Sounds like running on 5 cylinders

[sixlow]

98 Tourer  owned 8 months, always run non ethanol except for one tank, maybe 3k miles I have put on her. Ernie Schuster Dragon exhaust, (sounds great), belly tank. Ever since I poured some Marvel Mystery oil in the tank or maybe just a coincidence she has a misfire or lack of vroom, more of a Bah, Bah, Bah, and when I first noticed on my commute to work 50 miles each way she got 20mpg, she has always averaged 27mpg, yeah the worst mileage in my fleet but I attributed that to needing to run every gear up to 4 or 5k rpm just to hear that growl come from below me. a 2nd tank gave me 25 mpg, test run tonight didn't go far because it is getting worse and I feared a breakdown so I brought her home. She has not been desmog'd yet and the belly tank does use an inline filter. I did not properly mix the MMO just dumped in 2 or 3 oz.. could that have clogged the filter or fouled a plug ? previous owner had carbs done at a dealer about a year before I got her. any ideas on where I should start. Thanks

[BonS]

You have a belly tank and I don't know how that would or could change things. I do know that MMO is slightly heavier than gasoline so it can simply settle to the lowest place. In most cases, with gravity feed systems, that would be your carburetors. YMMV.

[longrider]

 The last 16 years and three valks I've had a cylinder drop out a few times.  Each time the bike had been running perfect.  The solution every time was installing new spark plugs.  Worth a try as it's inexpensive and easy to install

[Attic Rat]

does the miss clear up above 3,000 RPM

[Ricky-D]

You could start by draining a few ounces from each carburetor float bowl.

***

[Patrick]

I think I would first start by looking at the vacuum lines. The line between 3 and 4 is famous for going bad. Take a good at the vacuum tap caps also.
Also draining fuel from each carburetor would be a good thing to do. Each carb has a drain screw.

[Firefighter]

If you think it happened with the mystery oil, I would stop putting it in there, and check your spark plugs for fouling as long rider said. The oil is not going to burn like gasoline will, hence bad mileage, loss of power and possible filter clog or plug failure etc.

I used to work on outboard motors years ago when oil was mixed with gas. People would add a little extra oil every time they gassed up (just to be safe), and over time they accumulated too heavy a oil mix and caused loss of power and what you describe.

[sixlow]

The miss does not clear up with rpm increase. This morning before work I found the 6 new plugs I had, searched for gap tool awhile and got them gapped and a mark on top to hopefully index when installing. I also remember I had a smell of melting plastic after a ride prior to the MMO, I couldn't locate anything then. Next chance I get to work on her will be Friday night, then I got the weekend too. I will start with plugs and drain carbs, check those vac lines as well.

[Firefighter]

If you do find a fouled plug, look at that plug wire, boot and coil. Each coil fires two plugs, but possible coil or wire damage. If number six is fuel fouled (left rear), I believe that is the cylinder that gives vacuum to the petcock and could be possible petcock/ carb. problem. Let us know, someone will help figure it out.

[WintrSol]

As noted, the MMO could settle, but the pump in the belly tank should draw it up and push it into the carbs, just as the normal gravity-feed tank would. MMO makes a good 'top-end' lubricant, oiling and lubricating the intake valves and, to some extent the rings, but doesn't do much for sticky carburetors. If you think the MMO is the source, and want to clear it out better, a strong dose of a good fuel system cleaner, like Seafoam, Techron, or Lucas, should help - and clean sticky carbs at the same time.

[indybobm]

I also remember I had a smell of melting plastic after a ride prior to the MMO,

The problem might be related to the plug-in connector on top of the starter relay behind the right side cover. The smell of burning plastic could be the connector melting due to corrosion on the connections. This supplies power to all of the bike and could result in the poor mileage and running. Pull off the right side cover and check the connector for heat damage.

Does the plug look like this?
http://www.valkyrieforum.com/bbs/index.php/topic,74229.0.html

[sixlow]

I just got home and headed straight for the plug, it is o.k., a little white crusty in there but no meltdown.  More to check later this week. Thanks for all the advise so far.

[sixlow]

success story to tell here, today I went ahead and changed plugs and intake O-rings, also drained some gas from carbs while intakes where out, one thing about carb draining, left side came out the screw holes, (she was on kickstand) the right bank was different, as the screws were turned out fuel began to drain out a hose underneath by drool tube, is that normal ? or is this something set up for the belly tank to drain the upper tank ? anyway I found the vac line to #3 intake cracked and the vac line to #3 carb just resting on the end, not slid on the port. That one is right behind the idle adjust knob. So all is well, she is running like a bat outa hell,  I also swapped out the I/S ICM I had in my standard, I love the low end torque gained by this plug and play upgrade. Thanks to all for your advice.
this is an older pic.

[BonS]

There is (supposed to be) a drain hose attached on the bottom of each carburetor bowl. The six hoses tee into one and exit near the drool tube. Glad you got your vacuum leaks taken care of!