I have a 98 dyna wide glide that has 30,000 miles on it I have two problems that are recent, one is that gas mileage has dropped to about 105 miles out a tank of gas and I have vibration on the front end. I took the carb off and cleaned it good changed the intake manifold gasket put a new vacuum hose on, changed spark plugs, new spark plug wires. I had a problem with getting some backfire on deceleration before now that has seemed to go away. I drove the bike the other day i reached 95 miles before having to go to reserve tank. My mechanic thinks possibly because of mileage the valves are not seating correctly anymore, causing the poor gas mileage. If that was the case would that cause the vibration in the front end also. My gas mileage was not great before this but i was able to get about 110 miles before going to reserve. A few years ago I changed both rubber motor mounts myself.

 

If the motor mounts are after-market, then they may be transmitting too much energy back to you, from the power pulses. The stock mounts seem to work the best for smoothness at the seat/bars & pegs.

 

Your mileage is horrible

 

If your mechanic thinks you might have valve issues dual compression test that'll tell you.. Also check to make sure the VOES switch is operating properly.. If it's not that'll decrease gas mileage as well...

 

I am running a 180 and 48 jet in my carb and after 70 miles the plugs are black and sooty. My mechanic says that is expected because of the size of the jets .

 

I have no knowledge of correct jet sizes but if the plugs are that black it must be running too rich. That will kill your mileage and dump unburned fuel into the crankcase oil. Your mechanic may be mistaken, but hopefully someone here can offer a better set up.

 

The 48 is way too big.. A lot of people don't understand that on CV carb The pilot jet whatever you want to call it the low jet, does more than 30% of the cruising range up to about 3,300 RPM so that size of a jet is going to be rich all the way through to them RPMs..

 

For an example, my '01 88" twin cam is running stage I (air cleaner and exhaust) and I'm still at the OEM 45 jet....

 

With this updated info, I would start by having the carb corrected.
Here is the dyno from my old 2000 softail standard, 88 with a stage 1. I could get 200 miles on a 5 gallon tank. My stock carb upgrade was a Yost power tube https://www.yostperformance.com/
Pipes on it in the pics are stock headers with khrome weeks slip ons using the crossover pipe. It went through several pipe combos until this one. I never did a dyno after the final but would bet that torque dip is gone. Sometimes I miss that bike.

 

-I did a compression test on my bike both cylinders were about 120. I have a stage 1 and Kerker 2 in 1 exhaust. I don't know any other tests I should perform. I had a death wobble earlier this year on the freeway ,could it be the neck bearings. causing the vibration. Who out there has a 98 dyna like mine and what is your gas mileage. I have a 46 jet that a mechanic replaced with a 48 a few years ago should I put that back in to see if my plugs aren't black after 70 miles.