[SharpAndCunning]

I've spent a few hours the last couple evening digging through old carb archives, trying to find some guidance on my 2003 VTX carb issue. I have a long thread that has worked out a few bugs, but I'm still have issues. Looking to change my approach a little.

My current problem is the bikes wants to sputter and die at highway speeds. It happens around 85/90 mph. Closer to 85 when I'm going up a incline. I did put seafoam in it, and ran it about 100 miles, no real change.

It does it in 4th and 5th gear. The bike sputters and continuously pops (exhaust) like it's running out of fuel. You let off, and when the bike slows down around 65 it "catches back up" and runs okay. Get back up to 85/90 it'll do it again. I have a couple of times hammered on it and its gone up around 95/100 and still run okay, but those are few and far between.

This leads me to believe there is some problem in a transitionary area in the carb. Like, from moving from Pilot to main jet or something. My initial though was the fuel supply to the carb wasn't enough, but WOT in 1st and Second has to use way more fuel than highway cruising in either 4th of 5th.

The carb was fully cleaned and chem soaked last year middle of season. The carb is clean. The bike will run WOT in the first 3 gears just fine. I've brought it up to about 40 in 1st, around 60 in second, 80 In third all the way open on the throttle and it doesn't miss a beat. (Now, after researching the last couple nights I realize I could go higher in second and much higher in third. And when it's back together, I plan on trying that). There is no fuel strainer in the inlet of the carb, as this is a 2003 and it had an in-line fuel filter between the tank and the pump.

I've taken the bike apart to remove the carb bowl, and look at the float. I was wondering if maybe the float is sticking.

I've checked vacuum lines, removed, inspected the ends, and replaced. Everything looks okay. Is there some vacuum actuator, or spring controlled something that could be causing this?

It is a 2003. and the fuel pump was disconnected. The petcock is only 3 years old, but it's a 2003 version without the vacuum. I've checked it, and when its On or on reserve it's fully open. There have been countless people on the forums that ditched their fuel pumps and their bikes ran fine, but I wanted it here just for reference.

I checked the black rubber diaphragm that raises the needle last year when this was happening, and there are no cracks, tears, etc. I used a LED light in the dark all around it, multiple times, not light shinning through. I'm at the point, I don't know what else to check.

This is probably some stupid little thing, but I need a VTX expert to weigh in.

 

[SharpAndCunning]

No one?

 

[JPT]

What jets and A/F screw setting are you at? What air cleaner do you have?

 

[Silver F]

WHAT about the diaphragm.. 17 years old ... it most likely cracked and leaking
is a major vacuum leak.. and the diaphragm also lifted the throttle piston.

Honda recommend replacement every 7 years.
racers replace it every year.

 

[SharpAndCunning]

What jets and A/F screw setting are you at? What air cleaner do you have?

A/F is about 2 turns out. Stock size jet. Stock air cleaner, filter less than a year old.

WHAT about the diaphragm.. 17 years old ... it most likely cracked and leaking
is a major vacuum leak.. and the diaphragm also lifted the throttle piston.

Honda recommend replacement every 7 years.
racers replace it every year.

This was a thought I had, but when I had it out last summer, it looks almost brand new. I know the carb was cleaned/rebuilt about 4 years ago. I even took a flashlight and shined it around the diaphram to look for leaks/cracks/tears and it was fine.

The weird part is, it'll run fine, 1st, 2nd, and 3rd. It's a problem only at highway speeds If it was leaking, that would affect the idle too, right?

 

[IndyX]

Fuel filters frequently cause issues.
Most agree the filter at petcock is enough.
Carb screen probably good idea if you drop the extra filter.
Did you switch out to vacuum operated petcock?

See what it does in third gear? You'll be over 80 before you hit rev limiter. Speed limiter is north of 100 can't remember the exact number I never go that fast.

When I was doing carb work I'd wind out 2nd gear to limit. Anything that happens there will happen in all gears. You get the full RPM range.

 

[Silver F]

diaphragm ,, to check it.. hold a bright light behind it and look for small amounts of light thru the thin rubber...
looks has nothing to do with diaphragm's.

Charlie D and I talked all the time about Carburetors.
I was rebuilding carburetors much longer then Charlie D
Charlie was MUCH better at keyboard explaining
I started rebuilding carburetors on motorcycles in 1969
car engines and carburetors in cars in 1966

R.I,P. Charlie.
he was my age. SAD.

 

[SharpAndCunning]

Fuel filters frequently cause issues.
Most agree the filter at petcock is enough.
Carb screen probably good idea if you drop the extra filter.
Did you switch out to vacuum operated petcock?

