Weird power surge/loss 4th/5th gear

[Heathen]

I've noticed something I lack the mechanical knowledge to explain. hopefully someone here can point me in the right direction.

I'll be riding and it doesn't feel like the bike has full power.  I roll on the throttle and even with it cranked all the way, the bike struggles to make 4400 rpms.  I'm stuck at 88mph max..   AFter maybe 1-3 minutes of this situation all of the sudden something changes.  Something wonderful happens and the bike surges forward with new enthusiasm.  I yell encouragement, and the bike can then make 5500 rpms or more and I feel instant accelration.  Look down and I'm already passing 112 mph or more!!!   What in the heck is happening?   It FEELS like I'm runnning on 4-5 cylinders and all of the sudden the remaining cylinder or two starts working.  I don't even know if that's a physically possible thing.  I can change the oil and change a tire, but I'm no mechanics.  I've put 1100 miles on the bike since buying it a few weeks back, and I've run some carb/cleaner gas treatment (seafoam) through it in the prescribed amounts per gallon.    Still, once the bike sits for a few hours or days and I get a chance to open it up again, it will be right back to that 4400 rpm ceiling for a bit.  My previous Valkyrie never did  anything like this.  Can anyone help?

1999 Standard Valk.   (The guys I bought it from did a carb job and swear they synced the carbs with a "digi-meter", so it's probably not that..  I guess)

Thanks for any suggestions!!

I thought I'd start by inspecting the spark plugs and fuel screen, and then go from there.  just for lack of a better idea.

[98valk]

if oem petcock,? needs to be rebuilt with a new diaphragm.

[sandy]

if oem petcock,? needs to be rebuilt with a new diaphragm.

And new vacuum tube from #6 to petcock. 3.5mm ID

[Challenger]

if oem petcock,? needs to be rebuilt with a new diaphragm.

+1

[Steel cowboy]

Check off all the stupid things first, like tank creating a vacuum. Open the gas cap when it’s bogging, if it doesn’t improve then onto the vacuum lines. Then onto the petcock. If it’s not desmoged it could be one of many vacuum lines.
Just trying to save you some time with the diagnosis.

[Heathen]

Thanks for the pointers!!  I really had no idea where to start.  It IS the oem petcock, pretty dang sure.  Bike's got 37,400 miles on it and 1400 of those are mine!! LOL.  Can't imagine anyone replaced the petcock already. 

Sounds like easiest thing is to replace tube from #6 to Petcock, then try to gas cap thing.   It IS de-smogged, by the way.

So, when I'm cruising along and it's bogging.  I open the gas cap.  If it improves performance that tells me the tank is creating a vacuum?  Sorry to sound so ignorant, but I guess if that IS the case I'll come back on here and ask what to do about that.  Actually I'm somewhat embarrassed to admit that I just assumed the tank was sealed.  I never thought about it really. 

Okay, so I open the gas cap up and it doesn't improve, still bogged..  then it's either the diaphram in the petcock or the vacuum lines.   REbuilding petcock doesn't sound too bad, maybe I'll just do that too, just to be on the safe side.   

Again, thanks a million for the pointers.  I'm so freaking stoked about getting the bike running like it should!

Cheers!

[Jess from VA]

A way to improve gas cap vent without slosh (while riding) is to close it on a zip tie.

Perhaps you want to ride again (with it closed) to replicate the symptoms.  Then close the cap on a ziptie and see if you get immediate improvement.  This may help you find your problem.  Or not. 

[The emperor has no clothes]

Thanks for the pointers!!  I really had no idea where to start.  It IS the oem petcock, pretty dang sure.  Bike's got 37,400 miles on it and 1400 of those are mine!! LOL.  Can't imagine anyone replaced the petcock already. 

Sounds like easiest thing is to replace tube from #6 to Petcock, then try to gas cap thing.   It IS de-smogged, by the way.

So, when I'm cruising along and it's bogging.  I open the gas cap.  If it improves performance that tells me the tank is creating a vacuum?  Sorry to sound so ignorant, but I guess if that IS the case I'll come back on here and ask what to do about that.  Actually I'm somewhat embarrassed to admit that I just assumed the tank was sealed.  I never thought about it really. 

Okay, so I open the gas cap up and it doesn't improve, still bogged..  then it's either the diaphram in the petcock or the vacuum lines.   REbuilding petcock doesn't sound too bad, maybe I'll just do that too, just to be on the safe side.   

Again, thanks a million for the pointers.  I'm so freaking stoked about getting the bike running like it should!

Cheers!

You can put a zip tie (or something similar) under the tank filler to begin with. No need to ride first. I agree with others that it is likely the vacuum line to the petcock, or the petcock itself. When you go to order the petcock rebuild kit, it’s listed as a weird name. “Cover set petcock”

[Jess from VA]

This is the OE petcock rebuild kit (coverset) (and part number).  (it's gone up, like everything else)

https://www.partzilla.com/product/honda/16953-MBZ-B51

 

[Heathen]

HOLY GODS ABOVE!   You guys were so right.  Tracing the vacuum line to the petcock I found a pinch-clamp that wasn't centered on the fitting, it was just on the tube.  All I did was move it to where it should be and didn't think that would make ANY difference at all. 

But this morning riding to work it is like a brand new BEAST. The MONSTER unleashed, all 6 cylinders of pure power firing together in a perfect dance of power and soft humming!  I could NOT believe the difference.  The bike even started on the first try, which I didn't realize was related to my troubles.   WOW!

I'm still going to buy the rebuild kit under the assumption that I will need it at some point and I'll be hanging onto this bike for a long time.  There's a second pinch-clamp on another tube that is too small to fit over the part where it needs to be.  Today I'm going to pick up a larger hose clamp pinch-clamp thingie and replace that one too.

Thanks very much!

[Jess from VA]

And look for cracks in the vacuum line (on the back side), often right at the clamps/nipples.

[old2soon]

And look for cracks in the vacuum line (on the back side), often right at the clamps/nipples.

       What Jess said. That area under the fuel tank can git warm at times. And that vacum line should be checked or R & R while you are under there. Also the fuel lines Need to be as STRAIGHT as possible NO dips or kinks. The carbs on these things from the factory are gravity fed. Once you figure out How it SHOULD Perform Anything less Will rear it's ugly azzed head SOON! RIDE SAFE. And B T W these things Are ADDICTIVE!

[f-Stop]

Here's a pic of the Cover Set you need...


$38.58 here...https://www.rockymountainatvmc.com

They'll ship free on orders over $75.

Even at 40 some-odd dollars it's fairly inexpensive...seein' how you only need to change 'em out every ten years or so...

Also, it may be worth a look at the #6 carb's vacuum line.  It could be cracked and leaking slightly.  Maybe change it out and then see how she runs.

[98valk]

think about adding a Snubber in the vacuum line

If re-doing the oem unit, install a 1/8" npt snubber in the vacuum line to the petcock. The diaphragm will then take a very long time to wear out.
http://www.valkyrieforum.com/bbs/index.php/topic,3408.0.html

my oem petcock diaphragm went bad at 13k, I rebuilt it and installed a snubber in the vac line, now with 75k on the new diaphragm and no problems.

[-mike-]

Plus 1  what 98valk said.
Vacuum snubber installed and all potential issues are gone without loosing the security features of the stock petcock.