carb question....
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Junior User
Joined: Aug 2010
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From: hughesville
carb question....
ok so here it is the *** i baught the truck off of took the 4bbl off and dropped on a 2 barrel 750 cfm it still has the 4bbl intake and im wanting to put a 4bbl back on it would i be increasing gas mileage switching to a 550 cfm 4bbl from a 750 cfm 3 barrel (yes i know that all a 4bbl does is increase top end power and some midrange as well as increase fuel and air flow) !!! no i did not know he switched it till i got it home and was wondering why i was only getting 11mpg!!!
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Joined: Sep 2010
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I pretty much had the same kind of problem. I bought my 72 stepside a few months back, somebody had put in a 351w with a 2 barrel. I pulled the carb to go through and clean it onlly to find an adapter plate and a 4 barrel intake underneath. I was wondering why I was have trouble at higher rpms, and wouldn't accelerate past 55 very well. I just put a 600 holley on the other day, need to find my vacuum leak and hook up the choke. I haven't driven enough to see how the gas mileage is, but I do know that my mid range and upper HP seem to be alot stronger, and I can spin the tires at 1/2 throttle now. Got up to 80mph no problem. I was getting about 8mpg with the 2 barrel, will let you know how the 4 barrel works out.
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Joined: May 2006
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From: Kingsport, TN
I haven't seen WHAT engine is in question... A 600 CFM is big enough for a 390 FE. Only bigger if rou're pulling, racing or do a lot of high RPM stuff. I'd "tweek" the 600 CFM PRIMARIES for economy, "delay" the secondaries for opening at a higher RPM. <<Daily driving and cruising on primaries / set the secondaries to open at about 60-65 MPH. It takes a bit of 'finess', but can be done. Mash the go-pedal at any time and get FULL POWER. < then watch the gas gauge move]
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Joined: Sep 2010
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Lol, I couldn't believe how quickly my gas gauge hit E with just a 2 barrel on it, I think it will be a bit better with the 4. I had the 2 barrel running as rich as I could so I could get a bit more go at higher rpms, the engine was leaning out past 2700rpms.
Posting Guru
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From: Kingsport, TN
Not sure if it was the carb - or how it was adjusted (tuned). "More power" and "economy" are at different ends of the scale. That is why I have a dash-mounted manifold (vacuum) gauge. And a tach - and soon, a fuel pressure gauge dash-mounted. I like to see just what the engine is doing. IF it's tuned right, you can feel the secondaries kick in at freeway speed. ( I've got a flore-speed / NO slush-box to kick down)
Fleet Owner
Joined: Jun 1999
Posts: 24,552
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From: Creedmoor, North Carolina
BM, 11 mpg is decent for one of these old trucks, you might tweak up a couple more but you will hit the wall soon. A 550 cfm 4 barrel should be enough to run the old truck. Stay from a 750, you will be dumping raw gas on the road from the tail pipes.
Potto, You need some tuning on your whole setup, I think fire & gas are both off. I run a 390 CFM 4 barrel on my truck and ran mine up to about 5200 rpms in 2nd 2 weeks ago. I wanted to see how many Rs it was turning when it hit 100mph. GOD the band was playing sweet music, the notes were awesome.
John
Potto, You need some tuning on your whole setup, I think fire & gas are both off. I run a 390 CFM 4 barrel on my truck and ran mine up to about 5200 rpms in 2nd 2 weeks ago. I wanted to see how many Rs it was turning when it hit 100mph. GOD the band was playing sweet music, the notes were awesome.
John
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From: hughesville
well the 4bbl really helped, for the time being i found a 400 2bbl and took the jets out i was getting around 16 mpg but the 2bbl had to go and fast the truck ran like i had my hands around its neck choking it so shes got a 600 edelbrock with an edelbrock performer intake on her with street jets just installed the compcams 270/290 extreme street/strip cam, shift kit torque converter and headman headers to thrush turbos 2 1/2 inch all the way back and i switched to an electric fan lemme tell ya WHOOOOOOOOOOHOOOOOO!!!!!! lol pushing a nice 430hp and a decent 17-18 mph ....unless i play.... then its about 10, but it was all worth the 1300 bucks for everything, ended up sideways when i mashed it at 60 on the interstate
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Fleet Owner
Joined: Jun 1999
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From: Creedmoor, North Carolina
well the 4bbl really helped, for the time being i found a 400 2bbl and took the jets out i was getting around 16 mpg but the 2bbl had to go and fast the truck ran like i had my hands around its neck choking it so shes got a 600 edelbrock with an edelbrock performer intake on her with street jets just installed the compcams 270/290 extreme street/strip cam, shift kit torque converter and headman headers to thrush turbos 2 1/2 inch all the way back and i switched to an electric fan lemme tell ya WHOOOOOOOOOOHOOOOOO!!!!!! lol pushing a nice 430hp and a decent 17-18 mph ....unless i play.... then its about 10, but it was all worth the 1300 bucks for everything, ended up sideways when i mashed it at 60 on the interstate
John
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From: hughesville
shes got o/d not sure what the trans is out of but i know its not original to the motor but i didnt have the tank i thought i did i got the 18.5 so my mpg was off im getting 13-15mpg 11-12 playing
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Yeah, I know I need some adjustment, and I have a vacuum leak, and for some reason it needs choked for a minute when starting, probably because of the vacuum leak lol. With the 2 barrel I could just start and drive. I advanced the timing a little and got a tad more power, but after the motor is warm restarting kinda sucks.
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