Monojet carb question, accel pump

[pbjsammich]

Just picked up a 71 C10 with a 250 equipped with a 7040011 Monojet. Truck runs pretty poorly....there's a very evident missfire on idle. Stab at the throttle and it falls on it face. Slowly rev it up and it's fine. I think the points dist is showing it's age, so I have an HEI in the mail. Did the usual checks for vacuum leaks. Gas in the tank was old, so I bought a carb rebuild kit and ran into some issues:


One thing I found upon teardown was that the plunger for the accelerator pump (item 34 below) was stuck in the cylinder, off the piston. I removed the plunger from inside and installed a new plunger on the piston. Put everything back together and started it up. Ran a bit smoother this time around, but still having the same problems with stabbing at the throttle. Tore it down one more time and again, I found that the NEW plunger was off the piston. Both old and new are the blue colored plungers FYI.


The piston has a return spring located underneath, but no garter spring. Looking at the piston design, it appears a garter spring would be needed to hold this in place??

Or maybe my return spring has lost some of its gusto?

Do I need to "lube" the cup up a bit before I get some fuel in there?

Or am I missing something else here?

I'm an amateur at best when it comes to carb rebuilds. Any monojet gurus out there?

[Wrenchbender Ret]

Ive done many carb o-haul & repairs but had never had this problem till last week. It happened on a Q jet just like you describe. I found that the passages from the pump to the discharged were plugged up. It seems like the cups we get now are too soft. The Ethenol fuel probably doesnt help. I do put a little oil on the cup when I do one. It lubes it till the gas gets to it. I put some gas in the bowl this time & worked it quite a few times to hopfuly make sure it wont happen again. Use tag wires to clean out the passages.

[camrycurt]

I'm no monojet guru, but I did just complete my first carb rebuild on my Rochester BV monojet over the weekend. Different model from yours, but I'd guess they use a similar concept.

My accelerator pump did have two springs, one on top and the one underneath. You can see the top spring (#32) in the attached diagram for my BV carb. It's hard to tell in the diagram for your carb where they want spring #33 to go, but maybe that's your missing pump plunger spring?

In my carb rebuild I found I was missing the accelerator pump return spring and ordered one for $5 from Mike's Carburetor Shop online.

I also lubed everything up with some shots of silicone spray as I was assembling it all back together and haven't had any issues so far (knock on wood) with the rebuild.

Hope that helps.

[pbjsammich]

I appreciate the reply guys. Good to know that I'm not alone on this.

Ret, I did lube up the cylinder much more this time around and it seemed to help a lot. Thanks for the advice!!

Curt, hard to tell in my diagram posted but there is a spring on top and below the cup. If my problem persists, I'll likely try to beef up that return spring. Seems mine is the correct diameter to the seat of the cup, but has lost some of it's "spring-i-ness" hehe.


I must admit I cheated a bit last night. Since I ripped the new cup that came in my rebuild kit, I went ahead and reinstalled the old cup (still the blue one) as it was still in pretty good shape. After playing with it a bit, it appeared that once lubricated properly inside the cylinder, the bottom spring did it's job. So, I put a little dab of superglue on the cup to keep it in place, at a minimum, until I got the pocket filled up properly with fuel. Not sure if that will eat away at that cup or not, but we'll see. Truck is running considerably better....so far so good.

[coreybb13]

looks from your diagram like there should be a spring and clip above #34, it sounds like parts 32 & 33 are missing in your setup.

getting ready to dig into mine, where are you finding parts for yours?

[pbjsammich]

Originally Posted by coreybb13

looks from your diagram like there should be a spring and clip above #34, it sounds like parts 32 & 33 are missing in your setup.

getting ready to dig into mine, where are you finding parts for yours?

In the midst of rebuilding mine, one popped up on CL that was professionally rebuilt so I just bought a new one. Truck runs great now. The guy I bought it off of gave me another complete carb. I have 2 spares if you need parts. One was the problematic one I talked about above. The other appears in great shape, however I haven't run it. PM me if you're interested in either. I'd like to keep one as a spare but have no use for 2.

[PanelDeland]

The dist will probably help as much as the carb work did.I found that the Vac adv on mine wasn't working and even when I replaced it it still had a bit of hesitancy.Once I went to the HEI it worked great.

[shawno72]

Ok I just went through the same kind of problem with my 292. What I found when I disassembled the carb was the metal piece that pushes the accelerator pump down was bent upwards and no longer had a 90 degree bend to it, see pic I circled the problem area. Once I straightened it back out and made sure the point gap was good with the timing set at 4 degrees advanced she run like new.

[VetteVet]

Here's an excellent HEI 6 cyl swap by Shifty.
http://67-72chevytrucks.com/vboard/s...d.php?t=148346