Holley 2bbl carb
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Senior User
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From: Spokane, WA
Holley 2bbl carb
I have a 360 with a 2 bbl holley carb. I rebuilt it today and was surprised to find it's jets at #72. My truck is a 70 F250 Ranger with C-6 and my commute milage is 7.98 MPG Haven't done the highway milage yet. The engine has been rebuilt by previous owner. The pistons are .30 over and I think it may have a mild cam or RV type cam. The engine won't idle at factory specs in drive. It keeps dying out. My question is this: What jet size should I reduce to to get better fuel milage without starving out the engine? Thanks in adavance for any and all imput.
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Will it not idle after the rebuild? The main jet doesn't effect the idle and low speed operation. In a lot of driving in traffic at low speeds the engine is only running on the idle and transition circuit where the main jet has no impact. If you can't get it to idle after the rebuild I would look for internal leaks in the carb or vacuum leaks. The metering block gasket could be leaking.
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From: Spokane, WA
I think I wasn't too clear in my message. The gas milage was before the rebuild. I'm checking the fuel milage this week after the rebuild. The engine runs great but I need to keep the idle at 1000 RPM in park and about 850 in drive. Any lower and the engine stalls. I connected a vacuum gauge and it's steady at 15 " of vacuum. The timing is set at 8 degrees BTDC, 2 degrees more than what the book calls for. I would like to get around 10 MPG for city and about 12 MPG for the highway. Of course more would be better. I've upgraded to the pertronix ignition module but not the coil. Plugs are set at .35. So any all suggestions would be appreciated. Thanks
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From: Spokane, WA
Update: After the carb rebuild my gas milage for the week was 7.75 mpg. Yuck! Rebuilding it didn't improve mpg. I bought #70 and #69 Jets, removed the #73 jets in my carb (my post said 72's) and installed the #70. The first thing I noticed was the the engine seems to run/idle smoother. Tanks full again and I'll have the results next Saturday.
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From: Spokane, WA
The vacuum is at 15" with steady needle. I have a feeling that the cam has been changed to a mild one or an RV type cam. See my first post above. After driving today I must say that the engine runs better with the smaller jet size. Idle is better and accelleration is smoother. I'll probably drop jet sizes until the engine pings and then go up one size. Hope MPG goes up as well.
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From: Louisville,Ky.
15" is kinda low. My 460 that has a CompCams 268H that has a moderately lopey idle holds 17.5" at an idle. Where are you checking the vacuum at?
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From: Spokane, WA
Here's the update from the jet change. The gas milage improved from 7.77 mpg last week with the #73 to 8.24 mpg this week with the #70 jets. I'll drop down to #69 today and post results next week. The idle and accelleration is much better with the smaller jets.
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From: Spokane, WA
No, I haven't done the adjustment there. They where last adjusted when the smog check was performed back in October 03. The carb needed to be leaned out to pass. There are (I've heard) several ways to adjust the idle mixture which do you recommend? Thanks for your input. Jeff
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From: Spokane, WA
Oh, I also forgot to post that I checked the vacuum off the manifold and it is at 15" steady needle. I crack the throttle open and it goes to near zero and then bounces back to 19 and settles back to between 15" and 16" no vibration in the needl. I also disconnected the vacuum to the dist, plugged the vacuum line and adjusted the timing by vacuum to the highest reading which was 16". I then put the timing back at 8 degrees BTDC by way of timing light. I live at 5,000 foot level and the truck seems to run good at this setting. Startup, Idle and accelleration are acceptable. Just trying ways to improve the gas mileage.