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OT Rochester carburetor/ GM parts guys?

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ratface

03-30-2004 04:59:11




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Since I'm not working on the 8N thought I could get my hands dirty with the pick-up. Recent purchase 1986 GMC Sierra Classic, 305,automatic, no air. New rebuilt engine installed in 2002,have receipt, but man used old carburetor. Told me it needed carb when I bought it. Truck would not idle at all and sputtered and hunted, just ran like crap. Visibly see that everything has been replaced on engine, wires, hoses,starter, radiator, filters, Have pulled carb and near as I can tell from the rochester page its a 1977 carb I think from a buick. Everything lines up, linkages,pvc hose,gas line, vacumn advance, perfect fit except front two bolts had 1/2 of washers installed to take up slack. The first four numbers that I can make out are 1705 stamped on the side. The carb is a rochester quadrajet with electric choke. Anyone have a 1986 chev 305-350 where they can make out the carb number? I'm not inclined to rebuild since its the wrong carb anyway. Any parts guys out there know the carb matches for this vehicle? Anyone buy a carburetor recently?

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Joe Wiggins

04-01-2004 06:21:36




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 Re: OT Rochester carburetor/ GM parts guys? in reply to ratface, 03-30-2004 04:59:11  
I had a similar situation with a 1984 Olds 98 I bought a year ago. The engine only had 60K miles, but has set up since 1998, so it ran bad. Replaced all the usual stuff: plugs, wires, cap, rotor, ignition module, MAP sensor, water and fuel pumps, changed all belts, hoses and fluids. Still ran lousy at idle, poor kickdown when accelerating, and the CHECK ENGINE light was still on.

Tried rebuilding the carb; suffice it to say that rebuilding a feedback Quadrajet is a pain if you don't do it often, have the special tools and know all the tricks the pros learn from experience. Gas mileage was still about 10-12 MPG. Finally gave in and bought a Rochester Quadrajet rebuilt by Holley. Bottom line: I wouldn't buy any other rebuilt carb unless it is rebuilt by Holley. This thing was PERFECT out of the box. No adjustments needed, period.

I bought it from O'Reilly Auto Parts. It listed for about $500 from Holley, but I got O'Reilly to match the price of a Rochester rebuilt by some no-name company sold at Autozone for about $230. It was hard to bite a $230 bullet for a 20-year old car, but it was worth it after I got it installed. If you buy a rebuilt part, I strongly urge you use the Internet to get the competitor's price and get the store to match or beat the best price you've found.

Now the car runs like new, and I get 18-19MPG consistently, with about 85-95% of my driving done on the freeways at 60-75 MPH.

My advice is to buy the right carb with all the electronic controls hooked up properly, do all the other preventive stuff that could leave you stranded on the side of the road and enjoy your truck. It will have great power, very good mileage for a big vehicle, and not leave you stranded.

Joe

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FC

03-30-2004 12:34:28




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 Re: OT Rochester carburetor/ GM parts guys? in reply to ratface, 03-30-2004 04:59:11  
I agree with what someone else said about checking to see if you have the older distributor as well. Likely your best bet would be to use an older HEI distributor if part of your emissions are not hooked up or functioning (i.e. wrong carb without electronic feedback). The computer and associated electronics are pretty much an all or none deal....they work ok as long as everything is working and connected, however if this is not the case they will run like crap with only a few exceptions. One of the biggest problems I ever found with the quadrajet carb was the "paper" type floats they had. I had a pickup with one and fought it running bad until I finally gave in and took it to someone who weighed the float and found it to be saturated. Replaced with a new brass float and it ran like it should. You may also want to check to make sure the throttle shafts are tight in their bores. There is a kit to repair this as well by drilling out and bushing the throttle plate. Good carbs, just a bad rap because of lack of knowledge of what makes them work properly (such as the float deal). Good luck.

