305 QJet Problem

[76C10Stepside]

Finally got back around to working on my truck. It's a 1976 C10 Stepside with 305-V8 with 2Jet carb, transplanted from a 76 Caprice, and T-350 auto that I think is original to the truck. It ran like a champ for first 7 years. Rebuilt the top end second year and it runs very smooth.

Here's the problem: The engine wants to stall when I give it gas at a stop or in first gear. Once I get it going (like second gear), it runs fine. It's acting like there's a vacuum leak, but I can't find one anywhere, and no telltale sucking sounds. Pretty sure I already tried spraying starter fluid around to check for air leaks last time I worked on it.

Got a rebuild kit awhile back and replaced the throttle pump skirt, inlet valve and bowl gasket. Inside of carb is very clean and all parts look fresh. It squirts a nice twin jet of gas, so I've ruled out the throttle as the cause.
Today I cleaned the carb and carb base and replaced the carb-to-base and base to intake gaskets. The rebuild kit said to adjust the mixture screws on the separate base 2 turns out. It still stalls when I hit the throttle (did not try driving it yet). Tried adjusting out another half turn, no difference.
Crawled under the truck and checked the vacuum tube that runs from intake manifold (not on carb) to trans, and it seems fine. There is a short piece of hose that connects the steel line to the trans. I did not pressure test it yet tho.

All vacuum ports on the carb are either properly plugged or have good hoses on them. I don't think there are any vacuum leaks on the carb or intake manifold.

Can someone tell me what effect adjusting the mixture screws out too far will have on the throttle response, vs. adjusting them in too far? I would not expect mixture to have too much affect on throttle response, cause they are for idling. Any other ideas what the problem might be?

Thanks, and hope you all are enjoying this nice hot summer!

[beamn7]

Do you have power brakes? If so, disconnect the hose to the brake booster and plug it. See if you condition still exists. If it goes away, you need a new brake booster....

[76C10Stepside]

OK, will do. It it's bad, won't I hear a leak and lose brake pressure when I stand on the brake pedal? Seems fine, but will check anyway. Any other ideas?

[beamn7]

Not always. If anything, the pedal would seem slightly harder if it's a slow leak.
How about the vaccum advance? Hooked up properly?
When adjusting the idle air mixture screws be sure to go 2 to 2.5 COMPLETE revolutions. Complete meaning that if the slot is straight up and down with the srew completely closed, turning it to the next time it is straight up and down will only be 1/2 a turn....
May want to double check that as well....

[76C10Stepside]

OK, I'll test the power brake booster and the vacuum advance and distributor.
The VA is connected. I've been very fortunate with this truck. Hardly ever have to do anything to it. Thanks for the tips! BTW, that's a good looking Scottsdale you have, Jesse. :-)

[beamn7]

Thanks man! (that's the Silverado though, don't have any pics of the Scottsdale up yet, still doing all the body work....)
Hope you get 'er figured out!

[76C10Stepside]

Oh. Mine's a Scottsdale, but someone installed after-market body mouldings on it. Not sure what it looked like originally.

Last night plugged the line to the power brake booster. No change, still stalls when I hit the gas. Will check vacuum advance next.

[Corts60]

I would use a vacuum gauge to set the idle/mix screws. Hook it to manifold vacuum and adjust both screws individually until you get the highest vacuum reading at idle. That is the best way to do it.

As far the stumbling issue, is your vacuum advance hooked to port vacuum or manifold vacuum? I always go to manifold vacuum. Every smallblock I've ever owned ran wonderful this way. Afterwards, do you have a timing gun with an advance feature built into it? If so, I would check to see if you are mechanically advancing properly and at the right RPM. Usually, you want to be all in at 2000 rpm.

Just some thoughts and suggestions.

[motornut]

Originally Posted by 76C10Stepside

Oh. Mine's a Scottsdale, but someone installed after-market body mouldings on it. Not sure what it looked like originally.

Last night plugged the line to the power brake booster. No change, still stalls when I hit the gas. Will check vacuum advance next.

may want to check the sending unit
no leaks or cracks in gas line
maybe check the gas pump flow
or feed it from a gas can(is the gas old?)
i've found a buch of rusty crap stuck between the pump and the carb
drain the tube

[76C10Stepside]

Thanks for the suggestions, guys! What I got done today...

Tested vacuum advance (sucked on the hose): OK - "Points" plate rotates and returns freely. Also cleaned the electrodes on rotor and distributor cap. Note: This truck has electronic ignition (in distributor).

Couldn't find an easy/quick way to connect my tach/dwell meter to check ignition advance/retard, but I think V/A is OK.

Plugged the vacuum line to trans. No difference.
Sprayed starting fluid around the area to test for air leaks, none found.
Used a hose as a stethoscope to listen for leaks, none found.

