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Discussion Starter · #1 ·
Hey everybody, first post. Searched the forum for some answers, found some good material but my bike seemed to "pass" the electrical tests.

For reference, I primarily work on BMW Airheads. I'm used to points, carb tuning, air cooled, etc.

I picked up the CX500 because a friend asked me to help him find one, he'd pay for me fixing it up. It had been sitting a few years with gas in it.

I did the general maintenance things, ran the carbs through my ultrasonic a few times, set the float height according to the manual, checked the jets to ensure they were the standard (115 and 78 if I remember correctly), treated the intake boots with Shoe Goo because they were cracking, sprayed them with soap water to see if any air leaks were happening and to my knowledge there aren't any.

Put a new air filter in it, new spark plugs, compression test was within 10psi of each other, changed all fluids. Checked for spark and got a fat blue one out of each. Set the valves, stuck in a new battery and she came to life first push of the magic button. Idles fine, carbs are balanced within 5mg of each other.

I've been riding it around to get the cobwebs out and it bogs in mid range under load. Kinda bogs a second then catches all of a sudden and pulls you fast.

Now, if this was a points driven bike, my first course of action would be to ensure the timing was correct. Then set the valves, then tune the air and idle mixtures on the carbs.

The CX500 doesn't have most of those things. So I'm not sure what to look for? Does electronic ignition go bad or go "out of timing"?

Thanks for any input.
 

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Welcome to the forum. Please fill in your profile and add your location and bike year/model to your signature line.

I suspect I know your issue but need to verify which CX you have to know which ignition system you are using.
 

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Did you floss the jets?
The emulsion tube can still have the aeration holes plugged even if it looks clear.
Also, some of those carbs have an acceleration pump [depending on the year and model] that can have a pin hole in the diaphragm that will cause it to stumble on acceleration.
 

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Discussion Starter · #5 ·
Did you floss the jets?
The emulsion tube can still have the aeration holes plugged even if it looks clear.
Also, some of those carbs have an acceleration pump [depending on the year and model] that can have a pin hole in the diaphragm that will cause it to stumble on acceleration.
Man, why can't things be simple air in, gas in, exhaust out anymore?

It's a big no-no in Beemer world to run any wire through jets because they may not be replaceable. I did a few ultrasonic baths and checked for open passages, but it did not go so far as to guitar string them out.

What is an acceleration pump, and how do I identify it on the carb?

I did not look at the diaphragms although that is an excellent suggestion.
 

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If it has them, you will see it on the left {I think} side of the carb as a small sorta round plate with two screws holding it in place.

Did you download the FSM?
You can from the link below.
 

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Check the 8/9 and 9/5 here. It's an easy check. One thing that can cause this in a CDI bike is the low speed coil not developing enough power to rev through cleanly until the high speed coil takes over. Tell us your figures for these two but I'm guessing the 5/9 will show a low resistance. You can also test the voltage coming from the stator side of pin 9. Give us that one too.



If not here it could be in the carbs. These keihins are finicky beasties.
 

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Discussion Starter · #8 ·
I'll check some ohms and voltages tonight, thank you for that handy chart.

Also, it does have that little side plate with screws on it! I didn't know what that would be for, assumed it was for disassembling the butterfly valve. Do these carbs have idle and air mixture screws at all? The Haynes manual made no mention of benchmark settings for the carb.
 

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Idle screw is between the carbs toward the left front. Idle mixture screw is under the front venturi of each carb. The cover on the side of the carb is an aircut valve to prevent popping on the over run. Try 2 turns out
 

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It would be unlikely to see an 81 Custom with TI, wasn't until 82. All US GL's are TI. To check for sure on the Custom, look under the seat, there will either be one larger silver/gold metal box (CDI) or two small NEC labeled ignitor boxes with fins (TI).
You also might want to get a copy of Larry's Carb book, its more or less the bible for these carbs. Link below, along with the WIKI chocked full of great tips etc. including the FSM for free.

Joel in the Couve
 

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get a manual fpr the bike start reading

as for the carbs they are lean burn carbs and fussy and not a simple task to get right

you will need one of 2 books to fix them Mike Nixons or Larry Cargills

or pull them and send them out
 

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Discussion Starter · #14 ·
Check the 8/9 and 9/5 here. It's an easy check. One thing that can cause this in a CDI bike is the low speed coil not developing enough power to rev through cleanly until the high speed coil takes over. Tell us your figures for these two but I'm guessing the 5/9 will show a low resistance. You can also test the voltage coming from the stator side of pin 9. Give us that one too.



If not here it could be in the carbs. These keihins are finicky beasties.
I'm reading a little jumping around, but 9/5 settles on 308-310 and between 8/9 settles on around 205-210 in Ohms.

I cannot get a voltage reading on pin 9, not sure what "stator side" means but I tried a few configurations and nothing was hot. (Key is turned on, of course.)
 

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Discussion Starter · #18 ·
Also, with my experience in Bings, I'm thinking this might be carburetor related. Specifically fuel flow, THe vacuum works okay, I had to suck it out with my mouth first to get it into the carb bowls now it runs fine. But if I turn the throttle fast it will kill the bike. That's usually a sure fire sign of too much air and constricted fuel.

Are these carbs different enough from other CV carbs that I'll need to buy a $35 book to get correctly working? If it's a necessity, I can. I suppose I figured that you set your base settings on the screws, clean them out and tickle them from there and get them balanced would be enough. But frankly I'm used to carburetion technology that's 30 years older than this.
 
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