broken idle mixture screw
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Senior User
Joined: Apr 2002
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broken idle mixture screw
I was adjusting the idle mixture screws on my 2 barrel carb on my 390 and one of them broke off. It is a one and a half year old "remanufactured by holley" carb. I rebuilt it during the summer because I thought it was having troubles. Anyway, the drivers side screw seemed really really tight as I was adjusting it out. I had turned it in snugly fairly easily, but when I loosened it, it was really tought to turn. Finally, it just snapped off. I should be able to get it out with some pliers. Do you guys know what would make this happen? It was turning pretty easily when I tightened it snugly before loosening it, but it all of a sudden got really really tight when I did loosen it.
Also, I have another question. I get less around 6 miles to the gallon. I was wondering if there are hopes of getting better gas mileage with a four barrel carb/intake. If so, what size carb and intake should I look for? Would a good used carb and cast iron itake work well? The 390 is all stock and I am not looking to make it fast, I just want better mileage. I have tuned it up and tried everything else, and the engine is in decent shape in terms of vaccum and compression.
any advice you guys can give me would be great
thanks
brad
Also, I have another question. I get less around 6 miles to the gallon. I was wondering if there are hopes of getting better gas mileage with a four barrel carb/intake. If so, what size carb and intake should I look for? Would a good used carb and cast iron itake work well? The 390 is all stock and I am not looking to make it fast, I just want better mileage. I have tuned it up and tried everything else, and the engine is in decent shape in terms of vaccum and compression.
any advice you guys can give me would be great
thanks
brad
Post Fiend
Joined: Apr 2002
Posts: 7,157
Likes: 1
From: Central Kali
I have never had that happen with an idle mixture screw, don't really know. You might consider an MSD 6A ignition box. It made my 390 run much better. I have never checked my mileage but I know it is not that bad. The MSD will work with your points distributor and you can put in a Durospark distributor later to ditch the points. 
Post Fiend
Joined: Jun 2003
Posts: 8,209
Likes: 12
From: Black Hills of SD
In my opinion, the Holley remans are of generally poor quality so you need to ditch the pos. 6mpg is poor, even for a 390. With any prudence at all you should be getting double digits on average.
I have had pretty good luck with the stock cast iron 4 bbl. intakes, but be aware that many of the ones that you find at swap meets ect. are for the older cars and they use the larger thermostat. Some also have a road draft tube instead of a pvc port and you will need to do some mods to get it all plumed in. Also some guys think these are actually valuable. I disagree. I bought one at a large swap meet a couple years ago for $10, and it was all bead blasted and ready for paint. I bought a 600 holley 1850 with manual choke for $20 and kitted it for another $17 so I didn't have that much into the whole thing. I have been scouring the swap meets for 2 1/2 years for a used aluminum dual plane and haven't found one yet. The guys that specialize in used manifolds say they are hard to come by and sell quickly.
Also make sure that you have an engine hoist to change manifolds. This is no joke, those iron intakes are heavy. Also I reccomend the electronic ignition. A stock duraspark will do fine if you are on a budget, but the MSD seems to be popular as is the petronix. Just make sure that the vacuum and mechanical advance are working. If you can't find the specs or need the procedures to check out these functions just ask. I am sure the information is kurking around the site somewhere.
Also you will need a 1 inch carb spacer for most stock manifolds so the throttle arm on the carb will clear the manifold. For cooler climates the stock one with coolant flowing through it may be recommended, but they are getting scarce. I got one off ebay for $20 and was glad to pay that for it. You can also use the aftermarket spacers. They range in price from around $11 to almost $40.
I have had pretty good luck with the stock cast iron 4 bbl. intakes, but be aware that many of the ones that you find at swap meets ect. are for the older cars and they use the larger thermostat. Some also have a road draft tube instead of a pvc port and you will need to do some mods to get it all plumed in. Also some guys think these are actually valuable. I disagree. I bought one at a large swap meet a couple years ago for $10, and it was all bead blasted and ready for paint. I bought a 600 holley 1850 with manual choke for $20 and kitted it for another $17 so I didn't have that much into the whole thing. I have been scouring the swap meets for 2 1/2 years for a used aluminum dual plane and haven't found one yet. The guys that specialize in used manifolds say they are hard to come by and sell quickly.
Also make sure that you have an engine hoist to change manifolds. This is no joke, those iron intakes are heavy. Also I reccomend the electronic ignition. A stock duraspark will do fine if you are on a budget, but the MSD seems to be popular as is the petronix. Just make sure that the vacuum and mechanical advance are working. If you can't find the specs or need the procedures to check out these functions just ask. I am sure the information is kurking around the site somewhere.
Also you will need a 1 inch carb spacer for most stock manifolds so the throttle arm on the carb will clear the manifold. For cooler climates the stock one with coolant flowing through it may be recommended, but they are getting scarce. I got one off ebay for $20 and was glad to pay that for it. You can also use the aftermarket spacers. They range in price from around $11 to almost $40.
Lead Driver
Joined: Jan 2003
Posts: 9,893
Likes: 12
From: Ohio
Sounds like it got cross-threaded. The threads are very fine and if it was a reman. then it might have been a little used up to start with.
If you can't get it out, then replacing the metering plate is no big deal(costly, but no big deal). Drilling it out might take the rest of the threads out too. Those plates are SOFT.
I wonder what jets are in that carb. I just put a 750 Holley on my 352 and it went from 11.5 to 12.5 mpg. I did re-jet it(had 72 stock, put 68's in it). It purrs like a kitten and runs like a top. I used one of those 4-2bbl adapters and used it upside down. You have to use 1/2" spacer under carb for throttle blades to clear. I bought it for $10 at swap meet. It came with socket head screws to mount it to intake and then I used nut/bolts to hold carb to adapter.
Larry
If you can't get it out, then replacing the metering plate is no big deal(costly, but no big deal). Drilling it out might take the rest of the threads out too. Those plates are SOFT.
I wonder what jets are in that carb. I just put a 750 Holley on my 352 and it went from 11.5 to 12.5 mpg. I did re-jet it(had 72 stock, put 68's in it). It purrs like a kitten and runs like a top. I used one of those 4-2bbl adapters and used it upside down. You have to use 1/2" spacer under carb for throttle blades to clear. I bought it for $10 at swap meet. It came with socket head screws to mount it to intake and then I used nut/bolts to hold carb to adapter.
Larry
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