Author Topic: Emissions delete and tune up update  (Read 1216 times)

Offline Deathscythe

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Emissions delete and tune up update
« on: September 10, 2018, 06:30:19 PM »
Just wanted to give you guys a little update regarding my emissions delete and tune up work on my 86 GMC Sierra. I figured there could be some good information here for some regarding the effects on fuel economy and whatnot that some might want to know.

The truck currently has a carbureted 305 with a turbo 350 transmission and when I bought it, it was getting around 9.5mpg.
I replaced the plugs, wires, cap and rotor with some duralast replacements and autolite platinum plugs. Fuel economy went up to about 11.5mpg.

Next up I removed the EGR valve and replaced it with a Mr. Gasket block off plate, and pulled the heat riser vacuum diaphragm and wired the valve in the open position. I capped the related vacuum hoses and got an increase to 13.5mpg.

I'll still be replacing the thermostat housing to get rid of the TVS valve, related vacuum hoses, air pump and related hoses and lines here shortly. Once I get that done I'll be tuning the carb and will update with my results from there.

Online zieg85

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Re: Emissions delete and tune up update
« Reply #1 on: September 10, 2018, 08:27:49 PM »
Your best bang for the best fuel economy is proper timing throughout the power band once the carb it tuned for the highest vacuum reading.  305's with the ESC spark control either work spot on or it doesn't.   
1985 C30 Custom Deluxe Ext. cab 5.7L TH400 3.21 
1985 C20 Scottsdale 7.4L 4 speed 3.21
1986 C10 under construction
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Offline Deathscythe

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Re: Emissions delete and tune up update
« Reply #2 on: September 10, 2018, 09:19:50 PM »
That's what I figured through what research I've done throughout figuring out this process. I figured that tuning ththe carb now would be pointless for the most part, that doing so after getting all the vacuum lines straightened away first would make tuning the carb more accurate.
Finding the time necessary to get it done has been difficult.
I heard that tightening the mixture screws to closed, gently, then backing out 3 turns is an optimal starting point for tuning the carb. Is that accurate?
« Last Edit: September 10, 2018, 09:21:51 PM by Deathscythe »

Offline Henry

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Re: Emissions delete and tune up update
« Reply #3 on: September 11, 2018, 10:18:08 AM »
Hi DS:
If you have a 4-bbl Q-jet carb, 2-1/2 to 3 turns out of the mixture screws is a recommended starting point for carb tuning. What model of carb do you have?
Regards,
Henry

Offline Deathscythe

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Re: Emissions delete and tune up update
« Reply #4 on: September 19, 2018, 10:19:23 PM »
Sorry for the late response, I've been crazy busy.
I spent around 8 hours under the hood the other day fixing and changing a few things and did get to take a nice long look at the carb. I couldn't see and definitive markings as far as what make or model the carb is. The previous owner did say he replaced the carb recently and was a fan of O'Reilly's, at least that's where he purchased all the LED stuff for the brake and marker lights.
In any case, from what u understand there should be two screws at the front of the carb to adjust mixture, but my carb does not have these. What it does have is two round studs with flats where the screws should be. If it's a newer carb, could it be self adjusting or some kind of cover plugs for the screws?
I have as close to zero carb experience past what research I've done online and from other mechanically inclined people I know. Hopefully my explanation help some.

Offline Rapid Roy

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Re: Emissions delete and tune up update
« Reply #5 on: September 20, 2018, 06:20:46 AM »
Pictures of the carb can be a big help.
1974 Cheyenne 10 LWB 350/TH350/AC/4 BBL Quadrajet

Offline Deathscythe

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Re: Emissions delete and tune up update
« Reply #6 on: September 20, 2018, 09:34:13 AM »
I'll have to bust out an old camera next time I get under the hood. The picture quality is too high on my phone to post.

Offline Henry

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Re: Emissions delete and tune up update
« Reply #7 on: September 20, 2018, 10:48:05 AM »
Hi DS:
From what you describe, it sounds as if you may still have the plastic "no-tamper" caps on your mixture screws which is how the carb came from the factory. Are the "studs" plastic with a cross-pattern in them? These caps will still allow you to adjust the mixture by about 1/4 turn. They were originally adjusted at the GM factory (Rochester Division) on a flow bench for optimal setting. Experienced carb tuners simply pop off (or break) off the caps and discard them and set the mixture adjustment as they see fit.

Your carb identification number should be located on the center body of the carb (float bowl assembly) near the fuel inlet nut in a vertical orientation...it is critical to know what model carb you have as there were a lot of different versions of the Q-Jet.

Yup, pictures of the carb are important to help identify and help troubleshoot/rebuild.

Regards,
Henry

Offline Henry

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Re: Emissions delete and tune up update
« Reply #8 on: September 21, 2018, 11:30:40 AM »
Hi DS:
Correction for my last message: some Q-jet carbs have their part number marking on the passenger side of the carb towards the back on the float bowl. If your carb has "Carter Q-Jet" on it that is simply because Rochester could not keep up with production so Carter was contracted to help produce the Q-jets.
Regards,
Henry

Offline Rapid Roy

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Re: Emissions delete and tune up update
« Reply #9 on: September 21, 2018, 07:18:36 PM »
Hi DS:
Correction for my last message: some Q-jet carbs have their part number marking on the passenger side of the carb towards the back on the float bowl. If your carb has "Carter Q-Jet" on it that is simply because Rochester could not keep up with production so Carter was contracted to help produce the Q-jets.
Regards,
Henry

My Carter has name and number on left side (driver side).
1974 Cheyenne 10 LWB 350/TH350/AC/4 BBL Quadrajet

Offline Rapid Roy

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Re: Emissions delete and tune up update
« Reply #10 on: September 21, 2018, 07:30:09 PM »
I'll have to bust out an old camera next time I get under the hood. The picture quality is too high on my phone to post.

I use Irfanview to resize my pics. Free program. Just have to investigate how to use it.
1974 Cheyenne 10 LWB 350/TH350/AC/4 BBL Quadrajet

Offline Deathscythe

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Re: Emissions delete and tune up update
« Reply #11 on: February 18, 2019, 04:41:16 PM »
Sorry for the very late reply to this thread, life has me really busy and tuning the carb fell to the bottom of the list. But, I have been able to get a good picture of where the mixture screws should be, and what is there. So far I have no clue what is going on, I'm assuming a specialty tool will be needed, but haven't been able to locate one that will fit. Unless, of course, I'm missing something simple....

Offline VileZambonie

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Re: Emissions delete and tune up update
« Reply #12 on: February 18, 2019, 06:55:07 PM »
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Offline 75gmck25

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Re: Emissions delete and tune up update
« Reply #13 on: February 18, 2019, 06:58:34 PM »
AFAIK that is referred to as a double-D head on the mixture screw, and you need this type of tool. https://www.amazon.com/Thexton-350-Carburetor-Adjusting-Tool/dp/B0002SRCJ2

You can also go to cliffshighperformance.com (and other sources) and order replacement mixture screws with a standard flat tip screwdriver head on them.

Bruce

Offline Deathscythe

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Re: Emissions delete and tune up update
« Reply #14 on: February 18, 2019, 07:06:10 PM »
Thank you, much appreciated information. Now that I know what I need, I can get it adjusted. Since I did all the work to eliminate the majority of the emissions equipment, it's running very rich. I'll probably drive it more once it gets better mileage.