Hesitates and stumbles around 3k.

[John Schmidt]

A friend's bike is acting up....still. At first, it didn't want to start very good without using the choke and leaving it on. So, we changed out the slow jets and it kicks right off now. But....it still has a stumble if you hit it hard around 3k rpm. It will bog down like it's running out of gas, then catch and it's HANG ON. If you slowly accelerate up through 3-3500k rpm, it will feel like it going to bog down but will finally go on up the tach reading to as high as you want. It's just when you kick it, say you're running down the street in 2nd or 3rd and give it a twist....cough, choke, puke, buck, fart, then it catches and off you go. The owner has pulled the tank and the petcock filter in the tank, then installed an inline filter. At one point the vent was kinked, leaving him stranded a couple miles from home but eventually made it back. I told him about the vent tube and he found it pinched so that problem is cleared up. At one point before changing the slow jets(bike sat all summer while he was up north) when he tried to start it upon his return from up north, he had gas dumping all over the top of the engine from the overflow tubes. But, when I went over to do some preliminary checking, it was OK....no fuel coming out the overflow. Just got off the phone with him and suggested he order a new set of float valves/needles. Sometime next week he'll ride it over and the first thing I want to check is to drain each carb separately and measure the amount of fuel it dumps. Then when we get the carbs out, I want to check the pilot screws just to be certain I didn't accidentally mess up an o-ring in one or more of them. Then I'll replace the float valves as a precaution since he did have gas coming out the overflow at one point. Other than that, I'm stumped. While waiting for the new float valves, he's going to pull the top off the carbs and check the rubber diaphragm underneath...just in case. It appears to be a problem at the point where the carbs are switching over from the slow to high speed jets. Does anyone have other ideas what to check? This driving us nuts.

[John U.]

I guess you've checked hoses, O-rings and clamps to eliminate the possibility of vacume leaks.
Sounds like one of the floats was stuck and then freed itself up. Sitting all summer is plenty of time for ethanol to do it worst.
I think checking the diaphagms is a good idea.

Could be a pin hole or maybe one or more are out of kilter and not sealing to the cover properly.
Does the bike idle smoothly? Has the owner or a previous owner done any mods to the metering needles?
I'm curious to know what it turns out to be.

[Robert]

I have read your other posts about this bike and know how much work you've done on these carbs. There is a emulsion tube under the slow jet make sure thats clean, and of course all the passages in the carb itself. I have also seen the Dyna ignition cause problems in this range.

[Warlock]

A friend's bike is acting up....still. At first, it didn't want to start very good without using the choke and leaving it on. So, we changed out the slow jets and it kicks right off now. But....it still has a stumble if you hit it hard around 3k rpm. It will bog down like it's running out of gas, then catch and it's HANG ON. If you slowly accelerate up through 3-3500k rpm, it will feel like it going to bog down but will finally go on up the tach reading to as high as you want. It's just when you kick it, say you're running down the street in 2nd or 3rd and give it a twist....cough, choke, puke, buck, fart, then it catches and off you go. The owner has pulled the tank and the petcock filter in the tank, then installed an inline filter. At one point the vent was kinked, leaving him stranded a couple miles from home but eventually made it back. I told him about the vent tube and he found it pinched so that problem is cleared up. At one point before changing the slow jets(bike sat all summer while he was up north) when he tried to start it upon his return from up north, he had gas dumping all over the top of the engine from the overflow tubes. But, when I went over to do some preliminary checking, it was OK....no fuel coming out the overflow. Just got off the phone with him and suggested he order a new set of float valves/needles. Sometime next week he'll ride it over and the first thing I want to check is to drain each carb separately and measure the amount of fuel it dumps. Then when we get the carbs out, I want to check the pilot screws just to be certain I didn't accidentally mess up an o-ring in one or more of them. Then I'll replace the float valves as a precaution since he did have gas coming out the overflow at one point. Other than that, I'm stumped. While waiting for the new float valves, he's going to pull the top off the carbs and check the rubber diaphragm underneath...just in case. It appears to be a problem at the point where the carbs are switching over from the slow to high speed jets. Does anyone have other ideas what to check? This driving us nuts.

