Flat spot and stumble at ~4K?

[Simmy74]

Hey guys,
Need some suggestions
I rebuilt the petcock, replaced the petcock vacuum hose and changed the airfilter.
Now at 50_70% throttle the bike will climb nicely until about 3700rpm and then it struggles as if starving for fuel.
If I then twist to 100% it grabs and pulls through like normal.

Any help would be appreciated
Thanks

[rug_burn]

Sounds like it's struggling for fuel, does it not?    So you seem to have a blockage in that petcock, apparently.   May want to re-check that.

[Tfrank59]

clearly it's a fuel starvation thing.  longshot but did you fail to reattach the gas tank vent correctly?  And are you certain the petcock kit was installed correctly?  pretty sure what you changed--or some disassembly/reassembly related to the change--caused the issue, so do step-by-step elimination to inspect/reassemble each part you disturbed. 

[Jims99]

If it’s running up at full throttle I would say something may have gotten through fuel line when taking it apart. Maybe an old piece of rubber. If it was starving for fuel it wouldn’t clear up when you give full throttle. Could also be a vacuum leak keeping slides from moving right. Check vent as mentioned and vacuum leaks first.

[Bret SD]

Hey guys,
Need some suggestions
I rebuilt the petcock, replaced the petcock vacuum hose and changed the airfilter.
Now at 50_70% throttle the bike will climb nicely until about 3700rpm and then it struggles as if starving for fuel.
If I then twist to 100% it grabs and pulls through like normal.

Any help would be appreciated
Thanks

I've been struggling with the same problem, bought a 2002 CD and don't know the bikes history. Going down the highway it runs up to around 3500 rpm without missing a beat, then approaching 3700-4000 I get the hesitation. Cranking the throttle brings it back but it's frustrating as hell. In SoCal people drive fast..

What I've done:
- replaced air filter
- new petcock/screen with vacuum shutoff (old one didn't have it)
- synced the carbs (not related I know, but it needed it)
- pulled and checked spark plugs, they seemed ok but were gapped at .8, I
  re-gapped at .88 but it made no difference.

I need to:
- check for vacuum leaks (propane method)
- new fuel lines?
- check the vent line for partial blockage

When I had the tank off I checked for flow out of the petcock, for the small amount of fuel in there it seemed to dump out pretty quick.

What about crank sensor and coil related issues? Or is this type of thing a fuel starvation problem more often than not?

[The emperor has no clothes]

Sounds to me like a carb issue.

[Bret SD]

Sounds to me like a carb issue.

Thanks for the reply! (thinking replied to me, haha) Not trying to hijack the thread but I've been looking for solutions without success for the same problem as the OP.

Are you talking carb rebuild? I've run a can of seafoam through the tank, then 2 cans of techron successively to try and root out any crud in the carbs. It's frustrating because the bike runs so smooth, accelerates and cruises very well.. (up to the 3500 rpm mark)

One thing I've noticed is with a hard head wind it seems to bog down more, and it's not just the wind as I can hear it missing (previous owner punched the stock exhaust baffles). That got me thinking the extra air pressure is doing something with the fuel mix/flow from the jets? Where is the air intake? Maybe the PO messed with something, it's been de-smogged and has 61,000 miles on it. IDK if the float needles have been replaced, or if the jets were replaced.. flying blind right now on this thing.

I don't know alot about the bike but have a mechanical background, knowing enough to study and muddle my way through problems at least.

This is one I'd really like to solve. Thank you..

[The emperor has no clothes]

I’m no expert by any means. But it’s been my experience that if these bikes sit for any amount of time the carbs get clogged up. Usually it’s the slow jets that get the worst. It sounds like yours might be more of the main jets. Many here have had success using cleaners. I haven’t. It’s been a matter of removing the carbs and manually cleaning them for me. Pain in the butt, but not a terrible job.

[Bret SD]

Hmmm... That's what I was hoping to avoid, really appreciate the input.

