1982 CB900C slowly dies

ejether · 08-01-2012, 09:21 PM

Hey all,
I recently picked up a 1982 CB900C in non running condition. 44K
New battery, spray clean of the carbs and it started right up but was smoking and backfiring. Compression was low in the middle cylinders so I did a valve adjustment. After buttoning it back up, it started, less smoke, still smoking though (likely valve seals) and it had a bad vacuum leak somewhere and was till backfiring when it warmed up. I sealed things up and when I started it, it seemed happy until it started to warm up. It sputtered some but smoothed out as I decreased the choke. After idling 30-40 second with choke off, the idle dropped out and eventually it died and would not start again.

I pulled the plugs and they are black, and in the right two cylinders, the plugs were actually wet and, oddly, the pipes on those cylinders were actually cool.

Thinking coil fade, I checked the spark and they were all pretty good. Visible under fluorescent lights. I wire brushed the plugs, set the a/f screws to 2 turns out, let the engine cool some and tried to start again and it didn't start.

Compression #s: 160,180,120,150
The cold cylinders are the lower compression, but they should still be ok I reckon.

The jets are stock size and the air filter is a Uni. That filter may be gummed up with oily backfire though.

Thinking its 'flooded' I pulled the plugs and will let it rest for a while.

I examined the Air Cutoff valve on carb#1 and it appeared to be in fine shape. I have not torn into the rack to examine them all. I have been running a strong mix of SeaFoam during my tests, this last even was run with fairly fresh, 87 octane with no SeaFoam

Other than cleaning that airfilter, is there something obvious, or not obvious that I haven't checked yet?

Thanks for any thought or input!

RedKat600 · 08-01-2012, 09:33 PM

Float height, then float needle and seat condition. THey are probably leaking.

ApexHack · 08-02-2012, 01:07 AM

Seems like compression is low enough on that one hole to cause problems to me.

dragracer1951 · 08-02-2012, 04:26 AM

I had an 83 CB1000c
Basicly the same bike.
Be very carefull with the pilot air needles. They are a very long taper and VERY fragile.
Parts are not plentifull for those carbs either.
I would put in new plugs and give it a shot after cleaning the carbs really well.
2 1/2 turns out from GENTLY seated worked well for mine.
Also, pull the pilot air screws and have a serious look at the order of the spring, washer and Oring. Mine had them wrong.

ejether · 08-02-2012, 08:20 AM

Originally Posted by RedKat600

Float height, then float needle and seat condition. THey are probably leaking.

I think I'll be able to check that without pulling the bowls. I, as usual, was being optimistic about the condition of the seats.

Originally Posted by ApexHack

Seems like compression is low enough on that one hole to cause problems to me.

It may be bad enough for it to lose power, but its been running "fine" so I don't think its the big issue right now

Originally Posted by dragracer1951

I had an 83 CB1000c
Basicly the same bike.
Be very carefull with the pilot air needles. They are a very long taper and VERY fragile.
Parts are not plentifull for those carbs either.
I would put in new plugs and give it a shot after cleaning the carbs really well.
2 1/2 turns out from GENTLY seated worked well for mine.
Also, pull the pilot air screws and have a serious look at the order of the spring, washer and Oring. Mine had them wrong.

Forgot to mention that I also swapped the o-rings in the pilot screws. The screws looked good and the o-ring are new now. I'll go back to 2 1/2 turns.

I'll check the fuel level in the bowls like RedKat suggests and will probably end up pulling the needles and cleaning the carbs again.

strangerin · 08-02-2012, 09:53 AM

I wire brushed the plugs, - NEVER DO THIS

in the right two cylinders, the plugs were actually wet and, oddly, the pipes on those cylinders were actually cool. - you know that's because they are not firing?

spray clean of the carbs - not a real carb cleaning

RedKat600 · 08-02-2012, 07:38 PM

Originally Posted by ejether

I think I'll be able to check that without pulling the bowls. I, as usual, was being optimistic about the condition of the seats.

It may be bad enough for it to lose power, but its been running "fine" so I don't think its the big issue right now

Forgot to mention that I also swapped the o-rings in the pilot screws. The screws looked good and the o-ring are new now. I'll go back to 2 1/2 turns.

