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northboundtrain · 05-29-2011, 11:28 AM

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.

05-29-2011, 11:28 AM	#1 (permalink)
northboundtrain EcoModding Lurker Join Date: Apr 2011 Location: Boulder, CO Posts: 10 Thanks: 0 Thanked 1 Time in 1 Post	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.