I recently added cut outs to my exhaust and when I took it out this week for the first time the motor which was running fine through the Smitty's boggs and falls on its face when I give it gas. Not just punching it but when rolling on the gas. I have been told to check timing, raise fuel pressure, etc. I understand the lack of back pressure changes things but I need help understanding what exactly is causing this and how to fix it. Thanks...
You have been pointed in the right direction. I seem to remember years ago at the drag strip that we had to advance the timing and up the fuel pressure by a couple of PSI after uncorking the headers as the engine breathes better and needs the extra timing and fuel.
this as when opening the exhaust will make the motor A/F mixture lean out and if your using the Smithys with a small pipe ( 1 7/8-2") it will increase the back pressure substantually allowing the mixture to be leaner , raising fuel pressure won't help it, Richening up the mixture will basically 2-4 numbers up in the jet sizes ( not the idle needle mixture) . it will run rich when you close the cut outs so you have to find were the fine line point is at .
I knew my memory was missing something. We also pulled the bowls on the Holley carb to change jets as well, a pretty easy task on the double pumper center float Holley.
Yes it needs more fuel. Run into this problem on bikes all the time when opening up/ changing exhaust and that's the cure - bigger jets. Posted using the Full Custom H.A.M.B. App!
You'll need to change the jets for open headers or thru the cutouts. Then if you decide to close the cut outs you'll have other problems unless you change the jets back. The open headers will only improve performance at WOT if the engine is built to run at those Rpms. Running around town rolling into the gas isn't ever going to be right, you are supposed to suffer thru that on your way out of town to the place where WOT is needed.
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