Topic: Help, stumped - 76 CB550F idles OK, over 3k RPM stumbles, sputters, backfires

[Sable]

I have had My 76 CB550F for about 18 years now. My uncle was the previous owner before me. All I have ever done is change the oil filter and oil, add gas, tires and chain/lube. Never had a problem with it. Just over 18k miles on it.  Over the last 10 years, life was busy. It sat for a year at a time. I would get it out once a year and get her running, do 30 miles or so on her then put her back to storage. Now life has taken a turn where I can ride her daily.

I got her running and all was great. Did new tires, chain, plugs and points/condensers. She was running better than ever. Rode her for a couple months, 300+ miles. Then I let the tank get low enough that I had to use the reserve and everything went to hell.

The bike started to stumble as they do when it's time to go to reserve, when I flipped to the reserve it would not start back up, just died. So I got gas and filled the tank again, thinking maybe the reserve was plugged. I put the bike back to "on" (rather than reserve) and it started and I finished the ride to work with no issues. Thought I was good....boy was I wrong.  When I left work at the end of the day, all seemed fine until maybe 3 minutes into the ride. The bike started stumbling at cruising speed and I couldn't keep it going. I kept having to pull over and fiddle with it shaking the gas tank figuring the petcock was blocked. I barely got it home. Disassembled the gas tank and sure enough it was clogged with sediment. So I decided to rebuild the carbs, clean and repair the gas tank and put it all back together. Do it all right the first time.

Long story short, I got it back together and fired it up (rebuilt carbs, sealed gas tank, new gas filter etc). Started idling fine. Thought I was going to be ok. But as soon as I twisted the throttle it started stumbling and spitting, just like it did just before I tore it down. So I went to the internet. Searched high and low. Tried every trick and thought I could find. Adjusting point gaps, timing etc. Nothing seems to resolve the issue. I have swapped out fresh gas, advanced the timing which helps some but still it's not running right. I'm at a loss and too inexperienced with motorcycles to know where to go from here. My only thought is that the diachi points/condensers are the issue. I just find it really strange that it was running great, then suddenly just failed. Is that typical of bad points? What do you guys suggest? I have read as much as I can find, but if you got a link, send it and I will read it.

Bike idles fine, timing is reading right, once over 3k rpm it breaks up and spits sputters and backfires. It's almost like it's choking itself out. I tried removing the air filter also to get more air in and that was a no go.

I just want to ride. Been messing with this for a month now. No success. I am beyond frustrated.

Thanks in advance!

[TwoTired]

I just want to ride. Been messing with this for a month now. No success. I am beyond frustrated.

Probably best to find some who will fix it for you.    If you haven't the patience and determination to ferret out the problems of an antique bike and learn its intricacies, you're relationship with it is doomed, imo.  Yes, probably harsh, but given your ending statements, I think it must be said, as it focuses more on you than the machine's problems.  The machine doesn't care, it needs attention in the proper place to operate as you expect.  That said...

Have your read the spark plug deposits? 
http://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html

"rebuilt the carbs" is a nice blanket statement.  But, we have read many times about that, and then found critical segments of carb restoration were omitted/overlooked.
One example was dunking the carbs in solvent and expect all the internals to get clean without verification.  How can we gain confidence that the "carb rebuild" on this bike was done thoroughly/properly?  What exactly did you do?

Have you tried the old condensers?  Has the mechanical advance been checked for proper function?

Have you checked for head pipe heat differences?

Checked the plug caps?

Have you reviewed the entire 3k mile tuneup check list items?

We need to build upon the knowns to isolate the unknowns.
If you don't enjoy working on antiques and solving their special problems, you may never like or enjoy this bike again.  FYI

[Sable]

Carbs where removed, broken down, dunked over night. Scrubbed thoroughly, air blasted to clean out all the internals. Checked and verified thoroughly. New jets, seals and the like. All set back to OE spec. Triple checked the floats with the proper float tool. re applied the seals twice and I do have a couple float bowls leaking now  that I have put fuel back to them that I will need to revisit again, but figured I would get back to that once the bike is running better since taking the carbs on and off is just a blast to do!

Have not tried the old condensers, but I will.
I have not checked the mechanical advance, it seems as though that would have shown it's face during timing with the timing light, would it not?
I have checked the head pipe heat with an infrared temp gun, they all seemed to be about equal at 145 degrees when I put the sensor to them. I did not check while running but I can. Visually, I can see the that 1 and 4 seem to be bluing the exhaust while 2 and 3 are not, that would indicate to me that the outside pipes are likely running hotter.
When I pull the plugs, they are looking a bit wet. Like the bike is running too rich. But I can't get it to run well enough to run it under load at RPM to get a true reading. Based on how it's running, everything indicates running to rich.

