question on my q-jet

[Cornfield creations]

I put a 350 HO in my 79 blazer, the rest of the drivetrain is stock, 4 inch with 33's. I rebuilt my stock 79 q-jet using a performance rebuild kit from cliff's HP. I asked him about jetting and gave him my specs since he's been building these things for over 30 years, and he said to put in #76 jets. I thought by putting a engine that is new and more efficient I would be getting some better MPG. I am still getting around 10 when I baby it.

I was thinking if I changed the jets out and leaned it out a little bit would it be so bad for the engine? Maybe squeeze a MPG or two? What do you all think.

Like I said I gave Cliff all the specs and he told me all my sizings in the carbs are right and those were the jets to put in.

 

[Thunder]

Jets are only a part of the carb tune. Jets just get it close to where it is susposed to run The meetering rods are the most important thing for good MPG and performance.
The meetering rods are stepped for tuning at cruise and full throttle. To improve MPG you have to put in rods that are a little bigger diamater in the cruse step. and small at the full throttle step to give good performance.
Then you have to adjust the air valve so it opens at the proper RPM and not too soon or late..
If you want to tune your Q-jet properly get this book:http://cgi.ebay.com/Rochester-Carbu...goryZ378QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
I used the ebay link cuz you can get the book cheap there. They cost over 20.00 to buy new.
Great book. 15 MPG highway can be had out of a properly tuned q-jet

 

[Chevy911]

If that motor hasn't had many miles put on it yet, then your bad milage may just be that the engine hasn't broken in, the rings may not be sealing well yet, the last few motors I've rebuilt, you could feel them getting better as you drove them for a while. Also did you discuss the rods for the quadrajets as well, because jet size means nothing for normal driving unless the rods are correct. The main jets control full throttle mixture only, the position and diameter of the rods controls all your cruise mixture. I have a good article saved somewhere about setting these carbs up step by step, I'll see if I can find it again.

 

[dyeager535]

Is this an APT carb? I've not messed with adjustment of these yet, perhaps that is an issue?

Jets aren't the only deal when it comes to fueling, you've got a wide range of rods too. On my truck, when it was running lean, I could tell as it would surge just a bit while cruising at 35MPH or so. I can't recall which way I went, up a jet size or down a rod size, but that took care of it.

If you've got wrecking yards around, you may be able to scrounge some up from junk carbs.

 

[Cornfield creations]

Is this an APT carb? I've not messed with adjustment of these yet, perhaps that is an issue?

Jets aren't the only deal when it comes to fueling, you've got a wide range of rods too. On my truck, when it was running lean, I could tell as it would surge just a bit while cruising at 35MPH or so. I can't recall which way I went, up a jet size or down a rod size, but that took care of it.

If you've got wrecking yards around, you may be able to scrounge some up from junk carbs.

Yeah it's an APT carb. The primary metering rods are the stepped 52M. The motor has about 3,000 miles on it. It's been parked since the fall but I was just curious since I am going to start driving it again. I have to run 91 octane or higher in it so that doesn't help either. I was thinking about trying the fitch fuel catalyst the help with the fuel so I can run a lower octane and maybe increase fuel mileage. I hear some people get about 15mpg with these trucks with a 350, I just never see it. Even on the interstate I may see 11 and it still has high gears in it. I tuned the carb pretty close I felt with Cliff's book and what he said to do to tune it. I did an electric choke conversion but I think it needs adjusted some, and also one thing I noticed is after I drive it and it gets hot, I shut it off and try and start it and it feels like it flooded some. I have the cardboard type gasket, so I didn't think percolation would be the problem.