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Car coughs in High stress

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Old 04-19-2017, 08:34 AM   #1
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Car coughs in High stress

Hello, I swapped a 350 (333 hp) crate engine (with cam upgrade and holly 670 street avenger carb) into my 91 camaro. When I try to accelerate in higher gears it starts coughing and missing hard. If I press the throttle a little bit before I floor it this seems to help, although it still isn't perfect. I checked timing, plugs and switched distributors and nothing helped. Any input appreciated.
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Old 04-20-2017, 05:05 PM   #2
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Re: Car coughs in High stress

Sounds like a fuel delivery issue. Pump, filter, lines, etc.
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Old 04-20-2017, 08:01 PM   #3
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Re: Car coughs in High stress

Quote:
Originally Posted by sofakingdom View Post
Sounds like a fuel delivery issue. Pump, filter, lines, etc.
I agree.
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Old 04-21-2017, 09:17 AM   #4
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Re: Car coughs in High stress

Quote:
Originally Posted by sofakingdom View Post
Sounds like a fuel delivery issue. Pump, filter, lines, etc.
New filter when I did the swap. It's getting 7 psi of fuel pressure at the Carb. Ill check the lines tonight.
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Old 04-21-2017, 01:32 PM   #5
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Re: Car coughs in High stress

Is it getting similar pressure at WOT? Mechanical or electric fuel pump? Do you use a fuel pressure regulator? If so, what kind? Etc.
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Old 04-21-2017, 06:10 PM   #6
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Re: Car coughs in High stress

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Originally Posted by "John" View Post
New filter when I did the swap. It's getting 7 psi of fuel pressure at the Carb. Ill check the lines tonight.
But what happens at WOT?
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Old 04-22-2017, 09:30 AM   #7
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Re: Car coughs in High stress

Quote:
But what happens at WOT?
Zackkly:

Carbs don't work off of fuel PRESSURE. They need fuel VOLUME. The fuel bowl is at atmospheric pressure, for all practical purposes. All it cares, is that the bowls are full AT ALL TIMES. When they're not, it can't possibly work right, and consequently the engine can't run right either. Doesn't have the chance of a snowball in Hell.

If your fuel system could keep the fuel bowls full at .001 psi of FP, then .001 psi is all the PRESSURE you need. (not that it will ever happen like that; but the point is, if the fuel bowls are not being kept full, you could have 100 psi but it doesn't matter in the slightest) On the other hand, if you have 7 psi of FP idling sitting in your driveway, but at WOT the motor needs a gallon and a half per minute but the fuel system can only deliver a half gallon, then SOONER OR LATER, the thing is going to nose over and fall on its face. PERIOD. It's so simple it almost hurts to think that people fail to realize it.

Similarly, if the carb's tune is off, it'll run terrible no matter what else you do. Your fuel delivery problem will be internal to the carb rather than "fuel pressure" related.

First thing I'd suggest doing is to visit the Carb forum on this site, and read and follow the "Holley Tuning" sticky you'll find there. EVERY step, IN ORDER, from start to finish, skipping none, assuming nothing, and perfecting each. You may well find that the first thing you have to fix is Holley's crappy tune; namely that the carb has too big jets, too low of a PV number, and the pri throttles are open too far at idle; and that once you get that malfunction sorted out, it's A MILLION times better. It's ASTOUNDING sometimes how much better a carb will run after it's set up to give WHAT THE ENGINE IS ASKING FOR instead of somebody just "assuming" they know what's best for it. ASK THE ENGINE WHAT IT WANTS, be prepared to listen to what it tells you, and GIVE IT WHAT IT WANTS even if that doesn't seem to match your expectations. It tells you what IT LIKES by running better. Again, it's so simple, it makes my head hurt how somebody can be willing to ignore it and fool themselves with thinking THEY know better than THE ENGINE does and then complain that THE ENGINE isn't doing what they think it should. How boneheaded.

Remember the most basic of all tuning truisms:

"If it RUNS good, it IS good".

And conversely, if it DOESN'T run good, then it ISN'T good, no matter how much YOU THINK you "know" what the "timing" should be, what size jets YOU THINK it should have in it, and so forth.

Let us know how it works out when you go through what the sticky tells you to do.
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Old 04-22-2017, 03:55 PM   #8
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Re: Car coughs in High stress

Great answer sofakingdom.
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Old 04-26-2017, 08:37 PM   #9
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Car: 91 Camaro RS
Engine: 350 crate engine 360ish HP
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Re: Car coughs in High stress

Did some carb tuning and the bowls were set wrong. Thank you sofaking
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Old 04-27-2017, 06:37 AM   #10
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Re: Car coughs in High stress

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