I Need Help Solving My Backfiring Problem

[TPGause]

Hello. If anyone has a suggestion about stopping the backfiring on my 1999 Valkyrie Interstate, please reply. I have done a great deal of work on the bike including rebuilding the carbs, installing desmog kit, replacing 3 ignition coils, replacing the exhaust seals, cleaning and degreasing bearings, much, much more. Now the engine backfires when on the side stand with the tach held at 2100 rpm. Maybe popping 10 to 15 times a minute. When I disconnect the #1 spark plug wire, the popping stops completely. When wire is reconnected, popping starts again. Today I have checked the compression, valve clearances, and replaced the RH exhaust seals. Still the same problem. I am sure the popping is caused only by the #1 cylinder. Could this possibly be an ICM problem affecting only one cylinder? After the carb rebuilds and new coils the engine runs great. Just the Backfiring. 
Tom Gause
Mem No. VOC301

[Jess from VA]

Check the vacuum caps on the nipples on the back of the intakes.  Which are placed there after a desmog removes all of the vacuum lines (except the one to the OE petcock).  That caused my big backfires (several self destructed).

I have no idea about the number one spark plug wire.

[9Ball]

Are you sure the air box tube is properly seated.  Popping is sometimes caused by an air in leakage resulting in a lean condition.

Did you install an in-line fuel filter?

[3W-lonerider]

Since its backfiring while your on the throttle. I'd lay odds you don't have the air box tube on the carb correctly. Its probably curled up in the back.

[gordonv]

I believe from reading post, it is an air issue. Either a vacuum line or some other ingress of air. Like already mentioned, the air box, the air intakes.

Did you replace the intake o-rings?

How about the exhaust bolts? Check them for tightness, and only tighten a little. They are firm at 7 in lbs (NOT FOOT).

[TPGause]

Gordon,  Thanks for the suggestions. Several others also mentioned the air filter box to Carb flex hose. I will check that tomorrow. It's very tight along side the filter box and very possible the curved hose has partially slipped off. I believe the tank hold-down bolts will stand another removal!

The O-rings (all 6) have recently been replaced. I tightened the exhaust bolts today. However, I do not see how a torque wrench (even 1/4") could be used around the three pipe cluster on three of the acorn nuts.

It's nice to have some contact with Valkyrie experience.

Tom Gause 

[TPGause]

Jess, 9Ball, Lonerider,  Thanks for your input! seems like the consensus is an air leak between the filter box and the carb. I will check it out tomorrow. That sounds very likely to be the problem. The filter box is tough to get properly in place. A flex tube could easily be pulled loose.

Thanks to all who responded. I'll post again when the problem is solved.

Tom Gause

[Jess from VA]

I'm only reporting what I've read (here before), and that is some have run a ratchet strap over the air box once all hooked up, to pull it down evenly on top the connections.   And using alcohol for plastic lube, as it dries without residue. 

[9Ball]

While you’re in there, make sure your fuel line from petcock to carbs isn’t kinked, if you have a fuel filter in-line make sure it’s capable of flowing adequate fuel, and check that the vent line is clear.

Possible to be fuel related if not an air leak (most probable) since it is the front most and high side of the carb bank.

[Valkpilot]

Are you sure the air box tube is properly seated.  Popping is sometimes caused by an air in leakage resulting in a lean condition.

This is likely.  You may need a mechanic's mirror to see, because the tube "wrinkles" on the back side where it joins the down tube.


There are several other potential leak sources to check as well: http://www.valkyrieforum.com/bbs/index.php/topic,95448.0.html

[Jersey]

When you rebuilt the carbs did you test each of the six air-valves?  I had the experience of one of these going bad after I rebuilt my carbs.  Had to take it all apart and re-test and found it to be bad.  It was causing the same kind of popping you described.  Hope that helps.
Jersey

[Ricky-D]

There seem to be a lot of suggestions that your problem is an "air" problem and most say it's connected to the air box and it's installation. I really don't think that it's an air box installation problem at all.

I don't see where any diagnosis could be made at all, under the conditions you have reported.

On the side stand is no way to test anything, except the the side stand itself, pass or fail.

The bike should be upright in running position on level ground.

And your getting popping at 2100 rpm. That is a high idle, in neutral. It's difficult even holding the engine at that rpm with the throttle, while sitting in neutral, it'll want to go up and down, I know that.

I'm not going to say there could be a synchronization element to your problem, but this sure could be a possibility, that's for sure.

