TOPIC:

GatlinM wrote: ...Tube test checked out ok. Fuel filled level with the bowls....

Usually, fuel level should not reach the float bowl gasket, and is too high if level with top of the float bowl.

Specified fuel level is typically about 3 mm below the carb bowl gasket.

Would refer to the FSM for the fuel level spec for the particular model at hand.

Good Fortune!

Right, I should have stated more specific.

They were all at consistent levels, close to the top of the bowl. Guess it could explain some leaking at the seals, if it is too high.

Guess it's time to pull the carbs. Just confusing backfiring started after I cleaned up around the valve cover, and replaced the vacuum hose.

Fuel flow from the tank has been consistent.

GatlinM wrote: ...They were all at consistent levels, close to the top of the bowl. Guess it could explain some leaking at the seals, if it is too high....

And could also explain -- all by itself -- the issues reported in the original post.

Good Fortune!

Pulled the carbs out, bowls and jets are still clean from a rebuild last summer. Everything still firmly in place.

A screw is stripped on the throttle rod, so until I can get it tapped out, I'm not too worried about removing it to get to the needles. Visually they all seem fine.

I'm not convinced carbs are the issue, fuel flow to them is clear and clean. It's just perplexing that the power loss was not consistent and was remedied with the bike cooling off, jostling plug wires, and ultimately replacing the coils.

Valve clearance measured acceptable. Removing coils again, opening the cover, and clearing some junk from the vacuum hose simply gave the bike heavy backfire issues.

I'll have it back together soon and report in.

Maybe check the connections at the coils? I was having a similar issue. The wires were loose on one of the coils (the signal wires, not the plug wires) I had to crimp my female connectors down to make a solid connection with my teeth on the side of the road! It looked like I was kissing my bike, I'm sure. Works great now. Symptoms were just like you're describing. Just happened.
These little 550s are buzzy, and I think things work themselves loose more often if you're not careful.

Thanks D,
I'll doublecheck connections.

With the carbs back in, the same backfire is back.

I'll try to work on syncing and tuning carbs, as it's more prevalent.

Hard to replicate the powerloss without riding longer.

To clarify -- Is it still backfiring through the exhaust while under acceleration?

Would suspect an ignition problem being more likely than a carb problem.

The Dyna ignition coils have screws that attach the wires to the primary terminals.
Sometimes the screw heads are perilously close to grounding against the coil mounting spacers.
If not already done, would assure that the screw heads are insulated against shorting against the spacers.
Or could fit non-conductive spacers such as plastic or nylon.
I use a nylon tie-wrap as an insulator around the stock metal spacer.
And of course there are other ways to insulate the screw heads.











Good Fortune!

Thanks Patton,

Yes backfire out the exhaust.

Double checked and there seems to be adequate spacing on the coil mounts. I kept the original spacers.




Checked the vacuum cover again, as backfiring started after cleaning them up. There was some slight wetness and some white deposits.

Is the bike at hand fitted with a stock clean air system?





Good Fortune!

The bike has a straight hose between the vacuum covers. The air box has been plugged with a decent stopper.

Fit the carbs back on and cleaned plugs last week took care of backfiring. Think the plugs were just gapped wrong.

The bike ran fine again for a few days. Yesterday, I rode a block from the house, and the original engine bog down and throaty exhaust were back. I parked for a few minutes, and started it back up. I rode 30 minutes to a meetup I needed to make, then made it home fine at the end of the day.

Today, the problem was back, while on my way back home. I stopped for a moment, tried switching fuel line to prime, and choke, to see if it behaved differently. I throttled up in neutral, and after a few moments the bike returned to normal. I rode another few blocks, and it bogged down again, just before reaching home.

In the past, I've ran Seafoam through the fuel, I may give it a shot again, to help with some fuel peace of mind.

Only other recent change I can think of is changing oil/filter, months ago. I switched to a 20W-50, rather than 10W-40. The engine has run hot, and the change didn't really give me any noticeable difference.

Would double-check:
Fuel levels being 3 mm below bottom of carb body;
Air filter(s) clean and clear and unblocked;
Possible loose jets;
Crankcase oil diluted with gasoline;
Have battery load tested;
Assure ignition coils are receiving full battery voltage;
Remove and inspect spark plugs for too rich fuel mixture;
Condition of spark plug wires and plug caps.

Reported issues will likely be resolved by assuring strong correctly timed spark, correct fuel mixture, and acceptable compression.

Good Fortune!

Each carb float was leveling the same.

5 minute startup today, was normal.

All plugs were dry, with slight white (lean) dusting.

12 volts at battery. At the coils power was at 10 volts, though multiple readings were slightly diminishing, as I was leaving power on. Spark wires and caps are mostly brand new.

Air filter seemed normal, slight smell of fuel.

Went through plenty of wires, again, assuring connections, replaced all fuses, and started up. Bike ran for a minute, then same old problem.

Immediately checked plugs, and noticed cylinder 2 was wet. I went through checking spark and compression. I understand I have a cheap compression tester, and have read it may not be calibrated, so I wanted to try multiple readings. Manual cranking was getting tiring and inconsistent. I did running tests, and had consistent reads. I wasn't going to judge the running sound, but did notice between pulling plugs and checking cylinder 1 & 2, the engine sounded better. Putting it all back, it sat running fine for a couple of minutes.

Going to ride around the block and see what happens....