Poor Fuel Economy? Rebuilt with SR Kit

[OutlawDrifter]

Hello everyone,  first post.  I just rebuilt a 1984 model Q-Jet.  It was built to replace the 1406 Edelbrock that was on my 1982 2wd K5 Jimmy.  The Jimmy has a 350/TH350(lockup) combo, a very small RV cam, Edelbrock 2101 intake, 2.73 gears out back and currently a 26.6" tire.  I'm getting the same fuel economy with my rebuilt Q-Jet as it did with the 1406, about 10mpg.  It runs about 17" of vacuum while idling in park, I didn't test in in drive, however.  Idle screws were set with a vacuum gauge.

This baffles me, with a 1979 K10 Full-Time 4wd pickup I was making 15mpg with a similar setup.  Maybe I'm expecting too much, but I would think a 2wd full-size Blazer should do as well or better.

I don't drive with my foot in the carb, I have a 40 mile round trip to work, mostly highway @ 65mph.

After about a week of driving I pulled the top off and made sure the everything was in order, I did notice the primary rod hanger was bent up a little on the drivers side, I get everything back in order and reinstalled the top.

I installed the primary throttle shaft bushings, epoxied the well plugs, and followed Cliff's book while rebuilding.   

The Jimmy runs very well, I don't have any issues with bog, or tip in, WOT.

Any help would be appreciated.  What am I missing?

[OutlawDrifter]

Pulled the top again tonight when I got home.  The driver side primary rod hanger arm was bent again.  Swapped out the assembly with one from another carb.  Made sure to use the same rods.  Re-set every thing again.  17.5" of vacuum on this try at idle in park, 850rpm's.  Checked in drive @ about 650rpm's it had 14". 

The APT has never been adjusted to my knowledge.  I removed the aluminum plug out of the top when rebuilding.  It should be at the stock setting.....

Took it for a short drive, seems cleaner?  But sometimes, your efforts can be deceiving....

I took extra care when putting the top on to avoid any kind of unwanted contact with the top and the hanger.

[Cliff Ruggles]

I would tap the airhort APT access hole for 3/8-16 threads and install a removable plug.  Use the APT to fine tune the part throttle for best fuel economy.  Start at the lowest (leanest) setting.  Record the stock setting for reference.

Do some test driving and raise the APT up until you find the best fuel economy without any surging, hesitation, stumble, bog, or "flat" spot right off idle.

What jets and metering rods are in the carburetor?  Most later units used "M" series rods.

I would also verify the distributor is working correctly, and tune it as well, initital timing, timing curve, amount of advance added at cruise by the vacuum unit, etc.  You should EASILY get in the 13-15mpg range with that set-up.......Cliff

[OutlawDrifter]

Cliff I believe it has 49M rods and the jets are 72's.  The parts carb I have has a set of 52M's(if i remember correctly) and 73 jets. 

The APT was at 3.5 turns from seated.  I'm currently working my way down, I'm at 4 half turns down.

[Cliff Ruggles]

You should have full control from rich to lean with the 72 jets and 49 rods.  The upper section tapers from .049" down to .039".  If you find you need to lean it up more than the 49 rods will allow, install the 52's instead.

Also verify float level, as a high float setting will richen it up across the entire load/speed range. 

Don't forget to check the distributor.  Insure the mechanical advance is working correctly, and the vacuum advance is getting full vacuum at light throttle/cruise, and applying additional timing to the engine.....Cliff

[OutlawDrifter]

Cliff, after bottoming out the APT adjustment with no off-idle stumble or lean surge, I pulled the top.  I was wrong, the 52M rods were in the carb already.  I did make some adjustments to the hanger to get the rods to fully seat in the jets.

I also checked the float level and it was set properly as well. 

I checked over the distributor and the VA is not holding vacuum and advancing the timing, so I will have to replace the can.  The mechanical is working just fine. 

[Cliff Ruggles]

You just found one of the problems.  Without the vacuum advance, the timing is retarded at cruise, which kills fuel economy.....Cliff