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

[Sable]

No deposits on either set of plugs. Just coated in the black carbon. I can get you a good picture tomorrow.

[TwoTired]

Ok
But black carbon IS a deposit.   

[Sable]

Best I can do. Hard to get a picture of.

[TwoTired]

Those were the new plugs?  If I was covered with that much soot, I'd sputter, too! 

The pairs of spark plugs fire with opposite polarity.  Half of them with try to jump ground strap to the center electrode, the other half with try to jump from center electrode to ground strap.  Current will take the least resistive path.  But, why would the current jump the gap rather the simply travel the nice easy carbon bridge?

Further, the way this ignition system works, the coil's secondary will build voltage until a current path is established in both spark plugs.  Then it will drain the remaining power through the established current path.  Normally, the voltage rises until sufficient to jump the gap.  The spark gap sets the voltage peak where current begins to flow.  With little or no effective gap, the voltage doesn't get a chance to rise much, resulting in very weak power or heat kernel with which to ignite the air fuel mixture.

BTW, those plugs might be useful if the carbon was burned off of them.  But, they may confuse the issue when searching for the fuel mixture cure.

When you get inside the carb, do check the slide needle position.  From the rapid soot accumulation, it sure looks like the clip would be in the fourth position from the top rather the second position.

Let's review the choke lever operation.  Lever up is choke on.  Lever down is choke off.  With the choke on, operation over 3k will likely cause stumble, sputtering, and backfire.  Even more so when the engine get to normal operating temp.

[Sable]

Those where the older set of plugs. Choke is off.

[neilc]

Dude you did what I did and am going through now also,took carbs apart,bought replacement part,now trying to troubleshoot which rebuild parts are good,I put all primal parts back I could but am still dealing with carb issues,I also have the crease black soot problem my carbs are coming back off to change clip position and a total breakdown and cleaning and check all dimensions,my bike ran good till I started with carbs,luckily it's a hobby bike (good luck)

[lucky]

Better get the floats straightened out.
New float needles and seats. Correctly set by the book.

[TwoTired]

Those where the older set of plugs. Choke is off.

I asked about the new plugs where you said there were no deposits.  So, you show us the old plugs that clearly have deposits on them, and say there are no deposits on either set of plugs.  How can we possibly know what the heck is going on with your bike?  If the new plugs do look like the old plugs, you have a serious mixture issue with the carbs.

No deposits on either set of plugs. Just coated in the black carbon. I can get you a good picture tomorrow.

[Sable]

I'm not trying to confuse anyone. Those are plugs I just pulled out. I can pull out the new plugs I just put in and ran for maybe 1 minute if you would like. They already show signs of building black carbon. But they are one tenth as dark as these because I only ran them for 1 minute. Will post a picture if you like.

Would the floats cause it to run rich? I set them to OE spec of 22. When I set them, they took a fair amount of adjustment to get them right. They seemed bottomed out with the new float needle installed prior to adjustment.

[Duanob]

Would the floats cause it to run rich? I set them to OE spec of 22. When I set them, they took a fair amount of adjustment to get them right. They seemed bottomed out with the new float needle installed prior to adjustment.

Did you measure the floats with the float tang just touching the float needle spring? Not compressing it at all.

[Sable]

Yes sir.  Made sure both lobes of the float were at the OE spec height also.

[Sable]

Going to pull the carbs again tomorrow. Will take pics of this and that once I get it apart.

[Scott S]

 You said carb were broken down, cleaned, etc.

 Did you replace any of the O-rings in the carbs? The O-rings on the needle valve seats? On the main jets? Under the idle mixture screws?

 You also said you installed new jets. I strongly prefer genuine Keihin jets.
 *Do you still have the original jets? Did you use the needles from the jet kit?
 *Do you know how to pull the slides and check the clip position?
 *Did you pull the emulsion tubes?
 *Do any of the posts that hold the main jets have corrosion on them?
 *Where are your idle mixture screw settings.
 *You said choke is off...are you SURE? It can be confusing on the CB500/550 carbs and easy to get backwards.

[Scott S]

 Re-reading this thread...

 Check your timing again. Don't worry TOO much about timing at idle Use a strobe light and, at ~2500 RPM, set the advance so the two hash marks just to the right of the 4 and 3 line up with the timing mark.

[Sable]

I will take one more stab at it before pulling the carbs.

[Sable]

You said carb were broken down, cleaned, etc.

