adjustable part throttle on a qjet

[regal8r]

is there a special procedure in which to set this for the qjets? my 70 c10 with a 305 gets 11mpg on the highway and i dont think thats normal lol. its got long tubes and a tow cam...thats it for engine mods.

edit: trans/rear end:th350 with 3.73s

[regal8r]

up

[jhow66]

Answer=3.73 gears you are running. MPG about normal for your set up.

[WorkinLonghorn]

I get 15.5 highway w/3.73s (but manual trans and a 350).Check your vac.advance.Put a timing light on it and suck on the timing adv. hose and watch for considerable advance.

[regal8r]

Originally Posted by jhow66

Answer=3.73 gears you are running. MPG about normal for your set up.

is there anyway i could improve it other than changing rear end gears or tranny?

[Eddie H.]

Make sure your timing, vacuum advance, float level, and idle mixture is set right. There are no other fuel adjustments except changing jets and metering rods. What size eng was your q jet originally calibrated for?
Q jets were also bad about leaking fuel internally from the main jet well plugs.
If it hasn't been rebuilt recently, you need to rebuild it and seal those plugs with epoxy to prevent this problem.

Also, a 305 is working a lot harder to pull 2 tons of truck against the wind than a bigger eng would be. My 69 had a stock 307 with a 2 bbl carb, and would get 8-10 mpg hwy, at best. I replaced it with a mildly built small block 400 and got 12-14. The 400 has more torque at 1/4 throttle than the 307 did wide open , so it can run 70 without even breathing hard.

[regal8r]

Originally Posted by Eddie H.

Make sure your timing, vacuum advance, float level, and idle mixture is set right. There are no other fuel adjustments except changing jets and metering rods. What size eng was your q jet originally calibrated for?
Q jets were also bad about leaking fuel internally from the main jet well plugs.
If it hasn't been rebuilt recently, you need to rebuild it and seal those plugs with epoxy to prevent this problem.

Also, a 305 is working a lot harder to pull 2 tons of truck against the wind than a bigger eng would be. My 69 had a stock 307 with a 2 bbl carb, and would get 8-10 mpg hwy, at best. I replaced it with a mildly built small block 400 and got 12-14. The 400 has more torque at 1/4 throttle than the 307 did wide open , so it can run 70 without even breathing hard.

it smells REALLY rich at idle. im not sure what motor the carb was for to begin with, but it probably could use a rebuild. im intending on replacing it with a bigger motor in the near future. what kinda mileage do u think i would get with a 350, same trans and 3.42 gears?

[Eddie H.]

You could probably gain a couple of mpg on your 305 by rebuilding the carb and making sure it is jetted properly for your application. A 350 and higher gears might get you in the 13-15 range. Or keep your 3:73 gears and put in an overdrive trans would be even better. If you can afford it, go with a late model fuel injected vortech 350 and overdrive. My 99 Yukon gets 18-20 hwy with 3:73 gears and 4l60E overdrive. There are a lot of these later model fuel injected vehicles in the junkyards you could swap every thing over from.
The biggest thing affecting gas mileage is the downward forces exerted by the right foot on the get louder pedal.

[regal8r]

Originally Posted by Eddie H.

You could probably gain a couple of mpg on your 305 by rebuilding the carb and making sure it is jetted properly for your application. A 350 and higher gears might get you in the 13-15 range. Or keep your 3:73 gears and put in an overdrive trans would be even better. If you can afford it, go with a late model fuel injected vortech 350 and overdrive. My 99 Yukon gets 18-20 hwy with 3:73 gears and 4l60E overdrive. There are a lot of these later model fuel injected vehicles in the junkyards you could swap every thing over from.
The biggest thing affecting gas mileage is the downward forces exerted by the right foot on the get louder pedal.

i dont really do a whole lot of highway driving, so im not sure if i would benefit much from an OD tranny. the previous owner used this truck as a tow vehicle and 11 mpg highway is what he said he got. i was thinking about a 350 vortec swap though. how hard would the swap be?

[stllookn]

Qjets came with hundreds of different primary side calibrations. You can check the primary part-throttle calibration with the "tip-in" procedure after the engine is fully warm. Set the idle screw to 2000-2200 rpm. Gently tip in the choke flap, don't close completely and kill the engine, but move it in until you see a 50-100 rpm increase. This means you are just on the lean side and perfect. If there is no increase or the rpms fall off you are rich. Pull a small vacuum hose from the carb and see if the rpms increase due to the additional air coming into the carb. If it does you are running a bit rich. Set your APT rods to give a slight rpm increase while "tipping-in" the choke plate (slightly lean). Final adjustments can be done by checking your plugs after a medium length freeway run. Compliments of Cliff Ruggles "How to Rebuild and Modify Rochester Quadrajet Carburetors".

