starts missing in third gear?

Grumpy

Grumpy

The Grumpy Grease Monkey mechanical engineer.
Staff member
grumpy I just installed a ford 9" differential, with 4.10 rear gears, and a set of good wide tires on my ford 1966 fair-lane with a 460 ford v8 , ever since the install of the new differential ,rear gear change, and better, traction,tires it starts missing in third gear?
the engines not been touched, and its always run decent, with its old 3.08 or similar gearing, since we installed that rear end differential. the engines always run very well, and I did not touch a thing under the hood, so this comes as a big surprise. any ideas
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when your trying to isolate and test any issue it helps to mentally step back and ask yourself ,
"what condition has changed or been induced that might explain this"
then test your theory...Id start by installing a fuel pressure gauge sensor for the remote dash mount fuel pressure gauge at the carb inlet port and having someone watch it carefully as you made a few hard acceleration runs,
FuelFlowDiagram02a1.jpg


have you adjusted both the fuel bowl fuel levels so the idling engine shows fuel just level with the sight plugs lower edge.
no it does not mater if its a manual engine driven fuel pump or its an electric fuel pump, you need to verify fuel delivery consistent pressure at the carb inlet.
carburetor+plug_removal.jpg

lock_screw_removal.jpg

now it may be something else but Id suspect that either the fuel pump has a hard time over coming the initial inertial loads that are reduced once the cars launched,Id suggest you install a fuel pressure gauge where you can see it hooked up to the carbs inlet port and verify you maintain a consistent 6-7 psi , if the pressure drops noticeably at any time, during a run, that fuel system needs upgrading.
http://garage.grumpysperformance.com/index.php?threads/setting-up-your-fuel-system.211/

http://garage.grumpysperformance.com/index.php?threads/fuel-pressure-regulators.635/

or the oil pan contents sloshing to the rear uncover the pick-up and your oil pressure sensor is cutting electric to the fuel pump or ignition, until the oil pressure returns
Id sure throw an extra quart of oil in the engine and install a fuel pressure gauge to isolate those two potential sources before looking further

http://www.hotrod.com/articles/9-inch-ford-rearend-strongest-rearend-due-hypoid-distance/

http://garage.grumpysperformance.com/index.php?threads/rear-differential-width.2186/#post-5879


Fuel-Pump-Flow-Requirements.jpg


FuelFlowDiagram02a1.jpg

http://garage.grumpysperformance.com/index.php?threads/fuel-pressure-regulators.635/
lineroutefpj.png

fuellinesizer.jpg


Fuel Hose Sizes
AN SizeTube or Hose ODTube or Hose ID
-6 3/8” .297”
-8 1/2” .391”
-10 5/8” .484”
-12 3/4” .609”


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AN_fittings_1.jpg
 
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If I might chime in here. In addition to checking fuel delivery, it would be good to verify ignition KV capacity at each spark plug. An ignition scope would be good here as it allows you to see the KV pattern of all 8 cylinders from a single connection at the coil wire. Even without a scope, there are simple tests we can do to check ignition health.
First off, you say the engine hasn't been touched. How old are the plug wires? Are they stiff or still supple as when new. Stiff wires is a sign of failing insulation and wires that may arc to ground at high KV.
It's typical for an ignition system with aging wires or any other marginal secondary component to operate okay at idle or anytime required KV is low. At idle, required KV will be around 10K or less. As load and cylinder pressure increases, the KV required to initiate a flame front in each cylinder also increases. This is when we tend to see misfires occur due to KV leakage through old wires or other marginal components. This will usually show up at lower engine speeds, higher load. Like after the shift to 3rd gear, especially climbing a hill.

High voltage follows the path of least resistance. In a healthy ignition wire, that would be through the conductor and across the plug gap. In an aging wire, it will often be through a weak spot in the insulation to an engine ground close by. Or down the side of the porcelain past a poorly sealing boot. This kind of leakage can be checked using one or both of two simple tests. A KV tester, not the inline bulb type spark tester, but an adjustable KV tester like the inexpensive one from Thexton, will test each wire and the entire circuit back to the coil on each cylinder. The tester will have graduations for different KV levels from standard ignition at 10K, up to 40K for testing HEI systems. Set it according to your ignition system and connect it at the plug end of a wire and to engine ground. Start the engine and look for a steady spark across the gap of the tester with no missing beats. If you see missing beats, look for arcing along the wire. Look for leakage at the coil wire. Repeat for each cylinder.

The second test for KV leakage can be combined with the KV tester. This is using a spray bottle of water to spray a mist of water over the entire secondary ignition system. The water will encourage KV leakage at any weak spot. Some like to add a teaspoon or two of salt to the water to increase conductivity. I hesitate to spray salt on an engine. If you do add salt, be sure to wash down the motor afterward. Spray the wires and plugs with water with the plug wires connected to check for leakage at the boots.

Before performing these tests, a visual inspection of the ignition is a good place to start. Again look for stiff wires. Check the plugs for wear or any deposits. Analysing plugs tells us a lot about the condition of the engine. Oil ash tells us there is oil leaking into the cylinder either through the rings or valve guides. A plug that is light or dark means the cylinder is running lean or rich. Worn plugs push KV high by requiring more to initiate a spark. Look for any sign of leakage at the plug boots. A dark stripe down the porcelain is a KV leak. Pull the cap and look for wear on the cap contacts or heavy ozone build up inside the cap and at the rotor.
 
thats welcome and useful additional info, thank you!
yes your correct that you can,t ever assume you know the direct cause of any problem,
without going through the logical course of isolating and testing each potential source.
 
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