See what it does in third gear? You'll be over 80 before you hit rev limiter. Speed limiter is north of 100 can't remember the exact number I never go that fast.

When I was doing carb work I'd wind out 2nd gear to limit. Anything that happens there will happen in all gears. You get the full RPM range.

I dod re[;ace the petcock fuel filter towards the end of last riding season. Probably September/October. I was thinking that was the cause, and I broke the one that was in there when I removed it.

I don't have a carb screen at the moment.

I did not switch out the 2003 Petcock to a 2004+ Vacuum actuated petcock. I asked the forum here, and on VTXOA and the vacuum petcock only prevents fuel flow when the engine isn't running, so I opted to leave the 2003 one. When the Previous Owner of my bike had the tank replaced a couple of years ago (he let gas sit in it for 2-3 years, they replaced the petcock then.

diaphragm ,, to check it.. hold a bright light behind it and look for small amounts of light thru the thin rubber...
looks has nothing to do with diaphragm's.

Charlie D and I talked all the time about Carburetors.
I was rebuilding carburetors much longer then Charlie D
Charlie was MUCH better at keyboard explaining
I started rebuilding carburetors on motorcycles in 1969
car engines and carburetors in cars in 1966

R.I,P. Charlie.
he was my age. SAD.

That is exactly what I did last year (September/October) when I tried to check the diaphragm. I was thinking it might be causing my problems.It would kind of makes sense, but it shows no signs of deterioration. Maybe i can give you a call and chat sometime. I'm really out of ideas at this time for this bike.

It very well may be a vacuum leak in the carburetor somewhere, I just can't fathom for the life of me where.


I rode it today, and it's still happening. I had someone hold the throttle open after my ride, and I sprayed carb cleaner onto every vacuum line that touches or routes to the carb, and the bike never missed a beat. Granted this was in nuetral, but I was hoping I might find something.

The bike does have Hard Krome pipes, and I know the previous owner had the bike rejetted when he put the pipes on, and the carb was jetted back to factory when he put the OEM pipes back on. I know people here and on VTXOA swear they don't need to be rejetted for a simple exhuast change, but I'm tempted to put the OEM pipes back on and go for a ride just to see.

It's a speed thing at 85/90. It's not RPM related. So what changes on the bike at around that speed. That's what I need to figure out, to determine where to keep looking.

 

[SharpAndCunning]

Alright everyone, I just wanted to post an update. I was talking to the previous owner, and he distinctly remembers having to have the bike re-jetted when he put the Hard Krome system on.

So, instead of taking the carb apart again, and guessing, I put the OEM system back on. It only took about 40 minutes to swap it, so I said, what the heck, I'll try it.

It seemed better for a few miles. I was running 95+ in 4th, and then 5th and everything was fine for like 3-4 miles. I even brought it up to about 110ish. I was feeling pretty excited about it, Then all of a sudden the problem came back around 95 I tried it on RES just to be sure, and it happened again. It doesn't seem as drastic, but I think that has to do with the exhaust being quieter. I do think it is happening 5-10 miles per hour higher, consistently, than with the Hard Krome Pipes.

But now, again, I'm not sure if it's a fuel supply issue from the tank to the carb, or a jetting/vacuum signal problem to the carb.

 

[Silver F]

,
,
fuel filter at the carb inlet.

vacuum leak at intake manifold.

fuel tank vent is semi-clogged .

I agree air restricted.. or a vacuum leak...
fuel flow issues.

on the diaphragm ,, was any of the rubber "Stiff" ?
aka poor high speed performance .
remember the throttle does NOT directly control the carb... vacuum does.

if you inspect all the above... and the problem remains....
replace the diaphragm. its to OLD.
Honda recommend replacement every 7 years.
racers replace it every year.

 

[sumdatx]

Adding new pipes usually does not require any rejetting. Mess with the airbox requires rejetting. You need to investigate a little more: check your plugs to see if you're running lean; check your carb to see what rejetting took place. I'd replace the petcock with the correct one for gravity feed since that's what you're doing.
Now about your fuel tank. Is it clean and rust free? There's a vent hose attached to the right rear that is important to keep attached (makes sure air is in the tank allowing fuel flow - prevents cavitation) .
Appears you have a small problem since the bike runs well at lower speeds, meaning you will find the problem with a little patience. Remember that now all jet kits are equal. Dynojet kits a very different and hard to dial in. Same number main jet is actually smaller than Factory Pro. You could have the Scar tuned carb (washer under the main jet with a/f out some more).
Hopes this helps you get where you need to be . CharlieD always had one big piece of advice when trying to find the answer to a carb problem: check the small/easy stuff first. Chuck could very well be correct about that diaphram. We all know just how bad ethanol is for carb parts (rubber)....tom

 

[sumdatx]

one more observation,,,,, I had a similar problem that was caused by me: changed the needle position to too low and it spit and sputtered every time I gave it gas. Thought I'd moved it to #4 but had put it on #2. Go over the simple things again.
tom

 

[JPT]

What confuses me about this is, if it is carb related, it wouldn't matter what gear you are in or speed you are going, it would only matter what RPM you are at. The Carb doesn't care if you are in 1st gear or 5th gear. You would have the problem in every gear at the same RPM.

