Carb Tuning Or...?
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Senior User
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From: Nassau Co. NY
Carb Tuning Or...?
Motor:351m
Headers, full lenth
Weiand 8010
Holley 600CFM (80457-S)
Timing set unknown (assume stock) timed at 10 degrees BTDC
Cam unknown (assume stock)
Dizzy stock
So I recently install a new intake and carb on my 351m in a '78 F150. Everything I have read is that the 80457-S is pretty much plug n play with minor tuning needed.
I set the timing to 10BTDC with the vacuum advance disconected from the "spark" port on the carb. Valve cover decal calls for 6 degrees but 10 gave the best vacuum.
The problem I'm having is if I get on the throttle hard from a stop (4 speed so second gear) its kinda boggy and breaks up a bit. At a cruise if I floor it it just falls on its face sputtering. I'm thinking the secondary spring too soft and they are opening up too early. If Im gentle on the throttle she runns well with no sputtering at all anywhere in the RPM range.
I dont have a ton of experience with carbs and wanted to check if there wasnt something Im missing. Possibly with the timing. I'll cross post to the Carb section but figured there may be some help here.
Headers, full lenth
Weiand 8010
Holley 600CFM (80457-S)
Timing set unknown (assume stock) timed at 10 degrees BTDC
Cam unknown (assume stock)
Dizzy stock
So I recently install a new intake and carb on my 351m in a '78 F150. Everything I have read is that the 80457-S is pretty much plug n play with minor tuning needed.
I set the timing to 10BTDC with the vacuum advance disconected from the "spark" port on the carb. Valve cover decal calls for 6 degrees but 10 gave the best vacuum.
The problem I'm having is if I get on the throttle hard from a stop (4 speed so second gear) its kinda boggy and breaks up a bit. At a cruise if I floor it it just falls on its face sputtering. I'm thinking the secondary spring too soft and they are opening up too early. If Im gentle on the throttle she runns well with no sputtering at all anywhere in the RPM range.
I dont have a ton of experience with carbs and wanted to check if there wasnt something Im missing. Possibly with the timing. I'll cross post to the Carb section but figured there may be some help here.
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Joined: Mar 2008
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boggy stomp
The accelerator pump is a circuit designed to overcome the unavoidable lag that happens when either engaging the main circuits, or a rapid opening of the throttle valves at cruise. Given your detail oriented intro to your thread, I doubt you've overlook a fuel filter/pressure issue, and suggest that you need to step up a size on your accelerator pump nozzle and/or adjustment of the A/P cam.
Holley TV : Mr. CarbTune's Garage - Accelerator Pump Tuning
The carb number that you shared should already have a 31 AP nozzle, and the next size I'd suggest is a 35. On a side note, the idle air mixture will also affect the idle vacuum measurement. Most folks will adjust for best vacuum reading at idle, a little trick that I've learned is to adjust for best vacuum/smoothest engine speed at an RPM just slightly higher (300-500 rpm or a quarter turn faster) above idle. Since this is where the engine is actually beginning to work.
I was wondering if you have any experience with other intakes, and I hope the vid/tip helps ya.
Holley TV : Mr. CarbTune's Garage - Accelerator Pump Tuning
The carb number that you shared should already have a 31 AP nozzle, and the next size I'd suggest is a 35. On a side note, the idle air mixture will also affect the idle vacuum measurement. Most folks will adjust for best vacuum reading at idle, a little trick that I've learned is to adjust for best vacuum/smoothest engine speed at an RPM just slightly higher (300-500 rpm or a quarter turn faster) above idle. Since this is where the engine is actually beginning to work.
I was wondering if you have any experience with other intakes, and I hope the vid/tip helps ya.
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Senior User
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From: Nassau Co. NY
The video is great. Surprized I havent come across it yet.
I stopped down to the local speed shop yesterday and picked up the acc. cam kit and a .035 squirter as well as a 9.5 power valve. I figured the motor is pulling 20hG so it would be better suited than the 6.5 thats in it.
After installing the power valve and putting it back together I could not get the engine to stay running. The cause was the badly fouled plugs which I probably did trying to drive it the day before.
Tomorrow I'm going to start from square 1 with new plugs and make sure the float level is good and the fuel pressure is not too high. I noticed I had one idle air screw out 2.5 turns and the othe 3 so I'll set them back to 1.75 turns out also as a start point.
I failed to mention that in order to get it to idle I had to back the curb idle screw almost all the way out so I may check that throttle plates not hanging open also.
Thanks again for the video
I stopped down to the local speed shop yesterday and picked up the acc. cam kit and a .035 squirter as well as a 9.5 power valve. I figured the motor is pulling 20hG so it would be better suited than the 6.5 thats in it.
After installing the power valve and putting it back together I could not get the engine to stay running. The cause was the badly fouled plugs which I probably did trying to drive it the day before.
Tomorrow I'm going to start from square 1 with new plugs and make sure the float level is good and the fuel pressure is not too high. I noticed I had one idle air screw out 2.5 turns and the othe 3 so I'll set them back to 1.75 turns out also as a start point.
I failed to mention that in order to get it to idle I had to back the curb idle screw almost all the way out so I may check that throttle plates not hanging open also.
Thanks again for the video
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Joined: Mar 2008
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dem dang holleys
they get sentimantal when it comes to tuning, but some determination makes it worth it IMO. The rough idle suggest a power valve issue quick check is to seat(turn all the way in) idle screw on both sides, and if the engine stalls then you have a good PV, if the engine smooths out and keeps running you have a breach in the PV circuit - usually the diaphram, or gasket. Be careful to only use one gasket and to thoroghly inspect the gasket upon installation. I'll actually invert the metering block (block on top of PV) and use gravity to help the gasket stay where it needs to stay(grooves of PV). Running too high of a PV has been known to fowl plugs too. Should you see some black smoke cruising, I'd suggest a lower number. The usual school of thought is to measure your vacuum while cruising,(should be lower than idle) and use half of the cruise measurement (rounding down the next size for example if measured cruise vac is 14, half = 7, use a 6.5 if measured 16, half = 8, use 7.5) to choose a PV threshold. I'm interested in how your 9.5 works with your setup, when you get some fresh plugs.
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Senior User
Joined: Aug 2004
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From: Nassau Co. NY
The usual school of thought is to measure your vacuum while cruising,(should be lower than idle) and use half of the cruise measurement (rounding down the next size for example if measured cruise vac is 14, half = 7, use a 6.5 if measured 16, half = 8, use 7.5) to choose a PV threshold. I'm interested in how your 9.5 works with your setup, when you get some fresh plugs.
Thread Starter
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Senior User
Joined: Aug 2004
Posts: 389
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From: Nassau Co. NY
Turns out the company that reman'ed the carb did a horrible job. The throttle plates were left loose, secondary linkage was never set nor was the fast idle screw. Got it sorted but the engine jumped time so now I have to replace the timing set. Thanks for your help. I'll post my finding with the 9.5 pv when I get her going.
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