Corts60: The V/A is connected to a port on the right-front corner of carburetor. It's always been there since I've had this truck, always worked fine. The one port on manifold that has an available fitting is too big for the V/A hose. This truck is a Frankenstein I bought off a used car lot. Original 350 was replaced with a 305 with "76 Caprice" painted on it by the wrecking yard they got the engine from. There is no bracket for the accelerator cable (must have been mounted to firewall on the Caprice). I could not find one at the wrecker, so made a crude one.

Next thing I will do is pull the spark plugs and inspect them for problems, and probably replace with new.

[Weasel Diesel]

There is a check ball for the accelerator pump in the carb. If it is stuck, or loaded with trash, it will cause the problem you spoke about in the O.P.

If you are not familiar with it,... Pop the air horn off the carb, remove the float and it is located under this screw.


Unscrew it ( it's a plug, no hidden springs ) take a small thin magnet and the check ball should stick to it and remove it. Shoot some carb cleaner with a snoot straw on the can down in the hole. Cleaner should come out of the venturies and also bubble up in accelerator pump well. Reassemble and see what happens.

[bnoon]

Thanks @Weasel Diesel for that reply. I'm having a similar issue on my '86 and thought I may need a full rebuild (though it's probably a good idea in the near future anyway). Hope it helps you too OP.

[76C10Stepside]

Excellent! Thanks Rick!!!

[76C10Stepside]

Hey, Rick: Haven't got around to removing the check ball and cleaning the pump system yet. Thought I'd read up on the 2-Jet and how/where the check ball comes into play.
So, it's in the pump discharge passage, between the bowl/pump-plunger and the pump jets. Hmm...

In the OP, I said the pump jets are squirting two strong streams of gas. I'll go ahead and clean out this passage like you detailed, but seems like this is probably not what's causing my problem. Guess I should finish reading the descriptions of how this carb works! Anyway, if you think of anything else I should check... Thanks!

[Weasel Diesel]

If you are getting a good strong squirt, then your ball is probably ok...... OH GEEZ, YOU KNOW WHAT I MEAN.... Quit giggling.....

[bnoon]

Hahahahaha nice!

[Malo83]

Might find something that will help you out here
http://www.florida4x4.com/tech/quadrajet/

[76C10Stepside]

Originally Posted by Weasel Diesel

If you are getting a good strong squirt, then your ball is probably ok...... OH GEEZ, YOU KNOW WHAT I MEAN.... Quit giggling.....

Too close to the truth!

[76C10Stepside]

DAMMIT!


Part of the crummy new base gasket stuck to the manifold when I pulled the carb back off for another go. It's just thick paper with some black junk on the outside.

[76C10Stepside]

Uh-oh. Put "QJet" in the OP title when I meant "2 Jet" (2GC). Anyway...
The plastic throttle plunger is mangled, still works though. It was ok last time I had it apart, and no problems getting it back together. Strange. This one is not like the photos from the Rochester manual or seen online.
Local auto parts stores are useless. O'Reilly's ordered what I hoped was the right part, but turned out to be a totally different part for some other carb. Turns out they don't even list my part. No luck with Napa, Autozone or Baxter's either. Hoping to get one from Carbs Unlimited, 30-40 minutes away, when I get a kit for another carb that needs rebuilding.
I did managed to get drop shackles (Superior Automotive brand, powdercoated) and Monroematics, about 3/4" shorter compressed than the old air shocks, and a roll of silver pinstripe for another project.

[76C10Stepside]

Carbs Unlimited lists pump plungers (accelerator pumps) for Rochester "2G" carburetors. In the pictures, they look like the ones in "Section 6B - Engine Fuel Rochester" excerpt of the 1956 Pontiac shop manual that I found online. I called Carbs Unlimited today, and the guy told me they don't have any. I even had to explain that a "pump plunger" (this is the label it's given in the shop manual!) is an "accelerator pump". I asked if he knew where I might source this part from and he said "get it from an old carb". WTH???
Already tried all the local wrecking yards and craigslist, no luck. Guess I will try to mill a crude replacement from brass rod stock. Ebay has a rebuilt 2GC for a good deal, also a new repro unit for not much more. Both carbs are less than HALF the price local sources want for a rebuilt.

Anybody out there with Rochester 2GC 2 Jet carb know where I can get a new pump plunger?

[76C10Stepside]

Made my own part to fix the throttle plunger out of a jet from an old oil furnace. Had to machine the old plastic part that MELTED (no idea HOW) and the brass piece, using crude tools (drill press, grinder, file, sandpaper).

[raws67]

I think I found what you were looking for at the junkyard today.

[76C10Stepside]

That's it. Lucky you! Note how different my old one was. No idea where it came from. It had the metal retainer that holds the top spring in place, but it would not stay on the rod.
I have no idea what the purpose of that top spring is, maybe give a little "give" on the way back up? Anyway, mine didn't work before and now there is no spring. We'll see how it works...

[raws67]

It looks like the spring will allow the plunger to compress at full stroke. Let me know how your re-creation works out and if you need this one I am sure it wouldnt cost that much to throw this in an envelope and get it up there to you.