Author Topic: Cb500 running on 2-3 cylinders (Read 5980 times)

fevah · « **on:** September 06, 2009, 12:47:44 AM »

Bike was running fine on 4 when i got it. after a 4-1 exhaust and pods it was running off and then started on 2-3 cylinders.
got new plugs and all ok running on 4 again. got a 115 main jet and lowered the needle 1 clip to richen things up. runs heaps better but today it started on 3 again. pulled the offending plug and it was wet as. seems to spark ok when cranked it over while it was out. switched plugs with a good cylinder and the good one went bad and the bad good. makes sense apart from the plug looks sweet and its new. cleaned it all up and no joy. eventually it started working fine and the other went bad again. checked the points and that seems fine as does the timing. i cant work it out and its driving me nuts. the weather is getting good and i want to ride.
please help

haill · « **Reply #1 on:** September 06, 2009, 04:43:03 AM »

check your plugs are NGK D-7ES not DR-7ES which have a built in resistor. Which will lessen the spark output when paired with the resistors all ready built into the spark plug resistor caps.
new spark plug resistor caps for all 4 plugs takes out the guess work as well. They get old like everything else. around $15
mine did the same thing and this cured it.

fevah · « **Reply #2 on:** September 16, 2009, 12:19:16 PM »

Plugs are the right ones and are only a few kms and a bit of idling old. Pulled them out and earthed them against the head of the motor and cranked. the dropped cylinder plugs wouldnt spark unless they were unearthed and then the arc wouldnt be to the electrode. Grabbed some old plugs and tested till i got 4 working ones. put them in and fine it runs on 4.
now its dropped other cylinders and different floatbowls overflow at random and at random times.
Any ideas what could cause the sparkplugs to just stop working? they seem fine, cleaned them up and checked the gaps.

haill · « **Reply #3 on:** September 16, 2009, 02:27:45 PM »

yes the spark plug resistor caps. they are the connectors between the spark plug wires and the spark plugs. they have resistors in them and go bad. replace the resistor caps so you get proper spark.
as to gas overflow. the oring that lives under your valve seats are most likely dried/old and leaking. the float valve thinks it is shutting off the gas flow. but the gas leaks around the oring and does so regardless of your float height.
get yourself a oring kit for all the carbs and replace all the orings.

fevah · « **Reply #4 on:** September 16, 2009, 02:36:47 PM »

I replaced all the orings when i stripped and cleaned the carbs. its leaking out the actual overflow tube.
when i tested the plugs i used the on the same working lead/cap combo. some would work and others wouldnt. I am looking for a place to source new caps from as well.
Is it possible the points or battery could cause this intermittant cylinder dropping ?

1976CB550F · « **Reply #5 on:** September 16, 2009, 02:44:43 PM »

I have had many caps that arc out on the motor. I bought some new automotive wires and bought a spark plug wire splice kit and spliced them right out of the coils. I got the splice kits at my Honda dealership, about $8 each. As a bonus you can buy auto wires in a range of colors!

haill · « **Reply #6 on:** September 16, 2009, 03:01:33 PM »

leaking out of the overflow tubes would happen if either the float height is incorrect or the o'rings are leaking or the valve seats themselves are leaking.
the gas has to go somewhere and the overflow tubes are the place.
if you replaced the o'rings then it could be your valve seats leaking gas regardless of your float height then out the overflow tube it comes.
eBay for the resistor caps.

TwoTired · « **Reply #7 on:** September 16, 2009, 03:02:02 PM »

If the air fuel mixture is too rich, black soot deposits will form on the electrode insulator. Since these carbon deposits are conductive, it will eventually short out the spark plugs.

