Topic: CB550 Backfiring through carbs when trying to start

[Kenzo1979]

1974 CB550 stock carbs, stock air box, uni foam filter, stock ignition, stock stock. 

Ok, so my CB550K0 ran fine last year.  Over the winter I decided to get a brand new points plate complete with points and condensers (OEM TEC) and I cleaned the carbs.  They're the only changes.  I cleaned those freakin carbs more meticulously than ever before.  Reassembled using the existing hardware. 

Hooked everything up and BOOM --- Fireball in the air box!!!  WTF?  I thought to myself.  Never experienced this one before.  Can't even get it to start to check the timing with the strobe gun.  It's just constant pops out of the carbs.  Did some research and there is mention of excessive lean mixture as well as other things.

Checked the valve timing - spot on.  Points are gapped at about .35-.40mm Did the ignition timing statically-light comes on at F marks for both 1-4 and 2-3.  Checked for spark against the side of the engine with the plug -- all there.

Just trying to rule things out here:  Should I be going back through my carbs or is it possible these new points are somehow faulty even though my static timing with the test lamp seems to be accurate?

***Note:  My air mixture screws were set at 2 full turns out (usually they are 1 1/2 turns out)***  Could this 1/2 turn throw things off that much?

Another Note **I did the same exact thing to my 1971 CB500 this year and everything is fine with that. (Cleaned the carbs the same way and put a new oem (TEC) points system on.  Started right up and purred like a kitten).

Someone please point me in the right direction...Carbs or Timing?

[Airborne 82nd]

Sounds like your timing is off to me.

[fdbrat]

When you say uni filters in combination with stock air box,

You are running uni pods and a stock box?

Stock or pods?

Will it fire up with starter fluid?

How can you see a fireball if the stock air box is on??

[AJK]

Sounds like your timing is off to me.

+1. Check your ignition timing.

[Dimitri13]

Backfiring through the carb usually means retarded timing.

[Vinhead1957]

Did you accidentally swap wires on the point set

[Oldtech]

+ 1 for the swapped wires. This is the most likely cause. Seen it happen more than once.

[Kenzo1979]

I will definitely check the wiring.  I'm starting to think that is where the problem is, too.  Thanks.  Let you know what I find.


@fdbrat:  The air box is stock with a uni foam filter (a foam version of the stock paper filter - no rejecting - very popular alternative to original paper.)  First I only saw smoke, then I removed the filter and that is when I saw the fire in the box.

[Kenzo1979]

Although, I did go through and check the wiring while al of this was happening.  Here is what I have going on:  New points plate wiring from factory:  Blue to 1-4, Yellow to 2-3, that clips into respective Blue and Yellow down around my rear brake switch, from there it travels up to the coils where the Yellow connects to  yellow out of the coil on the Left side (controlling plug wires 2-3) and blue connects to coil on Right side (controlling plug wires 1-4).  Am i missing anything here?

[Kenzo1979]

Holy crap, I'm a dope...yup wires crossed!  Thx everyone.