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Old 05-29-2011, 12:28 PM   #1 (permalink)
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Chevy 350 Project.

Hello Ecomodders,

I'm in the process of getting a '77 GMC 3/4 ton truck put back together. I bought the truck from the original owner and it had only 67,000 miles. However the truck hadn't been driven much in the last few years, and the engine was full of sludge, so I tore it down and rebuilt it. I did a very budget rebuild with just re-ring and hone, mild pocket porting, and valve lap (i.e., no machine shop work).

I did replace the cam. The specs of the new one are 202 int./208 exh., .400/.410 lift, and 114 lobe separation. It's meant for strong low-end, high vacuum and good fuel economy.

I'm also upgrading the exhaust with long tube headers and dual pipes with a cross-over.

Otherwise, everything is stock. The carburetor is a Rochester Quadrajet.

I got the engine in last night, and at this point I'm looking to just get the truck running so I can break in the engine, but then I want to start modifying it further. I don't plan to do any aero mods at this point--it's a big old truck, afterall. Changing the gearing from 4-speed with 4.09 differentials to 5-speed overdrive with 3.73 differentials would be ideal, but it's not in my budget at this point. So the first place I believe to start with is the fuel and air system.

I'd like to try and lean it out as much as possible and do whatever I can to promote fuel vaporization, and I'm willing to experiment somewhat with this engine since I don't have a lot invested in it. With that in mind, I'm going to weld in an 1/8" NPT bung into a header tube for a future pyrometer, which seems like a good thing to have if I'm going to try to run the truck on the lean side. I'm also wondering if I should weld in any other bungs, like for an O2 sensor. (Is it possible to hook up an O2 sensor to a volt meter and use it to get useful information about the fuel-air ratio?) Where in the headers should these bungs be welded? I haven't installed the headers yet, so now is the time to do the welding on them.

I also haven't rebuilt the carb yet, but that will be happening this week. I'm wondering if I should go with some smaller jets right away (I live at 5,500 feet) or if I should gradually reduce the jet size when I have the instrumentation to monitor things better.

And then there's fuel vaporization. Pre-heating the fuel with engine coolant sounds like an easy mod to try. If so I might want to build a heat exchanger into the top radiator hose now so I don't have to drain the coolant to do it later. Any suggestions about that?

I will be using a vacuum gauge right away, which I know will help me monitor throttle position.

What if any other modifications could I do now?

Thanks for the help.

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Old 05-29-2011, 06:07 PM   #2 (permalink)
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The 350 is a notorious guzzler. Fine tuning the Quad, long tube headers, and that cam are all moves in the right direction I think. The 4 speed should be OK and so just getting a taller rear end in there shouldn't break the bank. Then do a/c and mechanical fan deletes, and see how much timing advance it'll take. Fuel preheat = waste of time.
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Old 05-29-2011, 07:39 PM   #3 (permalink)
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An O2 bung/3 wire sensor would be cheap. Using a multimeter you can tune your cruise pretty lean unless you are planning on towing a great deal.

Carburetor Tuning the Scientific Way

Good choice on the spreadbore carb. It can help the big pig get more reasonable mileage. The key with carbs is a very steady foot. Every time the pedal goes down the accelerator pump shoots additional gas.
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Old 05-29-2011, 09:10 PM   #4 (permalink)
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Whatever you do keep the quadrajet, a lot of people out there just toss them aside but they are great carbs for everyday drivers and fuel economy the front 2 bores are tiny and the back 2 are monsters as long as you keep your foot out of it and keep the secondaries closed you will get awesome mileage for the truck. changing the jets from stock may be a mistake. too small and the back bores may need to open sooner. Just play with the a/f screw if you think you can lean it make sure you keep a close eye on the temp gauge and look at your plugs listen for detonation and pinging.

My opinion get it running perfect in stock form a new coil set of wires cap rotor and pl ugs change the oil maybe drop the rear diff cover and fill back up with synthetic oil. then after that is done tweak the a/f on the carb next swap clutch fan for electric.

good luck looking forward to see how you make out I dig 70's GM trucks I restored or should I say fixed up 3 of them in past I sold my last one 4 years ago a 1978 chevy with a straight 6 2 bbl carb and 3 speed on the column. All the p/u trucks I had were column shift.
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Old 05-29-2011, 10:35 PM   #5 (permalink)
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Did you get any better mpg with the 6? I'm thinking of replacing the 283 in my '74 Cheyenne with a 6 in front of a T350.
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Old 05-30-2011, 01:45 AM   #6 (permalink)
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It was too long ago for me to recall mpg #'s back then I was not into keeping track. Depending on what you use the truck for Frank swapping an 8 for a 6 would be a waste of time. Its not like you will make it up commuting with it? around town an 8cyl gets almost identical and in some cases better stats than a 6 cyl.

If I had a 283 in a truck and wanter better fe I would #1 make sure I was running a quadrajet carb due to the small front bores or B. I would replace a 4 bbl with a 2 Holley makes a nice 500 cfm 2bbl that works nice on a small v8
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Old 05-30-2011, 02:20 AM   #7 (permalink)
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I'd focus on the wheels and brakes after you get it running. If it's sat for a while, you'll want to make sure all the tires spin freely. Anything that drags or sticks will destroy your mpg. Grease the wheel bearings, change the diff fluid, check the calipers, brake pads, rotors, drums and shoes, air up the tires to 40 or higher.
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Old 05-30-2011, 02:27 AM   #8 (permalink)
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Right now it has no engine in it, so it's not so much a swap as it is what do I plug in there next. The 283 has always had the intake and 2bbl off a 350 but it is to get a Quadrajet next.
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Old 05-30-2011, 10:44 AM   #9 (permalink)
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Thanks for the suggestions. The info on using an O2 sensor is very helpful. I'll rebuild the Q-jet stock and go from there.
No one has mentioned using a pyrometer. Would this be helpful to keep from running too lean and burning valves, etc.? They aren't cheap--couple hundred bucks--so I'd want to know it was worthwhile. If so, where in the header should I weld in the bung for it?
I do intend to do the brakes, wheel bearings, etc. That's all routine maintenance that needs to happen in time. I have already drained all the fluids, and will replace the rear diff with synthetic.
Thanks again,
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Old 05-30-2011, 01:55 PM   #10 (permalink)
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A wideband is a better tool than a pyrometer because both lean and rich conditions can cause egt's to spike. They are both in the same price range, the wideband being only slightly more expensive.

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