Engine Power Flat Spot

[Valk1500]

I have a 1998 Valkyrie in very good condition with 26,000 miles. It has developed a flat spot when I would be in any gear and I snap open the throttle. The flat spot occurs between 2,500 and 4,500 revs; once the engine revs past 4,500, it pulls strong again. During servicing, I had found the foam in the air box intake opening had deteriorated and almost vanished. I replaced it with a new item only to last a few months, deteriorating again. I think the new Honda part had been sitting so long in store that its lifespan was short just the same. So rather than buying a new one, I installed a piece of foam. However, the flat spot still remained. It's good to point out that if the throttle is not snapped open, but fed in normally as in normal riding, the flat spot does not occur.

 I believe the flat spot is due to the air box intake (varying size - becoming larger due to the deterioration of the foam) and that this is a common problem to Valkyries although I have never come across it in technical/problem forums.

 Has anyone encountered this problem and is there a solution to it.

[Blackduck]

See the post on Air cleaner foam above,
You can leave it out but will need to up the main jet size to compensate.
Where are you based? Did not think State side bikes ran the pad.
Cheers Steve

[Pete]

Sorry I have not seen the issue.
I have the Honda filter element but no foam.

If the foam is failing does it foul the carb slide or just accumulate on the main filter.
If on the main filter I assume you have cleaned that well to remove any foam particles.

[Jess from VA]

I cannot help with the flat spot, but...

The OE filter (green filter element) comes with no foam insert on top.  I would not add foam on top the OE filter. 

The K & N filter (pink with oil, or dirty over the pink) comes with a thin foam pre-filter and is recommended to be used with a stock Valk.  Lots of guys don't use it.  Not using it could cause a lean condition in some stock valks (and not others).  Many have diddled with their carbs, and of course this may change things a bit on the air filter side.

A foam pre-filter should not be able to penetrate any filter element. 

There is a thin rubber/plastic/urethane gasket between the airbox and it's top cover that may stay in place or come up/off on filter changes, and it needs to be there, and properly in place, with any filter. 

[Bagger John - #3785]

Your pilot jets may have just enough varnish buildup that they're restricted in flow slightly...contributing to an already lean (EPA mandated) mixture condition. You may also have a slight leak in one of the intake runner vacuum lines or the caps; this due to cracking with age.

Replace vac lines and caps, install the stock Honda paper filter element then add some Berryman's B12 or Seafoam to a tank of gas and run the bike until reserve...see if the problem persists.

[98valk]

only Europe bikes had an oem foam pre-filter.
all others as far as I know only have the std oem synthetic/paper filter, which is green.
maybe u have a lets in lots of dirt k&n filter?

[Chrisj CMA]

This has come up before.  If you have a European spec Valkyrie you need that foam.

I would also check the vacuum line to the petcock, and all vacuum lines.  And while you're under there make sure the fuel valve is dry and hold vacuum

[falconbrother]

Not much Valkyrie experience.  From working on Harley's I have learned a few things.  In theory I guess, the loss of foam would create a lean(er) condition, which could cause a flat spot for sure.  But, not as likely on CV carbs, In my experience, especially with the stock air box.  Any discoloration on the head pipes?  Gold is lean, blue is rich (as a general rule).  Intake leaks can cause a loss of power and flat spots.  If it's bad enough it will be hard to set the idle as well.  On the Harley there's a vacuum operated electric switch (VOES) that can cause a flat spot if it's bad so, you could have an issue in the ignition advance system.  I'm not sure if there is a portion of the ignition on a Valkyrie that works by vacuum like the carburated Harleys.  I would check for that though. 

In that mid range you're between the slow jet and the main jet.  The reason the flat spots ends at higher rpms (more throttle) is that the main jet kicks in and feeds the fuel you need.  Same with an intake leak.  Lord knows I've fixed plenty of intake leaks on carburated harleys.  With six carbs I would pull the plugs and compare them and see if you get any indication of what's going on from that.  The HD way of doing that is to ride it pretty hard then hit the kill switch before the bike can settle back into idle.  Pull the plugs and have a look.  You can really tell a lot by looking at plugs if you know what you're looking for. 

This link is HD but, the method is universal...and, in my experience, we flat six people have a lot less issues than the old Harleys do. 

http://www.hdforums.com/forum/engine-mechanical-topics/25572-plug-reading-101-properly.html

[Ricky-D]

CV (constant velocity) carburetors have no "flat spot". The best thing you can do is run some heavy doses of an additive (Berrymans) thru it and it should clear up.

***

[Blackduck]

A flat spot is a flat spot, does not matter what caused it.
Messing with jetting, messing with the air filter housing or junk in the carbs
Any carb can suffer a flat spot and CV's are no different to the rest.

[dago mooserider]

I'm a big fan of the berrymans. It straight up worked miracles on my bike before. I'd check for vac. and exhaust leaks and dose it with berrymans.

[Blackduck]

Go back and read the original post, sounds like his is a non US bike and has the foam silencer in the intake tract.
These bikes run 78 main jets and without the pad you need to go to 100 mains to cure the flat spot.
The design of the stock airbox causes a restriction in the intake, part of the reason why over 3000 RPM the economy drops off. It acts like and old style carby choke plate and draws extra fuel from the bowls.

[indybobm]

There are 3 people with the handle Valk1500 in the member database. One in Texas, one in Colorado, and one in Lija Malta. It always helps to put your location in your profile.
This was also his first post.

[Willow]

There are 3 people with the handle Valk1500 in the member database. One in Texas, one in Colorado, and one in Lija Malta. It always helps to put your location in your profile.
This was also his first post.

The OP is posting from Malta.