Topic: Sudden loss of power, backfiring.

[normalguy]

I took my 76 CB400F for a half-hour ride yesterday, parked it for fifteen minutes, then took off.  When I was getting onto the freeway, it suddenly had a lack of power.  I limped home on side streets.  It gives me low power, but it will sort of jump with a sudden split-second increase of power sporadically.  I was thinking of pulling the carbs this fall anyway (first time),  but does anyone have a general idea of where I should look first?

[750goes]

Are you running an inline filter - sounds like something is clogged up ? or some badly fouled plugs

 

[normalguy]

No inline filter...and I had just filled up the gas.

[750goes]

I would check the fuel flow from the tank and check the fuel itself for contaminants - like water......

[Bob Wessner]

Could be a sporadic ignition problem. Almost sounds like a cylinder of two is dropping out. Have you checked to see if you are getting good, regular sparking at all plugs?

[Uncle Ernie]

Yeah, sounds like fouled plugs to me too.  Why is another question.  Sounds like maybe a carb flooded while you were parked?

[oldbiker]

The last time that happened to me, I found that the central screw holding the points cam was loose so that sparking was intermittent.

[750goes]

If you just filled with gas - did it run dry or were you on RESERVE???

[normalguy]

I was not on reserve, but the bike had been parked for about a week.  I only wish this had waited another month so it would be a good fall project instead of an end-of-summer project.  Then again, it's always an ongoing project with these machines.  Zen and all that.  ;-)  I'll let you guys know what I find.  Thanks for pointing me in the right direction.  I'm going to chop the plugs then replace them (it's time anyway), and check the fuel for contaminants. There's a tiny amount of rust at the very top of the tank, looks very clean inside.

[volz1fsu]

Breaker points are a big cause of this, also have seen burnt valves on these engines quite a bit so basically it could be anything.  Start inspecting things and narrow down the problem.

[Jerry Rxman Griffin aka MuthaF'er]

Don't forget to check your battery voltage.

[Ernie]

check your connections at the coils my 550 did the same thing and that was the problem

[normalguy]

I'm still wondering why this would happen not immediately, but after the bike was warm, and then while cruising at freeway speeds.  Any theories?  This weekend I'm going to 1)drain the float bowls to check for contaminants, 2)clean the points and check the coil connections, 3)change the plugs.  Hopefully it can be fixed with everything on the bike.  Need to get it running well for ROCKERBOX.   

[OldSchool_IsCool]

Norm,

While you have the plugs out, unscrew the plug caps from the ignition wires and look for corrosion there.  If you see green, clean the cap screw then clip back the ignition wire until you strike clean copper.  Do this a little at a time (1/4 inch) and be careful not to clip so much that the wire is too short.

Also, check the resistance of ur plug caps.  If they all match, then they are probably all OK.  If they don't, then you may need to look up the resistance spec for your model & year and replace those that are out of spec.

I had power loss & starting problems when hot with my 550.  I changed the plugs, cleaned & gaped the points and clipped the corrosion off my wires.  One of those 3 fixed my problem.