From 10 to 15 mpg... - Pirate4x4.Com : 4x4 and Off-Road Forum

Lloyd · 02-14-2002, 08:16 AM

on my '77 M880 just by replacing the stock Carter BBD carb/manifold with a Q-jet and an iron spreadbore Mopar intake. Still full-time 203, 4.10's etc. Casting date on the manifold is '75 so it'd need to be off a 360 unless someone can tell me that the Police Interceptor 318's with the Thermoquads started that early. I messed around with an O2 sensor, and ended up fitting the Q-jet with 66 jets and 41 rods on the primaries, and a 4 in/Hg spring under the piston. I did use the Edelbrock accelerator pump for a stronger shot, but could have just shortened the rod on the original for the same effect. Left the secondaries alone, with BH rods on a K hanger, except for ditching the vacuum break diaphragm on the air valves (which was broken). Carb body casting is from a '70 Cadillac. It got that mileage on three trips to Albuquerque and back home (8200 ft) carrying sand once (2800 lbs of it) another trip for 110 gallons of paint, and another trip for a full bundle of railroad ties (a MUCH heavier load than the sand). There was some piddling around with it between trips, but mostly highway miles. It's still a tad on the rich side, I'd guess about 12.5:1, but makes very good power, is remarkably smooth and consistent, and generally runs much better. This was a pretty clean swap, with only two custom bits: an extension to raise/relocate the throttle cable clamp, and lengthening the transmission throttle pressure rod by 1-1/2" or so. Of course a new air cleaner, and different hose to the valve cover breather.
Main difference, and whole reason for the swap, is that the damn thing actually runs on steep inclines now.

GRMhick · 02-15-2002, 02:56 PM

Wow some great info to know.. now I know that cadiliacs had better equiptment on them than other chevys, but I wonder what the differences are between that carb and the one i got for $5 off a ghetto chevy van (brand new carb, or well, 5k miles or so). It doesent have any metering rods anymore (I thought there was something wrong with it, ended up being fuel lines). So what did you say you needed to do? you replaced the stock metering rods? and I take it you also put on the spring off the edelbrock Qjet... I have an edelbrock intake... hmm, sounds like something I am going to try before I go back to my fuel injection setup (burned the computor,and need to smog the truck). Oh, and I am assuming that your engine is a 318... I have a mildly built 360, so it soudns like your fuel setup woudl be about perfect for me... hmm, very cool... thanks.. and let me know if there is anything else you can add on this setup.
Garrett

Lloyd · 02-16-2002, 08:02 AM

Get an Edelbrock strip kit, about $100 or so from Summit. There are two, one fits 170xxxxxx series carbs and the other fits 70xxxxxx carbs. Get a generic oxygen sensor, about $20, and a "spark plug non-fouler" that fits the O2 sensor. Grind off the end of the non-fouler and fishmouth it, weld it to your exhaust pipe near the manifold or collector, and drill a hole for the sensor. Run a wire to the cab. Now you can either use a voltmeter, knowing that around 700 mV is optimal (14:1) stoichiometry, or buy a Nordskog panel-mount meter for about another $20. Also, see http://www.bob2000.com/carb.htm for photos and tuming method for Holleys - there's a lot of good info there. For Qjets, first wire the secondary air valves shut, and get the main jet sizes about right - since the primary rods all have the same size tips except for one special size motorhome rods. Different castings have different size vacuum ports and holes, which interact with rod and jet sizes, so what's great in one casting may not be very good in another. Once the main jets are where you want them, get the rods you need by experimenting and watching your meter at part throttle. You want decent driveabliity without the primary enrichment circuit getting too rich, or coming in too soon. This is also partially controlled by the spring under the power piston, which will be affected by altitude, cam size, available vacuum, etc. Once the primaries are right, do the secondaries. You may or may not want to adjust the air valve opening rate; if so, loosen the set strew and back off on the spring tension screw until they flop open, then bring it up 1/8 turn. If you floor it from a standing stop and there's a bog, bring it up another 1/8 turn; if not, you're done. Since the Thermoquad was used on 360's and the Q-jet is a CARB-approved replacement, you may go to the smog nazis with confidence.

