was all working ok until I switched from a solid ft cam to a hydraulic roller cam. I started bogging badly with the new cam and just figured out (I know I'm a little slow in the head) that the power valves were not opening due to higher vacuum from the hyd roller. Anyway the way I see it, The clutch was covering up a weak launch even when things were working ok with the old cam. 60' times were around 1.8. 45000-5000 rpm launch. Huge dual plenums so that's why I think I need more gas at launch.

If the carbs were originally pump inlet check ball style carbs then without the check balls a lot of the pump shot fuel will just return to the bowl vs going to the shooters. I hope that is not what you have. If you plug the pump inlet passage, as was mentioned) then you need the proper chebby style pumps in all of the carbs. Can't do a piecemeal deal on the carb pump strategy. Has to be one way or the other. If you do not have perfectly sealing end carb throttle blades you will get extra idle air flow and the pump shots will have to be larger to make up for that extra air flow on transitions.

Tom Vaught

End carbs are sealed up. Check balls weren't in the carbs when I got 'em but the discharge seems normal when the accel pumps are pushed. Looks like the pump wells fill up when fuel spills over a slot cut into the wall on the side of the pump well. Just wondering if anyone else has strengthened the pump shot somehow. Makes one appreciate all the options holley carbs have.

The bodies with the check valve design will function best with leather accelerator pumps. The cheaper neopreme pumps, especially those without an expander spring on the skirt, tend to flex when lifted, not allowing a full charge to be pulled into the pump well from the bowl.

As others have mentioned, the squirter size effects the timing of the pump shot, not the volume.

Also as others have mentioned, the height of the pump (changing the volume of the pump chamber below the pump) effects the volume of the pump stroke.

Different length pumps (pay attention to both the "free" length and the compressed length, not all articulations are the same dimension) can be used to effect the chamber volume. This volume may also be adjusted by bending the pump activation rod (from the throttle arm to the intermediate pump shaft) without changing pumps.

The check ball design tends to be much more reliable than the slotted design. The slotted design depends on the pump skirt flex to charge the pump chamber. If the rubber (or whatever the current buzzword is for rubber) skirt hardens due to the use of ethanol-laced fuels, then the flex is insufficient to fill the pump chamber, and the pump shot is adversely effected.

Jon.

So a rubber pump without an expander spring would be best for my slot fill carbs. Carbking, heard of anyone drilling pump discharge holes bigger? Thanks for all the help everyone, I'm understanding my carbs better now.

"The last few years,it has been almost impossible to get a decent accellerator pump for the 60-65 center carbs."

Same deal with the Q-jet.

The feed system isn't needed, and later carbs didn't use it. Q-jets never had a feed system, and they work fine.

I worked with the manufacturer and tested the pumps. We are able to get the good pump with the good seal, just like we do for the Q-jets.

Over the counter pumps have a low quality blue seal on them. It's soft and will swell up/fail once placed in service.

I don't stock much for the 2GC carburetors due to lack of interest, but usually have a dozen or so small and large pumps in stock.

The check ball feed system is just a poor design on those units, placing the aluminum check ball in the bottom of the pump bore. I used to bench test them, and 3 out of 4 will allow some fuel to go back to the bowl when the pump moves down.

I started blocking off the feed system and installing the latest design pumps, zero issues to date with any of them that we've set-up here......Cliff

Assuming that carbs have corrosion in the pump well area, it is totally understandable that the pump check balls would leak somewhat.

Like Jon said, a Pump check system IS more reliable if it is functioning properly.

That is why I eliminate the leakage with the new seat profile installed in the pump well and then continue to use the "check ball design tends to be much more reliable than the slotted design".

"Engineers do stuff for reasons" (Rochester Engineers doing work on Pontiac 2-BBL carbs knew that the pump check system was a better system otherwise they would have not designed it and put it into production.)

The pump check system was obviously removed on the Q-jet, for a reason. I was told by Rochester Engineers that the Q-jet was a very expensive carb to produce and they removed certain features to keep the cost down. (They still had warranty issues with the carbs that exceeded any of the 2-BBL issues, The well plugs leaking was one comment made).

Tom Vaught

Seems i remember years ago drilling a hole closer to the pivot point of the accell arm to relocate the linkage to provide more travel then shortening up the top of the accell pump rod to keep it from bottoming out premature and taking advantage of the longer stroke, thus providing more overall pump volume


But it was a very long time ago and might have that operation confused with another carb.

i might of just relocated the linkage hole closer to the pivot point

.

Thanks Formulas. The pump rod is curved so I think the best one can do is use a manufactured shortened one. I think I read the quadrajet one is straight and can be shortened.
OK, I pulled out all the secondary venturi clusters and used a pin vise to make the discharge holes bigger. They were around .024 originally and made them .033. Will get some shortened pumps to try also. Looks like they are 1/8" shorter. Will report back when I get my heads back and put top end back together. thanks for all the help everyone.

Tom, you are correct that a feed system would be better, IF it wasn't in the bottom of the casting and sitting in crud, water, and debris.

The engineers at Rochester knew this was a troubled spot, and later carbs do not use it, nor do they need it.

With a good pump using a high quality viton seal and garter spring, it will work flawlessly without the feed system. I tested them here, no lag anyplace in fuel delivery, and since all the parts are lower than the fuel level, the system stays completely full at all times.

Still very important to form a good seat for the steel check ball under the boosters, as it is higher than the fuel level, and will allow drain back and a delay in the pump shot if it is leaking.......Cliff

Back to the original posters question:

Drilling out the discharge holes will NOT give a larger pump shot, simply a quicker shot with less duration, with the possible result of a delayed hesitation.

The pump shot may be increased by increasing the pump cylinder volume. Increasing the pump cylinder volume may be accomplished by:

(1) Changing the height of the pump cylinder by: (A) using a shorter pump or (B) shortening the effective length of the pump rod or (C) changing the radius of the relation of the pump rod to the intermediate pump arm.

(2) Changing the diameter of the pump cylinder (more difficult than A above) by boring the pump well oversize, and fitting a custom made oversize pump.

Reiterating, the pump cylinder volume determines the pump shot volume; whereas the discharges nozzle orifices determine the length of time for the pump shot to be delivered.

As to the feed versus slot discussion: sometimes engineers are asked to resign a product so that the product will still work although less reliably at a lower initial cost. In the manufacture of a carburetor bowl, much less expensive to cast in a slot rather than machine a seat, a hole, and a cross passage after the casting step.

Jon.