This makes me thing it is either fuel related or electronic related (maybe the ECM wires, or battery).

But I am no expert...

 

[Vinish]

Perhaps it could still be the carb. It takes a lot more fuel to run the bike at 3000 rpms in 5th gear than it does in 1st gear. This is because the speed is so much higher in 5th gear. At higher speeds, the wind resistance is substantially greater (wind resistance increases as the square of speed). So this could be a carb problem where it "stumbles" when asked to deliver greater than a certain amount of fuel.

 

[JPT]

Perhaps it could still be the carb. It takes a lot more fuel to run the bike at 3000 rpms in 5th gear than it does in 1st gear. This is because the speed is so much higher in 5th gear. At higher speeds, the wind resistance is substantially greater (wind resistance increases as the square of speed). So this could be a carb problem where it "stumbles" when asked to deliver greater than a certain amount of fuel.

Probably true. So perhaps it would happen in lower gears if the the bike could rev higher.

 

[SharpAndCunning]

,
,
fuel filter at the carb inlet.

vacuum leak at intake manifold.

fuel tank vent is semi-clogged .

I agree air restricted.. or a vacuum leak...
fuel flow issues.

on the diaphragm ,, was any of the rubber "Stiff" ?
aka poor high speed performance .
remember the throttle does NOT directly control the carb... vacuum does.

if you inspect all the above... and the problem remains....
replace the diaphragm. its to OLD.
Honda recommend replacement every 7 years.
racers replace it every year.

thank for the advice. I took the bike out the other day, and I had someone hold the throttle open and I sprayed carb cleaner all around the vacuum connections hoping to hear a change in engine pitch, thats something I used to do on my 442 when I was trying to track down leaks, and the engine never missed a beat. I am going to try it again and aim directly at the boot. The boot is old, it very well could be in need of replacement.

When I checked the Diaphragm, It looked nice. I know the carb was taken apart 3-4 years ago after it had been sitting, and when I took it apart last year, the rubber looked and felt pretty new. The bike had probably only been ridden 1000 miles since the rebuild. At the time, I had to leave the choke on like 1/4 inch to get the bike to idle well. Turns out, there was a clog in one of the circuits, that was released after soaking it in chemdip. It almost looked like part of a cleaning instrument. Almost like the end of a Q-Tip. After that, the bike idled perfectly. It seemed like an expensive part and wasn't in the rebuild kit, so I didn't replace it at the time.

But very good to know about the diaphragm and high speed performance. Is there a place you recommed buying it from?

Adding new pipes usually does not require any rejetting. Mess with the airbox requires rejetting. You need to investigate a little more: check your plugs to see if you're running lean; check your carb to see what rejetting took place. I'd replace the petcock with the correct one for gravity feed since that's what you're doing.
Now about your fuel tank. Is it clean and rust free? There's a vent hose attached to the right rear that is important to keep attached (makes sure air is in the tank allowing fuel flow - prevents cavitation) .
Appears you have a small problem since the bike runs well at lower speeds, meaning you will find the problem with a little patience. Remember that now all jet kits are equal. Dynojet kits a very different and hard to dial in. Same number main jet is actually smaller than Factory Pro. You could have the Scar tuned carb (washer under the main jet with a/f out some more).
Hopes this helps you get where you need to be . CharlieD always had one big piece of advice when trying to find the answer to a carb problem: check the small/easy stuff first. Chuck could very well be correct about that diaphram. We all know just how bad ethanol is for carb parts (rubber)....tom

The tank if clean and rust free. It is a brand new tank installed in 2016. The petcock is a pretty expensive part, and I really don't want to replace it, unless if somehow could be causing my issues here. Thank you. I have checked that vent hose, and I even blew compressed air up the little nub of the tank to make sure it was clear. And I could hear air moving through the tank. Thats what you mean about making sure air is in the tank, right?

My carb hasn't been SCAR tuned. I remember that for a fact, and I remember wondering if I should do it when I had it apart for the 2nd time last year trying to get the idle fixed (was a blockage as I mention above).

I may just break down and buy the diaphragm. I will check needle position as you mentioned when I take it apart again.

