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Discussion Starter · #1 ·
Hello everyone I need help with my 1967 mustang coupe. I just recently got a Holley 650 double pumper for my car. The motor isn’t stock it’s got a cam, intake, headers, the motor has been bored 30 over and has aluminum racing pistons in it, but every since I put the carb on it every time I kick it the car hogs down and just stutters for a little bit then starts to go. Would changing the jets on the carb help.
 

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65 Fastback 289 4 spd, 65 convertible 5.0L 5 spd. 3.73 8.8
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Hello everyone I need help with my 1967 mustang coupe. I just recently got a Holley 650 double pumper for my car. The motor isn’t stock it’s got a cam, intake, headers, the motor has been bored 30 over and has aluminum racing pistons in it, but every since I put the carb on it every time I kick it the car hogs down and just stutters for a little bit then starts to go. Would changing the jets on the carb help.
possibly, check your plugs to see what they look like. does it do it with just the 2 barrel wide open, or when you floor it? do your accelerator pumps both work?
 

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You did not specify what motor you have or stick or auto. I will guess you are to large on carb cfm. For fun bump you timing advance up to around 16 if it will take it.
 

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Discussion Starter · #5 ·
possibly, check your plugs to see what they look like. does it do it with just the 2 barrel wide open, or when you floor it? do your accelerator pumps both work?
it only does it when the 4 battles are open when I punch it the plugs look fine
 

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65 Fastback 289 4 spd, 65 convertible 5.0L 5 spd. 3.73 8.8
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it only does it when the 4 battles are open when I punch it the plugs look fine
Well you should probably see if it wants that carb. Put a vacuum gauge on the intake manifold, go drive and floor it, what does the vacuum gauge read? go to 4secondsflat,com/ to see about reading spark plugs. If it ran good with the primarary, and poor with secondary, the 4 barrell must need adjusted
 

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It sound like a lean surge. The carb might be a little big, but can be tuned. It might just need a bit more fuel immediately from the accelerator pump. Check out the videos for tuning from Holley online-they really helped me with pump cams-they are different sizes and colors, and deliver different amounts of fuel over different amounts of time. If you really want to know what is going on, do what I did and get a wide band A/F gauge-they have gotten pretty affordable.They can tell you exactly what is going on with the mixture in real time and you can tune the carb that way. Best thing to have if not going fuel injected.
 

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Hesitation can also be the result of an incorrect power valve. Depending on how much vacuum your engine makes, the power valve may not be allowing fuel flow through the jets when you expect it to. A rough guideline is that the power valve’s value should be around half (maybe plus one or two) of the inches of Hg at idle. For example, if you’re making 18”, a 9-10 power valve is a good choice. If this is not matched to your engine, you will either have a momentary lean or rich condition on part throttle. Once the throttle is well opened, the power valve is freely flowing, so that can make for a “flat spot” that then frees up. Just something to consider.
 
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