Topic: CB5550F weird engine problem - any idea how to solve it?

[kristof]

I have been working on 1976 CB550F restoration for the last 7 months. I finally got the top end rebuilt and put together.
The cylinders were re-bored to the first oversize piston. I have new valves, valve guides, new valve seats etc.
I assembled it all and bike started like a charm on the first try. The carburetors were synchronized and I rode it, so far, around 30 miles. Worked great until the last weekend. The bike run great on Saturday. I started it on Sunday with no problems and it purred nicely, but in less than 3 minutes as soon as it warmed up and I turned the choke off, cylinders 1 & 2 stopped working.
Cylinders 3 & 4 are working fine.

I tested for fuel delivery and it seems it is OK.
Both sparks plugs on 1 & 2 are wet.
I have spark on all cylinders.

 I tested cold cylinder compression ( I did not have a compression gauge earlier)
1: 152psi
2: 152psi
3: 160psi
4: 162psi

While I am surprised to see this difference I do not have enough experience to know if this would cause problems with starting or if I should disassemble top of the engine and look for the problem there. Honda manual recommends engine re-boring if the warm engine compression falls below 142psi and states that proper warm engine compression should be 172psi.
At this point I am at the dead end and not sure how to solve it. Any ideas would be greatly appreciated.

[bwaller]

Your compression is fine. Did you change valve seats or seals? It is a good idea to recheck valve clearances sooner than later after a rebuild.

Check the colour of the other two plugs, maybe you're a little on the rich side and the bike idled too long?? Are your points well adjusted and timed?

Install a new set of D7's.

[kristof]

The head was redone so I have 5 angle seat grind, new valve guides and seals. I adjusted valve clearances when I assembled the top end. I rechecked valve clearance and it is correct for all 4 cylinders.
I checked (statically) timing and it is correct. The only connection between cylinders 1 & 2 is the common fuel line. Otherwise they run on a different condenser, breaker & coil. The spark plugs are nice mocha color on the firing cylinders.
Forgive my ignorance but what are D7's?

[moham]

The head was redone so I have 5 angle seat grind, new valve guides and seals. I adjusted valve clearances when I assembled the top end. I rechecked valve clearance and it is correct for all 4 cylinders.
I checked (statically) timing and it is correct. The only connection between cylinders 1 & 2 is the common fuel line. Otherwise they run on a different condenser, breaker & coil. The spark plugs are nice mocha color on the firing cylinders.
Forgive my ignorance but what are D7's?

I believe he's referring to NGK D7 (EA?) spark plugs...

Sound like fuel issue to me...besides noting a carb sync, you didn't say much about carb rebuild. How sure are you that all passages, jets, needles, floats, valves and fuel lines are operating properly?

[Pltfn]

Stupid question... how much fuel is in you tank?  I ask because I was stupid enough to think I had a real problem similar to this and it turned out that I was running out of gas ...

-Kelly

[kristof]

Carburetors were rebuilt with K&L kits. So all the jets & seals are new. Fuel valves and needles are new as well. Choke valve positions are adjusted and carburetors were synchronized using ProMotion tool. I had the engine running fine after all of this was done. I was thinking that it has to be fuel problem, as suggested, but cannot figure out where it is. I had the engine running very well. Both cylinders stopped working at exactly the same time.

My gas tank is full and fuel is flowing just fine. I tested it by draining fuel bowls and running fuel while the drain screw was opened.

[kristof]

I also tested if the fuel is getting to the inlet. I removed the connection to the air filter, covered the carburetor inlet with my hand and kick started. My had gets wet with fuel.

[kristof]

As for spark plugs I do not remember what they are, but I installed new ones (recommended) when I was assembling top end.

[bwaller]

You're getting fuel is the plugs are wet and may be fouled, so if you have spark I'd install new NGK D7EA plugs (not 8's)  Just don't let it idle too long.

[kristof]

Do you think this will be an issue even if I removed the spark plugs, let them dry and had cylinders venting for a whole night? I will remove carburetors and clean them again with compressed air. I just cannot understand why I would have a failure at the same time on both cylinders. I already checked all the jets for clear passage but I did not clean carburetors with compressed air.

[kristof]

You mentioned idling twice. What is the ill effect of prolong idling on the spark plugs?

[TwoTired]

If the spark plug center electrode insulator gets coated with carbon fouling, they short instead of spark.

Swap plugs with known working cylinders as a troubleshooting aid.

[kristof]

I did swap the spark plugs with the ones from working cylinders. No change. The spark plugs are clean, there are no carbon deposits on them. the spark plugs from not working cylinders work just fine in 3 & 4 cylinders.

[TwoTired]

I hate to mention this...

Take off the valve inspection caps of the cylinders not working.  Turn the engine over and see if the valves move up and down.
If you have an oiling problem at one end of the cylinder bank, the cam can seize, and break in half.

[kristof]

I did that as well. Everything is moving correctly. I did it when I was checking valve clearance.

[bwaller]

What happened when you switched plugs, did it start & run on #3&4 only?

You mentioned adjusting the choke butterflies, any chance there is a problem with those two or the choke shaft?

It seems things went bad the minute you turned the choke off...

[moham]

If you have an oiling problem at one end of the cylinder bank, the cam can seize, and break in half.

God wouldn't that suck!? I bet that would make a hell of a noise...I was thinking of something on the way home, probably nothing but what about the spark plug caps/boots? Were wires, etc changed out with the rebuild?

[kristof]

When I switched the plugs the engine started in 3&4 no problems. 1 & 2 did not fire at all.
I checked the butterflies and they open just fine. I do think you are right and there must a problem with fuel delivery of some kind.

I changed the coils and wires during the rebuild. I have spark on all 4 wires / plugs.
Unfortunately I am away next week so all experiments will have to wait until I come back.
I will take carburetors out and clean them with compressed air. I have noticed tonight that when I drained fuel from the fuel bowls there were small black particles in the fuel on the cylinders which did not fire. They looked like rubber but not sure if it was rubber. I did not start the engine as I drained the oil to see if there is anything in it. It is clean. I will update forum on my findings later.

Thank you so much for reading my desperate entries and providing your suggestions for solving the problem. It is greatly appreciated.

[traveler]

bad coil?

You have compression, you have fuel....it MUST be spark.

~Joe

[bwaller]

It's cylinders 1&2 so rules out a coil.

[scunny]

It's cylinders 1&2 so rules out a coil.

not if 1 & 2 leads to plugs have been swapped

[kristof]

The leads are not swapped. The bike was running beautifully for 30 miles after assembly. Spark plugs are correctly connected to respective coils ie. 2&3 together and 1&4 together.

[TwoTired]

Can't do much from here but suggest possibilities...

Have the jets fallen out of their home in the carbs?

[kristof]

Nope. The jets are correctly placed. Main jets have a screw based retainer holding them in place. Leak jets are firmly in place. So all is correct. I mentioned earlier that I have not made sure that passages in the carburetors are cleaned with compressed air after the problem surfaced. I will try to do it next. I will also put new spark plugs even though the ones I have in now were used only for 30 miles and look like new.

[moham]

So, what's the mysterious black substance? Old bowl gasket maybe? I'm trying to remember what else has a rubber gasket/o-ring in these carbs...main jets, float bowls...anything else?