What confuses me about this is, if it is carb related, it wouldn't matter what gear you are in or speed you are going, it would only matter what RPM you are at. The Carb doesn't care if you are in 1st gear or 5th gear. You would have the problem in every gear at the same RPM.

This makes me thing it is either fuel related or electronic related (maybe the ECM wires, or battery).

But I am no expert...

I was beginning to think this too, I checked the battery, and I even put it on a charger that does a load test/etc and runs the battery through a cycle to ensure the battery is good. Came back green 2x.

I've read about the ECM wires begining to degrade, etc, and I wiggled everything and checked, but I didn't see any evidence of breaking wires.

Perhaps it could still be the carb. It takes a lot more fuel to run the bike at 3000 rpms in 5th gear than it does in 1st gear. This is because the speed is so much higher in 5th gear. At higher speeds, the wind resistance is substantially greater (wind resistance increases as the square of speed). So this could be a carb problem where it "stumbles" when asked to deliver greater than a certain amount of fuel.

So would you agree, the diaphragm seems reasonable to be causing this?

 

[Vinish]

The petcock does not need replaced. Only the diaphragm inside it and maybe the spring. The parts that need replaced come in a fairly cheap petcock rebuild kit. As for what in the carb might be causing your problems, I am not certain.

 

[Silver F]

remember.. gravity feed.. Venting of the tank
vacuum controlled petcock...
small screen at the carb input of fuel
cracked vacuum hoses... cracked "T" fitting.
then the carb...

if needed do a flow test... fill tank.. disconnect fuel hose.. let gas poor into a bucket... using a clock to see how long it take to flow 1 gallon.

then you know.. it not the tank, petcock..

be thankful.. you have 1 carburator... unlike 4 or 6 carburetors...

Valkyrie 6
CBX 6
Benelli 6
Kawasaki 6

then all the 4 cylinders with 4 carbs. starting with the "1969" Honda CB750, 500, 550, 400, 350.. etc.
then the nightmare of the V four, Magna, Sabor, Interceptor, 500, 550, 750, 1000.

 

[Vinish]

If they do a fuel flow test, what is the minimum allowable flow rate?

 

[SharpAndCunning]

The petcock does not need replaced. Only the diaphragm inside it and maybe the spring. The parts that need replaced come in a fairly cheap petcock rebuild kit. As for what in the carb might be causing your problems, I am not certain.

I don't have a diaphragm inside my petcock. The 2003 does not have a vacuum operated petcock, and therefore doesn't have the replaceable diaphragm. The diaphragm we discussed above is the needle one, in the top of the carb.

remember.. gravity feed.. Venting of the tank
vacuum controlled petcock...
small screen at the carb input of fuel
cracked vacuum hoses... cracked "T" fitting.
then the carb...

if needed do a flow test... fill tank.. disconnect fuel hose.. let gas poor into a bucket... using a clock to see how long it take to flow 1 gallon.

then you know.. it not the tank, petcock..

be thankful.. you have 1 carburator... unlike 4 or 6 carburetors...

Valkyrie 6
CBX 6
Benelli 6
Kawasaki 6

then all the 4 cylinders with 4 carbs. starting with the "1969" Honda CB750, 500, 550, 400, 350.. etc.
then the nightmare of the V four, Magna, Sabor, Interceptor, 500, 550, 750, 1000.

I'm going to answer your comments/suggestions in order.

I'll check the venting of the tank tomorrow. The only way I know how to do that, is to blow compressed air into the nipple on the tank. Is there something else I should do?

I don't have a vacuum petcock. My bike is a 2003. It came OEM with a fuel pump. That has since been removed and disconnected. There is no different parts fische for 2003, but maybe there is some sort of restrictor in the fuel path in the carb because of the pump? I'm wondering, like if there i some reason gravity flow gets slowed down in a 2003 carb, that isn't there in a 2004+ carb.

I've checked and rechecked the metal fuel elbow in the carb. There is no metal strainer, and no remnance of one. I believe this is because the bike came OEM with an in line fuel filter between the tank in the carb, down by the fuel pump. I've removed that too.

As far as the fuel test, it seems to flow fine, and have a constant flow at the end of the rubber line that connects to the carb. But, if I unscrew the bowl drain, it comes out slow. I understand that hole is smaller, but it's much more sporadic. I suppose, to figure out if there was some sort of restriction in the carb, I could take the bowl off the carb while leaving it otherwise fully attached to the bike, put a large cup under it, and open the petcock and see if it pours out. If it doesn't I guess, I could either remove the carb and check the path, or blow max psi from the compressor into the carb backwards and hope the restriction comes out the fuel entrance.