I have the original shop manual for the bike that was obtained when my uncle bought the bike new and it has been followed. I have run through all the maintenance on it.

I know where you are coming from with the remark to not caring for the bike. I made that comment out of frustration as I have been twisting my head around backwards working on this bike. I have a two year old that I am neglecting to spend time with to work on this bike. A bike for which I never had a problem with before. So yes, I am frustrated.

[Sable]

Just re-pulled the plugs and took a picture. Dry and covered in carbon. Too rich indication. I have been running the bike all morning messing with it trying to come up with a solution. Here is a picture of one of the plugs.

[TwoTired]

The carbon says more than just running rich.  It also says that spark can be shunted along the carbon path defeating the purpose of the porcelain insulator.
  You'll need to fix both the cause of the carbon deposits and have clean plugs with which to test later.

You've shown one plug.  Are they all showing the same deposit pattern?  If all four operate the same, then all four carbs are suspect.  If the deposits are different among the four, then the difference makers have to be tracked down.

I will ask if you removed and cleaned the emulsion tubes behind/above the main jets.  These are often overlooked and are seldom clean properly with a dunking.  Plugged/fouled emulsion tubes can make the carbs deliver over rich mixtures.

[Sable]

Completely removed the emulsion tubes and cleaned them. Wanted to replace them but didn't find a source for them. all the jets and seals are newly replaced. I did keep all the old parts though as I knew I was taking out original stock components and wanted to hold on to them as you never know.

I only pulled the closest plug as we were running out the door. Come morning I will pull the other three as well to confirm. After tinkering with the ignition, at times I could get it to fire and run through the rpms, just not consistently. In order to achieve this, I would have to get the plate centered so the timing light would be reading just before the 1-4 and right of the F. I am a newb to timing on a bike, none the less an antique. I have no idea if this seems way off or not but from what I read, seams like every bike is a bit different.
But, in regards to your comment on the plugs, if the plugs are that sensitive t the carbon on them, it is possible that is why I was having issues finding the sweet spot since I wasn't pulling them and cleaning them each and every time I adjusted. I will pull all the plugs and clean them again. See where this gets me. (As you can tell they are new plugs, been on for a short time.)

Thanks for the help, keep the advise coming!

[TwoTired]

Cleaning plugs is an art form and not always successful at leaving the porcelain conductance free.  Try a new set while you are searching for the over rich culprit.  If nothing else, they can eliminate spark issues (for a while).

Stock exhaust?
Stock air filter?

There have been posts about aftermarket carb parts not built to the same specs as original.  Whose parts did you use?  Did you compare them to the originals.
I favor cleaning the original brass part and reusing them rather than doing the measurement verification.

What slide needle position did you use?  Stock slide needles?

You should have 069A carbs on a true F model bike.  (Some don't understand the difference between the F and the K model.)

I'll also ask if the pilot screws on yours are hollow tipped or solid tipped.  An F model normally has solid tipped pilot screws.

[Sable]

Stock Exhaust (4 into 1)
Stock Air box
Sudco Points and Condensers (dhachi I believe)
R&R Carb kit - I did compare parts and they seemed to be identical in all regards except for the pilot screw as you noted. The replacement had a hollow tip versus the original with a solid tip. Here is a link to the kit: http://www.ebay.com/itm/400461192629?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I used the same slide needle position as was on it originally. I chose to reuse the stock slide needles again since I reused the emulsion tubes again. Which was the second notch down from the fat end (short end).

I actually rebuilt the carbs reusing the stock parts once earlier this year. It ran great afterwards. But since I had to tear it down thinking that it sucked the crude from the gas tank into it I opted to replace the parts this go around hoping to not have to open them up again for a very long time.

069A Carbs on it.
Bike is all original, 2 owner (uncle and myself). I'm the first to crack into it. I can tell you the story behind every dink and scrap on the exhaust. Got with 8k miles, put the rest on it myself. Even road her for two years year round up in Cleveland, OH. We have some history.

What I don't understand is why it would suddenly just stop running well. I know I changed a lot of possible factors in doing the carb rebuild again after it went a rye and that now becomes suspect. But if after putting it back together it still exhibits the same problems as before I took it down, wouldn't that lean more towards me having resolved nothing (other then repairing the aging gas tank and potential fuel line clogs). Makes me want to think that I missed the problem elsewhere. Really makes me want to think it's in the timing somewhere.

I'm still listening though. Will give everything a shot. Got nothing to lose at this point. In 18 years, I want to hand this off to my son, running and with the knowledge on how to fix it. Too much fun to ride to stick it on a shelf.

[78 k550]

Did you try a new gas cap. Old one might not be venting.