What I do think is that the carburetor you say is the culprit, is starving for fuel at that particular throttle setting and that's why it is popping. Why is it starving? Well, that is what needs to be determined.
Somewhere in the low speed (idle) circuitry you'll find the culprit, whether it's a clogged orifice in the low speed gallery or a clogged idle jet should be investigated. 

But in reality, all bets are off because you're testing the bike on the side stand and it simply could be an obstinate fuel level in the offending carburetor.

***

[Regis]

I don’t mean to butt in, but my friend had this problem . I chased everything I could to find the problem and gave up. Adjusted valves , checked everything I could think of . Another guy (retired motorcycle mechanic) did an ohms test on the sparkplug wires and found one reading high . He replaced the wire and fixed the problem . You could probably swap a spark plug wire from the other side with a similar length and see if the problem switches too . Hope it helps .

[TPGause]

Many thanks to all who suggested fixes for my backfiring problem. I've spent three days working and test riding the bike. The backfiring is much lessened but still a problem. Here's the work:
1) The tube from the filter box to the #1 carb was completely detached. I removed air filter box with rubber (6) tubes attached, cleaned all tubes with alcohol, and marked the tubes inside the filter box so that I can check for proper orientation. There is a flat spot on each tube that matches with the four small holes on the bottom of the carb intake. When reattaching the tubes, I found that slightly rotating the tubes from inside the filter box greatly helped to seat the tubes on the carb intakes. All tubes had to be aligned before any would slip into place. Another 1/4 inch of clearance below the box would have been very helpful. Some 3/4 inch thick black foam strips were attached under the tank to push down on the air filter box and hopefully hold it in place. Test ran on side stand and at 200rpm doing slow speed turns: slight backfiring persists. Engine runs great at road speed and at idle. Minor loads around 2000-2500 rpm causes backfiring.
2) Adjusted all valves 0.006 intake, 0.009 exhaust: same backfiring. Will check s plug wires.

[TPGause]

I've checked out many of the ideas about backfiring. Here's what resulted:
1) When the plug wire on cylinder #1 is removed with engine running, backfiring stops. Verified spark ignition on #1 cylinder by removing plug wire, inserting new plug, and cranking engine. New plug sparks strongly and consistently.  Also, when plug wire is removed, the engine rpm at idle does NOT change. All other cylinders will drop about 100 rpm when their wires are removed. It seems to me that the #1 cyl is not producing power at idle.
2) All pilot screws were set at 6 1/2 flats out. I opened the pilot screw om #1 cyl to about 7 1/2 flats out. Now I get a slight drop of about 30 rpm when the plug wire on cyl #1 is removed. Still backfiring.
3) Thinking there may be a fuel restriction in carb #1, I added a full can of Seafoam to 1/2 tank.
Test ride this morning- Still backfiring. Thanks to Jersey and Ricky-D for carb suggestions.
4) I rebuilt all carbs about 8 months ago with a "Red Eye" kit. Still #1 carb may have a restriction.
Pulling the carb bank out for repairs is an undertaking. If anyone has ideas about carb/backfiring issues, please reply.

[turtle254]

I've checked out many of the ideas about backfiring. Here's what resulted:
1) When the plug wire on cylinder #1 is removed with engine running, backfiring stops. Verified spark ignition on #1 cylinder by removing plug wire, inserting new plug, and cranking engine. New plug sparks strongly and consistently.  Also, when plug wire is removed, the engine rpm at idle does NOT change. All other cylinders will drop about 100 rpm when their wires are removed. It seems to me that the #1 cyl is not producing power at idle.
2) All pilot screws were set at 6 1/2 flats out. I opened the pilot screw om #1 cyl to about 7 1/2 flats out. Now I get a slight drop of about 30 rpm when the plug wire on cyl #1 is removed. Still backfiring.
3) Thinking there may be a fuel restriction in carb #1, I added a full can of Seafoam to 1/2 tank.
Test ride this morning- Still backfiring. Thanks to Jersey and Ricky-D for carb suggestions.
4) I rebuilt all carbs about 8 months ago with a "Red Eye" kit. Still #1 carb may have a restriction.
Pulling the carb bank out for repairs is an undertaking. If anyone has ideas about carb/backfiring issues, please reply.

 6 1/2 flats out … ?
How many turns out are you running, should be 2 1/2 with 35 slow jets.

[Ricky-D]

To clear out the "low speed fuel delivery system" you do not need to remove the carburetor.

You do need to fabricate a "blow" gun that will have enough reach to go in the carburetor throat (venturi area) up past the butterfly valve.

