The Stovebolt Site Forums The Parking Lot Tons 'o Fun Accelerator Pump

I have a '48 3800 with a 216 and I am having a bit of trouble with the carburetor. I have rebuilt it and have replaced the accelerator pump plunger one more time after that while trying to address the issue but I am still getting a stumble off of idle. Curious if I am missing something in the carb that might help remedy this issue. I plan to check timing tomorrow but other than that I am a bit stumped.

On the bright side, I started it again for the first time this year and it still fires right up (once fuel gets to the carb) and has no leaks! Got a little bit coming out of the rad overflow once it got up to temp but I think that was just from topping off the radiator a bit too far. Before this it sat in a barn for 20 years and last year it fired up without much trouble (after I converted to 12 volt) and no leaks then either.

Several possibilities. One is the check valve for the fuel inlet. I have one here that the fuel enters through the center of the accelerator pump. The problem is that it's installed backwards and permanently pressed together, so there's no way to remedy the issue. If you apply vacuum to the port, it closes off, and if you apply pressure, it allows it to pass to the back side of the cup. Another possibility is that the plunger spring is too light, too weak, or too short. The plunger spring does the actual work, while the linkage returns it to the closed throttle position. You CAN add washers between the spring and the plunger cup to increase tension or decrease the release distance of the linkage.

You didn't say what happens if you move the throttle while looking down the carb. Does it begin to squirt fuel immediately, or does the throttle plate begin to open before fuel begins to flow? If there is a lag, then add washers to the spring or check the linkage to make sure that the hook that holds the pump plunger begins to drop as soon as the throttle moves. Without having one right in front of me. It's hard to say, because I don't know where the fuel inlet for the pump is. I know I have one around here somewhere, but it's not at my computer!

Lastly, if you have your idle set to about 400 RPM like I used to do on my '50 (just because I can and I love the sound of it!) Then you idle is set too low, OR your idle mixture is a bit too lean. There are transfer ports that utilize the idle circuit passages. They're the long slots just above the throttle plate. They provide additional fuel to make the transition to the primary metering system in the band between idle and the amount of air flow needed to pull fuel through the primary.

Alright, did a little more digging today and I get a squirt but only once I get to basically full throttle, sounds like washers are my next 'try' because everything on the outside seems to move right away like it should. That is for the spring at the accelerator pump, correct?

Almost sounds more like your float level is too low or your check ball is missing. Does your carburetor have the plunger with the flat post, slotted for the hook from the throttle linkage, or is it a round rod bent at a 90 at the top that goes in a hole in the throttle linkage and held with a clip?

The return spring goes under the plunger. The actuator spring goes above the plunger and does the moving of the plunger. The linkage from the throttle does not push the plunger down. If it has the slot/hook type, the hook drops and the spring pushes the plunger down. If it is the rod type, the linkage pushed the rod down and again allows the spring to actuate the plunger. This provides a constant, steady stream of fuel.

I found the carb I have, but they utilized the 2 aforementioned styles. The carb kit comes with the parts to rebuild either style.

I have the flat post style. One thing that might be a contributor though is I do not recall a second spring on the plunger; although it seems like the thing might not function at all without both springs? I guess I'll be taking the carb apart after work tonight to do some investigation and check float levels, etc. I am fairly certain the check ball is present because I distinctly remember putting it in (although my memory is semi-reliable, at best :D). Might just do one more rebuild kit for S&G; since they are only about $20 (IIRC), it seems worthwhile since it'll be partially apart already.

There are 2 check balls. One for the discharge, located in the carb body, the other for inlet which is located in the plunger itself, but if you don't have a return spring under the plunger, you certainly won't have proper operation!

Just ordered a rebuild kit with springs for both sides of the pump; needed some choke and fast idle springs anyway so figure I'll just replace as many as I can for good measure. Will be a good chance to check for my check balls too (no pun intended). Will update once things are back together!

Turns out the springs were indeed the most likely culprit!

The return spring was present but both springs were extremely worn. Rebuilt the thing again and put in some new springs (and took some wire wool to the pump well for good measure) and everything seems to work as it should. Not back on the truck yet but I put some fuel in the bowl and I can already see a MASSIVE improvement in the accelerator pump function.

Lighter spring goes on the bottom.

If you snap the throttle plate open in 5 millliseconds, the pump flow should last about a half a second.