no acceleration on throttle 1/2 and more

[squirrel9]

I am a bit confused... if someone can have ideas on diagnosis...
My problem: engine works fine so far if I do not turn throttle approx more than half. That means in normal city drives no problem but on highway I have no acceleration when turning full throttle. No matter what gear, RPM or speed but it seems to be dependent on throttle position. When closing throttle a bit it starts to accelerate again.

I searched forum about similar problem I have. General opinion regarding similar issue was in forum that bike runs too lean what I think also seems logic reason... just problem that I do not figure out why its lean. There were some advises like:
check petcock diaphragma, screen; fuel tank vent; check and clean main jets.
I have checked these and seems to be all in proper order. Also valves checked, ignition timing is fine.

Bike has standard exhaust and no modification with carburettors.

Thanks for your help! Tom

[wiseguy]

This is from the Honda Service manual

TROUBLESHOOTING
Engine cranks but won't start
No fuel in tank
No fuel to carburetor
- Clogged fuel strainer
- Clogged fuel line
. . .. - Clogged fuel valve vacuum tube
. . . . . . . - Disconnected fuel valve vacuum tube
-Clogged fuel tank breather tube (except California
type) - CLoyyed svapura Live arriissiurl (EVAP) tube No. I
(California type)
Too much fuel getting to the engine
- Clogged air cleaner - Flooded carburetor
Intake air leak
Contaminatedldeteriorated fuel
Improper starting enrichment valve operation
Improper throttle operation
No spark at plug (faulty ignition system-section 17)
Lean mixture
Clogged fuel ,'ets
Faulty float valve
Float level too low
Restricted fuel line
- Clogged carburetor air vent tube Restricted fuel tank breather tube (except California
type)
Restricted EVAP tube No. 1 (Caiifornia type)
lntake air leak
Faulty vacuum piston
Faulty EVAP control system (California type only)
- Faulty EVAP carburetor air vent (CAV) control valve
-Clogged hose of the EVAP CAV system
Rich mixture
Starting enrichment valve open
Clogged alr jets
Faulty float valve
Float leve[ too high
D~r tya ir clearer
Faulty vacuum piston
- -. , Faulty EVAP control system (Cal~fnrniaty pe only)
- Faulty EVAP purge control valve
- Clogged hose of EVAP purge system
Engine stalls, hard to start, rough idling
Restricted fuel line
Fuel mixture too leanlrich
Contaminated/deteriorated fuel
lntake air leak
Misadjusted id[e speed
Misadjusted pilot screw
Restricted fuel tank breather tube (except California
type)
Restricted EVAP tube No. I (California type)
Clogged air cleaner
Clogged slow circuit
Starting enrichment valve open
Faulty EVAP control system (California type only)
- Faulty EVAP CAV control valve
- Faulty EVAP purge control valve
- Clogged hose of the EVAP control system
Faulty ignition system (section 17)
Afterburn when engine braking is used
Lean mixture in slow circuit
Faulty air cut-off valve
Faulty pulse secondary air injection (PAIR) system
- Faulty PAIR control valve
- Faulty PAIR check valve
- Clogged hose of the PAlR system
Faulty ignition system (section 17)
Backfiring or misfiring during acceleration
Lean mixture
Faulty ignition system (section 17)
Poor performance (driveability) and poor fuel economy
Clogged fuel system
Faulty EVAP control system (California type only)
- Faulty EVAP CAV control valve
- Clogged hose of the EVAP CAV system
Faulty ignition system (section 17)

[R J]

Some where you are losing your fuel flow.

Start at the petcock and go forward.

Have a question however.

Have you added a damn external filter?
If so take it off.

[squirrel9]

Actually no, I've not added external fuel filter and original in tank seemed fine also diaphragma seemed ok. Didi not mention any small hole or something what should cause vacuum problem.

[Farther]

I would guess you have a dirty air filter or petcock issues.  New air filter and pet rooster rebuild.  change all your vacuum tubing too.  

