Fuel Trouble Diagnosing Hesitation from Timing OR Fuel Problem

Motorsport71

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Jun 3, 2020
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So i have installed a Blueprint Engine with 9:1 compression that dyno'ed 308 hp / 344 ft lbs torque. I installed a dual plane aluminum intake, a 1/2 phenolic spacer, a new Holley 4160 with vacuum secondary's, and an HEI distributor. For the drive line i have an AOD with 2400 stall torque converter and 3:73 gears.

I have a hesitation when i hit the throttle.

Would the mechanical fuel pump cause possible issues? I noticed when i changed the carb that the fuel had run back down the line and needed to pump fuel back up the line to fill the carb's bowl. If the internal check valve is bad could the pressure be low and that cause a hesitation? At and idle it appears to keep the bowl full. After the take off it seems to run fine.

For timing the HEI distributor will show my timing curve 14 degrees at idle and 32 at 3500 rpm. I had a local shop check it and he said with his timing light he was getting 16 degrees at an idle and 36 at 3500 rpm. But here's where i'm confused... I DON"T have the vacuum advance attached because of that curve (the mechanical innards are advancing it) . If i set the base idle at 14 degrees WITHOUT the advance hooked up (and yes, i sealed all vacuum ports etc). Could that be causing problems? Does having the vacuum advance connected affect initial timing on take off? Should i retard the timing further then hook up the vacuum advance?

Any thoughts or ideas would be greatly appreciated.
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
Aug 25, 2016
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Wow, a carb! What year mustang does this setup reside?
There are soooo many things that can cause an off idle stumble, fuel pressure, float bowl fuel level, power valve cam adjustment (if there is any, some carbs may have different ways of dealing with it), squirter size, throttle blade adjustment, even cam overlap can cause problems, vaccum advance amount and timing.
How educated are you with carbs? What kind of atomizer are you working with?
 

Motorsport71

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Jun 3, 2020
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Its a 1985 GT, sorry for that lack of info. It was originally a TBI with automatic car but someone had already yanked all the components and put a 500 CFM 2 barrel on it. If it had a MAF system or even a Speed Density i would have definitely stuck with that. In '85 the 5 speed cars had a 600 CFM carb so i figured that would be a good size considering it's mostly a street car and only a 302. The reason i put an HEI distributor in it is because the factory timing would not advance with the TBI components missing.

My knowledge of carbs is basic. I can set the float, adjust the accelerator pump properly to manufacturer specs, set the idle, check to make sure the fuel is squirting when throttle is applied for acceleration. I've never messed with the air/fuel mixture screws on the bottom but i know the basics of checking the vacuum while doing it. The car isn't equipped with an air/fuel mixture gauge. I only am running the stock instrument cluster. Again it's a street car. When the car hits peak operating temperature it does hesitate slightly less, but it is still there.
 

Motorsport71

Member
Jun 3, 2020
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Hesitation Fixed.
So I checked the fuel pump and confirmed it was letting fuel back down to line level. I put a new fuel pump made for a 351 H.O. from an 86 F150 since it originally had a Motorcraft 4 barrel which was basically a Holley. I put a new fuel filter in it. After replacing them it got marginally better.

Next I started over with the timing. I figured out that the mechanical advance inside the distributor does allow for the advance I wanted. However the 'curve' wasn't correct. Without the vacuum attached the initial advance wasn't enough. So after some trial and error I was able to get the vacuum advance adjustment correct by adjusting it internally through the vacuum port with an Allen wrench. I set the distributor with car running at 14 degrees idle / 32 degrees at 3500 rpm. It advances faster and hesitation gone.