The "blow" gun should, at the end, be turned down so it blows downward, and the turned down part should be short enough so that it can pass under the butterfly valve when the valve it fully opened. About 3/8 inch I surmise.

Now with the intake runner removed, you can look up inside the throat and see the butterfly valve, and if you turn the throttle "open" you will see the carburetor slide farther up in the throat.

Looking hard, you can see a few small holes spaced along the bottom of the throat starting in the area of the butterfly valve. These holes are the apertures for the low speed fuel delivery which is governed by the low speed jet in the fuel bowl. 

This is where your problem exists. The holes are not functioning properly because there is a clogged hole in the gallery not allowing fuel to pass.

To correct this problem you should remove the low speed jet in the fuel bowl. Then with the blowgun, blow out each aperture working back to the butterfly.

You may have to do this several times to insure all blockage is blown out of the gallery and exited thru the post in which the low speed jet is threaded, in the fuel bowl.

There are four apertures for the gallery, all of which are designed to allow fuel to flow depending upon the throttle position, and I would say that your problem would indicate, that the aperture which is blocked, is the one closest to the butterfly when the throttle is just about closed.

I hope this helps you.

***

[TPGause]

Turtle254 : The pilot screw that adjusts the slow speed carb mixture is set at 6 1/2 flats (or 1 5/8 turns) open. I have a small tool for turning the pilot screws that has a D shaped connector for gripping the pilot screws and a square block of brass for counting turns. Each turn is four flats as counted while rotating the tool. It seems to work well.

[TPGause]

Ricky-D: Today I finished cleaning the #1 carb as you outlined in your last posting. Everything went well. I also used spray carb cleaner to loosen the debris in the carb passageways. The #35 idle jet seemed to be clogged so I cleaned it out to be able to see light through the opening.  A piece of 1/4 " copper tubing soldered onto a standard air gun made a good 90 degree blower. Everything back together, I started the engine. Still backfiring. So here's where I'm at:
All pilot screws set at 8 flats (2 turns) open. Backfiring is loudest at about 2,500rpm. When the #1 sparkplug wire is pulled off, the backfiring immediately stops. I think this means that what ever the problem is, it is in the #1 carb. The ignition is firing all the plugs as it should. The #2 and #1 cylinder are firing off the same coil. Since the #2 cyl is running well, I assume the #1 cyl is firing just as well. When the #1 plug wire is removed with the engine running, there is no drop in rpm at idle. Therefore, the #1 cyl is not producing any power at idle. It seems I need to disassemble the carb and clean and adjust. Maybe the Air Cutoff valve is the problem as someone suggested.  Do you know what this valve does?                        (Ricky-See next posting for continuation)

[TPGause]

Ricky-D: Continuation of my previous posting:   I have compression and proper spark at cyl #1 but no power produced. So, the only thing lacking at idle is gas and air mixture. I assume the high speed jet and diaphragm is working fine because the engine runs very strong over 3,000 rpm on the road with no backfiring. I suppose there is a possibility that the valves are burned or stuck. However, the compression would be diminished or the rocker arms would be rattling. I put oil in the plug hole and got only a 5 psi compression increase. So, my question is: "Can you think of anything I need to check before I pull carb#1 out and disassemble and clean it?".

[TPGause]

Valkpilot: I read your post about Valkyrie carbs. If you care to have any input about my backfiring problem, please do so. I just test rode the bike this morning. The backfiring is somewhat more subdued (maybe Seafoam working) but there is still no drop in idle rpm when the #1 plug wire is removed.

[Ricky-D]

Since there is no dropping of rpms when the plug wire is removed, it would indicate that carburetor is not contributing to the effort of "running" the engine.

I would think this is a symptom of incorrect synchronization. Importantly tho, is that synchronization is only important at low rpms (ie very low throttle settings). Synchronization has very little affect "off idle" and no affect at all on higher throttle settings because the slide and needle are doing the main work.

 It still sounds like that cylinder is still starving for gas and the gallery and orifices may still be clogged. The air cut valve is a device to attenuate the air inducted into the cylinder under deceleration to mediate popping. It has no influence in the fuel delivery of the carburetor. You can Google "air cut valve" to learn more about how it works.

So synchronization would be the next recommended step, along with insuring the low speed fuel gallery and orifices are clear. You may want to insure the low speed jet is fully seated in the carburetor and not bottoming on the inside bottom of the float bowl. Also you can check the fuel level in the float bowl with a piece of plastic tubing attached to the drain outlet on the bottom of the float bowl. Naturally, it doesn't need to be said all this checking should be with the bike sitting upright.