Check the main jets. While working on Swoopie(sp) bike I changed out all the 35's and installed 38's. I also found all the main jets some almost clogged and some completely clogged. JAT
David

[John Schmidt]

Thanks for the idea Dave, that will be one of the things to check next week when he brings it over. As for hoses, clamps, etc., we desmogged it and I double checked the intake runner clamps when reassembling. It does idle smooth now with the slows replaced, but the high speeds....all I did was give them each a shot of carb cleaner. Since it sat all summer that may just be what is needed...pull the highs and clean them.

[Ricky-D]

It is folly, attempting to analyze a problem with no knowledge of what the present conditions of the bike are.

Like has already been said, ignition changes could be the source of the problem.

Whether there are stock components in the carburetors or aftermarket changes, can have a huge effect upon the running of the motor.

If the bike was running good before, then I'd say a good carburetor cleaning ought to get it back.

Start to make changes and adjustments before regaining the "good running characteristics" will put you on an unclear path with many pitfalls along the way.

Lastly, it just may be that it has always run, the way previously stated, just a little memory loss with the passing of time off the bike.

***

[Patrick]

I sounds to me like this monster still has a slight case of varnish.. I'm a big fan of carb cleaner and a lot of compressed air.. There was a very good point made about modifications, some mods do not go well with these engines..

[John Schmidt]

First...the bike ran fine prior to being parked from June until the Fall. As I understood him, no fuel conditioner was run in it before shutting it down. Before changing the slow jets, I pulled the OEM's out and they were totally plugged...4 of 6. There has been no other mods to the bike other than adding Cobra pipes by the prior owner, but it was running fine after my friend bought it and before parking fot the summer. You gave it a twist and it took off like any other Valk. When he returned home in the Fall and tried to start it is when he discovered the gas bath coming from the overflows...didn't say which side, and it wouldn't start or stay running without heavy choke. That's "usually" an indication of plugged slows. After we changed them, it fires up great, idles great, and no hesitation in the throttle response when sitting. When on the road and pulling is when it bogs down if you twist it, it will almost die before catching and taking off. As I said before, if you slowly advance the throttle it will increase the RPM through the trouble spot with only a hint of wanting to bog down. It literally acts like fuel starvation which leads me to agree with the idea of the high speeds still being partially blocked. It happens right around the RPM where the changeover to the hi's takes place. We'll know more next week, he'll ride it over and we'll work on it here...he's about 45 miles away.

I have an extra stock ICM which I thought I might stick in just as a quick check, but my gut tells me it's fuel....not ignition.

[Ricky-D]

Using RPM's to determine throttle setting is not accurate.

Index the throttle to determine where the throttle setting truly is: At any speed, the throttle setting cannot be determined by RPM.

That is: EXCEPT WOT!

***

[Patrick]

John, maybe your friend could try a couple heavy doses of your favorite fuel cleaner and see what happens.. It sure is easier than pulling those 6 critters apart again..

[Patrick]

John, maybe your friend could try a couple heavy doses of your favorite fuel cleaner and see what happens.. It sure is easier than pulling those 6 critters apart again..

[Patrick]

John, maybe your friend could try a couple heavy doses of your favorite fuel cleaner and see what happens.. It sure is easier than pulling those 6 critters apart again..

[John Schmidt]

Ricky, I'm not useing the throttle setting as a guide, I'm noting the rpm at which the problem occurs. It happens at the 3300-3500 range regardless of the gear I'm in or speed I'm traveling. The thottle setting is pretty much irrelevant, I'm watching the rpm for the anomoly.

Patrick....been there, done that.    It helped but didn't completely do away with the problem.

[Pete]

If it ran well before sitting and does not now, CLEAN the entire fuel system WELL. And check all settings

If the problem is still there, then it is time to debug the problem.

First question, is the owner sure the problem did not exist before the sitting?

Sounds like a transition problem that probably existed before and has to do with the pipe being changed, causing a lean or rich condition at transition.

You could open the pilot jets some and see how that affect it.

Good luck, not an easy task, keep at it you will find it.

[Thunderbolt]

before you do anything else.

[John Schmidt]

before you do anything else.

Terry, it was added after the trouble appeared. The owner was replacing the in-tank screen which was messed up. I've had an inline filter for about four years now with no trouble whatsoever.

