"It has always done this when it is warming up and sometimes within the first hundred yards or so, and on rare occasion on the road."
I'm gonna assume you bought your bike new and haven't added a better flowing air cleaner and exhaust. If you have added a better flowing air cleaner and exhaust...then richening up the carburetor would have been your next step, otherwise...your backfiring problem could be coming from a too-lean condition.
"When I got the machine back from the dealership I had to replace the intake and exhaust gaskets and the carburetor seal, but I can't see where this would cause this problem."
Let's clear this part up. You're saying that when you had the engine remanufactured by H-D you took the bike in, they pulled the engine, sent it to H-D, then reinstalled it and started it to make sure everything was hooked up right and they could sign off on responsibility? 'Cause when you buy a "new" engine replacement they won't warranty the engine unless they install and start it up themselves.
Like Mark mentioned, this is where the "I had to replace the intake and exhaust gaskets and the carburetor seal, but I can't see where this would cause this problem." concerns me.
First, if the dealership installed the engine and started it up...why would YOU think to replace those gaskets. Second, if you just "picked up" the engine at the dealership when it came back from the factory and installed it yourself...yes, you would have had to install the manifold and carburetor yourself. Last, anyone can fuck up an intake manifold and carburetor installation. Intake leaks are the main cause of lean combustion chamber mixtures.
Unless the dealership did the installation (which makes them responsible for the running condition of the engine) I suggest that you do an intake manifold vacuum leak test that includes the manifold-to-carburetor gasket. If you find no leaks there...then take the bike to someone who knows their way around an EGA (exhaust gas analyzer), and see where you stand.
If you have added a freer flowing air cleaner and exhaust...I suggest you do this:
Remove the cap over the air/fuel mixture screw and set the screw at 2 turns out as a base setting. You should be able to get the best low speed performance and a smooth idle within half a turn from there.
Install a #45 or #46 idle jet. For most bikes a #45 works fine but some bikes do better with a #46. Start with a #45. While you're in there check the float level height and adjust if needed.
Usually the stock main jet works fine so leave it alone, for now. If you find that WOT coughs on you...install a #180 or #185.
Now, use one .020" Yamaha needle shim, part #90201-03331, to shim up the slide needle. This puts it's taper in a better working range. Install new plugs while you're at it!
After all is done, fire the bike up, get it up to proper operating temp (a must with Evo engines) then check the idle and mid range operation with an EGA machine that tells if the air/fuel mixture is burning too lean or rich.
These mods give good performance all across the power band and decent gas mileage. By: 47Knuckle-Dragger on: 1-16-2008
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