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Discussion Starter · #1 ·
Is there a difference between ACV valves on different models of CX/GL500's and carburetors? I seen to recall one of the parts fiche sites showing the same part number but......



I have a GL400 with VB38A carbs and am getting some, though not bad, popping on deceleration.



Are there dimensional differences?If so what might they be?



Inspecting my ACV's does not show any obvious pin holes or other damage, but I have been getting fuel on the cover side of both AVC Valves (both carbs), which certainly does not seem correct to me.



What else might be at fault?



I just did a complete clean with ultrasonic and was truly amazed with the results. That moderate surge at 5k RPM is gone and mileage is back up to about 20k per liter.



Thanks in advance...



Mark in Thailand
 

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Mark,

There are differences in ACV's. An easy way to tell which ones you need is to look at the covers shape for the valve. If it's basically flat, it will take the valves with the coiled spring. 1980 and up CX/GL500's use those. If the cover has a raised outward section it will have a spring that's the same shape at both ends on it. That is used by the 78-79 CX's.

However, if your ACV's have a straight pin on them instead of a brass plunger, they will be the style of the CX/GL650's.



I'll try to get a picture up for the differences in a while for you.
 

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Is there a difference between ACV valves on different models of CX/GL500's and carburetors? I seen to recall one of the parts fiche sites showing the same part number but......



I have a GL400 with VB38A carbs and am getting some, though not bad, popping on deceleration.



Are there dimensional differences?If so what might they be?
Yes there are differences,,I believe the difference is in the length of the metal part with the ball. One type is longer that the other to properly cover the hole in the carb body. Blindstitch has posted a pic showing the two, but I don't know where it is.



I don't know off hand which bike takes which ACV.



If you did have the wrong ACV,,it may cause the popping.





Inspecting my ACV's does not show any obvious pin holes or other damage, but I have been getting fuel on the cover side of both AVC Valves (both carbs), which certainly does not seem correct to me.



What else might be at fault?


I do not know what would cause this. I know you posted before about it and I suggested that if there were pinholes or a leak around the diaphragm it might be the cause, but apparently that is not the case.



If there is no leakage around or through the diaphragm the only way for fuel to get there would be through the vacuum port that goes to the intake side of the carb.



I was thinking that if your bike was backfiring back through the carbs this might happen,,but I am doubtful that it would.



You could try blocking the hole where the little o ring sits,,just cut out a little circle of rubber the same size as the o ring and use it instead. If you try this you may get more popping on deceleration,,but if you turn your idle screws out a bit to enrich the idle mix it should take care of it.



The wrong ACV's may be the cause of this, If the metal ball does not properly block the hole it is supposed to,,maybe there is fuel getting sucked past the diaphram under maximum vacuum ,,seems unlikely, but something is causing your problem.



Having said all this I am really only guessing that what you are experiencing is abnormal,,I have never seen it happen on my carbs but maybe others have.



p.s. belated happy b-day wishes.
 

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Discussion Starter · #4 ·
Mark,

There are differences in ACV's. An easy way to tell which ones you need is to look at the covers shape for the valve. If it's basically flat, it will take the valves with the coiled spring. 1980 and up CX/GL500's use those. If the cover has a raised outward section it will have a spring that's the same shape at both ends on it. That is used by the 78-79 CX's.

However, if your ACV's have a straight pin on them instead of a brass plunger, they will be the style of the CX/GL650's.



I'll try to get a picture up for the differences in a while for you.


Larry (and all...)



Thanks for the replies!



This is a picture of the carbs in question, though, oh my I am so embarrassed, as this is a VERY OLD picture before I polished polished polished. They look and run great now, especially after ultra-sonic cleaning, but still have the popping issue. I'd have a newer picture, but camera was stolen a while back.









The springs inside are of different diameter at each end, and have a brass plunger.



Thanks,



Mark



BTW: no how clean you might think your carbs are, a proper ultra-sonic clean will send rivers (literally, you can see it happen) of gunk out of them and bring them as close to factory spec as you can possibly get. ALSO: Follow Larry's book...a fellow GL'er here has it and it was invaluable.
 

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Here is a picture of the difference between each valve. It's hard to see but the 78-79 valve on the left have a shorter brass plunger than the 80 & up in the middle. Notice the raised section of the covers for the 78-79 too. The 650 valves push pin opens and closes a valve that's pressed into the body. So they don't use a plunger.







