Hesitation during acceleration at higher speed

[superjoe37]

Hello everyone,

Quick question...I just finished a two day, approximately 600 mile ride. Towards the end I appeared to develop a slight hesitation during (attempted) rapid acceleration when speed was around 65-75. Was trying to pass someone and the hesitation was definitely a shocker. That got me jacking around at all speeds trying to see what was going on. At the lower speeds she still accelerates like she ever did (bat outta hell). But the whole rest of the ride the top end pretty much consistently had the initial hesitation. I have about 37,800 miles, bought it last year with around 33,500 so I don't have much history on the bike. I am in Texas and it was pretty warm today mid-90's, but I had the bike last year when it was hotter and never noticed the hesitation. Any ideas from the wealth of knowledge out there?

Thanks 

[WintrSol]

Had  you filled the tank a few miles earlier? IOW, was the tank mostly full, or closer to empty? The symptoms indicate to me that there was a drop in the fuel levels in the carbs. That could be due to a restriction in the fuel lines, or something blocking the tank vent.

[MarkT]

Sounds to me like starvation due to high speed high use then when the carb bowls are already nearly depleted you ask for more.  I've had this happen with the smaller orifice Dan-Marc esp while pulling a heavy trailer and aggravated by high heat, lower fuel levels in the tank (less head pressure) and uphill long climbs.  Is there a Dan-Marc installed?  Bet it's the smaller orifice one.  Or an aftermarket fuel filter?  Also ensure the fuel line is straight, all level or downhill no dips or loops.

[RWhitehouse]

I'd agree it sounds like a fuel starvation issue. These things are thirsty at higher speeds, and are pretty sensitive to any restriction in the gravity feed.

-OEM vacuum petcock could be failing and not opening fully, known issue. Rebuild, or replace with a manual Pingel. Also, potential vacuum leak in the line between the OE petcock and the manifold.
-Obstructed tank vent. After it's been hesitating, pull over and open the gas cap. If you get a whoosh of air going in, it's not venting and causing a vaccuum, which prevents proper flow. A trick I've seen around here is to get a bit of drinking straw and put it in the tank, then close the cap. If the problem vanishes, you know that's the issue.
-Any aftermarket inline fuel filters between the petcock and carbs? These are notorious for causing issues.

I actually ran an inline filter for a while and seemingly noticed no problems, even at extended high speeds. Where I did start to notice issues was higher speeds AND the fuel tank was getting to it's last quarter or so. Also, flying down the highway at 90 in 115* heat into Vegas last week, I also started to vapor lock and cylinders were cutting out as if it was running out of fuel even though the tank was mostly full. I stopped and could see fuel literally boiling inside the clear filter, I mean it was HOT as hell in there (no temp light or anything though). Let it cool off a bit and slowed down and it was fine, but when I got home from the trip I took the advice of most people here and chucked the filter. Just trying to blow through the used filter had a surprising amount of resistance, definitely see how when the fuel level is low and there's less head pressure it would only allow a dribble of fuel through, not enough to keep up with demand at freeway speed. I've had absolutely zero issues since.

[gordonv]

Towards the end I appeared to develop a slight hesitation during (attempted) rapid acceleration when speed was around 65-75. Was trying to pass someone and the hesitation was definitely a shocker.

Where you using wide open throttle? Crank the throttle quickly fully, trying to pass someone.

But if you where to just gently continue to apply throttle will you got to a higher speed, not sputtering?

Mines doing the same. Drove for about 2K miles, only issue was when trying to rapidly accelerate, like when passing. But to just accelerate till I was going 95 MPH, no issue.

It's sputtering like it's running out of fuel. But since it happens immediately at any speed, when I twist to WOT, I don't think it is.

Suggestions where to verify that the hoses are on each end of the carbs properly. Then next to sync the carbs. Since I just had the carbs and tubes off, to do a lot of work, and replace the intake o-rings, it all makes sense. Will post when I know more.

[Paladin528]

65-75 is the lower end of the power band in 5th gear.  some small hesitation should be expected but should be short lived as the power picks up right in that area.

[Ricky-D]

I expect, that what you are reporting about the performance of your Valkyrie, has been present all the time, and you have finally noticed, it for some unseen reason.

I say this because your Valkyrie is not stock and most likely has been modified by the previous owner.

If you want advise, it would be to indicate all the modifications done to your Valkyrie, so that those riders that have done some of the same modifications would be able tell you what they have done, to overcome the poor performance of their Valkyries because of the modifications.

***

[Gavin_Sons]

I use to have the same issue. Did everything under the sun to try to fix it. New fuel lines, new petcock, new tank filter, which none helped.  Decided once daddy to pull the carbs and clean them.  What I found was plugged main jets. Looked like specks of rust stuck in them but tank was not rusted.  Pull as bowl off and take out the jets and look at it,  that should be a quick way to find out for sure. 

[WintrSol]

I just had the opportunity to test mine under similar conditions: cruising at ~65 and ~70, racked the throttle open, she lept forward with no hesitation (as expected). Engine-wise, mine is fully stock.

So, something is restricting fuel flow for those that have a hesitation.