[Blackduck]

A little more info may help us figure this out.
How long have you had the bike and when did it start to show this problem?
Did it gradually get worse or just started suddenly?
Does it run to the same rpm through the gears and what is the maximum rpm it will go to?
Does it change between being cold and when it warms up?
Have you tried pulling the enrichener (choke) on when it does this?
Cheers Steve

[9Ball]

an over-oiled K&N filter will cause this type of problem.  It's either air or fuel, so you need to start eliminating potential issues.

[Patrick]

Generally these monsters do not have a high jet problem.. Your's sure sounds like a fuel delivery issue..

[squirrel9]

Thanks for your input! I will check petcock once again.
I have tried also to pull choke when accelerating but no change - still acceleration restricted.
Can it be also issue with ingnition?
I hope that not because problem is not dependent on engine temprature and RPM. Only throttle position seems to cause problem.

Can I order some kind of repair kit for my original petcock or it makes sense to install some other petcock?

Thanks.

[R J]

Let's find the problem before ya start sending $$$$$ off for possibily unneeded parts.

[Attic Rat]

have you had your air box off or did you modify it? Ck your tubes on the air box to carb see if they are pulling air or not sealed this will cause what you are experincing. Have you ck the carb piston diaphragm could have a whole in it

[Ricky-D]

Generally speaking, when you roll-off the throttle at an RPM close to being "on the cam" and experience a "surge" of power, it is usually from a too lean condition at the carburetors.

The "roll off" reduces the air flow and momentarily the motor experiences a more rich mixture condition and produces an increase in power.

Indexing the throttle will help in analyzing any carburetor problems.

There are quite a few possible reasons for a lean condition with your Valkyrie.

The petcock is the first place to start looking.

And by all means check that you have no vacuum leaks, anywhere!

***

[DFragn]

If all else fails check all 6 carb diaphragms for pinholes or small tears. Be gentle they're tender little devils. You can hold them up to a bright light with lite manipulation to inspect for damage.

[rodeo1]

i gotta agree with the dirty or partially covered or over oiled air filter, sounds like it does fine till it needs more air and aint getting it.

[Dag]

Check if you have lost one of the plugs on the intakes...

[squirrel9]

Will check air filter and air filter box tomorrow. Maybe some air leak...
Probably it is easier to change all vacuum tubes than to check them all...

By the way previous owner had K&N but now I am having new original filter. But it should not be the reason of my problem I think.

Tom

[Patrick]

When installing the fuel tank, be careful with the vent tube.. Its fairly common to 'kink' that little bugger which leads to a fuel starvation problem..

[squirrel9]

Thanks! I will check these plugs also on intake... so far i have not paid attention on these.

[PhredValk]

I would start with the simple stuff. If you removed the tank, you may have a kink in the fuel line.
Fred.

[Blackduck]

Pull a sparkplug and check the colour. Very dark/black and sooty would suggest restricted air intake.
Light to white lack of fuel.
Would still like some answers on my earlier questions.
Cheers Steve

[squirrel9]

Thanks. Also answers to Steve:
I am new owner of bike and I guess previous owner had this issue but forgot to tell me...
What i know is that bike has been sitting for long time and has driven only 1000miles in last 3 years.

No difference of rpm : basically I have it with 2nd gear on high rpm and also on 5th with low rpm. But it is always when turning about 3/4 of throttle.

No difference is it warm or cold.

Choke makes difference generally but it does not elliminate flat spot of acceleration.

When operating in slow speeds (normal city drive) and do not turn throttle much, bike drives ok.
Tom

[Blackduck]

OK,
With that history it could be several things.
That is the effect a modified air box has. Could also be plugged jets but usually it is the smaller slow speed jets not the mains that plug up.
Would still have a look at a couple of sparkplugs first  then think about pulling the fuel tank to check the air box and filter. Can also check the filter screen on the petcock as well. Just need to make sure the petcock lines up correctly when replacing the extension knob.
You could remove the fuel line from the petcock, attach a longer hose and apply vacuum to the petcock to check for good fuel flow before removing the tank.
Cheers Steve

[squirrel9]

Thanks! Have checked petcock once again and it works I think. I just need to apply weak vacuum and fuel flows very well.
Checked air box and mentioned that cover just consist of plastic and no any foam pad on it. I heard that there should be some foam pad also in original box cover.
Could it be that this influences my bike fuel mixture? I have never seen cover with that pad so I can not estimate how much it could make air flow different...