I'm not up on the problem your having, but from reading Allan's post I would suggest checking the float height. If they are too high they will feed excess fuel into the carbs. That excess fuel can be drawn into the ACV chambers and cause them to be wet with fuel. Do your carbs have the black or plastic floats?

You also need to make sure that the recessed ring that the edge of the valves seal sits in is really clean. If there is debris built up in them it will cause a vacuum leak there too just like having a pinhole in it. Also check to see if the two metal flanges that press on each side of the rubber diaphragm is still sealing good. I have seen them loose a few times. You won't see any holes there, but it can leak out from under the flanges causing them to keep pulling fuel in like you were on deceleration.



Also check for exhaust leaks. Excess air being drawn into the exhaust on deceleration will cause popping too.
 

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If you have the tapered springs in those carbs you have the wrong ACV valve in them. It should be like the one on the left in my picture.
 

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This is a pic that stitch had posted at the old forum,,I think it is also one of Larry's pics.



This is not the one I was looking for,, but it shows the difference in the length of the metal piece a bit better. The brass piece on the left one is shorter.



 

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Thanks Allan, that's a little better even though it's blurry.



One more thing I forgot to mention. Be sure the covers are flat and not warped. If they are it will also cause a leak in the acv system. I sand these down on a flat plate to ensure they seal good once I put the covers back on.

If you look at the cover on the left in the picture you posted you'll see a crack on the farthest tab. That cover caused sealing issues too.



 

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Discussion Starter · #9 ·
Here is a picture of the difference between each valve. It's hard to see but the 78-79 valve on the left have a shorter brass plunger than the 80 & up in the middle. Notice the raised section of the covers for the 78-79 too. The 650 valves push pin opens and closes a valve that's pressed into the body. So they don't use a plunger.







I'm not up on the problem your having, but from reading Allan's post I would suggest checking the float height. If they are too high they will feed excess fuel into the carbs. That excess fuel can be drawn into the ACV chambers and cause them to be wet with fuel. Do your carbs have the black or plastic floats?

You also need to make sure that the recessed ring that the edge of the valves seal sits in is really clean. If there is debris built up in them it will cause a vacuum leak there too just like having a pinhole in it. Also check to see if the two metal flanges that press on each side of the rubber diaphragm is still sealing good. I have seen them loose a few times. You won't see any holes there, but it can leak out from under the flanges causing them to keep pulling fuel in like you were on deceleration.



Also check for exhaust leaks. Excess air being drawn into the exhaust on deceleration will cause popping too.


Larry,



I stand corrected. I have the springs as shown on the left in your picture (just went out and checked them (it's 1:30am now). Thus, it look as if the ACV assemblies are the correct ones, tho if from a 78-79 that a bit strange as the carbs are off the old wrecked bike which was a' supposed to be an '82 TI bike. Hmmmmm.....who knows whats been cobbled together over the years by PO's. The left side again had gas on the outside towards the cover.....could not check the right side.



At any rate, the float bowls should be 15.5mm. If one were to err on either side of that number, which should it be? Plus towards 16mm or minus towards 15mm? Oh, the Floats are Black and I adjusted them as best I could to 15.5 upon cleaning two weeks ago. Also installed new float valves at the same time.



The Metal Flanges around the ACV's are clean, at least on the left side. I'll see if I can check the right side tomorrow.



As for other excess air, I do note that even covering BOTH mufflers has little or no noticeable effect. The H-Box does have condensation drain holes (maybe 3/8") near the muffler connection on each side, but I would not think that would be enough to release all the pressure. I do not feel any releasing gasses at the H-Box connectors, but will check more closely soon.



Thanks everyone, for the help...



Mark in Thailand
 

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Discussion Starter · #10 ·
Thanks Allan, that's a little better even though it's blurry.



One more thing I forgot to mention. Be sure the covers are flat and not warped. If they are it will also cause a leak in the acv system. I sand these down on a flat plate to ensure they seal good once I put the covers back on.

If you look at the cover on the left in the picture you posted you'll see a crack on the farthest tab. That cover caused sealing issues too.







I'll sand mine tomorrow.



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The left side again had gas on the outside towards the cover.....could not check the right side.


Are you positive that the fuel is not on the inside of the acv diaphragm , but leaking through/out to the cover side when you loosen the cover off?
 