[WintrSol]

I’m no expert by any means. But it’s been my experience that if these bikes sit for any amount of time the carbs get clogged up. Usually it’s the slow jets that get the worst. It sounds like yours might be more of the main jets. Many here have had success using cleaners. I haven’t. It’s been a matter of removing the carbs and manually cleaning them for me. Pain in the butt, but not a terrible job.

Or, depending on how much throttle is really being used to reach that rpm, it could be the transition ports, the ones fed by the pilot jets, but are upstream of the throttle plates. They fill in the fuel gap between just above idle, to when the CV lifts and fuel feeds from the needle jet. I know the OP said 50% throttle, but I rarely reach that much throttle, even when pushing it a little. If they are only partially clogged, a tank or two treated with something like Seafoam may clean them out.

It also seems like there may an issue with the needle jets, though - maybe a delayed lift of the CV pistons. Air jets that feed the bubble chambers could cause this, too, if the flow is reduced.

[Simmy74]

Ha!
Guess I'm not alone!
Thanks guys..
In my case the symptom was there before I rebuilt the petcock  and changed the filter.
After that it got much better,  but is still there. I was extra careful on the rebuild,  and mine also runs perfectly except at that rpm/throttle.

I have had 6-6 pipes for about a year without issue.

[9Ball]

You mentioned that you changed the filter...is it a stock filter or K&N?

Also, do you have an inline fuel filter?  Not the OEM petcock screen, but an aftermarket fuel filter.

Have you replaced the fuel tubing from the petcock to the carbs?  If the tubing is too long it may be kinking and cutting off the full flow of gas.  This wouldn’t be noticeable until you hit the higher revs.

[Simmy74]

Yes it is a K&N filter
I took the tank off again and inspected the hoses, but I see you point how it may bend or kink after its buttoned up.  I would think that if that was the cause, however, it wouldn't recover and take off at full throttle

[Ricky-D]

Probably the first thing that should be done when experiencing a carburetion problem is to verify the carburetor(s) are in OEM spec, and clean, with no clogged passages. Usually this will takes care of any bad running problems that are due to carburetor(s) problems.

If this fails to correct the problem, then the next angle of attack would be to determine exactly where in the range of carburetor function the problem raises it rotten head.

So, most experts, when approaching problems such as these, would then suggest to index the throttle.

That's the next step: Index the throttle.

This step will give a real heads up to correcting a running problem that is carburetor(s) based.

***

[WintrSol]

So, most experts, when approaching problems such as these, would then suggest to index the throttle.

That's the next step: Index the throttle.

This step will give a real heads up to correcting a running problem that is carburetor(s) based.

***

I meant to say this with my post about actual throttle position - without an index on the throttle, you don't really know where it is. These engines run so strong that it is easy to guess it's at 50% when it's really about 20%.

[Tfrank59]

Hey guys,
Need some suggestions
I rebuilt the petcock, replaced the petcock vacuum hose and changed the airfilter.
Now at 50_70% throttle the bike will climb nicely until about 3700rpm and then it struggles as if starving for fuel.
If I then twist to 100% it grabs and pulls through like normal.

Any help would be appreciated
Thanks

I didn't see this in your first post, so I'll ask now: was the bike running good before the parts replacements?  

If not, then it could be anything in the fuel system (e.g., carbs), or it could be something else altogether.

If previous to the parts replacements the engine ran well, then that's logically where you begin diagnosis.  Step by step elimination means just that--you inspect/reassemble each item that was changed, then run the bike to see if that was the cause of the issue.  If the issue remains after doing that for everything that you initially changed, then explore other areas, beginning with anything related to what was changed (i.e., fuel system). 

BTW, your first post didn't include that you have aftermarket pipes, a fact which could come into play with your fuel/air delivery issue. 