I'll check the fuel level in the bowls like RedKat suggests and will probably end up pulling the needles and cleaning the carbs again.

If you can check and clean those without pulling the bowls, you need to come do mine. You HAVE to pull the bowls to check float height...no way around it.

Oh, and what strangerin said. Spraying crap at carbs isn't cleaning them....need to dip them if they are questionable.

Wrench · 08-02-2012, 08:10 PM

You can revive wet-fouled spark plugs very easily. Cook the tips clean (center electrode) with a propane torch. Get them red-hot. This mimics the cleaning that happens under normal combustion conditions.

Are you absolutely certain you have the valves adjusted correctly?

james1300 · 08-02-2012, 08:31 PM

Is it getting fuel?
Pull the petcock and check the screen FIRST. Before opening the carbs up.

RedKat600 · 08-02-2012, 08:33 PM

Originally Posted by james1300

Is it getting fuel?
Pull the petcock and check the screen FIRST. Before opening the carbs up.

Two spark plugs are fouled black with fuel.....I'd say his petcock is working just fine.

ejether · 08-03-2012, 09:11 AM

Originally Posted by RedKat600

If you can check and clean those without pulling the bowls, you need to come do mine. You HAVE to pull the bowls to check float height...no way around it.

Oh, and what strangerin said. Spraying crap at carbs isn't cleaning them....need to dip them if they are questionable.

Untrue, you can check you float heights wet. Attatch clear tubing to the drain port on the bowl, open the screw. Not all carbs have that port but if they do, the height of the fuel in the bowl will be the saw as in the tube. As long as you have free flowing fuel to the carbs, you can check, not adjust. It's actually more precise than measuring the float height.

Originally Posted by Wrench

You can revive wet-fouled spark plugs very easily. Cook the tips clean (center electrode) with a propane torch. Get them red-hot. This mimics the cleaning that happens under normal combustion conditions.

Are you absolutely certain you have the valves adjusted correctly?

I'm certain about the valves. Thanks for the tip on cleaning plugs!

Originally Posted by RedKat600

Two spark plugs are fouled black with fuel.....I'd say his petcock is working just fine.

Very astute RedKat! Also, the petcock and tank is sitting on the floor of the garage. I'm using a plastic bottle and length of fuel line hung from the handle bar. Yes it's above the level of the carbs :D

I won't be able to work on this til Monday, but thanks for the thoughts!

RedKat600 · 08-03-2012, 09:16 AM

Originally Posted by ejether

Untrue, you can check you float heights wet. Attatch clear tubing to the drain port on the bowl, open the screw. Not all carbs have that port but if they do, the height of the fuel in the bowl will be the saw as in the tube. As long as you have free flowing fuel to the carbs, you can check, not adjust. It's actually more precise than measuring the float height.

I'm certain about the valves. Thanks for the tip on cleaning plugs!

Very astute RedKat! Also, the petcock and tank is sitting on the floor of the garage. I'm using a plastic bottle and length of fuel line hung from the handle bar. Yes it's above the level of the carbs :D

I won't be able to work on this til Monday, but thanks for the thoughts!

Fuel height =/= float height. I've tried this method.....I don't know why you think it's more accurate than a float gauge as you are measuring the fuel in the bowl, NOT the height of the floats. The two are not the same......

strangerin · 08-03-2012, 10:59 AM

Originally Posted by Wrench
You can revive wet-fouled spark plugs very easily. Cook the tips clean (center electrode) with a propane torch. Get them red-hot. This mimics the cleaning that happens under normal combustion conditions.

Are you absolutely certain you have the valves adjusted correctly?

ejether:
I'm certain about the valves. Thanks for the tip on cleaning plugs!

normally maybe, but tracking has ruined your plugs

a chanc to say it again:NEVERNEVER DO THIS

ejether · 08-03-2012, 11:45 AM

Originally Posted by RedKat600

Fuel height =/= float height. I've tried this method.....I don't know why you think it's more accurate than a float gauge as you are measuring the fuel in the bowl, NOT the height of the floats. The two are not the same......