Paul

[Sable]

Thought about the gas cap but it does the same thing even with the gas tank removed from the bike running on the gas left in the bowls. I did just replace the gromet on the gas cap though just for piece of mind.

[Sable]

All plugs look identical. 1, 2 and 3.
In the past I have always just done a quick clean with a brass brush and some brake cleaner or PB blaster. Am I crazy for this?

[Sable]

Advancer looks good, here is what we are working on.

[TwoTired]

The advancer must function properly.  Can't normally tell that by looking at it physically.  Pretty, isn't it?  jk/ 
It's function is checked using a strobe timing light.  The 1.4 F timing light aligns at idle, and the Hash marks align when the engine reaches over 2500 RPM.

At this point, I'll make the guess, that the cut out symptom is due to the fouled plugs.  Replacing those will make it run ok until the new ones foul, making the spark unreliable.

Second, if the carb kit came with hollow tipped needles, it was for a K model, which employs higher fuel delivery rates than the F model parts.  I recommend cleaning the original brass parts and putting them back in the carbs at the stock settings.  The slide needles are clipped in the second notch from the top as they ride in the the bike.

The internal parts are made with micrometer precision.  It is rare to find someone who can see part differences between .015 and .017 inches,  However such differences can make fuel delivery orifices flow different amounts of fuel.  Aftermarket components are cheap, in part, because costs have been cut in part precision, part measurement checking, or allowing tolerances greater than what the factory specified.  While you can get good parts in aftermarket, the variability among them makes them less desirable.

I surmise it ran well with the old parts in the carbs until fuel tank contamination fudged things up.  While fixing that the carb was altered with different parts.  It ran ok until the plug insulators fouled due to improper fuel mixtures.  I predict that new plugs will make it run ok again for a while, and then the symptom will return again, until the plugs are again replaced.  This recursive cycle will repeat until the carb issue is corrected.

 I would question why the tank's filter did not prevent carb contamination.  There are opportunities to change the filter's effectiveness via incorrect assembly. However, if the bike sits, evaporative fuel deposits can form in the carb bowl.  Later use allows the solvent action of the gas to break off these deposits from the bowl walls and clog the fuel orifices.   An upstream filter won't prevent this, of course.

Here's my 76, if your interested.  Though I think yours is in better shape, the fairing on mine allows me to ride it year round in these parts.  I still have the parts to put it in stock form, when the time comes.  It's a sweetie and made a very nice commute bike.

[Sable]

Alright, I tried cleaning up the plugs and reinstalling. Made no difference. Thus I will try new plugs and see what that does.

I also tried cleaning up the original old TEC points and condensers.
Installed the points first and tried that to see what it did. I will say hand down this made a considerable difference in trying to establish the timing, but it did not resolve my issue at all.

Next I swapped back in the original TEC Condensers.
This didn't work so well, fired back up instantly but then it seemed to exaggerate the issue and if I touched the throttle it died almost immediately.

So I think by deduction, I will leave the TEC points on and go back to the daichi condensers for now. Thinking maybe I have two sets of bad condensers.

1. will try new plugs
2. will drop the coin for OEM Condensers

try all over again.

Now in reference to the timing. Let me explain what I am doing and maybe I am doing it all wrong.
I get the bike to idle just over 1k rpm (attempting to be at 1100).
I set the timing light to 0 plug it on #1 cable and shoot. What I have been trying to do is to get the double hash marks to line up that are located to the left of the F1-4 inscription. RPMs will increase if I shift timing plate counter clockwise. Is it right to be aiming for those hash marks or should I be aiming for the line to be directly over the F itself?
Then I repeat the process for 2-3 using the #2 cylinder.
Maybe I just fail at setting timing?

I know what you are saying about the parts. I too think I have a K kit. Guess all I can do at this point is swap out the jets back to stock and see what happens from there. Was trying to avoid doing that but looks like that may be the only option. It's choking when I throttle up so this seems the most logical solution if the K parts are going to do as you described. Makes complete sense.

When validating the advancer, when in full advance the timing light should align with the hash marks to the right of the F1-4 marks correct? (I've read through all of this so many times I am confusing myself at this point)

[Sable]

I tried swapping out the hollowed tips for the original solid tips. Not much change if any. Was worth a shot.
Looks like this kit came with 100 jet and the stock are 98. Will pull the carbs and swap those out also.

Are the float needles any different? I know one of my stockers was buggered up and wouldn't spring in and out properly so I will likely have to leave these in.

New plugs first.
Then pull carbs and swap in original parts....will need all new O rings to do this though. Have to order them in.

Done for today I think.

