Flat Spot
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Freshman User
Joined: Jun 2004
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Flat Spot
My 70' F-250 has a 390, with the original holley 4-barrel carb, intake, and aftermarket headers. When I get on the gas, at about 1/4 throttle theres a flat spot; I'll keep sqeezing the pedal gently and then the truck will jump as the engine gets a burst of juice. The carb just had a rebuild kit put in it, but the flat spot is still there. I have very limited knowledge of carb tuning etc so can anyone point me in the right direction? Thanks alot.
Senior User
Joined: Feb 2004
Posts: 185
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From: Crownsville, MD
I am no expert either (just an addicit of this board), but two things that folks often link to "flat spots" are ignition timing and carburator jetting. If the ignition advance curve for your engine is wrong, you will get a flat spot (or no power at all). If you do a search on setting ignition timing you may find some info here.
Based on your description, it sounds related more to the throttle than the RPMs (and ignition advance). I would guess the jets are too small in the primaries on your carb. I have heard folks say that the jets that come with most 600-650 cfm carbs are too small for a 390 (something like a #65 I think). I know someone (Rusty?) recently recommended using #68 jets in the primaries and #72 in the secondaries.
Hoepfully someone will stop by soon and provide more detail than this, but those seem to be the two main reasons for acceleration flat spots.
Hope that helps,
Bob
Based on your description, it sounds related more to the throttle than the RPMs (and ignition advance). I would guess the jets are too small in the primaries on your carb. I have heard folks say that the jets that come with most 600-650 cfm carbs are too small for a 390 (something like a #65 I think). I know someone (Rusty?) recently recommended using #68 jets in the primaries and #72 in the secondaries.
Hoepfully someone will stop by soon and provide more detail than this, but those seem to be the two main reasons for acceleration flat spots.
Hope that helps,
Bob
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