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Discussion Starter · #1 ·
As above - I use my UK Spec 1981 CX500B every day for commuting.



Replacement CDi, re-wound Alternator, new Honda (water pump) oil & coolant seals fitted by me in 2009.



The following process has repeated itself over the last week or so & has become gradually worse:



Engine starts reliably & runs from cold with no problems.

Starts to splutter slightly every few minutes.

Starts to stall when idling & won't start on the starter.

Ignition off - pretend to jump-start it - ignition back on & it usually starts first time on the starter motor !?

Gradually worsened over the last few weeks when most of my riding has been in the rain.



Thought the head gasket might be the culprit ?

Cold compression test = 110/120 psi (left/right cylinders).

No bubbling in coolant reservoir.

A tiny bit of condensation in the oil (last changed 1200 miles ago).

A slow build-up of coolant/oil beige gunge in the air box - presumably condensation ?



I'm changing the oil/filter tonight - saw a few water bubbles but nothing serious.

I'll know better after the weekend.
 

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being not consistant,work or no work,its a hard one.im sure youl get lots of help.

why not put your location etc into your signiture line.




just a first thought,as you say you have had it out in a fair bit of rain...make sure your plug hollows,and the drain holes are clear.

i dont think its a head gasket problem
 

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ok been thinking more.it really sounds like a wet/rain related problem.

problem is,it could be electrics shorting out..or,is it possible water has got into your fuel?
 

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I would also check to be sure the fuel petcock is operating properly,,I think you would have a vacuum petcock,,if it was not delivering enough fuel it may act the way you describe. Check the vacuum hose for cracks or loose fitment at the tap and the carb.
 

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can you post the condition of the plugs,wet? colour?
 

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Might also be a vapor lock, try clearing out the tiny vent hole in the gas cap.
 

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Discussion Starter · #8 ·
Bandit - Left spark plug = black soot all up the thread but grey on electrode.

Right spark plug = grey (normal).

Plugs changed two days ago with oil & filter.



Sadly no change in engine behaviour - same symptoms persist in dry or wet weather.

Always starts first time when cold & always starts to falter once engine has reached normal operating temperature.



Added - Shep:

main fuse holder, HT leads & caps are original to my knowledge - why should the reliablilty of these components worsen as engine temp reaches normal ?



Added - ramprat06 - I'll try that idea - thanks.



Added correction - just remembered - HT leads & caps replaced in 2009.

Feels like a compression or a fuel problem to me.
 

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Bandit - Left spark plug = black soot all up the thread but grey on electrode.

Right spark plug = grey (normal).

Plugs changed two days ago with oil & filter.



or a fuel problem to me.
Kawi,ill comment on the plugs,forget soot on threads,imo irrelevant

grey electrodes tells me you are running lean.tan[brown]is a good burn.

when you start up,choke out,yes.?.warmed up choke in....

try running on a variable choke.....1/4 out....1/2 out,play with it even when warmed up.

itl cost nothing to try.

are you sure you dont have an air leak at one of the 6 joints?
 

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Added - Shep:

main fuse holder, HT leads & caps are original to my knowledge - why should the reliablilty of these components worsen as engine temp reaches normal ?


Coils can break down if weak/old as they heat up.Also a couple of times over the years the AirDam



umber 9 on here,

http://www.cmsnl.com/honda-cx500a-1980england_model16465/partslist/E05.html



that is fitted to some models has been left off and that caused the engine heat to affect old coils and cause them to break down.
 

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Discussion Starter · #11 ·
Kawi,ill comment on the plugs,forget soot on threads,imo irrelevant

grey electrodes tells me you are running lean.tan[brown]is a good burn.

when you start up,choke out,yes.?.warmed up choke in....

try running on a variable choke.....1/4 out....1/2 out,play with it even when warmed up.

itl cost nothing to try.

are you sure you dont have an air leak at one of the 6 joints?


Already tried varying the choke & idle speeds - makes no difference sadly.
 

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Discussion Starter · #12 ·
Coils can break down if weak/old as they heat up.Also a couple of times over the years the AirDam



umber 9 on here,

http://www.cmsnl.com/honda-cx500a-1980england_model16465/partslist/E05.html



that is fitted to some models has been left off and that caused the engine heat to affect old coils and cause them to break down.


Your suggestion doesn't match the symptoms completely.

As above - the engine can stall - I can pretend to jump-start it with the ignition off - switch the ignition back on & it starts immediately - all within 30 seconds during which the engine temp stays hot.

Then the engine can run for a minute or two until the next time the revs drop & it stalls again despite increasing idle-speed to 1300-1400 rpm.



I'll explore an air leak next - fingers-crossed.
 

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Your suggestion doesn't match the symptoms completely.

As above - the engine can stall - I can pretend to jump-start it with the ignition off - switch the ignition back on & it starts immediately - all within 30 seconds during which the engine temp stays hot.

Then the engine can run for a minute or two until the next time the revs drop & it stalls again despite increasing idle-speed to 1300-1400 rpm.



I'll explore an air leak next - fingers-crossed.


That does sound more fuel related.Also check fuel cap vent holes are clear and in-line filter,if fitted,is correctly orientated?
 

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I would also check to be sure the fuel petcock is operating properly,,I think you would have a vacuum petcock,,if it was not delivering enough fuel it may act the way you describe. Check the vacuum hose for cracks or loose fitment at the tap and the carb.


Also the fuel diaphragm itself could be rotten and/or have a small hole/leak so I'd take the petcock apart and check it out while reviewing the fuel system
 

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Quick test, when the bike dies kill the petcock, drain the float bowls, if nothing comes out then you have fuel starvation...simple but effective...oh and if there is, must be quiet a bit of fuel, not just a few drops as some can still drain from the fuel pipe...



Sounds like a simple fuel starvation issue....



I'd even go as far is a to fit a temp fuel jerry, put it ontop of the tank, somthing like a plastic coke bottle, start and take a short ride...just remmeber to vent the jerry.
 

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Discussion Starter · #17 ·
Had to park the CX for a month due to other commitments.

Had another look at it this evening - valve clearances spot on, fuel tap working fine, carbs were fine with fuel in the float bowels - now emptied - so nothing obvious there.

Ignition coils failing when hot or an internal head gasket leak remain possibilities but difficult to test without spending money - I'll look into the costs of both soon.
 

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Discussion Starter · #18 ·
Bought after-market replacement coils - no nothing about electrics.

New coil has two input connections but the original only has one - see photos.

What do I do ?



 

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Bought after-market replacement coils - no nothing about electrics.

New coil has two input connections but the original only has one - see photos.

What do I do ?





I'm thinking one terminal could be a ground. I believe the CX coils have a small wire leading over to one of the bolts that mount them. Did your coils come with a wiring diagram? What are they off of? Perhaps you could look up a diagram for that bike.



Secondly, you haven't stated whether or not you've tried the other suggestions, such as running it until it quits then quickly shutting off the petcock and checking the float bowls. Mine had this exact problem and it was my vacuum petcock. This is a great way to test it. Being that the plugs are gray (lean) and the bike restarts shortly after dying I think this has a good possibility of being the culprit.



Mine was also plagued with bad coils as well, along with a bad starter. There may be more than one problem, so you need to do elimination testing. If you eliminate things one at a time, you eventually find the solution.
 

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