The saga continues.

***

[TPGause]

Ricky-D: Thanks for the guidance. I have synchronized the carbs many times, most recently two days ago. I am certain that the #1 and #3 (reference ) carbs are pulling the same vacuum at 900-1000 rpm. So, we have air coming into the #1 carb, spark, but, likely, no fuel. I checked the gas flow to the carb bowl by removing the drain screw and opening the petcock/ vacuum shut off. Fuel flowed freely out of the open drain hole. The idle jet is securely screwed into the carb body. I could feel it hit bottom when it was tightened. It's sounding more and more like the #1 carb is clogged or restricted in some manner to prevent fuel flow at idle speed. If it needs to be removed, I will do so next week.
 
I intend to take a couple of days off from working on the bike and go ride it some.
Thanks again for your contact. It's been a great help.

[Thunderbolt]

Did you replace the small vacuum lines on each carb, I think they may go to what is called a cut valve?  Been a while since I looked at a Valkyrie but those would develop small cracks that would only be visible if you took them off and sort of squeezed the ends flat. 

[TPGause]

Thunderbolt: It has been 8 or 9 months since I rebuilt all six carbs. Most of the tubing was removed by installing the Desmog kit. The rubber tubes on the air cutout valves were replaced using new bulk tubing (about 1 1/4 " long). Also, I used the Bike Bandit test kit to check each of the air cutout valves. All seemed OK.

I see that you are in Worthington Springs, FL. I live on Hwy 121 about three miles south (where the new metal barricade is on the outside of the curve). Maybe we will meet one day. My Valkyrie is a Green and Gray Interstate.

[Valkpilot]

Valkpilot: I read your post about Valkyrie carbs. If you care to have any input about my backfiring problem, please do so. I just test rode the bike this morning. The backfiring is somewhat more subdued (maybe Seafoam working) but there is still no drop in idle rpm when the #1 plug wire is removed.

For one thing, your pilot screws aren't open enough, at least as a starting point according to the manual.  On a 99 with 35 jets, they're supposed to be set at 2 1/4 turns out (9 flats by your tool.) 


The air passages all need to be clear for fuel to be drawn up from the bowls through the jets.  This is why really gummed up carbs can't always be cleared by fuel additives. One way to make some headway here is to pull the diaphragms and use spray carb cleaner to douche out every little opening and passage you can see.  Watch for splashback!  Wear eye protection.


Also, Ricky D is right.  Re-check your carb balance.  Any time you make any change to the air or fuel mixture (which you've done by correcting vacuum leaks and by adjusting pilot screws, you have to rebalance.


If you continue to have a problem at operating speeds, look upstream to fuel supply rate.

[pancho]

While the bike is running continue opening the # 1 pilot screw 3, 4, 5 full turns to see if that reduces the the problem which it probably will. That will confirm that you still have some blockage in the carb. There are some air passages in the air funnel area that can be cleaned and sprayed out with the air box removed I believe, but it is easier with the carbs on a bench, which may solve the problem.

[TPGause]

Pancho: Thanks for the tip. I'll try opening the pilot jet on #1 cylinder several turns. I'm ready to remove the 6 carbs and clean #1 if necessary. I put a plastic tube on the vacuum nipple on the backside of the curved intake manifold. When the tube is closed, the engine backfires as usual. When the tube is open, no backfires! I really do not understand this but it is a fact.


About your wobble problem: My bike shook badly at road speed. It was out of control above 75 mph. The steering head bearings were loose as the adjusting nut was not restrained by the broken (missing a tab) lock washer device. I cleaned and regreased the bearings ( solid with old grease), adjusted the nut, and all the shaking problem was gone.

[pancho]

At 2100 RPM the carb is just transitioning from the idle circuit and so you are getting a bit of fuel, but not enough to fire every time at that RPM so the backfiring. When you open the tube and admit more air, the already lean mixture becomes more lean and doesn't fire at all, so no backfire. I believe you have a gum plug somewhere in the idle circuit,,, try opening the pilot screw and see what happens.


Have you tried what RIcky-D suggested to clear the idle fuel delivery holes in the carb throat?

You can also try removing the pilot screw and fit the tube on a can of cleaner or WD in there and give it a couple of shots.

[Ricky-D]

I guess I should have said this in the beginning, but the idle jet needs to be removed so that when you blow out the low speed gallery the air can escape thru where the idle jet is removed.