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Discussion Starter · #12 ·
Are you positive that the fuel is not on the inside of the acv diaphragm , but leaking through/out to the cover side when you loosen the cover off?


Allan,



Not positively certain, but I think so....they were complete wet top to bottom. Not dripping wet but wet.



Mark
 

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As for other excess air, I do note that even covering BOTH mufflers has little or no noticeable effect. The H-Box does have condensation drain holes (maybe 3/8") near the muffler connection on each side, but I would not think that would be enough to release all the pressure. I do not feel any releasing gasses at the H-Box connectors, but will check more closely soon.



Thanks everyone, for the help...



Mark in Thailand


A 3/8" hole in the H box could be causing the popping. I have never seen holes in any of the air boxes this side of the pond. Cover the holes with something and see what happens. Usually the condensation holes are in the mufflers near the connections.



And if you were to err on either side I would go toward 16 for testing. But 15.5 leaves you with no doubt that it's correct.
 

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Discussion Starter · #14 ·
A 3/8" hole in the H box could be causing the popping. I have never seen holes in any of the air boxes this side of the pond. Cover the holes with something and see what happens. Usually the condensation holes are in the mufflers near the connections.



And if you were to err on either side I would go toward 16 for testing. But 15.5 leaves you with no doubt that it's correct.


Thanks Larry and Allan,



I'll go through things again tomorrow (actually later today as its 2:30now and I'm about to bed down) or tomorrows tomorrow a time permits.



Will try to plug the H-Box hole and seal things up there and see what happens. Sand the ACV covers for a tighter seal. Floats are probably nearer to 16mm as I adjusted to center line of the valve, not the end of the floats themselves.



Will check back in soon.



Thanks again for the help!!!



Mark in Thailand
 

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I would check the diaphragm itself to see if you have the correct ones,,someone may have swapped them but kept the correct springs.



It may not be abnormal for the acv to be wet,,the intake tracts are wet if you take the carbs off shortly after driving the bike. I thought you meant there was a bit of gas spilling out when you removed the cover. It's too cold to ride yet,,but hopefully I may get a ride in this week as we are supposed to get some warmer weather. I may pull the cover off one of my acv's and have a look if I do get out.
 

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In all these discussions we are assuming that you hace correctly replaced the small o ring that seals the vacuum port. Have you?
 

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Discussion Starter · #17 ·
In all these discussions we are assuming that you hace correctly replaced the small o ring that seals the vacuum port. Have you?


John W,



An often overlooked item, indeed.



But yes, they are in place.



Thanks



Mark in Thailand
 

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Mark,

Although you mention having cleaned the carbs, I would also make sure that the vaccuum port in the carb is clear by blowing air in the hole and witmessing it coming out in the discharge side of the carb. The last time I had a popping on compression problem on one of my GL500 I solved it simply by cleaning that duct.
 

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Discussion Starter · #19 ·
Mark,

Although you mention having cleaned the carbs, I would also make sure that the vaccuum port in the carb is clear by blowing air in the hole and witmessing it coming out in the discharge side of the carb. The last time I had a popping on compression problem on one of my GL500 I solved it simply by cleaning that duct.


I am assuming that by Vacuum duct you are referring to that which controls the Petcock. If that is correct, then it should not be an issue as I do not have a Vacuum Petcock.



Thanks again..



Mark
 

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Allan,

You are correct in that the cover side of the ACVs do get a little wet as well as the intakes. It's a normal function of the system to draw in the wet fuel when it is revved and then you let up on the throttle. It draws some of the fuel into the vacuum passage and gets on the cover side of the valves. I too thought he was saying it was excess fuel until I reread the post's above.

Besides this, popping in the exhaust is caused from too little fuel and too much air. That was the intent of the manufacturer when they integrated the acv valves. The exhaust gasses were so hot and lean when you let up on the throttle that the small atomized amount of fuel is exploding in there. By adding more into the exhaust through the acv, it would keep the gasses cooler in the exhaust and not let it explode.



Mark,

John was referring to the vacuum port that is feeding the acv chamber. You'll see the small hole in front of the throttle plate. It's on the engine side and leads to the acv cover where the small o-ring goes.



BTW, I would suggest that if you were to block that vacuum hole in the acv cover that your popping will get more pronounced by leaning it out more.
 
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