Need some advice - 9 MPG is painful!!!

[86BlackBurban]

Greetings!

I'm new to the forum but have been tinkering and modifying cars for about 25 years now. Back in September I bought a 1986 Chevy Suburban 2WD with the 350 coupled to a 700R4 transmission. The original motor had 71K miles at the time, and on the way home from buying this truck, the oil pump went out and the engine threw a rod about 100 miles from home :-(

Fast forward to February 1st - Found a freshly built 350 with a 260H Comp Cam using roller rockers (the kind that you have to add ZDDP at every oil change). This engine has 9.0:1 pistons, double roller timing chain, and a dual-plane edlebrock intake. It has true dual exhaust with an X-pipe, dual cats, and uses 2" piping. All in all, it's not a very aggressive build, nor did I want it that way. Just wanted a motor that would breathe a lot easier pulling the occasional trailer and whatnot around town, not really interested in roasting the tires on this beast!!!

So I got everything together using the stock Q-jet carb which uses size 73 jets, not sure about the metering rod size though. Idles well when it is warm (160* thermostat) and sounds great. Runs a bit rich - but at WOT the secondaries don't open and it barely accelerates...

My main problem now is fuel mileage...

We drove 500+ miles on Thursday and averaged only 9 MPG. We stopped at various points and I adjusted the ign timing...a few degrees advanced, a few retarded, but still managed only 9 MPG. My next thought was the carb needed some adjusting, so I found some size 52 jets at pull a part - the truck cranks, but does not stay running (too lean Im guessing). I can always go back to the 73 jets to get us home, and by now I need a new carb kit to replace the gasket. But before I open up the carb today I want to get opinions on the next step.

Haven't worked on anything carbureted in a long time (been playing with ricers for several years!) and need a little help here guys! Thanks so much...will post pics of the Beast later today.

[Shark Racer]

Metering rods and metering rod height are the two main components for getting good MPG.

First off, I must say that I've never heard of using 52 jets in a quadrajet. They must be for another carburetor. Go back to the 73.

What are the casting numbers on your carburetor?

How did you determine the secondaries don't open? The only real way to do this is to video the carb as you're going down the road. As long as the choke lockout lever isn't bound up or the choke stuck on the secondaries will open. (well, the secondary airvalves could be locked up with grime)

This guy posted a great video of the secondary air-valve system in action(about 1 minute in): http://www.youtube.com/watch?v=nRK9n4NQvOA

[86BlackBurban]

Thank you for the help! I think maybe the metering rods were not working correctly because at high RPM the motor had no power. May just put the stock jets back into the carb for now and make sure the metering rods are correctly installed.

How do you ensure the metering rods stay put when putting on the top third of the carb? This may be my problem - the metering rods pop out when I try to set the airhorn down on the carb. And the accelerator pump plunger wants to pop out of place which sometimes tears the gasket. If there's a video that shows the RIGHT way of carb assembly it would be of great help...

If I get 12 MPG going home it will be a lot less costly...thanks again!

[Shark Racer]

There should be a black plastic retainer around the neck of the power piston. After you have the metering rods correctly set in the jets, you press the plastic retainer into the bore and that will hold it tight.

If the metering rods come out and do not go back into the jets when you put the airhorn on, you are going to bend the rods and potentially piston arms when you tighten the airhorn down.



This photo should give you some idea of how the setup works.

Cliff offers a great replacement retainer in his kit. You cut the stock one off and slip the new one around. It fits VERY tight. Once you have the piston retained, you should lightly tap it down to make sure it travels freely. In general, if it doesn't travel freely you should try to smooth out the walls of the power piston bore.

Cliff's book is a Godsend for this stuff.

Good luck!

I've been thinking about making videos on how to rebuild an M4M and how the various components work, although I'm nowhere near as knowledgeable with these things as many others...

[Mr.Pontiac]

I don,t really push any part at all but i came across a power chip that truely gave me 2 miles per gallon right off the bat!! I own a 2010 Chevy Silverado 296 i believe and the gas milage was ok butyou know. I heard of a power chip called G - Force and installed it in 15 min. on my truck and for 70.00$ with shipping i figured give it a try!! AMAZING RESULTS right off the get go!! I figured im now getting from 18 miles per gallon to 20 eaasily and if i ad a bottle of Lucas it even gets better!! Try it man its only 70.00$ WORTH EVERY DIME!!

[Cliff Ruggles]

One problem I see is using a 160 degree thermostat.  That will hurt engine efficiency and fuel economy.  I'd put a 195 stat in it and tune the APT screw as described in our book using the stock jets and metering rods.....Cliff

[86BlackBurban]

UPDATE:

Again, thank you for the wonderful help!

I got everything back together WITHOUT a power piston retainer and it runs much smoother now. Didn't notice a retainer on the old piston so chances are this carb had been opened before...hmmm...

Sorry, no power chip will help me - I'm computer-free!

Anyway, got 11 MPG on the way home which is loads better, but still have a lot of ground to cover. Looks like I may order one of Cliffs rebuild kits along with the manual...this is exactly what I was needing.

But why does a colder running temp hurt fuel economy? This was not my experience in the ricer department - seems like those smaller engines got a little BETTER fuel economy with a colder stat...Something to do with fuel atomization in a carb vs temperature? I'd love to hear your theories and I'm open to all suggestions - it's very nostalgic for me to be working with carbs again and hopefully will be more rewarding as time goes on...

[omaha]

Some heat is better for fuel economy. 180-190~(more??). For all out power, maybe cooler is better but not always. Also, an original iron intake might get you a few mpg's better. Yup, the fuel stays "suspended" in the air better in its long path (comparatively speaking) to the valve. With the FI its a much shorter path and the injectors are closer to the heat source of the engine. SO...other than installing F.I., the QJ would be the closest thing to it as far as carburetors are concerned (maybe some high dollar weber setup or a 1 barrel might beat it but really...??) I think there were some other dual booster carbs design out there, maybe ford made them IDK, they might get close to the QJ for mpg.

[Cliff Ruggles]

The key to best efficiency is heat, not cold.  Fuel "globs" up on cold parts, and it takes heat, pressure and spark for combustion.  You'll find the best power made right around 200 degrees engine temps, and cool/dense air coming into the engine from an outside source.

For best efficiency, get the engine hotter, it will require less fuel to do the same amount of work.

Get one of our kits, the PP retainer is standard in them, and we just added the secondary cam/spring to them as well.  Had to raise the price a few dollars, and EVERYTHING went up this year, except our wages!.......Cliff

[86BlackBurban]

What are your thoughts on gently heating the fuel (110-130 degrees) vs raising the operating temperature of the entire engine? A cooler running engine keeps the incoming air more dense, and I know liquid fuel does not burn...but atomizing warmer fuel *should* be more efficient...thoughts?

[68 Firebird]

The explanation of heat vs effecincy is very true.  When is swapped from a 160 t-stat to a 180 degree stat I had to slightly lower curb idle and found in gear idle smoother.  Buddies thought I was crazy to go higher but saw no ill
effects.

Gerry