[Jims99]

I know it’s more of a pain, but if running cleaners through it and it hasn’t cleared up, time to pull and clean carbs. Make sure all air passages and jets are clear. Sometimes it’s a very small piece of gunk that will screw up everything. Take your time and do it once. They’re pretty simple carbs. Good luck.

[rug_burn]

Sounds like, from all the discussion that the transition ports referred to by the guy in the P-51 (can't see his name right now) would be a good place to look.

   Keep in mind, you can get  to some of the carb without pulling the entire carb carrier, or rack, or whatever it's called:   Just carefully pull off the chrome carb covers, taking care not to let that spring jump out and fall into a pile of sawdust, as in my garage...
   Personally, I've run a drill bit very lightly down the needle hole to clean out whatever may be there (a .098 drill clears, but a .101, #38 would probably be a little better.)   You may have to rig up a pin vise or some type of holder.     Also, you want to make sure those about .375" id o-rings in the needle carrier aren't leaking.   If the vacuum which is supposed to pull up the diaphragm leaks out, the mixture goes lean.  Sounds familiar. 
   I'd also clean whatever else you can get to from there.

[9Ball]

Is the k&n filter over-oiled.  Too much oil on the filter will actually block proper air flow which might be your issue at higher demand, such as you are experiencing.

I would consider putting an oem filter into the air box instead of the k&n and seeing if this solves your problem.  Might be an easy fix.

[Simmy74]

Is the k&n filter over-oiled.  Too much oil on the filter will actually block proper air flow which might be your issue at higher demand, such as you are experiencing.

I would consider putting an oem filter into the air box instead of the k&n and seeing if this solves your problem.  Might be an easy fix.

I thought about this too.  Yes it has a freshly oiled K&N, and the problem was there BEFORE I cleaned it and rebuilt the petcock.  Both of these seemed to help a little  but I think I will take the good advice e given and get to cleaning the carbs.
Thanks guys

[98valk]

at that rpm, the bike is on the needles, pilot jet is out of the picture basically.  if it pulls cleaning after that rpm then the main jet is good.

check the slides, springs and needles for correct installation and proper free sliding movement. also check the slide diaphragms are correctly seated and not torn/damaged.  check that the bleed holes in the bottom of the slides have no obstructions.

[9Ball]

I rebuilt my carbs (again) last summer.  Replaced everything.  Bike now runs like the day I bought it off the showroom floor in early 2000.

I bought a second set of carbs off a wrecked bike about 12 years ago with plans to rebuild it as a spare for an easy one day swap if needed in the future.  Finally getting around to it.

Started the cleaning and disassembly this past week.  I’m replacing all the jets, pilots, o-rings and gaskets.  I bought the redeyetech big kit plus all new screws and tubing.  It’s a tedious process but it is the only way to do it right.

I found one tech tip that is priceless, white vinegar (5% acidity) of all things.  A quick soak followed by a rinse and carb cleaner and the brass parts come out looking factory fresh. I didn’t clean the jets because they are being replaced, but will soak the old jets and pilots and store them in my parts bin as a backup.

Good luck.  I can highly recommend the redeyetech rebuild kit as it’s worth the money and Rich’s attention to quality is second to none.  Hope you have the Honda service manual as a guide, along with the rebuild kit instructions this will make your effort successful.

[WintrSol]

Why replace the jets - are they damaged, or irreversibly clogged? Jets don't wear out; gas does not dissolve them.

[9Ball]

Why replace the jets - are they damaged, or irreversibly clogged? Jets don't wear out; gas does not dissolve them.

Because cleaning them is sometimes a hit or miss proposition.

Good luck with whatever you decide to do...

[WintrSol]

Why replace the jets - are they damaged, or irreversibly clogged? Jets don't wear out; gas does not dissolve them.

Because cleaning them is sometimes a hit or miss proposition.

Good luck with whatever you decide to do...

Then make sure to get OEM if you can; aftermarket jets are often not made to proper spec.