Honda CX 500 Forum banner
1 - 20 of 37 Posts

·
Registered
Joined
·
37 Posts
Discussion Starter · #1 ·
I always wanted to be rich, but this isn't what I had in mind. My bike is running well enough that I can take it out for runs to the beach and around town, but it is eating gas and coating my plugs with carbon on even a short trip. I'm looking for help in identifying the most likely causes.



This was the project bike that everyone dreams about. The PO simply gave it to me because he thought there was no value in it and that it would never run again. The carbs were handed to me in two boxes containing a number of nicely labelled plastic parts bags. The bike had been parked in a heated garage for 15 years. The PO was the original owner and had kept everything stock and maintained. Using the supplied manual, I put the carbs together best as I could and the bike fired up. With help from Shep and others on this forum I removed and cleaned the carbs a couple times (using high pressure air and carb cleaner), I replaced the spark plug boot innards with brass rods (no corrosion in the boots and the resistor etc came out without issue). I tested the ignition system as directed and all is good. Air box is the original, air filter is clean, there are no air leaks at the carb boots. My idle screws are set to 1.75 turns out. Clean oil and filter, new spark plugs, etc.



Strong spark, great compression and lots of power. Well, lots of power now that I have put low speed jets where the high speed ones are supposed to go!! Before that I was losing all my power around 4500 rpm, but could raise that ceiling to 6000 if I removed the air filter for better flow. Now, with 78 jets where the high speed 112s are supposed to go, I can easily hit 9000rpm in short order.



Everything points clearly to running extremely rich (dry black plugs after a short ride, poor mileage, smell of fuel, runs better with smaller jets, etc etc). Based on my observations, I don't believe the issue is restricted air flow. This leaves me with the conclusion that it is simply feeding too much gas into the mix. I suspected the jets were worn out and replaced them with new ones (78/112) and there was no change in performance. That's when I took my old low speed jets and put them where the high speed jets go. The bike ran well right through the rpm range 1100 to 9000. I was still fouling the plugs to dry black though and I could still smell fuel at idle (there is no fuel leaking from the float bowls, so the smell is coming from the exhaust). I removed the air filter again for better flow and the bike ran smoother and gained at least 15 mph top speed (it can easily hit 90mph. I haven't had the courage/foolishness to go higher), but still fouled the plugs with a dry black coating after a single ride.



So... with 78 jets where my 112s are supposed to go and no air filter, I am still getting about half the gas mileage I should be and fouling my plugs. Where do I go from here? New throttle needles? New sliders? Is there something else I should be looking at?



Sorry this is so long winded, but I wanted to give as much detail as possible. I have searched the forum extensively looking for information on this and haven't found any clear answers or other complaints of such highly rich condition. Any suggestions will be appreciated as I would love to get this great machine running well enough to spend my summer travelling on it.



John
 

·
Registered
Joined
·
3,999 Posts
A couple of possible causes may be if you have bad air cut off valves,,

and do you have the slide needle seats installed,,they are a small brass piece that go in the high speed tower before you screw the brass emulsion tube in.
 

·
Registered
Joined
·
4,104 Posts
air filter is clean,



this can be deceptive.

Trev had similar symptoms and the filter looked fine

when he changed it the bike ran better and stopped guzzling juice

have you tried running minus a filter for a short while to see if there

is a noticeable difference?



All this assumes the carbs are AOK of course
 

·
Registered
Joined
·
37 Posts
Discussion Starter · #4 ·
A couple of possible causes may be if you have bad air cut off valves,,

and do you have the slide needle seats installed,,they are a small brass piece that go in the high speed tower before you screw the brass emulsion tube in.


I believe the slide needle seats are there, but I will check to be certain. After having the carbs out a few times, it becomes a pretty simple job to remove them. I'm hoping to get a few more responses this morning and then pull the carbs this afternoon.



