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Carb diaphragm symptoms

13K views 16 replies 7 participants last post by  redwing52 
#1 ·
Hi all, been away a while, trying to get back into it. My bike sat for a long while with multiple problems. I finally got to working on it and rebuilt the carburetors with the kit from Randakk's, replaced all the coolant hoses (despite the OEM Honda lower radiator hose not fitting properly), replaced all the vacuum hoses under the rubber mat, intake gaskets, etc. Got it together, tuned and synced the carbs, and she ran like a bat outta hell, smoked the back tire and left a black streak 30 feet long. Rode about 20-30 miles, then had to leave her sitting for another week or so. Got on for a short ride, and she ran like crap.

Finally got to her again, seemed like she was lean on the left bank. A little propane near the intake smoothed her right out. At first I thought I had somehow gotten a piece of trash in the bowl, despite new Honda filter and fuel pump, so I tore into it and found... NOTHING. Spotless as I'd left her. Blew through the passages with compressed air just to make sure, back together we went, and ran great... for about 60 seconds. Didn't have the air box fully sealed so I was able to confirm that propane in the right throat bogs it down, propane in the left throat smooths her out. Ran the hose from the propane around the intake gaskets, up under the mat, and all around, only makes a difference at the opening to the carb. The thing is, it comes and goes. Is that typical of a small tear in the diaphragm on the CV slide? That is the one part I didn't have to replace so at $100 a pop, I didn't. The slides seem to move in relative unison, though without the air box fully installed, it doesn't rev much very well anyway. I also checked the fuel petcock with the propane, nothing there (as I recall from when it failed many years ago, it caused a lean miss on just one cylinder, rather than the whole bank). Just wanting to make sure I'm thinking right before tearing the carbs back out, AGAIN.

Thanks in advance.
 
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#6 ·
I recall when mine went out (on just 1 of the 2), the bike suddenly Lost Mid-Range Power... could barely ride it the last 10 miles home on the freeway, while splitting lanes.


Mine went out at around ~100k miles; I replaced both, and it's gone another 100k... So, the cost was worth it, in my view.


That said, I'd first look real close at the Vacuum Line that feeds the Ignition Timing... I know you said that you replaced all the vacuum hoses, but if you have a leak somewhere, it may still mess that up. I install metal clips/clamps on all my vacuum lines, just to ensure no leaks.
 
#7 ·
Thanks for the replies so far for confirming what I thought I knew about the diaphragms. After checking nearly everything else, completely disconnecting the vacuum lines from the left intake manifold, and even double checking the diaphragm anyway (I forgot I actually did replace it/them all those months ago; found the receipt where I bought them), I'm down to it being in the carburetor. If you're sneaky enough, you can turn the assembly around and find that the lean condition follows the carb to the other bank.

With the bowl and low-jet clean, I have to assume there's something in a passageway somewhere that shifts occasionally causing the randomness of it's timing. Which brings me to my next question: do the pilot screws flow fuel, or just air? The manual doesn't say, and a search didn't reveal the answer. Thanks again for any useful answers.
 
#10 ·
One other possibility is the fuel level in the carb is just tickling normal. When the ambient air is right and barometric pressure is right the bike likes it and runs good. Any change in weather that lowers oxygen levels to the engine and the left side balks. I would really scrutinize the spring loaded needle and double check (triple check) float height. If you have to fail make sure you fail on the high side. Just some more ideas to chew on. If you go to the link below and open full screen there is a pretty good video. If you click off and on you can watch the video second by second to understand how your carbs work.
Earlier you asked if the mixture screws control gas or air and Dave answered. What the screws control is actually an emulsion of fuel and air mixed together. You should see that in the video. The emulsion is indicated by the dots in the emulsion tube above both jets. I am not positive but I think this is where the air solenoids do there thing. They increase or decrease the amount of air in the emulsion by opening and closing the air jets to the emulsion tubes. I'm not absolutely sure of that so don't count it as gospel. I will say that on my bike I can take the hoses right off and there is very little change in the way the bike runs. If I blow in the hoses toward the carbs it shuts off immediately. Hope some of this helps.
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https://www.youtube.com/watch?v=wyspAHrMbb8http://
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#17 ·
For anyone who cares I found just a little more info on the air jet control solenoid valves. There are two of them located just in front of your right knee as you sit on the bike. They are behind the right fairing inner cover. From left to right they are celled #2 (left) and #3. (right) Though they look the same they have different part numbers so I will guess they are not the same or interchangeable. ????? The #1 air jet (air bleed, air bleeder etc.) is part of the carburetor like all carbs everywhere. It is a fixed orifice in the air horn of the carburetor. The air control solenoids work by opening and closing the air bleed passages to atmosphere. Pretty simple really. They are controlled by the ECU. They are open and closed based on throttle position, engine temperature, engine RPM, engine load etc. When both are closed the engine is full rich. No air is coming to the emulsion tubes from the air jet solenoids #2 and #3. The only air is from the fixed air bleed #1 in the carburetor itself.
There are several testing procedures in the service manual. The process is confusing. It seems the #3 solenoid never opens during the testing procedure. That makes me wonder if there isn't a mistake in the manual? I checked both of my manuals and they are identical???? If you are having a major issue with the bike don't waste time looking at the solenoids. They are fine tuning only. If you force them open or closed the best you can do is make the engine tone change slightly. That is all I know after and hour or more of searching.. I know it is not much but it might help someone. :)
 
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