***

[9Ball]

I haven’t seen where you answered whether or not you have an in-line fuel filter or whether you replaced the fuel tubing from the petcock to the carbs.  An in-line filter may be restricting some fuel flow or a too long fuel line can become kinked.  These may not be the problem but are worth checking if applicable to rule out any fuel delivery issues.

Does the popping occur when the bike is on the side stand?  Does it happen when running stationary and also when riding the bike?

I’ve never been a fan of trying to clean the main and secondary jets and I replace them with new jets during carb cleaning or rebuild.

[TPGause]

Pancho: I have tried cleaning the slow speed holes and passages using the Ricky-D method. I modified an air gun to shoot 90 degrees using a I/4 inch copper tube. Also removed the Pilot screw and sprayed inward with carb cleaner. Still same popping problem at 2,500 rpm while under no load. I'm going out now to try opening the pilot screw.
Ricky-D: Idle jet was removed when I a blew carb cleaner into the four holes near the butterfly valve.
9-Ball: I have no in-line fuel filter, only the filter in the tank (new). Fuel flow was checked by removing the drain screw on #1 carb. Engine pops at 2,500 rpm just the same when standing straight up or on the side stand. It only makes weak pops when decelerating from road speed. I can live with the weak pops.
I am very thankful for all the help and suggestions. This problem will get solved.

[TPGause]

OK. End of the line. I have tried every suggestion in an attempt to avoid pulling the carbs off and cleaning carb#1. Today I opened the pilot screw five turns. Still backfires as always. With the time I have to work, it will take two or three days to complete this job. I hope the problem is actually a clog in the slow speed passages. I'll soak the carb body overnight and then blow out the passages.


Thanks everyone.   Tom

[nogrey]

I tried to read all of the comments on this issue and there have been a few suggestions I don’t believe you’ve tried yet, and a few suggestions that have not been made.
One person asked if you tested each of the air-cut circuits. Each carb has an air cut valve on the side. It is a black round plastic device with a short rubber tube going to it. These are known to malfunction and their entire purpose in life is to enrich the fuel input to the carbs when you let off the throttle, thus eliminating backfiring. If they are defective, they can cause backfiring. You should be able to maintain a vacuum on each of the 6 of these. If not, the valve is defective.

Also, I recently cleaned a set of carbs on a valk that I was reselling. Had the very issue you’re describing. In that case it turned out to be a bad O ring on the fuel bowl drain. You’re ahead of the game already because you know which carb it is. Any air leak of any kind in that carb will cause this problem. Culprits I’ve run into in the past that have caused this issue have included the fuel bowl drain O ring, pilot screw O ring, fuel bowl gasket, downdraft tube gasket, airbox connection (you’ve already checked), diaphragm gasket and split diaphragm, leaky air-cut valve vacuum hoses and defective air cut valves. You’ve already eliminated vacuum tubes if I read right.  
Good luck.

[JohnG]

I recently had a backfire problem that I fixed by replacing a spark plug.

[Ricky-D]

I tried to read all of the comments on this issue and there have been a few suggestions I don’t believe you’ve tried yet, and a few suggestions that have not been made.
One person asked if you tested each of the air-cut circuits. Each carb has an air cut valve on the side. It is a black round plastic device with a short rubber tube going to it. These are known to malfunction and their entire purpose in life is to enrich the fuel input to the carbs when you let off the throttle, thus eliminating backfiring. If they are defective, they can cause backfiring. You should be able to maintain a vacuum on each of the 6 of these. If not, the valve is defective.

Also, I recently cleaned a set of carbs on a valk that I was reselling. Had the very issue you’re describing. In that case it turned out to be a bad O ring on the fuel bowl drain. You’re ahead of the game already because you know which carb it is. Any air leak of any kind in that carb will cause this problem. Culprits I’ve run into in the past that have caused this issue have included the fuel bowl drain O ring, pilot screw O ring, fuel bowl gasket, downdraft tube gasket, airbox connection (you’ve already checked), diaphragm gasket and split diaphragm, leaky air-cut valve vacuum hoses and defective air cut valves. You’ve already eliminated vacuum tubes if I read right.  
Good luck.

Sorry, but how you describe the action of the "air cut valve" is misleading in what you imply as an enrichment. 

Actually, the aircut valve's job is to momentarily block the air side of the idle circuit (its air bleed, in other words) to make conditions for exhaust afterburn less likely.

The net effect is the same, but how you describe, the carburetor gets there, is incorrect.