[TwoTired]

I've had spark plugs that were uncleanable, as I couldn't remove a thin conductive film atop/within the porcelain. Sometime the carbon arc path actually permeates the porcelain, which is porous.  Only way I got these plugs to work was with an abrasive blaster and a good run in a lean burn environment.  My 78 Cb550K is kind of a spark plug cleaner/corrector.  LOL

There are numbers stamped on the slide needles.
273004 is what I found in my 069A carbs.
Unless you measure the replacement aftermarket needles, diameter at distance from clip position, and/or taper angle, I'd assume a replacement from a K kit is wrong.   If you are bold, or wish to experiment, you could swap back all the originals apart from the buggered up one.  If the spark plug deposits correct on all but the replacement, then shop for the correct needle.

Because of the stock higher pressure muffler and air induction set up, the main jet has to be 98.

Timing.
The two marks near the 1.4 stamp apply to T for top dead center, and F for ignition Fire at 5 degrees before TDC.  This is the mark where the spark should fire at idle.  As the engine revs, two hash marks appear when above 2500 RPM, and the timing light should fire in between those marks.  I think that's 35 degrees before TDC.  I don't care, as long as it is in the vicinity of those marks, it proves the advance is working well enough for street use.
I only use static time for my bikes anymore.  Any gains from precision don't make any difference during a legal street ride, imo.

Since your bike appears to be near stock, I would try to keep all the parts like when Honda delivered it.  They made them run pretty good with those parts. 

[Hot500]

Ive just experienced similar problems with my stock 500. Well internals are stock but running 4 into 1 and pods. The bike started well and seemed to rev willingly, but once riding and under load would not rev past 3000.

Checked all the electrics, plugs, points, timing etc. No difference. Pulled the carbs and found a lot of gunge. These carbs were completely rebuilt about 12 months ago, but the rest of the bike was not finished until recently. (Note to self: Next rebuild - leave the carbs to last.) Deposits had built up in the carbs and fouled almost everything. One main jet was completely blocked. Cleaned and reset everything, and bingo  she lives. 

[Sable]

The slide needles are OK. Its one of the silver floats that is no bueno. Sorry for being unclear.

[TwoTired]

The slide needles are OK. Its one of the silver floats that is no bueno. Sorry for being unclear.

Ok, I misread your post. But, you'll need a heads up about the float needles. 
I recall a post by Hondaman about some aftermarket float needles, where the spring on them was stronger than the original type.  This altered the desired/proper fuel level in the carb bowls, and difference messed with even tuning across the four carbs.
He mentioned the spring stiffness was obvious once it was scrutinized for rebound pressure compared to the originals.
If you have one of these stiff spring float valves, note that the factory float height setting won't be correct.  You will have to use the clear tube method to adjust the float height toward the goal of having all four bowls fill to the same level.

I haven't personally witnessed the wrong spring pressure.  But, it makes perfect sense.

If you find this applies to your float needles, you might try to give the one you have that is stuck an overnight soaking in carb cleaner to free it.  If that doesn't work, try some heat form a soldering iron or boiling water to soften the stuck deposits. 

It helps if you happen to enjoy tinkering with bits.   

[Sable]

Swapped in new plugs just to see what she would do.
Literally one solid pull on the throttle and right back to breaking up over 4k rpm.
Was worth a try for $8. LOL

Will be pulling the carbs (which finally stopped leaking), swapping in the stock parts.

In all honesty, I have little hope that this will resolve my problem since it was doing the same thing with those parts in to begin with.

[TwoTired]

Swapped in new plugs just to see what she would do.
Literally one solid pull on the throttle and right back to breaking up over 4k rpm.
Was worth a try for $8. LOL

Did you look at the plugs to see if they sooted up that quickly?

[Sable]

No, not yet. Will pull them after work. I did notice the old set looked cleaner when I pulled them out.Still with carbon but not as bad. Will let you know.

[iron_worker]

How do your points look? Do the contact surfaces look pitted/burnt? If so, this can be an indicator of a failing condenser. The condenser is there to absorb the energy that is attempting to make the current arc back across the points instead of going through the primary side of the coil.

IW

[Sable]

I pulled the new plugs, still semi clean. Not black but they definitely already started to get a film. 2 and 3 may have been just a little bit worse than 1 and 4 but ever so slightly. 2 was the dirtiest but just on the very tip of the plug.

Does this with new diachi points and right now I have the Tec points back on it. But I filed the points down before reinstalling them. Still with the newer diachi condensers. The points do arc a strong blue. Figured this was typical though.

[TwoTired]

I'd be more interested in deposit descriptions regarding the porcelain insulator, particularly near the center electrode.

The points arcing is a product of the coil's electromagnetic field collapsing to fire the spark plugs.  The collapsing field crosses both the secondary and primary wires within the coil, which produces a voltage in both.  The primary's voltage development has nowhere to go but back to the points and condenser.  The condenser staggers the phase of the voltage and current, to reduce peak heating effects of the arc.