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Bob

03-30-2004 07:08:02




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 Re: OT Rochester carburetor/ GM parts guys? in reply to ratface, 03-30-2004 04:59:11  
A Chevy pickup of that year would have had an electronically controlled "feedback" carburetor originally. Along with the carb, the ECM (computer) would have controlled the spark (ESC).

If the carburetor has been replaced with a non-feedback, older carb, and the electronic spark control distributor has not been replaced with a simple centrifugal/vacuum advance unit, the system will be operating in limp-home mode, and will run terrible with terrible mileage because the spark advance won't be functioning. If the distributor has a multi-pin connector connecting it into the engine harness in addition to the large red power supply wire, it is an ESC distributor.

I am not offering advice on the legalities of tampering with the original emissions equipment, just stating what I have seen people do, and retrofitting with and older carburetor, without also replacing the distributor with the older type DOES NOT WORK.

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Hobo, Nc

03-30-2004 08:10:51




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 Re: Re: OT Rochester carburetor/ GM parts guys? in reply to Bob, 03-30-2004 07:08:02  
ya wrong, a 86 full size pic up did not have a ECC carb. They went to Fuel injection in 87. The 86 did look like a ECC carb but was not. Unless it was used in Cal a forn i a emission stdandard, that i do not know.



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Bob

03-30-2004 08:33:11




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 Re: Re: Re: OT Rochester carburetor/ GM parts guys in reply to Hobo, Nc, 03-30-2004 08:10:51  
I have a 1986 sold new in North Dakota that has the feedback carburetor. There is a duty cycle controlled solenoid in the carburetor that pulses the metering needles up and down. The % of time they remain down (closed) sets the fuel rate. They used that setup several years back from 1986, also. Probably the HD 3/4 ton may not have had the feedback carb, but the 1/2 ton did!



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SteveB(wi)

03-30-2004 06:43:24




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 Re: OT Rochester carburetor/ GM parts guys? in reply to ratface, 03-30-2004 04:59:11  
If it's a quadrajet 4 bbl. there are some really good articles out there on tuning them, also good books available. I included a link for a good one to start.



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Hobo,NC

03-30-2004 06:37:59




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 Re: OT Rochester carburetor/ GM parts guys? in reply to ratface, 03-30-2004 04:59:11  
Good parts man can look up carb number by year, make and modle. Try NAPA. Very rare to not be able to fix a quadrajet, The last one I did i could have got a rebuilt one for what I paid for the parts. Don't take long to run up the price when choke pull off's run 7 $75.00 a piece. Big problem I have with them is the throttle shaft gets worn and unable to get to idle at a low RPM.
The elderbrock carb may be the trouble free way to go. Almost all ways find the distributor vaccum controll not working, so check that. if it has electronic spark controll you need to bypass that. It will have a four wire flat conector that comes out of the base of the distributor, unplug it and jump A to C on the dist. side. (I think its A to c would have to look up) use propane to search for vaccum leaks. if all fails have see a few that someone had adj. the valves to tight.

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Tim...OK

03-30-2004 05:35:03




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 Re: OT Rochester carburetor/ GM parts guys? in reply to ratface, 03-30-2004 04:59:11  
This is just my opinion,probably not worth much.. The q-jets were good years ago,but I've found it hard to get one right,rebuilt or used. Last couple I've bought were the aftermarket Edelbrocks,they work great on a stock engine,no gaskets below fuel level (like original),they're made by Weber,they are nice carbs.

Tim



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Hobo,NC

03-30-2004 07:01:13




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 Re: Re: OT Rochester carburetor/ GM parts guys? in reply to Tim...OK , 03-30-2004 05:35:03  
ya rite about the Weber part but for some reson they changed the name, Any way it's still a engilish name,



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Hobo,NC

03-30-2004 06:45:41




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 Re: Re: OT Rochester carburetor/ GM parts guys? in reply to Tim...OK , 03-30-2004 05:35:03  
They mak'em rite cheer in Sanford ,NC. Magnetti Marelli Powertrain USA INC. Also make yer fuel injection system fer ya Harley.



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