67' 250S Accelerator pump question - PeachParts Mercedes-Benz Forum

flacollect · #1 12-28-2008, 08:09 PM

Hi guys.... My accelerator pumps both work, but one seems to push more at a regular rate than the other. I tried bending the adjustment, but that doesn't seem to make it squirt any better.......any suggestions? they are both new pumps... I'm trying to get that stumble to quit on take off...I think if i can get more gas AS SOON as I touch the gas, I can get that stumble to quit; however, I keep messing with these pump levers, and can only get so much out of them........any suggestions on how to get more gas out of the acc pumps? thanks

Dave

Todd Miller · #2 12-29-2008, 12:21 AM

I'm not familiar with the accelerator pump adjustment on these "big" carbs. But with any dual carb set up, it is absolutely essential that both carbs be in perfect sync with one another. There is no substitute for this and it's the first step in dual carb "tuning."

Once you have them sync'd correctly, then see if you still have a stumble on acceleration, and if you still have one pump supplying more fuel (or fuel for a longer duration, or whatever your current issue is).

230/8 · #3 12-29-2008, 06:55 PM

Like Todd said, balance the carbs then carefully adjust the idle mixture evenly between both carbs. If your stumble persists then check and replace your ignition points if they show the slightest bit of pitting. Set the dwell and timing and you should be good to go. The only way to measure accelerator pump volumes is with a special tool that catches the pump shot at the nozzle. No one has these tools and you really don't need to check it that closely. Just make the previously mentioned checks and you should be fine. Also, if you are bending something then you are doing something wrong. Inside the carb is a clamping screw on the pump lever shaft. Remove the carb top, check that both levers are contacting their pump rods at the start of the throw and if not adjust the offender accordingly by moving the lever on the shaft.

230/8

ja17 · #4 12-29-2008, 08:06 PM

Hello Dave,

If your pumps are working correctly you should get a good strong stream out of each tube as soon as the linkage is activated. Bend adjustment of the arm over the pump is to adjust the begining of pump action but should not effect volume of the fuel stream.

I like to take the discharge tubes completely out (they have a tappered base and are just pressed in). They are easily removed with a slight tug using needle nose pliers. Next you can run a fine wire down the tubes to clean any hard deposits. Blow some compressed air through the jets (backflush). Activate the carb linkage to eject fuel from the open discharge ports before re-installing the discharge tubes. Re-install the discharge tubes aim them and test, next set them firmly in place with a "tap" on the ridge at their base (screwdriver and small hammer be gentle not much force required).

Early Zenith carbs like in your 250S had leather pump cups. These wear from use or dry out from lack of use. Replacements are neopreme rubber and are more resiliant. Hopefully you have the later.

Good luck,

flacollect · #5 12-29-2008, 09:09 PM

Thanks guys. I do have the new neopreme pumps cost me 108. each. I have 4 older ones now I'm thinking might be better. No leather. The pumps are squirting. The one isn't squirting as hard as the other ..I was thinking it wasn't adjusted correctly. I have been putting shims under the pump lever on the outside to keep from going inside and see if it helps, but to no avail. I have a pertronix and the ignition timing is spot on 6 degree btdc.

When I take off in the car, if I ''wiggle'' my foot on the gas pedal a little it runs fine. It seems like the accelerator pump isn't engaging enough fuel. I can't seem to figure what else it could be.

I'll keep at it....Car runs good though. Pretty amazing for sitting for a long time, and I magically made it go lol. Kinda : )

thanks again

Dave

1967 - 250S

ja17 · #6 12-30-2008, 12:06 AM

Hello Dave,

If timing is right, squirters are right, I would try a fresh set of plugs. Even a fairly new set can begin to cause problems if they hve been subjected to poor mixture conditionds during set-up. This an inexpensive check, and you can always re-use the originals latter if the new ones do not solve the "stumble".

Try some non-resistor NGK 5BPES . In addition use an ohm meter on all the spark plug wires and ends. The resistor ends on the original type wires do go bad. Original spark plug wires are tin coated, solid stranded, copper core, with screw off ends. Do not use carbon wires.

Arthur Dalton · #7 12-30-2008, 12:36 AM

That would be BP5ES.......

flacollect · #8 12-30-2008, 10:24 AM

Do not use carbon wires?? How do I know? I got some wires on ebay they seem to work fine.......not sure I understand why these wires would be bad...???

thanks

Dave

Todd Miller · #9 12-31-2008, 02:16 AM

Dave, Dave, Dave...
You need to sync the carbs, then if you still have a problem, pursue it further.

By synchronizing, not only are making the carbs have their throttle plates open the same amount at idle, but you are making the throttle plates open at the same time. You are also setting the idle air screws at exactly the same position (if equiped) and the mixture screws at exactly the same position. Both carbs need to be identical in operation and setting to one another.

Once you accomplish that, then you make 100% certain that the linkage which connects both carbs together, does not hold one, or both carbs partially open once it's reinstalled.

This is a big deal, and the car will never run properly until the carbs are synch'd correctly.......no matter what other parts you throw at it.

There are half a dozen things that will cause your lean stumble on acceleration. Wiggling your foot simply proves that the problem is due to a lean mixture. That momentary lean condition can be caused by......wait for it......half a dozen things.

Re: the carbon core plug wires. Carbon core wires are typically very high resistance. Carbon core coil wires typically fail internally and can cause your problem. In a points ignition car, you want it the way the factory had it. Solid core wires with resistor ends, and non-resistor plugs. Unfortunately, you can no longer buy non-resistor plugs in the US, unless you find someone with some new old stock.

Use caution when buying non OE parts. I can't tell you how many times I've fixed drivability problems (like yours, and others) by simply removing all the wiz-bang, aftermarket, look-a-like, Bosch wanna-be, cr@p that people buy and put on their cars.

One last thing. Are you sure 6BTDC is correct for your idle timing? My '66 Coupe is set to 30BTDC at 3,000rpm+ and that winds up being 2-1/2ATDC at idle with the vacuum retard. I'm not saying your 6cyl is set up the same way, but you might check the book spec.

Arthur Dalton · #10 12-31-2008, 07:32 AM

One of the things I always like about Benz using resistor plug connectors with solid wires is all plugs get the same uniform feed with the same resistance , regardless of wire length..carbon wires have resistance by the foot, so a short plug wire is not even close to the resistance of a long plug wire, and all the wires are different length.

B/c of the difficulties of getting non-R plugs lately, some guys are having luck on vintage stuff using the R replacement plugs by simply eliminating the original R plug end connectors to standard ends, while still using solid wires...the results are not in yet.........but it still gets you an R factor at the plug while still being able to use solid wires ...which are the best, specially for these older, low output ignition systems............

mbgingerbread · #11 12-31-2008, 10:02 AM

I am currently running the Pertronix unit in my 71 280se M130. Is there an ideal setup for the ignition wires using this point's replacement setup?
I have a good set of carbon wires from one of the reputable (?) import houses and NGK's gapped at .028. The coil is the 3 Ohm Pertronix flamethrower wired direct to the distributor. I also have a separate ground from the distributor to the chassis..

Todd Miller · #12 01-01-2009, 01:06 PM

With the Pertronix ignitor and their coil, you can run your gaps waaaaay out to .035" or wider. You'll know when your gap is too wide because it won't start as quick cold.

Use the advice above with regards to your plug wires. Personally, I wouldn't use carbon core, at all.

You can buy copper and steel core wire in bulk, from places like NAPA (Beldon wires) and make your own using new coppe connectors and rubber boots. Then get online and find any number of sites selling OE/Vintage looking screw on resistor ends.