I know they are not the same and I did not say more accurate.
I said more precise. Float height, as you say, does not equal fuel height so a float height of, say, 14 mm *may not* be the same in all bowls. With the wet height, you can check to see how close they are to being the same (http://en.wikipedia.org/wiki/Accuracy_and_precision)

But, in anycase, but doing this in this case, I'm not trying to determine if they are set correctly, only if they are stuck open. If they are stuck open then the fuel level in the tube will rise to the level of the air jets on the carbs (at least) If they are not (stuck open), then the level in the tube will max out at some, lower level. Since, often, its easy to find a measurement of where the level of the fuel in the tube should be in realtionship to the top of the bowl, I may actually be able to tell if the floats are set correctly, though I haven't looked into that for this bike yet.

Mostly I'm just using it as a qualitative measurement of the health of the float needle valve.

Originally Posted by strangerin

NEVERNEVER DO THIS

You live up to your signature strangerin, but fear not, thou crusty codger, I have purchased new plugs and will not wire brush them in the future.

When you say tracking, what do you mean exactly? I gather you mean damaging the electrodes with the brush?

RedKat600 · 08-03-2012, 12:00 PM

Originally Posted by ejether

I know they are not the same and I did not say more accurate.
I said more precise. Float height, as you say, does not equal fuel height so a float height of, say, 14 mm *may not* be the same in all bowls. With the wet height, you can check to see how close they are to being the same (http://en.wikipedia.org/wiki/Accuracy_and_precision)

But, in anycase, but doing this in this case, I'm not trying to determine if they are set correctly, only if they are stuck open. If they are stuck open then the fuel level in the tube will rise to the level of the air jets on the carbs (at least) If they are not (stuck open), then the level in the tube will max out at some, lower level. Since, often, its easy to find a measurement of where the level of the fuel in the tube should be in realtionship to the top of the bowl, I may actually be able to tell if the floats are set correctly, though I haven't looked into that for this bike yet.

Mostly I'm just using it as a qualitative measurement of the health of the float needle valve.

You live up to your signature strangerin, but fear not, thou crusty codger, I have purchased new plugs and will not wire brush them in the future.

When you say tracking, what do you mean exactly? I gather you mean damaging the electrodes with the brush?

I know the difference, thanks.

I also am familiar with the test you are doing. You specifically stated "You can check your float heights wet". When in fact you cannot, it's the fuel height you check.

Also, it can't be more precise than measuring the float height, as there is no specification nor measurement for fuel height in the bowls. It's only good for checking if there is fuel, or if the needles are leaking. Good luck with the carbs.

ejether · 08-03-2012, 12:20 PM

Originally Posted by RedKat600

I know the difference, thanks.

I also am familiar with the test you are doing. You specifically stated "You can check your float heights wet". When in fact you cannot, it's the fuel height you check.

Fair point.

Originally Posted by RedKat600

Also, it can't be more precise than measuring the float height, as there is no specification nor measurement for fuel height in the bowls. It's only good for checking if there is fuel, or if the needles are leaking. Good luck with the carbs.

On some bikes, there is a specification for fuel level.
On my 92 Seca II there is a line on the back of the bowl that you use to set the floats wet.

Thanks for the well wishes.

strangerin · 08-03-2012, 05:39 PM

Originally Posted by ejether

I know they are not the same and I did not say more accurate.
I said more precise. Float height, as you say, does not equal fuel height so a float height of, say, 14 mm *may not* be the same in all bowls. With the wet height, you can check to see how close they are to being the same (http://en.wikipedia.org/wiki/Accuracy_and_precision)

But, in anycase, but doing this in this case, I'm not trying to determine if they are set correctly, only if they are stuck open. If they are stuck open then the fuel level in the tube will rise to the level of the air jets on the carbs (at least) If they are not (stuck open), then the level in the tube will max out at some, lower level. Since, often, its easy to find a measurement of where the level of the fuel in the tube should be in realtionship to the top of the bowl, I may actually be able to tell if the floats are set correctly, though I haven't looked into that for this bike yet.

Mostly I'm just using it as a qualitative measurement of the health of the float needle valve.