***

[TPGause]

It's a Grand and Glorious morning! At 5:00 am yesterday I started taking parts off the bike to remove the carbs. This morning (5:30 am) I had everything back together, pilot screws set at 1 1/2 turns open, and carbs synchronized. It runs very smooth and powerful. No backfiring. #1 carb drops rpm when plug wire is removed. A 30 mile test ride when well. The bile is amazing. I don't think it has ever run this well since I have owned it ( 3 Years). Pulling off from a stop is smooth and positive.

What I found: The air cutout valve on carb #1 was hanging loose. The mounting screw seemed to bottom out before the valve was tight. May be defective threads. I put a flat washer on the screw and used force to tighten. Valve is now securely in place. The carb was disassembled and cleaned while it was off. It has been many days of work to find such a small defect. All is well.

Thanks to all who made suggestions. It is good to have you knowledgeable people out there!

[JohnG]

Good job. Keep it simple go through all the checks. Ride on.

[9Ball]

It's a Grand and Glorious morning! At 5:00 am yesterday I started taking parts off the bike to remove the carbs. This morning (5:30 am) I had everything back together, pilot screws set at 1 1/2 turns open, and carbs synchronized. It runs very smooth and powerful. No backfiring. #1 carb drops rpm when plug wire is removed. A 30 mile test ride when well. The bile is amazing. I don't think it has ever run this well since I have owned it ( 3 Years). Pulling off from a stop is smooth and positive.

What I found: The air cutout valve on carb #1 was hanging loose. The mounting screw seemed to bottom out before the valve was tight. May be defective threads. I put a flat washer on the screw and used force to tighten. Valve is now securely in place. The carb was disassembled and cleaned while it was off. It has been many days of work to find such a small defect. All is well.

Thanks to all who made suggestions. It is good to have you knowledgeable people out there!

Per Rich at Redeyetech, The OEM screws holding the aircut valves are called “triobular thread rollers”.  They roll threads into blind holes and they stick in place well the initial time they are used. Once unscrewed they are deprecated and not guaranteed to stay when reused.”  It’s very easy to strip out the threads if even slightly tightened.  The solution is to use the stock thread cutting screws to cut bottom threads and then use a longer SEMS replacement screw that has double washers.

This is what probably led to the aircut valve coming loose due to thread failure in the carb aluminum mount.

[nogrey]

I tried to read all of the comments on this issue and there have been a few suggestions I don’t believe you’ve tried yet, and a few suggestions that have not been made.
One person asked if you tested each of the air-cut circuits. Each carb has an air cut valve on the side. It is a black round plastic device with a short rubber tube going to it. These are known to malfunction and their entire purpose in life is to enrich the fuel input to the carbs when you let off the throttle, thus eliminating backfiring. If they are defective, they can cause backfiring. You should be able to maintain a vacuum on each of the 6 of these. If not, the valve is defective.

Also, I recently cleaned a set of carbs on a valk that I was reselling. Had the very issue you’re describing. In that case it turned out to be a bad O ring on the fuel bowl drain. You’re ahead of the game already because you know which carb it is. Any air leak of any kind in that carb will cause this problem. Culprits I’ve run into in the past that have caused this issue have included the fuel bowl drain O ring, pilot screw O ring, fuel bowl gasket, downdraft tube gasket, airbox connection (you’ve already checked), diaphragm gasket and split diaphragm, leaky air-cut valve vacuum hoses and defective air cut valves. You’ve already eliminated vacuum tubes if I read right.  
Good luck.

Sorry, but how you describe the action of the "air cut valve" is misleading in what you imply as an enrichment.  

Actually, the aircut valve's job is to momentarily block the air side of the idle circuit (its air bleed, in other words) to make conditions for exhaust afterburn less likely.

The net effect is the same, but how you describe, the carburetor gets there, is incorrect.

***

I don’t mind being corrected. I always try to make it known that I’m no mechanic.
Here is what I read that lead me to the enrichment concept “The air cutoff valve is a simple but vital component of the carburetor system on GL1000s. It’s function is to eliminate backfiring on hard deceleration (throttle fully closed). Hard deceleration creates a high vacuum signal in the intake tract which activates the valve. When activated, it restricts the flow of air to the external idle circuit air jets thereby richening the mixture. This slightly richer mixture is calibrated to eliminate backfiring in this operational mode.” The GL1000 carbs are similar in function to the Valk.
I don’t see the need to argue. I got him to the issue and I figure that’s what counts.
Cheers.