 Did you replace any of the O-rings in the carbs? The O-rings on the needle valve seats? On the main jets? Under the idle mixture screws?

 You also said you installed new jets. I strongly prefer genuine Keihin jets.
 *Do you still have the original jets? Did you use the needles from the jet kit?
 *Do you know how to pull the slides and check the clip position?
 *Did you pull the emulsion tubes?
 *Do any of the posts that hold the main jets have corrosion on them?
 *Where are your idle mixture screw settings.
 *You said choke is off...are you SURE? It can be confusing on the CB500/550 carbs and easy to get backwards.

I have the original jets. I currently have the original needles installed at original setting. Second clip down.
Yes, second clip position from the top.
Yes, I pulled the emolsion tubes.
Corrosion no, dirty yes.
One full 360 turn out on idle mixture screw.
Positive on the choke. She's so rich she won't run when choked at this point.

[Sable]

Picture of the new plugs run for about a minute.

[Sable]

Made a cellphone video. https://www.youtube.com/watch?v=e3yV8Ymyd24&feature=youtube_gdata_player
I can ride it around the neighborhood underb3k all day. Put that is no fun.

[TwoTired]

First thing I noticed was that the plugs have different deposits on them.  The far right is not running the same as the others.  Not the current RPM problem.  But, it will be an issue later on.  The new plugs should all be sparking as required in their present condition.
Has there been a vacuum sync?

It sure does idle poorly.   

While I've never seen a CB550 operate with that specific behavior, were it mine, I would check the fuel height in the carb bowls with the clear tube method to find exactly how high the fuel level is.  A too high level could explain the the symptoms.
Did you say you had to readjust the floats a lot during the rebuild?

Another possible check, is drain each carb bowl in turn (After turning off the fuel valve) and measure the volume each holds.  I don't know what it is, off hand.  But, if you choose do do this, I will measure mine for comparison.

Not to insult, but I'd check some of the following if you brought it to my garage.
Have the main jets fallen out?  They have a spring clip that holds them in when the bowl is installed.  Still have all four spring clips?
Are the choke plates staying in proper position?  They have a spring/ball detent which is supposed to hold them at set position.
Is there a rag in air box or air filter box?

The picture below shows the jet retainer spring and the float installed properly.  Please verify that the float is right side up.

Be aware, it is frustrating for us too.  We want to help, and are searching for clues that you must provide to solve the issues.
I'd venture to say that your bike has an issue none of us have seen exactly before.  Whether it is mechanic induced or a part failure remains to be seen.

Have you tried different gas?

[Sable]

I appreciate the patience.
I have found on the #1 cylinder it looks as though the head gasket has begun to fail and this most certainly would not help the situation. But would that be the cause of the issue?

Once she is done cooling down, I will pull the carbs again and start going over everything. I did have to adjust the floats a decent amount, yes. The jet spring clips are all still there.

Fresh gas, 93 octane.

This time around I had the air screws at 1.5 turns back.
Focused the timing at 2500 rpm. Was able to 2/3 dead on but ran out of adjustment to get 1/4 perfect, but it was close.

I'll post some pics of the carbs once I open it back up.
Thanks for the help!

[TwoTired]

I have found on the #1 cylinder it looks as though the head gasket has begun to fail and this most certainly would not help the situation. But would that be the cause of the issue?

Nothing to worry about.
At each end of the cylinder block is an oil passage from lower end to camshaft/top end.  These passages are sealed with orings at the head interface, and because of the cylinder heat, eventually harden and weep. 
I think my 74 has had that weep for 20 years.  The head gasket is fine.  I just wipe the oil off it on a weekly basis.

Personally, I have never timed ANY of my SOHC4s using only the full advance marks.  I prefer to have it correct at idle for smooth idle.  (It's what Honda specifies, too.)  If I was racing instead of street driving, then maybe I'd place greater emphasis on full advance timing.

[Sable]

It's not the small seepage that caught my attention, it was what looks to be a missing chunk of gasket.
But I still plan on tearing the carb off and going through it.

[Scott S]

 Sounds like the carbs are way out of synch, too. Have you bench synched or vacuum synched them?

[Sable]

Sounds like the carbs are way out of synch, too. Have you bench synched or vacuum synched them?

No sir. I will try syncing them after I go through the carbs again.

[Sable]

Here are the same plugs after me riding it around the neighborhood for a bit this morning.
Tearing her down to get to the carbs now.