You live up to your signature strangerin, but fear not, thou crusty codger, I have purchased new plugs and will not wire brush them in the future.

When you say tracking, what do you mean exactly? I gather you mean damaging the electrodes with the brush?

Yeah, thought maybe I'd read New plugs somewhere in this.
it's bad enough to fight the original gremlins wo/ putting more in as you're working.

If you look closely at the insulator on the center electrode you will see what look like little tracks-pieces of metal that the wire brush left behind, it's a path to ground for the spark.
The unlucky people may get thhe engine to start, but have a mystery miss.

fyrejunkee · 08-03-2012, 08:57 PM

Originally Posted by RedKat600

* as there is no specification nor measurement for fuel height in the bowls *

The manual for my bike actually gives the procedure and the measurements for doing exactly that. I used it to double check that I set the float heights correctly.

strangerin · 08-04-2012, 07:53 AM

Originally Posted by fyrejunkee

The manual for my bike actually gives the procedure and the measurements for doing exactly that. I used it to double check that I set the float heights correctly.

I'd really be interested in hearing that number, part because I am working on some Ninja carbs, part just comparison to my old KZ (1mm below).

ejether · 08-08-2012, 08:46 AM

I confirmed last night that the needle valves are operating properly and the floats aren't set so badly that the bowls are overflowing. That said, it was still only running on two cylinders. 1+2 were running 3+4 were not. Since the coils run 1+4 and 2+3, I think I can still look at something other than bad coils. If I open the throttle wide open with the choke off, 3+4 will *just* start to puff but the engine won't rev. Confirms a super rich condition in those cylinders.

Ordinarily, I'm used to clogged carbs running lean, not super rich, but these carbs have that air cutoff valve enrichment circuit thing and I think they are stuck open letting in extra fuel.

Regardless, I give up on getting it running as it is so I tore the carbs down and am doing a full cleaning (like I should have done in the first place). The air cutoff valves were pretty gunked up like I expected.

The PO said the carbs were rebuilt by a pro last year and except for the gunked up air cut off valves, they look pretty clean so thats good. They'll be even cleaner soon.


08-01-2012, 09:33 PM	#2
RedKat600 Joined: May 2011 From: LaCenter, WA I Ride: 2003 Copper SV1K	Float height, then float needle and seat condition. THey are probably leaking. __________________ Follow PNW Riders on Facebook and Twitter


08-02-2012, 01:07 AM	#3
ApexHack Peg Dragger Joined: Jun 2007 From: Seattle I Ride: YZF750, HawkGT, YSR50, VT1100(parting), '73 CB350F, SV650, CB550F, CB175	Seems like compression is low enough on that one hole to cause problems to me. __________________ Follow PNW Riders on Facebook and Twitter


08-02-2012, 04:26 AM	#4
dragracer1951 Joined: Sep 2005 From: Bremerton I Ride: 05 KTM 625smc, 03 Are See fiddy one, 05 DRZ470SM, 95 FZR1040, 69 Combat Commando Roadster, 73 Commando Interstate, 67 BSA B44, 71 BSA B50	I had an 83 CB1000c Basicly the same bike. Be very carefull with the pilot air needles. They are a very long taper and VERY fragile. Parts are not plentifull for those carbs either. I would put in new plugs and give it a shot after cleaning the carbs really well. 2 1/2 turns out from GENTLY seated worked well for mine. Also, pull the pilot air screws and have a serious look at the order of the spring, washer and Oring. Mine had them wrong. __________________ Follow PNW Riders on Facebook and Twitter


08-02-2012, 09:53 AM	#6
strangerin Knee Dragger Joined: Jul 2005 From: spokane I Ride: other peoples when they let me, working on Honda VF750	I wire brushed the plugs, - NEVER DO THIS in the right two cylinders, the plugs were actually wet and, oddly, the pipes on those cylinders were actually cool. - you know that's because they are not firing? spray clean of the carbs - not a real carb cleaning __________________ Follow PNW Riders on Facebook and Twitter