The air cut off valves - are those the rubber diaphragms that you can get to by removing the 3 screw side cover on the carbs? They look ok, but how would I know if they were bad?
 

·
Registered
Joined
·
37 Posts
Discussion Starter · #5 ·
air filter is clean,



this can be deceptive.

Trev had similar symptoms and the filter looked fine

when he changed it the bike ran better and stopped guzzling juice

have you tried running minus a filter for a short while to see if there

is a noticeable difference?



All this assumes the carbs are AOK of course


yes, I did remove the air filter and it ran 'better' but only slightly improved the issue.
 

·
Registered
Joined
·
8,423 Posts
Air cut off valves can be checked by holding up to a strong light.Any holes replace or bypass.They can be bypassed using a small circle of rubber(very small) where the small O-ring goes.



The hardest part of carb cleaning is getting the four brass tubes(two under the,"Black Banana" in the top) clean).I use a thin guitar string/wire and copious amounts of WD40 and Carb Brake cleaner even after a cold vinegar soak.Then high pressure air blow out.Also the the tiny holes poked clean near the Throttle plates and high pressure air blown out.



Correct float heights are paramount to the correct operation of the carbs as well.



http://globalcxglvtwins.hostingdelivered.com/viewtopic.php?f=19&t=110



If your type of carbs also have the,"Accelerator" pump then their diaphragms,like the Air cut-offs,may be damaged and overfeeding the carbs when not needed.



Basically it was the EPA rules around the 1980s when the bikes were exported to the US market that made Honda have to over-complicate the later Keihin carbs IMHO.



1:There was a locking tab on the idle mixture screws to stop people adjusting the mixture(Redundant now so remove if there).



2:They added an accelerator pump to enrich the lean mixture when the Throttle was first opened from low speeds.





3:(and same on UK Carbs) they have the Air-cut-off valves for when the Throttle is quickly closed to enrich the mixture to prevent,"Popping".



I know the Air-cut-offs can be bypassed with no ill effects as one of my CX runs this way.Not having an Accelerator type set of carbs I don't know if that can be bypassed if need be.



Also you can run without an air-filter for a short while to see what effect it has and adjust your mixture to suit.Do NOT do this if you live in a sandy/dusty environment like off-road tracks/desert etc.Then get a new one if required or gas soak/clean and high pressure blow out the old one if the vanes are ok.
 

·
Registered
Joined
·
37 Posts
Discussion Starter · #7 ·
Air cut off valves can be checked by holding up to a strong light.Any holes replace or bypass.They can be bypassed using a small circle of rubber(very small) where the small O-ring goes.



The hardest part of carb cleaning is getting the four brass tubes(two under the,"Black Banana" in the top) clean).I use a thin guitar string/wire and copious amounts of WD40 and Carb Brake cleaner even after a cold vinegar soak.Then high pressure air blow out.Also the the tiny holes poked clean near the Throttle plates and high pressure air blown out.



Correct float heights are paramount to the correct operation of the carbs as well.



If your type of carbs also have the,"Accelerator" pump then their diaphragms,like the Air cut-offs,may be damaged and overfeeding the carbs when not needed.



1:There was a locking tab on the idle mixture screws to stop people adjusting the mixture(Redundant now so remove if there).

2:They added an accelerator pump to enrich the lean mixture when the Throttle was first opened from low speeds.

3:(and same on UK Carbs) they have the Air-cut-off valves for when the Throttle is quickly closed to enrich the mixture to prevent,"Popping".



I know the Air-cut-offs can be bypassed with no ill effects as one of my CX runs this way.Not having an Accelerator type set of carbs I don't know if that can be bypassed if need be.


I will try the air cutoff bypass and if that works, then I'll order new set of diaphragms.



I've cleaned under the black bananas a couple times, following your instructions, so I'm going to assume that's ok at this point. I measure the floats every time I'm about to reinstall the carbs. There is no locking tab on the idle mixture screw, but I do have the accelerator pump on one carburetor only. I'll look at that while the carbs are out today, but won't try to bypass it until I have looked at everything else that has been suggested.



What about the throttle needle? It looks undamaged, but a guy at a bike shop told me that they can wear out and cause a rich condition.



Thanks again Shep. This gives me enough to get started.
 

·
Registered
Joined
·
8,423 Posts
Needles can wear so compare left with the right and see what you think.I've got various spare carbs and a crappy one and even the worse ones the needles are fine.



Re-check here,



http://www.spark-plugs.co.uk/pages/technical/diagnosis.htm



Stuck choke plates not opening completely when off?



Can be caused by many different conditions. Carbon deposits build up when the plug fails to fire correctly and burn them off. Air/fuel mixture too rich, choke stuck on, electrical problem, extended periods of low speed driving, plug heat range too cold. All should be investigated
 

·
Registered
Joined
·
37 Posts
Discussion Starter · #9 ·
Needles can wear so compare left with the right and see what you think.I've got various spare carbs and a crappy one and even the worse ones the needles are fine.



Re-check here,



http://www.spark-plugs.co.uk/pages/technical/diagnosis.htm



Stuck choke plates not opening completely when off?



Can be caused by many different conditions. Carbon deposits build up when the plug fails to fire correctly and burn them off. Air/fuel mixture too rich, choke stuck on, electrical problem, extended periods of low speed driving, plug heat range too cold. All should be investigated




Choke plates are fine. You pointed me to that in another post when I was trying to figure out why my spark plugs were failing, and the plates are working 100%. BTW - my spark plugs stopped failing when I switched to the smaller high speed jets, so we were on the right track in terms of it just being excessively fast carbon buildup. Unfortunately, changing the jets just lessened the symptoms and didn't address the cause.



The picture below, from your link (top one - dry black - of the two below) is exactly what my plugs look like. I have ruled out or almost ruled everything on the plug diagnosis list other than a rich mixture.







I'll let you know results of my air cutoff bypass. Seems the most likely culprit right now, so I'm going to make time for that later today.
 

·
Registered
Joined
·
37 Posts
Discussion Starter · #11 ·
Just to recap.You should have the 78s in the carb body and the 112s in the Brass Raised tube.


yep. but for now, it's 78's in both until I get this resolved!! The new 112s are sitting on the shelf waiting for their turn.
 

·
Registered
Joined
·
309 Posts
The carbs were handed to me in two boxes containing a number of nicely labelled plastic parts bags.



John


Are the needle jets installed?



These are the brass jets (not the 78`s or 112`s) that poke out into the bottom of the venturi and what the needles slide in`n`out of when the piston rises/falls.

If they`re not there the bike will run rich...



Another possible cause of using so much fuel could be the vacuum tap.

If both diaphragms are perished (unlikely, but possible - especially after such an extended lay-up) the engine`ll draw in neat unmetered gas via the vac tube to the r/h carb, although that`d probably only show up on your right hand plug..

Are both plugs showing signs of running rich?
 

·
Registered
Joined
·
37 Posts
Discussion Starter · #13 ·
Are the needle jets installed?



These are the brass jets (not the 78`s or 112`s) that poke out into the bottom of the venturi and what the needles slide in`n`out of when the piston rises/falls.

If they`re not there the bike will run rich...



Another possible cause of using so much fuel could be the vacuum tap.

If both diaphragms are perished (unlikely, but possible - especially after such an extended lay-up) the engine`ll draw in neat unmetered gas via the vac tube to the r/h carb, although that`d probably only show up on your right hand plug..

Are both plugs showing signs of running rich?


Just checked and the brass piece that the needle slides into is in place in both carbs.



I have previously done the petcock bypass and plugged the tube to the r/h carb, so that isn't an issue. Yes, both plugs are showing signs of running rich.
 