Sounds like you have more work to do on the carbs after you replace the spark plugs.

fevah · « **Reply #8 on:** September 16, 2009, 03:13:46 PM »

Thanks for the info guys gives me more to go on.
Excuse my ignorance but what part is the valve seat ? ive only ever really worked on EFI cars and carbs is a whole new ballgame to me.
from what ive read the main jet and needle clip position should only affect from 1/4 - full throttle and the air screw on the carb is for idle? is this what i need to play with to tune the idle mixture?
so once the soot buildup shorts the plug the plug is dead forever?

BVCB650 · « **Reply #9 on:** September 16, 2009, 03:48:17 PM »

Quote from: haill on September 06, 2009, 04:43:03 AM

check your plugs are NGK D-7ES not DR-7ES which have a built in resistor. Which will lessen the spark output when paired with the resistors all ready built into the spark plug resistor caps.
new spark plug resistor caps for all 4 plugs takes out the guess work as well. They get old like everything else. around $15
mine did the same thing and this cured it.

Why run the hotter plug?

1976CB550F · « **Reply #10 on:** September 16, 2009, 04:09:52 PM »

A Dr-8ES would be the hotter plug, the R stands for resistance. Haill is correct.

BVCB650 · « **Reply #11 on:** September 16, 2009, 04:29:13 PM »

Quote from: 1976CB550F on September 16, 2009, 04:09:52 PM

A Dr-8ES would be the hotter plug, the R stands for resistance. Haill is correct.

Wrongo!!!! An 8 is the standard heat rang for my 650. A 7 is hotter for colder weather, and a 9 is for extended high speed riding.

http://www.jasonkent.ca/manuals/79%20CB650%20Service%20001.pdf

1976CB550F · « **Reply #12 on:** September 16, 2009, 04:43:46 PM »

Sorry BVC, blonde moment. Your heat range is correct. As well, Thought you were taking about the r in the part # not stock plug numbers for your 650.

BVCB650 · « **Reply #13 on:** September 16, 2009, 04:46:38 PM »

No problem. I got a set of 7's I am getting ready to put in for winter but it's supposed to warm up this weekend so I'll hold off. I ride up here year-round after they plow the roads. Plugs as hot as it will take without a meltdown is a lifeline.

haill · « **Reply #14 on:** September 16, 2009, 05:37:57 PM »

the gentlemen with his concern for his bike has/rides or would like to ride his CB500. it takes a d-7ES as a stock plug.
your soot filled plugs can be cleaned and reused. kerosene and a toothbrush to get the carbon build-up off which is caused by burning all that excessive fuel in your float bowls. get that fuel problem sorted first. get a cb500 manual....really it's the best thing you could buy for your bike.
The valve seats are a brass tapered plug that live under your carb floats arm pin. Controlling the amount of gas let into the carb bowl by the rise and fall of the float.
Get new spark plug resistor caps put your newly cleaned NGK D-7ES into them and things should on the mend.
TwoTired is the one who helped me with this problem originally, for had the same issues. he's a wealth of knowledge.

TwoTired · « **Reply #15 on:** September 16, 2009, 09:05:55 PM »

Quote from: fevah on September 16, 2009, 03:13:46 PM

Excuse my ignorance but what part is the valve seat ?

The float valvet seat is a brass part with oring that fit into the carb body.

Quote from: fevah on September 16, 2009, 03:13:46 PM

from what ive read the main jet and needle clip position should only affect from 1/4 - full throttle and the air screw on the carb is for idle? is this what i need to play with to tune the idle mixture?

For the most part, yes. But be aware that the slow/idle/pilot system delivers in parallel with the mains needle jet system. There is some interaction.
Further, these carbs do run rich at idle. They have to, as there is no accelerator pump on these carbs.
Both fuel delivery paths rely on carb throat vacuum to pull fuel through their jets. When the slides are suddenly opened, much vacuum is lost and the jets reduce their delivery leading to a lean condition. To compensate, the pilot/idle/slow system is set on the rich side, so there will be enough fuel for the engine to pick up and make the venturi develop the needed pressure drop to restore the fuel flow through the jets.
The engine should still respond well (smooth acceleration) from low rpm loaded power, when giving it up to one half throttle travel. This is my litmus test for correct Idle Air Bleed setting.