08-02-2012, 08:10 PM	#8
Wrench Railer Joined: Feb 2005 From: West Richland I Ride: Pitster Motard	You can revive wet-fouled spark plugs very easily. Cook the tips clean (center electrode) with a propane torch. Get them red-hot. This mimics the cleaning that happens under normal combustion conditions. Are you absolutely certain you have the valves adjusted correctly? __________________ Follow PNW Riders on Facebook and Twitter


08-01-2012, 09:21 PM	#1
ejether Permit Joined: Jul 2010 From: Portland, Or I Ride: 2007 Yamaha FZ6	1982 CB900C slowly dies Hey all, I recently picked up a 1982 CB900C in non running condition. 44K New battery, spray clean of the carbs and it started right up but was smoking and backfiring. Compression was low in the middle cylinders so I did a valve adjustment. After buttoning it back up, it started, less smoke, still smoking though (likely valve seals) and it had a bad vacuum leak somewhere and was till backfiring when it warmed up. I sealed things up and when I started it, it seemed happy until it started to warm up. It sputtered some but smoothed out as I decreased the choke. After idling 30-40 second with choke off, the idle dropped out and eventually it died and would not start again. I pulled the plugs and they are black, and in the right two cylinders, the plugs were actually wet and, oddly, the pipes on those cylinders were actually cool. Thinking coil fade, I checked the spark and they were all pretty good. Visible under fluorescent lights. I wire brushed the plugs, set the a/f screws to 2 turns out, let the engine cool some and tried to start again and it didn't start. Compression #s: 160,180,120,150 The cold cylinders are the lower compression, but they should still be ok I reckon. The jets are stock size and the air filter is a Uni. That filter may be gummed up with oily backfire though. Thinking its 'flooded' I pulled the plugs and will let it rest for a while. I examined the Air Cutoff valve on carb#1 and it appeared to be in fine shape. I have not torn into the rack to examine them all. I have been running a strong mix of SeaFoam during my tests, this last even was run with fairly fresh, 87 octane with no SeaFoam Other than cleaning that airfilter, is there something obvious, or not obvious that I haven't checked yet? Thanks for any thought or input! __________________ Follow PNW Riders on Facebook and Twitter


08-02-2012, 08:31 PM	#9
james1300 Track School Dazed Joined: Oct 2005 From: CENTRAL I Ride: When I can	Is it getting fuel? Pull the petcock and check the screen FIRST. Before opening the carbs up. __________________ Follow PNW Riders on Facebook and Twitter


08-03-2012, 10:59 AM	#13
strangerin Knee Dragger Joined: Jul 2005 From: spokane I Ride: other peoples when they let me, working on Honda VF750	Originally Posted by Wrench You can revive wet-fouled spark plugs very easily. Cook the tips clean (center electrode) with a propane torch. Get them red-hot. This mimics the cleaning that happens under normal combustion conditions. Are you absolutely certain you have the valves adjusted correctly? ejether: I'm certain about the valves. Thanks for the tip on cleaning plugs! normally maybe, but tracking has ruined your plugs a chanc to say it again:NEVERNEVER DO THIS __________________ Follow PNW Riders on Facebook and Twitter


08-08-2012, 08:46 AM	#20
ejether Permit Joined: Jul 2010 From: Portland, Or I Ride: 2007 Yamaha FZ6	I confirmed last night that the needle valves are operating properly and the floats aren't set so badly that the bowls are overflowing. That said, it was still only running on two cylinders. 1+2 were running 3+4 were not. Since the coils run 1+4 and 2+3, I think I can still look at something other than bad coils. If I open the throttle wide open with the choke off, 3+4 will just start to puff but the engine won't rev. Confirms a super rich condition in those cylinders. Ordinarily, I'm used to clogged carbs running lean, not super rich, but these carbs have that air cutoff valve enrichment circuit thing and I think they are stuck open letting in extra fuel. Regardless, I give up on getting it running as it is so I tore the carbs down and am doing a full cleaning (like I should have done in the first place). The air cutoff valves were pretty gunked up like I expected. The PO said the carbs were rebuilt by a pro last year and except for the gunked up air cut off valves, they look pretty clean so thats good. They'll be even cleaner soon. __________________ Follow PNW Riders on Facebook and Twitter

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode