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Discussion Starter #1
Hi All,

I hope everyone is staying safe through these crazy times. I had some questions maybe some of you could possibly help me with. My 67 mustang has had throttle stumble & hesitation for a long time and now that I have a little time on my hands, I am determined to fix it. Here is what my mustang is experiencing:
  • Hard starting during cold and warm weather.
  • Throttle stumble and hesitation when the car is in neutral and I blip the throttle.
  • Smells like raw fuel constantly.
Here is my Car/Engine setup:
  • 289 bored .020 over (Approx a 294)
  • 5 Speed Manual Trans
  • 3:55 Gears
  • Headers to Dual Flowmasters
  • Holley 650 Double Pumper (Model 4150)
I think a 650 Double Pumper is probably too much carb for my application but I don't want to buy another carb for now. Before completely rebuilding the carb, Is there an order of things to check and troubleshoot before? I was thinking to check spark plug gaps, timing, adjust rich/lean mixture, adjust the accelerator pump assembly and then if all of these don't solve my problem, rebuild the whole darn thing from scratch. Is there something I'm missing or should be looking at first before diving in?

Thank you in advance for your help.
 

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Important questions:

1) Did the car always do this since you installed the carb or did the car develop issues over time?

2) Have you made any ignition or fuel changes since you installed the carb?
 

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I think you’ve already figured out that it’s running too rich.
At a minimum you’ll have to remove the fuel bowls to change jets and also see what power valve is installed.

Absolutely start by adjusting the idle mixture screws. And you can look at the accelerator pump adjustment but there isn’t much to do there without changing the pump cam or diaphragm.

It would also be a good idea to check ignition timing and manifold vacuum.

In the end, you’ll wind up taking the carb apart just so you can see what jet sizes are in there.
Once you’ve got it apart, order some smaller jet sizes, maybe a different power valve, and a good gasket kit with the blue reusable gaskets.
 

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Consider installing an AFR gauge so that you'll be able to see the actual air/fuel ratio instead of just guessing what the engine is getting from the carb.
 

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Down load,print and read this, and do what it says, anything else is like trying to give you a haircut over the internet.

bob2000.com/carb.htm
 

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  • Throttle stumble and hesitation when the car is in neutral and I blip the throttle.
  • Holley 650 Double Pumper (Model 4150)
I think a 650 Double Pumper is probably too much carb for my application but I don't want to buy another carb for now.
Pity, because you already know the answer to your problem. A 650 is too big for a 289, and the double pump only makes it worse.

You don't mention what cam you have, which is much more important than your .020 overbore. However, at most you should be looking at no more than 600 cfm if you have something like a 289HP cam. If you have the stock C3OZ-V, 500 cfm is the absolute limit for driveability.

You could fix this problem for $248

749229
 

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Discussion Starter #7
Important questions:

1) Did the car always do this since you installed the carb or did the car develop issues over time?

2) Have you made any ignition or fuel changes since you installed the carb?
Hi there. The car has always had this hesitation on a throttle blip. The starting problems have gotten worse over the years as this wasn't always the case. I have not made any ignition changes since the install of the carb.
 

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Discussion Starter #8
I think you’ve already figured out that it’s running too rich.
At a minimum you’ll have to remove the fuel bowls to change jets and also see what power valve is installed.

Absolutely start by adjusting the idle mixture screws. And you can look at the accelerator pump adjustment but there isn’t much to do there without changing the pump cam or diaphragm.

It would also be a good idea to check ignition timing and manifold vacuum.

In the end, you’ll wind up taking the carb apart just so you can see what jet sizes are in there.
Once you’ve got it apart, order some smaller jet sizes, maybe a different power valve, and a good gasket kit with the blue reusable gaskets.
Thank you. I was trying to avoid a rebuild if it wasn't needed but I may need to just go for it. Timing & spark plugs is something I will definitely check first. The car does sit a lot (drive it once a month usually) so maybe the sitting fuel is clogging up the carb? Thanks for the info.
 

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Discussion Starter #11
Pity, because you already know the answer to your problem. A 650 is too big for a 289, and the double pump only makes it worse.

You don't mention what cam you have, which is much more important than your .020 overbore. However, at most you should be looking at no more than 600 cfm if you have something like a 289HP cam. If you have the stock C3OZ-V, 500 cfm is the absolute limit for driveability.

You could fix this problem for $248

View attachment 749229
I think a 650 is too big as well but according to Holley's recommendations, it's not. I used their tool here: Carb Selector

After plugging in the details, it shows that a 650 Double Pumper is an option. In either case, I believe I have a Ford Racing B Camshaft (from the previous owner) and I don't know the specifics of the cam, unfortunately.
 

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As a baseline, I would check intake vacuum, timing, spark plug gap, float level and adjust the idle needle valves. If you are using 10% ethanol gas and with very limited driving, than your float needles and power valve are likely toast. I would get a rebuild kit. Put the correct power valve in it. See what primary jets are in it. adjust the floats, adjust the idle screws and than tackle the main jetting. I use an Innovate AFR unit to dial in my jetting.
 

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Discussion Starter #14
As a baseline, I would check intake vacuum, timing, spark plug gap, float level and adjust the idle needle valves. If you are using 10% ethanol gas and with very limited driving, than your float needles and power valve are likely toast. I would get a rebuild kit. Put the correct power valve in it. See what primary jets are in it. adjust the floats, adjust the idle screws and than tackle the main jetting. I use an Innovate AFR unit to dial in my jetting.
Sounds like a plan. Appreciate the help!
 

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Also, FWIW, that carb is a 650 cfm unit... the same on I run on my 5.0HO.
 

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A 650 double pumper is fine for a 289 when tuned right. The lean stumble can be fixed. It would be hard without an AFR gauge. I've found that to be one of the most important things when tuning my car. They aren't too expensive and you can get someone to weld in a bung if you don't weld for pretty cheap. I'd start with a different accelerator pump cam that gives more fuel immediately. But it's hard to say if that is the problem. Even having an AFR unit I had a very hard time getting the lean stumble out. I think in the end it was the IFR's that I changed that did the trick. I went up in pump cams and then back down. I have a 750 cfm with a 347-no hesitation, runs great-no bog etc. I messed with it a lot though.
 
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