Quote from: fevah on September 16, 2009, 03:13:46 PM

so once the soot buildup shorts the plug the plug is dead forever?

No, they can be cleaned. The porcelain is porous, however. To clean as new usually requires an abrasive blaster. Or, normal combustion with the correct mixture and some loaded, higher RPM usage to attain self cleaning temps.

fevah · « **Reply #16 on:** September 17, 2009, 01:01:26 AM »

did a quick clean up of the faulty plugs and got it running on 4 but it will only idle with the choke on, as soon as i turn it off it dies, even when i had it warmed up for a while. how big a gap should the slides have when you bench sync? I followed the FAQ in here and gapped them to 1/8th" but i think thats far bigger than they were to begin with.

TwoTired · « **Reply #17 on:** September 17, 2009, 12:04:33 PM »

Quote from: fevah on September 17, 2009, 01:01:26 AM

did a quick clean up of the faulty plugs and got it running on 4 but it will only idle with the choke on, as soon as i turn it off it dies, even when i had it warmed up for a while. how big a gap should the slides have when you bench sync? I followed the FAQ in here and gapped them to 1/8th" but i think thats far bigger than they were to begin with.

I've not reread this entire thread. (So, maybe I'm covering old ground.)
I am beginning to hate the drill bit bench sync method, as it does not ensure that the slides can be fully closed and the opening solely determined by the idle stop knob only.

Still, this recent complaint seems to point to plugged/restricted slow/pilot/idle jets. Or, too low of a fuel level in the carbs. Which is at odds with and engine that carbon fouls plugs.

Is your gas tank clean? Is the fuel getting to the carbs clean?

Have you done the entire 3000 miles tune up check list? Is the air filter clean as new?

Do all the head pipes seem to be the same temp?

fevah · « **Reply #18 on:** September 17, 2009, 12:15:32 PM »

The fuel is getting to the carbs fine, tank is clean and so is the gas comming out.
Airfilters are brand new pods.
Where is the 3000 mile checklist?
I checked the points gapping and timing seems fine. im going to replace the points anyway and check the valve clearances this weekend.
1 cylinder seemed cooler than the other 3. Also i noticed that there was some decent sized backfires through the carbs every now and then. (on choke)
Thanks heaps for all the help so far i appreciate it.

BVCB650 · « **Reply #19 on:** September 17, 2009, 12:30:14 PM »

That usually mean you are lean.

fevah · « **Reply #20 on:** September 17, 2009, 01:30:31 PM »

yea i read that which makes no sense as to why its fouling plugs ?
guess i'll wind the idle air screws in a bit and try and phase that part out. The not idling off choke thing is new and weird. I didnt think older bikes would be this difficult. At the moment i want to open the gad lid, kick it over and throw a zippo at it

TwoTired · « **Reply #21 on:** September 17, 2009, 01:56:41 PM »

Quote from: fevah on September 17, 2009, 12:15:32 PM

Airfilters are brand new pods.

You have my condolences. Are these the type that can be over oiled?

Quote from: fevah on September 17, 2009, 12:15:32 PM

Where is the 3000 mile checklist?

In the owner's manual. I've posted the schedule on this forum before...somewhere. I'll see if I can find the pics.

Quote from: fevah on September 17, 2009, 12:15:32 PM

1 cylinder seemed cooler than the other 3.

Ought to track down cause for that for sure.