·
Registered
Joined
·
37 Posts
Discussion Starter · #14 ·
Air cut off valves can be checked by holding up to a strong light.Any holes replace or bypass.They can be bypassed using a small circle of rubber(very small) where the small O-ring goes.


I just removed the left side air cut off cover and sure enough there is light coming through. It appears to have a small tear about 3/8" long right where the rubber meets the metal. I made a rubber plug to replace the rubber o-ring.



Unfortunately I don't have a screw driver that will get the second cover off. Taking the 2 carbs apart and back together looks like a lot of work. I'm going to run out this evening and see if I can find a small angled phillips to finish the job.



Here's hoping the other one is damaged too, as it would seem that both sides would have to have the same problem in order to foul both plugs. I don't see anything that is shared by both sides of the engine after the air filter.



John
 

·
Registered
Joined
·
8,423 Posts
There are two different sets of Air-cut-offs used in these carbs AFAIK.I think my CX500 carbs use ones with a slightly shorter or long brass middle bit seen in these pictures but I think this set is for you,



http://cgi.ebay.com/ebaymotors/2-AI...4108070QQptZMotorcyclesQ5fPartsQ5fAccessories



I don't know how to be sure.



The little O-rings in the picture go in the holes that you block if you want to bypass but you leave the damaged Air-cut-offs in if you get my drift?
 

·
Registered
Joined
·
3,999 Posts
I use a Phillips bit with a small wrench (1/4" I think) to turn it. I am not sure if this will work on GL carbs though, they are closer together than the CX carbs.



As Shep said, to block the ACV, leave everything in place except the tiny O-ring, replace it with a solid piece of rubber the same thickness and diameter.
 

·
Registered
Joined
·
37 Posts
Discussion Starter · #17 ·
When the angled phillips driver wouldn't quite fit, I tried Allan's suggestion of the phillips bit with a 1/4 inch wrench but I couldn't get the screws to turn. In the end, a pair of needlenose pliers did the trick. I just grabbed the screw head from the side, squeezed hard and turned.



I followed your instructions Shep and replaced the o-ring with a solid rubber plug the same size. Initial tests were promising - I actually had to use the choke to start the bike for the first time, and the right side carburetor wanted the idle mixture screw turned richer (not sure why the left side didn't really care). I took it for a short ride with the air filter back in place and it ran great at speed, but rough at idle. I'll make a few adjustments tomorrow and then go for a longer haul. I am still running with the smaller jets in the high speed seats, but I want to know what kind of a difference this has made before going back to spec on everything.



One point of note regarding the air cut off bypass - there is definitely some difference to get used to in throttle reaction. It is much less smooth. The bike jumps forward on throttle and cuts out quicker on letting the throttle off. I'll definitely be ordering replacement parts, but the bypass at least will give me an idea of whether that was the main cause of my unwanted richness.



I'll update tomorrow after a better test run so that you can add this to your knowledge banks.
 

·
Registered
Joined
·
37 Posts
Discussion Starter · #18 ·
Back to square one.



Bypassing the air cut off valves seemed to make a small improvement on the RH carb, but no improvement on the LH carb and overall caused rough idle and rough low speed issues. And it was still running rich on both cylinders.



I have put the ACV o-rings back in until I can replace the acv diaphragms as the bike was running well before, but just too rich. So I'm pretty much at the point where I either need to do a complete carb rebuild or try even smaller jets (I'm already at 78/78 so I would probably try something like 50/78). Right now she runs great without an air filter, but rough with a filter. I have a hunch that going with smaller jets will resolve the issue for now, but obviously there is a problem somewhere that I can't find and I have no idea where else to look.
 

·
Registered
Joined
·
3,999 Posts
Do you have the black rubber plugs in the middle tower? If you do, are they fit tightly in the tower?
 

·
Registered
Joined
·
226 Posts
Have you tried confused.com? .....sorry....not being very helpfull am i...but an interesting read, i hope you get it sorted
 
1 - 20 of 37 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top