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I have a 98 ZL 500 and i recently added black magic twin pipes to it. I also added UFOs to the carbs last year and I was getting horrible gas mileage. I jetted the main jet down a size to a 350 and this helped a little but not much. I was getting gas mileage equal to a 99 ZR 600 EFI around 8 or 9 miles per gallon even when taking it easy on the gas. I moved my needle jet clip position up one to lean out the midrange a little and I added the pipes. I did nothing to the 350 main jet when I added the pipes. It still seems a little rich. I took my pipes off and there was gas in them where the pipes enter the silencer. Does anyone have any advice on carb jetting for this? Black magic says up one main jet size but it already seems rich with the 350? Could i go down a size main jet or just leave the main jet and change my needle jet to a leaner one? On my 98 it has the box type coolant reservor and the pipe comes within an inch of it. Has any one experienced any problems with this maybe melting the reservor? I know these are a lot of questions but any advice would be great and very helpful. And it is kinda hard to give me a good answer without knowing the temp and altitude(2000ft) and all that for jetting but just if anyone has ever added them to their sled and has any advice would be great. Black magic says 110 hp with these pipes, I hope so. Thanks.
 

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Try a 330 or 340 main jet. Only real way to see if it will work is to try and test ;)
 

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Well for starters 8-9 is pretty good mileage for a 500. You will probably never get much more than that unless you change the gearing and clutching for conservative driving and drive it conservativly but whats the fun in that. Its the price we pay for having a 500 that will run with most 600 twins. What you really need to be checking is piston wash and plug color to determine were your at with jetting. On a stock 500 at 0-1000ft in 10-30farh you can run 320s or 330s the stock size was 350s. I don't think you should be getting below a 340 safely or otherwise i cant see that the pipe would be making a HP improvement.
 
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