The Stovebolt Site Forums The Shop Area Engine Shop Rochester Monojet Question 250 6 Cylinder

My 71 C10 inline 6 has the factory monojet carb and is weeping gas after startup. Appears the gas is leaking from the pivot plate assembly for the accelerator pump. Does not happen until the truck is running for about 10 seconds...interestingly enough, the truck idles well and has some throttle response while weeping significantly. I though the float/needle were sticking.

I reset the float level a bit lower after cleaning the needle/seat and set it too low and not enough fuel to run more than a second or so. Reset to spec. After I separated the carb top from the float bowl assembly and removed the in between gasket I checked the accelerator pump on the bench it pushes gas out of the top of the float bowl assembly where the ball/spring/T seat for pump discharge. That opening is normally covered with the gasket between float bowl and carb top when put together and no evidence of leaking gas from there.

I presume the gas from the filled up float bowl is spilling over or something is blocked. Does the metering rod or idle tube relate to this type of condition? A rebuild is gonna happen but I sure like having some idea of problem before throwing stuff at it. It idles during the weeping...obviously not for long as I quickly shut it off as it"s raw gas on the manifold. Any ideas on the possible issue? And its been 30 years since I rebuilt a carb...:-) Thanks

Your accelerator pump arm looks fuel stained like my B was. It’s possible your over filling the float bowl and flowing out there. Check the needle and seat for debris. And verify your fuel pressure,

Make sure the float is not full of fuel.

I took a look at the new needle and seat in place and out of the bowl..... interestingly the "NEW" little wire that is in the rebuild kit that is offered for clipping onto the needle and connect to the float to "help" pull up the needle when the float rises seems to interfere with the seating of the needle. The original carb didn't have one and the idea is sound with today's gas and lack of lube, but the tiny wire has to sit in the screwdriver slot of the seat or it creates a bit of interference between the float tang and seat? I can't imagine that little floppy wire would stay in that small seat slot each and every time. Guess I can not install it and determine if that shuts down the float better. I am thinking the needle sticking and starving is better than raw gas running out on a hot manifold... yikes.

Well.... that was wishful thinking. Ran good for about 20 seconds warming up . God throttle response and suddenly gas weeping out again and pooling. Guess the rebuild may show me the culprit along the way. I am getting pretty good at removing the monojet and putting it back on though.

Lots of times when folks search online for a similar problem the end resolution is missing..... this is how this one got resolved. I did a fuel pump pressure test on the new Delphi "exact replacement" for the factory 6 cylinder fuel pump. Well dimensions are the same but the new pump is much more powerful (over 12 + lbs) compared to the old pump. The ethanol was getting frothy and the brass float in the little Rochester Monojet could not fight the pressure. I was told by the gentleman at the FLAPS that the new Delphi (China made I learned) pumps are all higher pressure than stock and with original equipment may create too much pressure. I cleaned up the old pump and did nothing else but swap them and the old 6 cylinder purred...20 mile run out and back. So much for thinking a new fuel pump would go good with the new brass float and bits. Aw Dang and Joe H thanks.

That float looks like it is way too high in the photo. It generally is about parallel to the top of the bowl when set to specs. I have never seen a float like that in a Monojet. I dought you use the wire clip with that float.

George

Problem solved, you are now officially an expert on Mono-jets next time it comes up. Take them apart a few times for the same problem, does get old, but you tend to learn on the way.
Thanks for the update.

Thank you for posting the update, you are now the forum resident expert on Rochester type M carburetors, congratulations!

The Rochester single barrel carbs, whether the type B or the type M (monojet) get a lot of negative press (most of it deserved), but occasionally, like this time, they are not guilty.

The import from China fuel pumps, fuel regulators, and carburetors all have one redeeming feature. They are quite useful if one has a strong right arm, and a rabbit problem in one's garden. The imported from China carburetor rebuilding kits are too light to do any harm to the rabbit. Maybe someone can find another "repurpose" for the carburetor rebuilding kits.

There is a company in the Boston area, Then and Now Automotive, that manufactures fuel pump rebuilding kits. I am pretty sure the kit components are made in the U.S.A. They also have reproduced many older fuel pumps. I do not know if they produce a pump for the stovebolt.

Jon.

If those new pumps can be dismantled, changing the spring inside will adjust the pressure. The camshaft lobe and the lever that rides on it provide the vacuum that draws the fuel from the tank. The spring and the size of the diaphragm it's moving is what determines the pressure in the line going to the carburetor. Apparently the manufacturers of the new pumps are using too strong a spring in their products. BTW- - - -shortening the length of the spring WILL NOT help- - - - -that actually makes it stronger. A spring with a smaller diameter wire would need to be substituted for the original one.
Jerry

Jerry - the ones I have seen are crimped together, not bolted. They could still be disassembled, repaired, and re-crimped, but why bother? If the manufacturer has no more integrity than putting out pumps with the wrong springs, why should we believe the rest of the pump is any better?

Good ones CAN be obtained from Then & Now.

40 years ago, I hand-built some special tooling to open the crimp on the vacuum motors used to operate the secondary carburetors on GM tri-powers, and then more special tooling to re-crimp them. While the arms were different, the diaphragms used in the vacuum motors which opened hidden headlights were the same. One could open a tri-power cannister, re-electroplate the cannister, remove the arm from the dead diaphragm, open one of the headlight cannisters, remove the arm, put the re-plated original arm on the new diaphragm, and crimp the cannister back together. I probably did 75~100 when we were restoring the tri-powers.

Jon

I was surprised by the China part of Delphi as I thought it was a GM mfg here....guess the buyer just has to sort it all out. Offshore isn't always bad...I'm told.....but I'm still willing to pay for USA quality. For the vendors..... Some tech specs even would help with pump pressure etc. Especially for the hobby guys like me. I thought " correct " replacement for a 6 cylinder 250 would take into account what the mfg. designed for use. That poor little mono jet float and needle seat combination was used to a gentle flow and suddenly it had a tsunami of pressure working on trying to start an underhood fire. Must be setup for those tons of aftermarket carbs used by many. I like the idea of giving the stock stuff some care and respect if I can. I should have paid heed to the old saying....if it ain't broke......