Toomas

[9Ball]

stock filter has no pre-filter "pad" as you call it.  If filter is OK then you need to start looking elsewhere.

[Blackduck]

The foam pad I refered to is glued onto the inside of the top cover. It is for sound suppresion. It is not a "pre-filter"
I found out the hard way that removing it will cause a flat spot when applying throttle. These fuel systems are designed with restricted air flow through the intake and jetted to suit. Opening anything up on the intake causes a lean condition that gets worse as the RPM go's up.
My example is I opened the airbox right up and needed to go to 127.5 mains to get the mixture back to correct specifications.
If you know someone else with a Valk you could try their cover on the airbox before spendinga lot of money and time chasing your problem. Pinwall always have airboxes for around $10.
Cheers Steve

[John U.]

I think the pad you refer to is not OEM for US or Canadian bikes. I believe the main jet sizes are different as well.
though I'm
not sure about California bikes.

[squirrel9]

OK. Quite interesting... definety would like to try box with that foam pad on cover.
If I only would find another Valkyrie here in Estonia... this is hard task.

How this soft element on box cover looks like? Can I just replace it with some home made thing?

Toomas

[Blackduck]

Maybe some US bikes did not have the pad but I have purchased a new cover from the US and it had the pad in there.
It is around 4" square and 3/8" thick fine bubble acoustic foam.
Cheers Steve.

[squirrel9]

Thanks Steve.
I found also topic about that airbox cover pad...
http://www.valkyrieforum.com/bbs/index.php/topic,5882.msg43871.html#msg43871

It matches very well with problems I have with my Valk. Unbelivable if this pad makes such difference!!!! Lets see, I try to glue similar pice of pad on box.

Please describe what material is that pad? Is it filter material or just some kind of usual foam?

Toomas

[9Ball]

I think the pad you refer to is not OEM for US or Canadian bikes. I believe the main jet sizes are different as well.
though I'm
not sure about California bikes.

you're definitely on to something here, no foam in the airbox cover for US bikes.... also, if we had to go to 127+ mains then there's something really wrong.  Even with K&N and Cobra 6x6, the mains are normally only increased from 100 to 105.  Some of the supercharger guys use the large main or I've heard of those removing the airbox completely and running separate filters for each carb using really large mains to get the A/F mixture right.

We need to be talking apples to apples here...and European and Australian specs serve little to help those working on US bikes....

[Ricky-D]

Trying to relate experience with Valkyries made for US consumption with Valkyries made for foreign consumption is folly and could possibly lead to poor decisions and worse performance.

Carburetors, and ignition are just a few of the things that may be so different that comparison is not practical nor possible.

***

[squirrel9]

Yes, I am completly agree that comparison makes sense only in case of "apples to apples"... Just before I started this topic I even did not know that there is any difference on carburettors and airbox between US and EU export Valks. Anyhow I have got lot of useful information and definetly will check vacuum tubes, intake o-rings replace etc. So thank you very much for advises.

My airbox cover foam is glued and tomorrow will try how it feels on road.

Regards,
Toomas

[9Ball]

Good luck and please update us with your success.....

[squirrel9]

Hello! Thank you all of good advises and help! I cut approx 10x13cm and 10mm thick filter element and glued it on the top of box cover intake. Went to ride and it runs perfect without any hesitation in full throttle. So this piece of foam decides a lot in EU bikes - you drive or you do not drive.

Regards,
Toomas

[Blackduck]

Glad to hear you have had success.
You may have to fine tune the size a bit if it runs a little rich but at least you are headed in the right direction.
Cheers Steve

[lawlessmob]

Re ;Topic: no acceleration on throttle.   My thanks to Blackduck  for his advice to squirrel9  back in Sept 2011
Re the acoustic pad in the air filter cover. After a full service on the Valk it had a bad flat spot and lacked acceleration. Spent the last 3 days pulling the bike apart to find out what I stuffed up. I thought the foam pad was a bit crumbly so I cleaned out all the traces. After reading your suggestion I replaced the missing pad with a similar one ( used a non abrasive tilers sponge ) and the bike goes like you have been fired out of a cannon.
Thanks again,
Steve in NSW Australia