If the jets are clogged, it usually will not idle, or there is a big loss of power just above idle.
Other than trying new spark plugs, for a flat spot only at 8,000 rpm my guess is that the exhaust changes already done require a rejet.
Dynojet, Factory Pro, Ivans and Dial-a-Jet make jet kits. It will be easier to use an aftermarket slip-on muffler and that company's recommended jetting changes for their exhaust and any air filter changes, or going back to a stock muffler that is not modified.
It usually takes a lot of experimentation to get the jetting right for a modified exhaust, especially if you use an aftermarket air filter that flows more air at the same time.
For suddenly worse running, it is often either bad fuel, bad spark plugs, or clogged carb jets.
“We need randomness, mess, adventures, uncertainty, self-discovery, near-traumatic episodes, all those things that make life worth living; compared to the structured, fake, and ineffective life of an empty-suit corporate leader with a preset schedule.” From Dr Nassim Nicholas Taleb, Antifragile: Things That Gain from Disorder
Last edited by Jeff in Kentucky; 12-25-2012 at 04:00 PM.