Don't shoot the stinking messenger...LOL!

BTW...those 3.73's are pretty deep...find some 3.08's.

[WorkinLonghorn]

Just out of curiosity,did you verify the proper operation of the VACUUM ADVANCE unit on the distributer? If the diaphragm in it ruptures,or if the linkage gets jammed up the results will be lousy highway mileage.So easy to check,you might as well do so before talking about engine swaps,overdrive trans.or even re-tuning the carb.

[68speedalert]

Originally Posted by stllookn

.....Set your APT rods to give a slight rpm increase while "tipping-in" the choke plate (slightly lean). Final adjustments can be done by checking your plugs after a medium length freeway run. Compliments of Cliff Ruggles "How to Rebuild and Modify Rochester Quadrajet Carburetors".

Very nice! I've been rebuilding Qjets more years than I care to tell, and have never heard of a good what to adjust those rods.

Excellent post!!!

regal8r, many have touched on your timing, and that is HUGE. Make sure the vacuum and MECHANICAL advance are both working.

If you don't have a clue what the APT rods and these timing items are, tell us.

[jacobs]

Here's an excellant website for tuning a q-jet http://www.corvetteclub.org.uk/files...une_a_qjet.pdf

[Eddie H.]

Originally Posted by stllookn

Qjets came with hundreds of different primary side calibrations. You can check the primary part-throttle calibration with the "tip-in" procedure after the engine is fully warm. Set the idle screw to 2000-2200 rpm. Gently tip in the choke flap, don't close completely and kill the engine, but move it in until you see a 50-100 rpm increase. This means you are just on the lean side and perfect. If there is no increase or the rpms fall off you are rich. Pull a small vacuum hose from the carb and see if the rpms increase due to the additional air coming into the carb. If it does you are running a bit rich. Set your APT rods to give a slight rpm increase while "tipping-in" the choke plate (slightly lean). Final adjustments can be done by checking your plugs after a medium length freeway run. Compliments of Cliff Ruggles "How to Rebuild and Modify Rochester Quadrajet Carburetors".

Don't shoot the stinking messenger...LOL!

BTW...those 3.73's are pretty deep...find some 3.08's.

That sounds like an excellent method for fine tuning.

I'm showing my age here, but back in the old days when I worked for Chevy dealers, we used the "propane enrichment method " which accomplished the same thing. We would richen the mixture slightly with propane and watch the rpm to tell if the carb was running rich or lean. We also used a 4 gas analyzer to fine tune things even further.
Now all the engines have fuel injection, oxygen sensors, and computers that can adjust fuel mixture and ign timing on each individual cylinder several times per second. Carbs were "state of the art" in their day, but fuel injection is the only way to go for the best performance and economy.

[68speedalert]

Originally Posted by Eddie H.

T..... We would richen the mixture slightly with propane and watch the rpm to tell if the carb was running rich or lean. We also used a 4 gas analyzer to fine tune things even further......

Ah but I believe that was at idle and used the idle mixture screws.

stlookin is talking about the APT, which is the screw that adjust how far the metering rods go into the main jets.

[regal8r]

Originally Posted by 68speedalert

Very nice! I've been rebuilding Qjets more years than I care to tell, and have never heard of a good what to adjust those rods.

Excellent post!!!

regal8r, many have touched on your timing, and that is HUGE. Make sure the vacuum and MECHANICAL advance are both working.

If you don't have a clue what the APT rods and these timing items are, tell us.

when u say apt rods, are u referring to the metering rods? we 'rebuilt' qjets in fuel systems in auto tech which was almost a year ago or so, so i have a good understanding of the qjet (considering im 19 years old). also, to stllookin, what do u mean by tip in? move the choke plate so that its closing, or opening?

[regal8r]

all this seems pretty complicated. how much would a shop charge me for tuning the carb?

[68speedalert]

Originally Posted by regal8r

all this seems pretty complicated. how much would a shop charge me for tuning the carb?

Don't even try to find someone.....there are very few that were that good with Qjets in their day, much less now. What you learned in autotech is as far as SOME guys could go. Most didn't even get that far. This type of tuning is very specialized.

You could ask at a NAPA parts store if there is anyone they know who is good with the carbs. Then ask the guy about adjusting the apt rods. You'll probably get a blank scare.....

And yes, those are the metering rods. Also can be referred to as the 'power piston or valve". They have a spring that pulls them up when vacuum drops. So there are 2 things that effect where they are when cruising. The screw that sets the stop and the strength of the spring pushing the rods up.

jacobs link is very good. Maybe a little hard to follow but try reading it a few times, it might help.