02-14-2002, 08:16 AM	#1 (permalink)
Lloyd * Join Date: Mar 2000 Member # 538 Posts: 1,794	From 10 to 15 mpg... on my '77 M880 just by replacing the stock Carter BBD carb/manifold with a Q-jet and an iron spreadbore Mopar intake. Still full-time 203, 4.10's etc. Casting date on the manifold is '75 so it'd need to be off a 360 unless someone can tell me that the Police Interceptor 318's with the Thermoquads started that early. I messed around with an O2 sensor, and ended up fitting the Q-jet with 66 jets and 41 rods on the primaries, and a 4 in/Hg spring under the piston. I did use the Edelbrock accelerator pump for a stronger shot, but could have just shortened the rod on the original for the same effect. Left the secondaries alone, with BH rods on a K hanger, except for ditching the vacuum break diaphragm on the air valves (which was broken). Carb body casting is from a '70 Cadillac. It got that mileage on three trips to Albuquerque and back home (8200 ft) carrying sand once (2800 lbs of it) another trip for 110 gallons of paint, and another trip for a full bundle of railroad ties (a MUCH heavier load than the sand). There was some piddling around with it between trips, but mostly highway miles. It's still a tad on the rich side, I'd guess about 12.5:1, but makes very good power, is remarkably smooth and consistent, and generally runs much better. This was a pretty clean swap, with only two custom bits: an extension to raise/relocate the throttle cable clamp, and lengthening the transmission throttle pressure rod by 1-1/2" or so. Of course a new air cleaner, and different hose to the valve cover breather. Main difference, and whole reason for the swap, is that the damn thing actually runs on steep inclines now.

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02-15-2002, 02:56 PM	#2 (permalink)
GRMhick Pirate4x4 Addict! Join Date: Feb 2000 Member # 87 Location: I live in hotels tear out the walls I have accountants pay for it all Posts: 7,203	Wow some great info to know.. now I know that cadiliacs had better equiptment on them than other chevys, but I wonder what the differences are between that carb and the one i got for $5 off a ghetto chevy van (brand new carb, or well, 5k miles or so). It doesent have any metering rods anymore (I thought there was something wrong with it, ended up being fuel lines). So what did you say you needed to do? you replaced the stock metering rods? and I take it you also put on the spring off the edelbrock Qjet... I have an edelbrock intake... hmm, sounds like something I am going to try before I go back to my fuel injection setup (burned the computor,and need to smog the truck). Oh, and I am assuming that your engine is a 318... I have a mildly built 360, so it soudns like your fuel setup woudl be about perfect for me... hmm, very cool... thanks.. and let me know if there is anything else you can add on this setup. Garrett

02-16-2002, 08:02 AM	#3 (permalink)
Lloyd * Join Date: Mar 2000 Member # 538 Posts: 1,794	Get an Edelbrock strip kit, about $100 or so from Summit. There are two, one fits 170xxxxxx series carbs and the other fits 70xxxxxx carbs. Get a generic oxygen sensor, about $20, and a "spark plug non-fouler" that fits the O2 sensor. Grind off the end of the non-fouler and fishmouth it, weld it to your exhaust pipe near the manifold or collector, and drill a hole for the sensor. Run a wire to the cab. Now you can either use a voltmeter, knowing that around 700 mV is optimal (14:1) stoichiometry, or buy a Nordskog panel-mount meter for about another $20. Also, see http://www.bob2000.com/carb.htm for photos and tuming method for Holleys - there's a lot of good info there. For Qjets, first wire the secondary air valves shut, and get the main jet sizes about right - since the primary rods all have the same size tips except for one special size motorhome rods. Different castings have different size vacuum ports and holes, which interact with rod and jet sizes, so what's great in one casting may not be very good in another. Once the main jets are where you want them, get the rods you need by experimenting and watching your meter at part throttle. You want decent driveabliity without the primary enrichment circuit getting too rich, or coming in too soon. This is also partially controlled by the spring under the power piston, which will be affected by altitude, cam size, available vacuum, etc. Once the primaries are right, do the secondaries. You may or may not want to adjust the air valve opening rate; if so, loosen the set strew and back off on the spring tension screw until they flop open, then bring it up 1/8 turn. If you floor it from a standing stop and there's a bog, bring it up another 1/8 turn; if not, you're done. Since the Thermoquad was used on 360's and the Q-jet is a CARB-approved replacement, you may go to the smog nazis with confidence.