Quote from: fevah on September 17, 2009, 01:30:31 PM

At the moment i want to open the gad lid, kick it over and throw a zippo at it

Seems an unfair condemnation, considering you don't know when it had it's last 3000 miles routine tuneup.
Machines need routine maintenance. You should blame the mechanic, not the machine.

fevah · « **Reply #22 on:** October 07, 2009, 03:03:55 PM »

Been a while, bought a house and have been packing/moving so the bike hasnt had any attention.
i did check and regap the valve clearances. regap the points and timing. and that all seems fine.
I had to move it to the new house so i fired it up, idling on about 2 cylinders ( no pods just straight off the carbs ). new house is only a mile or so away so i thought id just see how i went. got a bit of a ways down the road and it died. Started it again, flicked the choke on and boom all good, rode really well but if i flicked the choke off it died. Got home, felt the pipes and it was indeed running on all 4.

Cliff notes - idles like rubbish and drops cylinders but runs fine when riding and choke open.
thoughts ?

TwoTired · « **Reply #23 on:** October 07, 2009, 05:04:43 PM »

Had a few people on the forum get this wrong, so I'm just covering some bases.

Do you know the choke lever up is on, and choke lever down is off, right?

fevah · « **Reply #24 on:** October 08, 2009, 11:11:12 AM »

Will have to have a test ride to remember how i had it. I think you may be right though and im confused about choke setting. I figured down with the butterflies opened was choke on because it allowed more air. Am i totally wrong and butterflies closed is choke on and allows more fuel than air ?

TwoTired · « **Reply #25 on:** October 08, 2009, 01:39:25 PM »

The lever is mounted 90 degrees w.r.t. the butterflies.

Look carefully at the stampings on the choke lever.

Lever down: choke off, butterflies open.
Lever up: choke on, butterflies closed.

fevah · « **Reply #26 on:** January 31, 2010, 03:13:23 PM »

Long time no update.

you were dead right about the carbs, i was an idiot.

I put the needle jet back to stock position and took it for a ride with no pods. it idles better (aside from #4 backfiring thru the carb sometimes) has a small flat spot from 4000-5000 ish but it reved out a lot better. Hesitates a bit but generally better than before. pulled the plugs when i got back from ride and:

#1 - black, little wet - so rich ?
#2 - brown - all good ?
#3 - brown - all good ?
#4 - white - lean ?

all the carbs are set the same, main jets are 2 sizes larger than stock i think. i forgot to check the idle screw settings. Any ideas on the differences and the cause of the backfire on #4 ?

TwoTired · « **Reply #27 on:** January 31, 2010, 04:29:44 PM »

Air filter effects the mixture. Will run lean without air filter. And, will wear out faster, with the pistons scraping dirt off the cylinder walls. Maybe your air is always clean and without dust/dirt?

Last part of the tune up is to check/adjust the carb's vacuum sync, so all the carbs and cylinders are working together. Weird idle/backfires can be caused by this alone.

Have you verified float height in all carbs?
Are all the orings inside the carbs soft and sealing?
Did you ever check the tappet gaps?
Are you certain that their are no air leaks in the carb couplers?

fevah · « **Reply #28 on:** January 31, 2010, 05:39:57 PM »

i just ran it with no pods to see if that helped the issue. i can put them back on and have another go.
i have only bench synced the carbs with a drill bit so i really shud get a friends manometer thing and check it properly.
i reset all the float heights when i 1st stripped the carbs but it probably wouldnt hurt to check them again.
i did check and regap all the tappets.
havent checked the carb couplers for leaks, can that be felt or do i need to spray something around and listen for change in idle ?

rogue6 · « **Reply #29 on:** January 31, 2010, 07:08:41 PM »

It doesn't seem like anyone has suggested you might have an issue with the coils themselves. One fires cylinders 2/3 and one fires cylinders 1/4. Your symptoms combined with that fact create a coincidence too large to ignore. Swap out the coils between each side and see if the problem follows to the different cylinders.

fevah · « **Reply #30 on:** January 31, 2010, 09:08:22 PM »

i forgot to add that ive put in some cb750 coils and it is now running on 4 cyl after 1 or 2 new plugs as well

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Author Topic: Cb500 running on 2-3 cylinders (Read 5980 times)

1976CB550F

1976CB550F

1976CB550F