The trouble didn't exist prior to being parked for the summer...about five months. I rode it and so did he, no trouble at all. It started when he returned in the Fall and tried to run the bike.

[Thunderbolt]

Sounds fuel related to me too.  Or maybe lack of air entering the tank to replace the fuel going out the bottom.

[John Schmidt]

Sounds fuel related to me too.  Or maybe lack of air entering the tank to replace the fuel going out the bottom.

Yes, he did get caught about 2 miles from home. Just shut down, couldn't figure out why but noticed a hissing sound from the gas cap so opened it to see why and whooosh. Needless to say, it ran fine going home as long as he was above 3500 rpm. Found the vent line kinked.

[Ricky-D]

Trying to determine the reason for a riding problem is definitely hard.

Doing this for someone else when you don't have that firsthand experience makes it even more difficult.

If the problem is rpm related: Meaning it happens at the same rpm in all gears you can discard any ideas that it is carburetor based.  If you index the throttle it becomes very easy to determine if it is truly rpm related. This is a fine point here! Many times you may think it is rpm related where it may not necessarily be so.

The idea is that you should be trying to eliminate the possibilities.  Indexing the throttle is such an easy thing to do, so simply discarding this analysis tool is foolish for many reasons.

However, if it is truly rpm related, the answer lies in the air pump dynamics and is not fuel related.

If it is rpm related, look for the problem being related to the exhaust system and/or the air box and intake area.  That is if it is truly rpm related.

***

[Chiefy]

I'm no where near the level of mechanic as some of the other guys here, so keep that in mind.....

Sorry if I missed this....... But is there fresh gas in tank?  It's not the same stuff from last year I hope.

Also, my Valk did that once out of the blue.  I had gotten a bad tank of gas.  I ran it out and refilled and all was well.

I know that's really basic stuff, and if you've checked these things, I'll go shut up..... 

[John Schmidt]

We think alike. When I found out he had left it sitting for 4-5 months with no fuel stabilizer, the first thing I told him was to siphon as much of the old gas out as possible and start afresh...with some Techron, a full bottle.

Since it seems to happen at about the same rpm each time, the first thing I plan to do next week when he brings it over is subsitute my spare ICM...just in case. Don't really think that's the problem but it's any easy check. The engine will accelerate with no hesitation when just sitting in the garage...one quick twist and it rips right on up the tach...no bogging down. Get it on the street, different story. From around 3300-3500 rpm is when it starts acting up if you twist it quick...unless you eeeeaaaase it on up past that point, then it acts pretty normal.

[custom1]

I'll start off by saying I have NO experience with these Valks(just got mine). But could this be vacuum related? The idea stuck me when you said it runs good in the garage but not on the street where it would be under load. Do these beasts have vacuum advance for the timing? Just throwing that out there.

[fstsix]

Check you petcock diaphragm, sounds like just enough fuel for good idle but starts leaning out under load, put a vacuum pump on it or give the poor man check on the vacuum side with a hose and mouth  and see if it is sealed and no gas coming through. 

[John Schmidt]

Check you petcock diaphragm, sounds like just enough fuel for good idle but starts leaning out under load, put a vacuum pump on it or give the poor man check on the vacuum side with a hose and mouth  and see if it is sealed and no gas coming through. 

Tried that....no leak down of vacuum, on either the vac. hose or the petcock.

[pvan]

 Quote from JS "Before changing the slow jets, I pulled the OEM's out and they were totally plugged..."


I'd pull all 6 carbs, take em apart, soak em in a new can of carb cleaner and do carb kits... this exercise will likely save you time in the end. 

[fstsix]

I have read your other posts about this bike and know how much work you've done on these carbs. There is a emulsion tube under the slow jet make sure thats clean, and of course all the passages in the carb itself. I have also seen the Dyna ignition cause problems in this range.

Good thought on the Dyna..mine has gave me intermittent problems also, in the past dropped 2 cylinders once, and another time felt like it was way out of timing, with a strange tone through the Exhaust, and down on power big time,, pulled over unplugged the Dyna and reconnected reset all modes and has been fine for the last 7 years, i do carry the stock ICM in my bag and i have now 2 electric fuel pumps....so far i have always made it home.