I think it's time for a new carb..

[agers20]

So, I just got my truck back yesterday from the mechanic. He did basically a tune-up for me, as well as replacing the water pump and fuel pump. I have a problem with the carb. It's rusty, idles high until you push the lever down some, and it seems to be running super-rich. So I need to figure out if a rebuild is in order, or to just order a new carb. It didn't help that the last time it ran was 2013. If it's a rebuild, what type of carb is on it? its an 84' 305. Or if a new one is in order, does a 500cm seem right? Also, electric or manual choke?

 

[75gmck25]

Your original carburetor should be a Quadrajet on a spreadbore manifold, but you need to verify. If it is a Quadrajet, take off the air cleaner and clean up the carb enough to get the numbers that are on it.

Rebuilding a Quadrajet is usually a good idea, but it sounds like yours may be in rough shape and need quite a bit of work. A good rebuild will not be cheap. When I considered replacing my Quadrajet I found that the 600 cfm Quickfuel Slayer carburetor like this one seemed to have a good reputation and price point. However, I kept the Quadrajet and retuned it. https://www.summitracing.com/parts/qft-sl-600-vs/overview/

If you do have a stock GM spreadbore manifold (for Quadrajet), most newer carbs are squarebore, so you will need a new manifold or a carb adapter. Use one with an electric choke so that you don't have to figure out linkage for a manual choke.

Bruce

 

[agers20]

well, this is what it looks like. the smaller butterfly is open almost all the way unless you push it back. that explains the high idle.

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[75gmck25]

The butterfly/flap on top toward the front of the carburetor is the choke. With an electric choke it should be open shortly after the engine is started. The two flaps behind it are the secondary air doors, and they are controlled by engine vacuum and tension on the air door spring. The carburetor venturi are down inside next to the intake, but you can see them if you look straight down into the carburetor.

Your Quadrajet looks very similar to the original on my '75 GMC, but yours is in even rougher shape. It is an older model that might not be original to your '84 engine. I made a trip to the pick and pull and managed to find a Quadrajet on an '86 3/4 ton pickup that was one of the last ones with no electric connections. It was in good shape, so I got a rebuild kit from Cliff's Performance and rebuilt and adjusted it for my truck.


Bruce

 

[Doppleganger]

There's a guy in Arkansas called Mountainman Fuel Injection and Carb - he does it all but his specialty is Quadrajets. Guys on the hot rod sites all talk about him - very highly rated.

This is mine before / after he did it. IIRC was about $239. I haven't put this one on as I'm leaning towards a TBI setup instead.

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[trukman1]

I've had really good luck with National Carburetors but I had to call service/tech. They asked every question you could think of before rebuilding the carb such as engine, modifications, tranny, vehicle weight, etc. (Just another option.)

The gentleman I talked to was named Eddie Obie. He even asked if I was wanting better gas mileage or performance. I used to rebuild Q-jets and wanted the internal plugs epoxied, new shaft bushings, etc. Cost was around $150 or so. Carb was absolutely incredible and runs excellent to this day. Just an option.

BTW, National rebuilds most carbs sold in auto parts stores or at least they used to. Not sure how much the cost is now since mine was 20 years ago.

Note: I wouldn't send them the carb unless they understand exactly what you want done. They will send you a box to ship the carb in since boxing incorrectly can damage the carburetor.

Q-jet carbs came in 750 and 800 cfm versions. Search online will show you how to tell which one you have at a glance.

Yours is probably a 4MV 800 cfm unit since all 4 bbl carbs were supposedly 800 cfm after 1972.

Haynes makes a manual for Rochester Quadrajets that will help you rebuild your carb yourself. They are very easy to rebuild as long as the housing isn't worn where the shafts goes through. If it's worn, you will be better off sending it out to be professionally re-bushed, IMO.

Hope this helps. Good luck!

 

[bucket]

High idle until tapping the go pedal is normal operation. Just wanted to make the OP aware.

 

[bucket]

Also, let's not throw money at it just yet. From what I got out of the original post, the choke operates normally and the truck "seems" to run rich. What makes it seem to run rich? Are the plugs fouling? Have you calculated mpg and it's terrible? Or does the exhaust smell?

Does it run ok or poorly?

 

[Super-Dave]

Buy Doppleganger's for $239 so he can do his TBI, and call it done!

You're welcome!

 

[Doppleganger]

Buy Doppleganger's for $239 so he can do his TBI, and call it done!

You're welcome!

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heh heh

 

[Doppleganger]

double post

 

[agers20]

The mechanic is who told me it's running rich. plugs are brand new. I haven't calculated the mpg because it runs like crap. it has a whole lot of problems that have to be fixed to be able to find out the mpg. Doppleganger, are you willing to sell yours, sir? and the high idle doesn't subside when you hit the pedal. It revs up and comes back down to the high idle. I think the small spring that pulls that small butterfly closed is rusty. I can push the flap down some and it goes back to what I consider a normal idle.